Text File

Newsgroups: sci.space.news From: yee@atlas.arc.nasa.gov (Peter Yee) Subject: Altas Program Brochure (Forwarded) Message-ID: <1993Apr7.231156.6297@news.arc.nasa.gov> Organization: NASA Ames Research Center, Moffett Field, CA Date: Wed, 7 Apr 1993 23:11:56 GMT Lines: 1978

ATLAS PROGRAM BROCHURE TEXT TO GRAPHIC DESIGN

OCTOBER 16, 1992

TABLE OF CONTENTS

The Heritage of Environmental Inquiry

The ATLAS Program

Requisites for A Habitable Planet: Earth's Atmosphere and the Sun

Warmth and Protection for the Blue Planet: The Middle Atmosphere

Energy for the Blue Planet: The "Solar Constant"

The ATLAS Instruments

Mission Operations

Beyond ATLAS

Fact Sheets

This document was prepared under the auspices of the Marshall Space Flight Center Payload Projects Office by Essex Corporation.

Design: Brien O'Brien Illustration: Frank Kulczak, David Johnston, Brien O'Brien, and MSI, a Division of the Bionetics Corporation

THE HERITAGE OF ENVIRONMENTAL INQUIRY

Since its emergence, the human race has been fascinated by its environment, particularly the weather. Early peoples developed elaborate explanations for the natural phenomena they encountered, learning to predict the weather by the "signs" they came to recognize over years and years of observation. Furthermore, their abilities to react to long-term environmental changes preserved the species. When the glaciers of the ice ages approached, for example, they migrated toward the milder, equatorial latitudes. When the glaciers retreated, they moved into more temperate climes.

Today, we are still intrigued by our surroundings and seek to understand all there is to know about our world. Like our ancestors, we observe and measure, then theorize and test the theories. In doing so, we have refined many of the explanations they developed and have generated a more scientific framework from which to interpret the physical world. In modern times, we have even succeeded in some of our strategies to reverse human-induced changes that despoil our environment. We have levied controls on toxic emissions, for instance; we have reclaimed nearly lifeless waters around major population centers of the Great Lakes and cleaned up the Hudson River.

Our heritage of environmental focus has grown beyond its beginnings as a familial and tribal concern to a regional investment to today's multinational awareness. We acknowledge that humans now play a pivotal role in determining the intricate balance of environmental conditions that sustains life on this exquisite blue-and-white planet.

Today, peoples of every nation are expressing concern over the health of Planet Earth. As a result, a variety of scientific, economic, and political interests have formed alliances to develop a better understanding of the workings of our home planet. One such cooperative effort is the National Aeronautics and Space Administration's (NASA's) series of Spacelab missions called ATLAS, the Atmospheric Laboratory for Applications and Science. The ATLAS program investigates the relationships between Earth's atmosphere and the Sun.

THE ATLAS PROGRAM

The primary goal of the ATLAS program is to help characterize the chemical and physical components of the middle part of Earth's atmosphere, including the energies of sunlight that affect the middle atmosphere. Because no single space mission can provide enough information to accomplish this goal, a series of ATLAS missions, carrying a core of seven instruments, has been planned. With these instruments, an international team of scientists will gather data under a variety of atmospheric conditions over both the Northern and Southern Hemispheres. Some of the instruments focus primarily on atmospheric observations; others measure solar energy. Both sets of data are indispensable if we are to develop a comprehensive understanding of the middle atmosphere, and each set contributes to the interpretation of the other.

The ATLAS research projects have been selected primarily because of their abilities to make very sensitive measurements. The instruments chosen are calibrated before and after each mission so that the information they gather about the middle atmosphere and Sun will be accurate and precise.

The ability to recalibrate equipment regularly is an important part of the ATLAS concept. The longer an instrument operates in space, the greater is the chance that high-energy radiation or other harsh conditions can damage the hardware and skew the data gathered. As a Space Shuttle-Spacelab payload, the ATLAS experiment hardware is returned to Earth after each mission for recalibration, refurbishment, or refinement. This benefits not only the ATLAS program but also related space investigations, since the ATLAS Shuttle instruments have identical or similar counterparts on free-flying satellites. Scientists can use the precise ATLAS instrument measurements to refine the satellite data.

ATLAS and Spacelab

The ATLAS program depends on Spacelab, a set of modular hardware that is arranged to form laboratories carried into orbit in the Shuttle's payload bay. The ATLAS instruments themselves are installed on Spacelab's U-shaped pallets, which are directly exposed to space. A pressurized canister, the Spacelab igloo, along with nearby pumps and power boxes, provides the science equipment with power, communication links, and environmental control.

From the Shuttle's orbit high above the middle atmosphere, the ATLAS instruments will look down through the atmosphere to gather chemical and physical data. The orbiter's altitude also benefits those experiments that observe the Sun and measure solar output. To accomplish their science objectives, these instruments must operate above the relatively dense lower and middle atmospheric regions, which absorb many wavelengths of solar radiation.

As a Spacelab mission, each flight also offers the ATLAS investigators the opportunity to interact with their experiments from the ground. Before the mission, the teams develop lists of commands that can be issued by the Spacelab computer system at specific times. Controlled by these commands, the science instruments perform their operations automatically and according to schedule. If the scientists want to modify their instrument's operations, they can change the list of commands and transmit the changes from the ground control center to the Spacelab computer. They can also consult with the trained science crew in space to modify the preplanned instrument operations, having the crew either change the command lists or perform the operations manually. In the Spacelab concept, crewmembers in space are integral parts of the science team.

Mission Configuration

On each mission, six ATLAS instruments will be mounted on a Spacelab pallet in the bay of the Shuttle. A seventh instrument and its electronics, a co-manifested payload, will be located in two canisters attached to the payload bay wall. Because the pallet, igloo, and canisters occupy less than a third of the space available in the Shuttle bay, the ATLAS mission may share Shuttle resources and space with other payloads.

An International Endeavor

The ATLAS experiments are sponsored by space organizations and institutions around the globe, underscoring the international nature of atmospheric concerns. Each of the seven core investigations provides information that will complement and enhance the interpretation of data gathered by the other investigations. The data gleaned on ATLAS missions will also be available to the larger science community involved in related studies.

ATLAS and Mission to Planet Earth

The ATLAS series is a crucial component of NASA's Mission to Planet Earth, a comprehensive initiative to study the interactions of Earth's life forms, waters, atmosphere, and land masses, known collectively as the Earth System. Investigations on Mission to Planet Earth spacecraft make short- and long-term worldwide measurements of these global environmental components to determine how such systems interact to produce our environment and how they are affected by natural and human-induced changes.

ATLAS investigations aboard 7- to 10-day Shuttle flights augment Mission to Planet Earth's longer term, free-flying satellite missions. On the Shuttle, instruments collect complementary, highly calibrated information from which scientists can construct "snapshots" for "albums" of information -- data that provide a quick look at how conditions have changed from one mission to the next and that validate or complement satellite data. For instance, ATLAS 1 and 2 are underflights of the Upper Atmosphere Research Satellite and provide correlative measurements. These periodic checks on atmospheric conditions will also supplement both the early Mission to Planet Earth satellite flights, such as the Upper Atmosphere Research Satellite, and the later programs, such as the Earth Observing System.

Mission to Planet Earth is NASA's contribution to the United States Global Change Research Program, a unified study of the planet and its components, from its deepest interior to its outermost atmospheric regions. In turn, the Global Change Research Program is a cooperative element of the International Geosphere/Biosphere Program, one of the most comprehensive scientific undertakings of all time. This program focuses on the ocean-atmosphere-land-life system, studying its interactions on timescales from decades to centuries.

Coincident and Complementary Measurements

While the ATLAS instruments make atmospheric and solar measurements in support of their individual investigations, the data also enhance other ground- and space-based programs, particularly those using free-flying satellites. Coincident and complementary measurements are especially valuable in solar and atmospheric studies.

In coincident measurements, an ATLAS atmospheric instrument and an instrument that surveys the same parameter from another orbiting craft view the same air mass in the atmosphere at approximately the same time. These observations are especially important when comparing the calibrations of the two instruments. When analyzing data taken under these observing conditions, the scientists do not have to consider spatial, temporal, or temperature variations in the atmosphere. On the other hand, for the ATLAS solar instruments, it is critical only to make measurements of the Sun at the same time as the satellite instrument does, since atmospheric conditions do not affect space-based solar observations.

Complementary measurements are compiled to add to our total body of knowledge about the atmosphere and the Sun. For instance, if an ATLAS instrument records the concentration of a particular trace gas at sunrise and sunset, while an instrument in a polar orbit makes the same measurement at local noon and midnight, scientists can better establish the diurnal variation of that trace gas by combining the two sets of data. If a satellite instrument studies a few related chemical species and the ATLAS instruments observe additional ones, the combined data define the chemical processes better than either set of information by itself.

REQUISITES FOR A HABITABLE PLANET: EARTH'S ATMOSPHERE AND THE SUN

As far as we know, Earth is the only planet in our solar system that supports life. The atmosphere, in combination with the Sun's energy and the magnetic fields emanating from our planet, creates the conditions making life on Earth possible. By warming the planet and transporting water and heat, the atmosphere moderates Earth's climate -- its long-term and prevailing meteorological conditions, such as temperature, wind patterns, and precipitation.

Our climate is driven both by the solar energy reaching the planet and by the Earth System's responses to the infusions of energy. These combined influences produce global temperatures that are sufficiently warm to incubate and sustain plant and animal life. It is theorized that small, though significant, long-term changes in either solar output or atmospheric characteristics might produce noticeable changes in Earth's climate. For example, changes in the Sun's output would be reflected in the response of the Earth System and could be manifested as changes in mean global temperature and as disturbances in the water cycle. It is far from certain, however, what the magnitude of these changes might be and even less certain how they might affect life on Earth.

Is there evidence of solar or atmospheric change? While further investigations are needed, measurements of solar energy indicate that it does indeed fluctuate in both amount of energy and wavelength distribution. Moreover, there is overwhelming evidence that humans exert significant influence on the atmosphere through the actions of our sizeable population. One impact of our activity can be traced to the beginning of the Industrial Revolution. As the industrialized nations developed, burning more fossil fuels and clearing land for industrial and agricultural expansion, the atmospheric concentrations of certain important trace species -- particularly carbon dioxide, one of the compounds that is effective in trapping the heat reradiated by Earth -- began to increase dramatically.

Although we have no influence over changes in solar irradiance (the Sun's energy received by the planet), we can control our actions and choices that affect the atmosphere and our climate. To do this responsibly, we must first understand the cycles of the Sun and the role sunlight plays in atmospheric processes. How much energy does the Sun pour upon our protective atmospheric layers? Does the amount of solar energy reaching Earth's upper atmosphere change? If so, what portions of the radiation change and by how much? Where in the atmosphere are different wavelengths of radiation absorbed? How do solar variations affect chemical reactions in the atmosphere?

Next, we must be knowledgeable about the intrinsic characteristics and behavior of the atmosphere under various solar conditions. What trace gases are present in Earth's atmosphere? Where do they originate? How are they distributed in the atmosphere and in what concentrations? How are the chemicals transported upward, downward, or over the globe? How do they interact with one another?

NASA's ATLAS program seeks to provide answers to these questions to help distinguish human-caused changes from natural variations. With this knowledge, we will be able to make more responsible decisions about our use of resources and our strategies for protecting the atmosphere.

The Greenhouse Effect

Planet Earth moves through space within its own protective mantle: the atmosphere. This mantle contains gases and clouds that absorb or reflect certain portions of sunlight. Other solar wavelengths penetrate the atmosphere and reach Earth's surface. The warm Earth radiates energy back toward space, but clouds and atmospheric gases can prevent some of these energies from escaping. The balance of absorbed sunlight and trapped radiation determines Earth's temperatures.

The gases most involved in the planet's radiation balance are called "greenhouse" gases. The primary ones are trace gases -- water vapor, carbon dioxide, methane, and nitrous oxide. Like the glass panes in a greenhouse, these gases let sunlight through to Earth's surface, yet prevent heat from escaping to space. This natural warming is called the greenhouse effect, and it makes life on Earth possible.

Historical records show that the concentrations of greenhouse gases -- particularly those of carbon dioxide -- fluctuate naturally and moderate Earth's temperature accordingly. When greenhouse gas concentrations drop, the planet experiences a cooling period; when these gases increase in concentration, Earth's temperatures rise. Because of complicating factors, such as changes in cloud cover, this possible cause-and-effect relationship is not yet completely understood.

In recent times, human populations have grown exponentially and, with them, the basic needs of an increasing population: more food, more potable water, more shelter. One of the impacts of our efforts to feed, clothe, house, and transport our growing numbers is that we increase emissions of the greenhouse gases into the air, thus affecting the chemical balance of the atmosphere.

Using atmospheric models and historical temperature records, scientists are trying to determine whether these growing concentrations of gases will enhance the natural greenhouse effect, changing our climate and global temperatures. They are asking whether Earth's mean temperature is changing, and if it is, they want to know how rapidly it is rising or falling. What they discover in their investigations may have a far-reaching impact on our lives.

Consider what warmer or cooler mean temperatures could mean to plant and animal populations. We might see one result along our coastlines. Sea levels could rise if polar ice caps melted, or the levels could fall if the glacial ice caps grew. How would possible changes in sea level affect the economy of people who live along today's coastlines? Would the coastal cities be inundated, or would their communities be landlocked? Other consequences of a mean global temperature change might be evident in vegetation patterns. Would plant populations be able to migrate toward a suitable environment? Would farmers have to grow different crops or adopt other agricultural practices?

By characterizing temperatures and trace gas concentrations in the middle atmosphere, the ATLAS missions will contribute to our understanding of the natural greenhouse effect. With its measurements of solar energy, the ATLAS series will help us anticipate possible climatic changes and distinguish the changes that might be related to varying solar output.

WARMTH AND PROTECTION FOR THE BLUE PLANET: THE MIDDLE ATMOSPHERE

Characterizing processes in the middle atmosphere is the major focus of the ATLAS science program. In the middle atmosphere, the trace gas ozone, a form of oxygen made up of three oxygen atoms, shields life on Earth from the Sun's ultraviolet radiation. Scientists believe that certain chemicals introduced into the atmosphere by human activity are harming that protective ozone shield.

Stratospheric Ozone

Stratospheric ozone absorbs and scatters ultraviolet radiation from the Sun and, in doing so, performs two important functions. First, the interaction of ozone and ultraviolet radiation heats the middle atmosphere and contributes to the thermal balance of the planet. This profoundly influences atmospheric circulation and, hence, Earth's climate. Second, the ozone layer protects life by preventing ultraviolet radiation, which is lethal or damaging to plants, humans, and other animals, from reaching the planet's surface.

Stratospheric ozone is an extremely effective ultraviolet filter, absorbing nearly all of the solar radiation reaching Earth's atmosphere in the wavelength range between 210 and 340 nanometers. Yet, if all the ozone molecules located mainly within the 40-kilometer (km) [25-mile (mi)] expanse of the stratosphere were under the pressure and temperature conditions of Earth's surface, they would form a layer only 2 to 5 millimeters (0.08 to 0.2 inches) thick.

A part of the ATLAS mandate is to study the photochemistry of ozone and changes in global ozone levels. Since 1985, scientists have noted that ozone concentrations over the Antarctic have decreased significantly during spring in the Southern Hemisphere (September, October, and November) when light from the Sun begins to reach the polar regions. The cold conditions, sunlight, and atmospheric chemicals present over the South Pole combine to destroy large numbers of ozone molecules, thus permitting more ultraviolet radiation to penetrate through the lower atmosphere to Earth's surface.

Much research has been directed toward determining the risks that ultraviolet radiation poses to living creatures, but the exact effects are not yet known. Of particular concern is the danger to oceanic plankton in the southern seas; because these tiny plants and animals form the basis of many food webs, they are important components of the ecosystem. Increased exposure to ultraviolet radiation can also reduce crop productivity, and health threats to humans include increased numbers of skin cancers and cataracts.

The photochemistry of ozone is extremely complex. In the daylit atmosphere, solar ultraviolet light splits apart the three-oxygen ozone molecule, freeing a single oxygen atom and a two-oxygen molecule to react with other trace gases. Ultraviolet radiation also splits the oxygen molecule to produce two oxygen atoms. Simultaneously, some of the oxygen atoms and molecules recombine to produce ozone. The net result of this cycle of ozone destruction and production -- the number of ozone molecules -- determines how much ultraviolet light is blocked in the stratosphere.

Some chemical reactions in the atmosphere, particularly those involving nitrogen and its compounds (nitric oxide and nitrogen dioxide, for example) and chlorine and its compounds, especially chlorine monoxide, not only destroy ozone but are also catalytic. One chlorine atom, for instance, may cause the destruction of thousands of ozone molecules before it is incorporated into a compound that does not react with ozone.

In addition, water vapor plays an important role in ozone chemistry in the stratosphere. Ultraviolet radiation dissociates the molecules of water vapor, forming hydrogen and hydroxyl radicals. (Hydroxyl radicals are highly reactive molecules consisting of one oxygen and one hydrogen atom.) Both of these chemically active species can react rapidly with ozone or produce other compounds that help destroy ozone molecules.

Further complicating ozone chemistry are the reactions between different families of compounds, such as nitrogen and chlorine species. In one case, for example, nitrogen dioxide will combine with chlorine monoxide to produce the relatively stable molecule chlorine nitrate, temporarily removing both the reactive nitrogen and chlorine compounds from the ozone-destroying cycles.

Sources of Atmospheric Chemicals

How do the trace chemicals that are so involved in ozone photochemistry enter the atmosphere? Many are transported from Earth's surface. The specific chemicals and their concentrations can be changed both by unpredictable natural occurrences and by the activities of daily human life (anthropogenic sources). Wind patterns, for instance, carry sea salt and dust into the atmosphere; volcanic eruptions spew sulfur, ash, and carbon dioxide and other chemicals into the middle atmosphere; and thunderstorms move water into the stratosphere. Among the anthropogenic sources of atmospheric trace gases, chlorofluorocarbons are of particular concern. Chlorofluorocarbons are used as refrigerants, foam-blowing agents, cleaners, and aerosol propellants. These compounds are stable at Earth's surface, but when they are carried into the middle atmosphere, sunlight breaks the molecules apart, freeing chlorine to destroy ozone.

Sunlight and the Middle Atmosphere

Many gases filter solar radiation as it passes through the atmosphere, absorbing or reflecting particular wavelengths. The components of sunlight that penetrate the upper regions to reach the mesosphere and stratosphere fuel the chemistry of the middle atmosphere, with the absorption of ultraviolet radiation by ozone being the primary driver. When the amounts of these radiant wavelengths vary, the chemical reactions in the atmosphere reflect the changes. Furthermore, if changes in the middle atmosphere are observed, it is important to understand which changes might be related to variations in the amount of ultraviolet sunlight.

Characterizing the Middle Atmosphere

To improve our understanding of how the atmosphere responds to chemical and solar variations, we must develop an extensive and thorough compilation of atmospheric and solar measurements. The suite of ATLAS instruments will investigate the chemistry of the middle atmosphere, studying atmospheric constituents and quantifying the solar radiation that drives middle atmospheric chemistry.

The ATLAS investigations, along with ground-based measurements and others made by instruments aboard free-flying satellites, aircraft, balloons, and rockets, will provide a compendium of data that will improve our mathematical models of the middle atmosphere and its relationships with solar energy. These models are important tools that atmospheric scientists use to test and refine their theories of how Earth's atmosphere functions; they are also useful for predicting future changes.

Conditions and dynamics of the middle atmosphere are of primary interest to ATLAS scientists, but what is the middle atmosphere?

Earth's atmosphere is not uniform. It has several layers, four of which are distinguished by thermal characteristics, chemical composition, movement, and density. The layer in contact with Earth's surface is the troposphere, which extends to altitudes ranging from 8 to 15 km (5 to 9 mi). It is the most dense of the layers, and its temperatures drop with increasing altitude. Essentially all weather occurs within this region.

Extending from just above the troposphere to 50 km (31 mi) is the relatively dry, less dense stratosphere, characterized by temperatures that gradually increase with altitude. The stratosphere contains the thin, but crucial, layer of ozone, which absorbs and scatters solar ultraviolet radiation, the major source of stratospheric heating.

The next layer is the mesosphere [50 to 85 km (31 to 53 mi)]. In the mesosphere, temperatures again decrease with altitude. The region is also characterized by an abundance of chemicals that exist in excited states. In an excited state, atmospheric components contain more energy than in the conventional state; they act as energy reservoirs and play a major role in mesospheric chemistry. Atmospheric scientists refer to the stratosphere and mesosphere as the middle atmosphere.

The fourth layer is the thermosphere, extending to about 600 km (372 mi). Here, temperatures soar as the Sun's energy ionizes atoms and molecules. Chemical reactions in this region also occur at a much faster rate than they do in the cooler layers closer to Earth.

The transition zones between each of these layers is called the pause. The border between the mesosphere and the thermosphere, for example, is the mesopause.

Nitrogen, oxygen, water, and a myriad of other elements and compounds make up the atmosphere. Nitrogen and oxygen constitute the bulk of the atmospheric ingredients, approximately 79 and 20 percent, respectively; yet, the other, less abundant components are far more influential than their concentrations might indicate. These trace elements often play very important roles in atmospheric chemistry. The ATLAS science teams are seeking to quantify and study specific trace elements and their roles.

There are two other major atmospheric divisions: the ionosphere and the exosphere. The ionosphere is characterized by the presence of charged gases, or plasmas. Although the ionosphere is associated with and embedded in the thermosphere, plasmas move closer to Earth or farther away, depending on the time of day and magnetic conditions, and it is not uncommon for plasmas to cross borders of other atmospheric regions. The exosphere is the outermost layer of the atmosphere. It begins at the top of the thermosphere and extends until it merges with interplanetary gases. Hydrogen and helium predominate here, but their densities are very low, accounting for the extreme vacuum conditions of the region.

Atmospheric Models

Once the ATLAS experiment teams complete their measurements and data analysis, scientists from around the world will have access to the results of the ATLAS observations, such as the solar flux in a given wavelength region or the three- dimensional distribution of ozone in the stratosphere. For these data to be most useful, scientists usually interpret the information in the context of numerical models, which allow the researchers to test their understanding of atmospheric processes and to make predictions about the atmosphere. Models can calculate any number of occurrences, such as the expected change in stratospheric ozone and temperature in response to increases in chlorofluorocarbons, small variations in solar ultraviolet output, and large volcanic eruptions, which inject massive quantities of sulfur dioxide into the stratosphere. Such models are especially important because their results can be used to understand the atmospheric significance of international agreements, such as the Montreal Protocol, which regulate the production and emission of chlorofluorocarbons and their substitutes.

Consider the case in which certain ATLAS instruments simultaneously measure the concentrations of several trace species in a given region of the atmosphere. How can scientists tell whether all these data are consistent with each other and with current knowledge of the chemical and meteorological processes in the atmosphere? They construct a numerical model that incorporates the known processes, such as chemical reactions, solar-driven dissociation of molecules, atmospheric heating and cooling, and meteorological events associated with winds. Each model is very complex and requires large, fast computers. Using both the model and data, scientists can search for inconsistencies that might identify errors in one of the measured quantities of the trace species or in their understanding of the rates of one or more of the processes assumed to affect that molecule's concentration.

If the future state of the atmosphere is to be modeled, an accurate understanding of atmospheric processes is necessary. Models are validated by real measurements of the atmosphere, and the space-based measurements from the ATLAS missions will be an important part of this validation process.

ENERGY FOR THE BLUE PLANET: THE "SOLAR CONSTANT"

Sunlight provides the energy for many atmospheric processes; yet, the Sun's radiant output -- in terms of its total energy as well as its distribution of wavelengths -- fluctuates, from a maximum to a minimum and back again over an 11-year cycle. Within this 11-year cycle are embedded the short-term variations of the 27-day solar rotation period. Earth's atmosphere reflects both cycles, and one of the goals of the ATLAS program is to characterize these influences and the related atmospheric responses.

The Sun's energy arrives at the top of Earth's atmosphere with a broad wavelength distribution that includes gamma rays, X-rays, ultraviolet radiation, visible light (where most of the energy is contained), infrared radiation, microwaves, and radio waves. For years, the total energy in this solar irradiance was thought to be unchanging; thus, it was named the "solar constant." As more sophisticated and sensitive equipment measured the solar constant with greater precision, it became apparent that the term was a misnomer: solar energy does indeed fluctuate.

From our vantage point in the solar system, we receive visual evidence of the solar activities that influence total solar irradiance. Dark sunspots appear, move across the face of our star as it rotates, and disappear; tremendous and fascinating flares erupt from its surface. Historical records indicate that these phenomena occur in 11-year cycles. Recent measurements further indicate that the solar constant also may vary by about 0.1 percent over these cycles.

While a 0.1-percent variation may seem inconsequential, such a small change over a long period may have great climatological significance. For example, the general cooling of the planet in the 400 years spanning the 15th and 19th centuries --a period known as the Little Ice Age -- is thought to have been caused by a 1 percent or less variation in total solar irradiance. It is, therefore, important to measure the total solar irradiance and its variations as precisely as possible so that we can identify evidence of long-term change and can anticipate the effects of these changes on Earth's climate. Before the ATLAS missions, the best estimate of the total solar irradiance was 1,367 watts per square meter; however, the precision of the measurements producing this estimate was not sufficient to predict climatological responses reliably.

Two ATLAS core instruments will measure total solar irradiance. Using slightly different techniques, these highly calibrated instruments will gather data at selected times during each mission to identify short-term irradiance changes. Even though the ATLAS measurements will not be made over a long enough time to identify long-term changes, these readings will serve as a point of comparison for future solar irradiance measurements.

Identical or similar instruments on satellites are also measuring the solar constant at the same time as the ATLAS instruments. By comparing the irradiance values determined by these instruments with ATLAS values, scientists will be able to obtain more accurate and precise measurements of the solar constant over the longer periods in which the satellites are active.

Electromagnetic Spectrum

To probe the mysteries of the atmosphere and the Sun, the ATLAS experiments rely on the information contained in radiant energy. Each instrument is designed either to record specific ranges of energy that reach its sensors or to quantify the total amount of that energy.

The entire range of radiant energy -- radio waves, microwaves, infrared radiation, visible light, ultraviolet light, X-rays, and gamma rays -- is called the electromagnetic spectrum. The human eye is sensitive to the visible light portion of the spectrum. When visible light passes through a prism, we can discern its many component colors -- red, orange, yellow, green, blue, indigo, and violet. Each of these colors represents a specific segment of the visible light energy range, and our eye- brain connection interprets the different segments as different colors. In a similar fashion, the ATLAS instruments interpret some of the electromagnetic spectrum energies produced by the Sun -- particularly the ultraviolet, visible, infrared, and microwave ranges. The instruments distinguish the various "colors" of these spectral ranges and thus identify the chemical species producing (or absorbing) the energy.

Radiation enters each instrument in the form of photons, or packets of energy. Photons are described in terms of their wavelengths, which are directly related to their energies. For example, some radio photons move in waves that crest at distances of hundreds of thousands of kilometers. These low-frequency photons contain less energy than higher frequency photons, such as ultraviolet photons that crest as often as every 100 millionth of a meter.

The measuring nomenclature of the electromagnetic spectrum is based on photon wavelength. The radiant wavelengths of most interest to ATLAS investigators are those that range from the lower energy infrared, microwave, and visible to the higher energy ultraviolet. These ranges, in order of decreasing wavelength, are measured in millimeters [(mm), thousandths of meters], micrometers [(5m), millionths of meters], and nanometers [(nm), billionths of meters].

THE ATLAS INSTRUMENTS

Seven instruments form the heart of the ATLAS program. Five of these --ATMOS, MAS, SSBUV, SOLSPEC, and SUSIM -- gather data critical to our understanding of the middle atmosphere; the other two -- ACRIM and SOLCON --measure total solar irradiance; their readings are complemented by the spectral measurement of SOLSPEC. Fact sheets located in the folder on the back cover of this publication describe each experiment and its specific mission objectives in more detail.

Chemical Constituents of the Middle Atmosphere

Three investigations determine the concentrations and distribution of trace gases by measuring either the radiation these gases emit or the wavelengths they absorb or scatter.

The Atmospheric Trace Molecule Spectroscopy (ATMOS) and Millimeter- wave Atmospheric Sounder (MAS) instruments primarily study the composition of the middle atmosphere. Their findings will be used to characterize global, seasonal, and long-term changes in the concentrations of trace atmospheric species, particularly the chlorofluorocarbons and their resultant chlorine breakdown products and other chemicals involved in the photochemistry of ozone. Data from this research will help validate models of stratospheric chemistry.

As the Shuttle's orbit carries the spacecraft into or out of Earth's shadow (orbital night), the ATMOS instrument views the Sun as it sets or rises through the atmosphere. The spectrometer measures changes in the infrared component of sunlight as the Sun's rays pass through the atmosphere. Because trace gases absorb very specific infrared wavelengths, the science team can determine what species are present, in what concentrations, and at what altitudes by identifying the wavelengths that are "missing" from their spectral data.

Some species or reactions within Earth's atmosphere also emit characteristic wavelengths of radiation. The MAS instrument measures trace chemical concentrations using a parabolic antenna to receive millimeter-wave emissions from atmospheric gases. The information the MAS instrument gathers from these emissions will allow the determination of middle atmospheric temperatures and pressures and of the concentrations and distribution of chlorine monoxide, water vapor, and ozone.

The primary objective of the Shuttle Solar Backscatter Ultraviolet (SSBUV) experiment is to measure stratospheric ozone concentrations. It does so by measuring the ultraviolet radiation scattered by the ozone layer and comparing that radiation with other readings it makes of the ultraviolet spectrum coming directly from the Sun. The SSBUV instrument is virtually identical to Earth-observing satellite instruments that monitor ozone globally and will make coincident measurements with them. Because SSBUV flies on the Shuttle, it can be rigorously calibrated after each flight and before the next to maintain its high precision; a built-in system also allows inflight calibration. From the data gathered by this highly calibrated instrument, scientists can adjust for any drifts in data received from the satellite instruments.

Solar Radiation and the Middle Atmosphere

Two core investigations study specific wavelengths of sunlight, particularly the ultraviolet energies, which are the primary energy source for chemical reactions in Earth's middle atmosphere. Their measurements are also compared with the SSBUV ultraviolet readings.

The Solar Spectrum Measurement from 180 to 3,200 Nanometers (SOLSPEC) experiment collects solar ultraviolet, visible, and infrared radiation to determine how solar energy is distributed at these wavelengths. Data from this study will also help scientists understand how the energy distribution varies and help identify and quantify the connections between variations in solar energy and atmospheric changes. By comparing SOLSPEC data from one flight to another, these solar spectral ranges can be monitored. Since SOLSPEC's spectral range covers most of the Sun's energy output, its data can also be used in combination with ACRIM and SOLCON to study the solar constant.

The Solar Ultraviolet Spectral Irradiance Monitor (SUSIM) instrument team studies solar energy variations in the ultraviolet spectrum. It is important to know the absolute intensity of this wavelength range, since the ultraviolet light flux varies widely, affecting middle atmospheric chemistry. Of particular interest are the shorter ultraviolet wavelengths, which exhibit the greatest variation over a solar cycle. The SUSIM also undergoes a rigorous calibration program before, during, and after each flight, enabling the science team to assess changes in instrument accuracy. Data from the ATLAS SUSIM will be compared to data gathered by identical and similar satellite instruments: these comparisons can improve the quality of the longer term measurements since the satellite instruments are not able to undergo rigorous postflight calibration tests.

Total Solar Irradiance Measurements

Two other investigations seek to refine the value of the solar constant by measuring total solar irradiance, an important factor in the study of Earth's climate. Data from these investigations will also help establish the total solar radiation scale for the International System of Units.

The ATLAS Active Cavity Radiometer Irradiance Monitor (ACRIM) is part of an ongoing effort to measure the total solar irradiance. The ACRIM provides measurements of the energy contained in the ultraviolet through infrared spectral ranges with a high degree of accuracy and precision. Its ATLAS readings are combined with those from identical instruments on satellites to compile a database of total solar irradiance measurements. To determine the total solar irradiance, the ACRIM measures the power required to maintain a specific temperature difference between two cavities when one is open and exposed to sunlight and when both are closed. The ACRIM is recalibrated after every mission to provide a benchmark of accuracy for satellite data.

The Measurement of Solar Constant (SOLCON) instrument is also designed to improve the accuracy of measurements of total solar irradiance and to identify long-term variations in the value. In contrast to the ACRIM, the SOLCON instrument measures solar irradiance by determining the difference in power required to bring two cavities into thermal balance when one is open to the Sun and the other is closed. To ensure the accuracy of the instrument, the science team characterizes it precisely in a laboratory setting. They also compare the instrument to a set of ground-based models and other absolute radiometers before and after each flight.

MISSION OPERATIONS

The global relevance of the ATLAS series is reflected in the ongoing efforts of the participating international space agencies to focus the expertise of their science, engineering, and management communities on characterizing the middle atmosphere. The ATLAS missions themselves have evolved through years of careful planning, coordination, and preparation.

The ATLAS program is sponsored by NASA's Office of Space Science and Applications, directed by the Earth Science and Applications Division, and implemented by the Flight Systems Division, all located at NASA Headquarters in Washington, D.C. Here, the program managers and program scientist define the overall science goals, select experiments, and budget funds for instrument teams, hardware development, payload integration, mission operations and planning, and the publication of scientific results from the U.S. instruments.

The management of each ATLAS mission is the responsibility of the Marshall Space Flight Center in Huntsville, Alabama. Working closely with the program offices, the mission manager directs a civil service and contractor team effort to match science objectives, instrument requirements and capabilities, and Shuttle-Spacelab resources so that each flight is engineered to gather the maximum amount of science information. This effort also includes the preparation of a minute-by-minute schedule, called a timeline, that combines crew activities, experiment requirements, Spacelab resources, and Shuttle maneuvers into an efficient operating plan. The mission manager and team work with other NASA centers that prepare the Shuttle and Spacelab for launch and landing, conduct flight operations, and collect, process, and distribute data. The team also ensures that each crew is well trained in the mission's science objectives and hardware operations.

The ATLAS science community, too, has certain mission planning responsibilities. The principal investigators of the experiments form an Investigator Working Group that meets regularly before the mission to advise the mission manager's team on science- related issues and payload operations. The working group is chaired by the mission scientist, a member of the mission manager's team. During the mission, the working group becomes the Science Operations Planning Group, meeting twice a day to evaluate science activities, solve problems, and recommend ways to take full advantage of any unplanned opportunities.

After the investigator teams have qualified their instruments for flight with comprehensive tests ensuring that they can survive and operate in the space environment, each team sends its experiment hardware to the Kennedy Space Center in Florida for installation on the Spacelab pallet. Once integrated, the instruments are again tested as a payload to ascertain their readiness for flight. The payload is then fitted into the orbiter.

By launch day, all the elements for a successful mission are in place, awaiting the flight. The ATLAS missions launch from Kennedy Space Center. Shortly after the Shuttle reaches its orbiting altitude, the payload bay doors are opened, and the crew activates the Spacelab systems and makes sure each is operating properly. Next, the master timeline of science commands is loaded into the computers, and the ATLAS instruments begin operations. The computer timeline and the minute-by-minute mission schedule can both be revised, if desired, to respond to additional science opportunities or to any problems encountered.

Every ATLAS flight crew is divided into two teams, each of which works a 12-hour shift so that science operations can continue around the clock. At least one member of each team will have special training in both Spacelab and experiment operations and will oversee science activities on the shift. Most of the ATLAS instruments operate automatically, commanded by the Spacelab computers; however, the crewmembers can use keyboards to enter observation sequences if necessary. Another crewmember on each team is part of the orbiter crew and is responsible for maneuvering the Shuttle when an instrument requires precise pointing or must be operated in a specific attitude.

During the mission, the management and science teams maintain a 24-hour vigil in the Spacelab Mission Operations Control/Payload Operations Control Center at Marshall Space Flight Center. This facility contains banks of computers, monitors, and communication consoles that keep the people on the ground and in space in contact with one another to monitor the payload, collect data, solve problems, and reschedule science activities as necessary. The mission management team at Marshall is also in contact with personnel at Johnson Space Center's Mission Control Center in Houston, Texas, who are responsible for the orbiter operations and the flight itself.

The ATLAS instruments will gather data from about 4 hours after launch until about 12 hours before landing. At this time, the crew will deactivate the science hardware and Spacelab, and the payload bay doors will close for reentry.

After landing, three other important phases of the ATLAS program begin: flight data processing, concentrated data analysis, and preparation for the next mission. The Goddard Space Flight Center is responsible for distributing the flight data to the scientists, who further process the data and start their analyses in earnest. A few months later, the investigators are ready to begin reporting their findings, publishing their results in journals and magazines, presenting papers at science and technology symposia and workshops, and releasing their information through database networks. The body of knowledge gained through their ATLAS efforts is thus shared with investigators worldwide. The teams also inspect and recalibrate their instruments to ensure that the hardware is accurate, in good operating condition, and ready for the next flight. If the instrument must be repaired or updated, it can be refurbished and reintegrated quickly.

BEYOND ATLAS

Throughout history, people have placed great importance on changes in the environment, making significant investments of physical, mental, and spiritual energies to adapt to these changes and to determine their causes and effects. Unlike our ancestors, we have the benefit of hundreds of years of record keeping that points out long-term trends in our environment. Today, we can explore within the atmosphere itself to characterize the intricate tangle of chemicals and sunlight that influences and determines our environment. We now occupy an historically unique position: we are one of the controlling factors in the evolution of our atmosphere. To understand our role in environmental change, we must know how the Sun, the atmosphere, and the rest of the Earth System are interwoven.

The ATLAS program, with its emphases on Earth's middle atmosphere and the Sun's influence on climate, is but one of the multidisciplinary endeavors to conduct systematic and continuing studies of our environment. The ATLAS missions are laying the groundwork from which an accurate description of the atmosphere and the Sun's influence on it can evolve. As our understanding of the Earth System becomes more refined and detailed, we become better prepared to carry out our responsibilities as guardians of Planet Earth.

ATLAS 2 MISSION

ATLAS 2, the second of the Atmospheric Laboratory for Applications and Science missions, is scheduled for launch in 1993. This mission characterizes the chemical and physical components of Earth's middle atmosphere and the solar energy injected into the atmosphere, studies that began on ATLAS 1. Like its predecessor, ATLAS 2 is an integral part of the Spacelab contribution to NASA's Mission to Planet Earth.

Seven instruments comprise the core payload. The Atmospheric Trace Molecule Spectroscopy (ATMOS) and Millimeter-wave Atmospheric Sounder (MAS) study atmospheric constituents. The Solar Spectrum Measurement from 180 to 3,200 Nanometers (SOLSPEC) and the Solar Ultraviolet Spectral Irradiance Monitor (SUSIM) characterize the solar radiation that drives chemical reactions in the middle atmosphere. The Shuttle Solar Backscatter Ultraviolet (SSBUV)* experiment measures both solar ultraviolet output and stratospheric ozone. Two other instruments -- the Active Cavity Radiometer Irradiance Monitor (ACRIM) and the Measurement of Solar Constant (SOLCON) -- measure the total solar irradiance. The objectives of each ATLAS 2 experiment program and its instrument complement are described in detail in accompanying fact sheets.

Scientists from six nations are participating in the ATLAS 2 mission, underscoring the worldwide importance of atmospheric and solar research. In addition to the United States, investigators represent Belgium, Germany, France, The Netherlands, and Switzerland.

MISSION FACTS

Flight number: STS-56 Launch: 1993 Launch site: Kennedy Space Center Prime landing site: Kennedy Space Center Shuttle altitude: 296 km (160 n. mi.) Orbital inclination: 57 degrees to the equator Shuttle attitudes: payload bay toward Sun payload bay toward Earth Number of crew members: 5 Mission duration: 8 days Payload operations: 24 hours/day Configuration:igloo, 1 pallet, 2 Get-Away-Special canisters

The mission will fly while the Upper Atmosphere Research Satellite is also gathering atmospheric and solar data. The orbits of the two spacecraft will allow instruments to make a number of independent measurements of the same regions of the atmosphere at the same time. Data gathered during these opportunities will be compared to ascertain satellite instrument calibration.

*SSBUV is co-manifested with ATLAS 2.

Unique Science Opportunities

The ATLAS 2 mission is scheduled for a night launch in the Northern Hemisphere's spring. The Shuttle's path will then carry the payload over Earth's Northern Hemisphere during darkness on the ascending part of the orbit. This orbit will allow scientists to study daily variations in the chemistry of the middle atmosphere over high northern latitudes. (ATLAS 1 flew over the high southern latitudes at night and over the high northern latitudes during daylight, detailing those atmospheric conditions.) ATLAS 2 measurements are particularly crucial in light of recent data from the Upper Atmosphere Research Satellite and the Second Airborne Arctic Stratospheric Expedition, which indicate unprecedented levels of chlorine monoxide at high northern latitudes during the winter of 1991-1992. Given these high concentrations, atmospheric models suggest that, under appropriate meteorological conditions, significant ozone depletion over the Arctic is possible.

Many important chemicals involved in the photochemistry of ozone exhibit strong diurnal variations. For example, in sunlight, chlorine reservoir compounds break down and release chlorine, which destroys ozone; at night, the concentrations of the reservoir species increase, reducing the amount of chlorine available for chemical reactions. The best time to measure the effects of the nightly buildup is at sunrise; so, for scientists to measure the nighttime chemical concentrations over the Northern Hemisphere, the Shuttle must be entering orbital sunrise from a high northern latitude. The ATLAS 2 orbital path will permit measurement of these and other key species in the atmosphere over northern regions, and what is learned from measurements of the buildup of these concentrations will improve our understanding of ozone chemistry and our ability to test models of atmospheric behavior.

Science Operations Plan

The ATLAS 2 science operations plan calls for periods of atmospheric data gathering interspersed with orbits dedicated to solar observations. During the orbits designated for middle atmospheric measurements, the Shuttle will fly with its payload bay toward Earth, and the atmospheric instruments will operate almost continuously. ATMOS will take solar radiation absorption readings during each orbital sunrise and sunset, MAS will measure microwave emissions from Earth's limb throughout each orbit, and SSBUV will make its measurements of backscattered ultraviolet radiation in the daylight portions of these orbits. The ATMOS and MAS instruments will be inactive during solar observation periods.

For solar observations, the Shuttle's payload bay will point toward the Sun in the daylit portion of each orbit. At these times, ACRIM and SOLCON will measure total solar irradiance, SUSIM and SOLSPEC will make solar spectral measurements, and SSBUV will gather its data on solar ultraviolet radiation.

MISSION MANAGEMENT TEAM

Mission Manager: Program Manager: Ms. Teresa Vanhooser Mr. Earl Montoya Marshall Space Flight Center NASA HQ

Assistant Mission Manager Mr. Gerald Maxwell Marshall Space Flight Center

Mission Scientist: Program Scientist: Dr. Timothy Miller Dr. Jack Kaye Marshall Space Flight Center NASA HQ

Assistant Mission Scientist: Dr. Steve Smith Marshall Space Flight Center

Chief Engineer: Ms. Angie Jackman Marshall Space Flight Center

ATLAS 1: AN ENCOUNTER WITH PLANET EARTH

In March of 1992, the Shuttle Atlantis carried the first Spacelab flight of the National Aeronautics and Space Administration's Mission to Planet Earth into orbit. In its payload bay were 13 sophisticated and complementary instruments designed to identify the chemical species in our atmosphere, to measure the Sun's energy falling on and entering the atmosphere, to study the behavior of charged particles in the electric and magnetic fields surrounding Earth, and to gather ultraviolet light from stars and galaxies. This first mission in the series of Spacelab flights called the Atmospheric Laboratory for Science and Applications is known as ATLAS 1. The very successful 9-day mission established a voluminous and historical scientific database on the middle atmosphere. Science teams will be analyzing this database for years to come; however, some preliminary results are available.

Six atmospheric instruments were aboard Atlantis: the Atmospheric Lyman-Alpha Emissions (ALAE) instrument, the Atmospheric Trace Molecule Spectroscopy (ATMOS) instrument, the Grille Spectrometer (Grille), the Imaging Spectrometric Observatory (ISO), the Millimeter-wave Atmospheric Sounder (MAS), and the Shuttle Solar Backscatter Ultraviolet (SSBUV) instrument.

*The ALAE team has reported the first detection of deuterium atoms at 85 km (53 mi), the mesospheric altitude at which ultraviolet light dissociates hydrogen or deuterium from water vapor molecules. The deuterium observation is an important indicator of the hydrogen chemistry of the mesosphere, and the data will be useful in determining the height of the turbopause, the altitude below which atmospheric constituents are distributed by turbulent mixing.

*ATLAS 1 crewmembers observed a layer of tiny droplets of sulfuric acid and water in the atmosphere. This aerosol layer was produced by the eruption of the Mount Pinatubo volcano in 1991. The ATMOS team is studying how concentrations of ozone and other gases are affected by the presence of this aerosol layer, which is located between 20 and 28 km (12 to 17 mi) and coincides with the ozone layer. ATMOS data also indicate that the concentrations of all chlorofluorocarbons have increased significantly since 1985, when ATMOS measured the concentrations of the compound during Spacelab 3.

*The Grille Spectrometer successfully mapped the distribution of 10 target chemical species, and early analysis is focusing on methane (a source of hydrogen with concentrations that have been increasing from 0.5 to 1 percent each year) and hydrogen chloride (the dominant chlorine reservoir in the stratosphere).

*The ISO team acquired a database on upper atmospheric spectral emissions, ranging from the extreme ultraviolet to the near infrared. The instrument recorded spectra of the atomic oxygen green line, which had not been measured before in the daytime mesosphere, and also obtained basic measurements of the hydroxyl radical in the same region. Atomic oxygen, along with molecular oxygen, plays a major role in ozone production in the stratosphere and mesosphere. The hydroxyl molecule is another dominant player in mesospheric chemistry; it especially affects the concentrations of ozone and carbon monoxide.

*The MAS instrument obtained excellent global coverage of middle atmospheric chlorine monoxide, water vapor, and ozone measurements over the latitudes ranging from 80 N to about 40 S. In general, chlorine monoxide data agree with previous measurements and with models of undisturbed stratospheric chemistry. As predicted, a large diurnal ozone variation occurs above 70 km (43 mi), with greater concentrations on the night side of the orbit. Also, the data indicate a large latitudinal gradient in mesospheric water vapor, which appears to be in higher abundance over the tropics than at mid-latitudes.

*During 7 of its 40 Earth-view orbits, the SSBUV gathered unique data on ozone in the upper stratosphere. By looking at 3 ultraviolet wavelengths 4 times in a row (rather than at 12 wavelengths for a whole ozone profile) and by surveying an air column between 40 and 60 km (25 and 37 mi), rather than between 15 and 60 km (9 to 37 mi), scientists were able to study small-scale features of ozone. This special wavelength scanning mode yielded higher spatial resolution of ozone [50 km (31 mi) versus the usual 250-km (155-mi) detail]. These features should provide data on how ozone is transported. The SSBUV also measured solar ultraviolet radiation over four orbits. These data are being analyzed to identify changes in solar irradiance related to changes in solar activity measured on SSBUV's previous flights in 1989, 1990, and 1991.

Scientists are using data from two of the solar instruments aboard ATLAS 1 --the Active Cavity Radiometer Irradiance Monitor (ACRIM) and the Measurement of the Solar Constant (SOLCON) -- to refine measurements of the total amount of the Sun's radiant energy that reaches the top of Earth's atmosphere. ACRIM and SOLCON scientists are now analyzing data from the 23 solar-viewing orbits of ATLAS 1 and will be comparing their measurements of the solar constant with one another and with information from the ACRIM II instrument aboard the Upper Atmosphere Research Satellite. The results of SOLCON's solar observations will also be compared to data from instruments aboard the Earth Radiation Budget and EURECA Satellites. These measurements are crucial to our ability to identify possible trends in solar irradiance variability.

Two other instruments -- the Solar Spectrum Measurement from 180 to 3,200 Nanometers (SOLSPEC) and the Solar Ultraviolet Spectral Irradiance Monitor (SUSIM) -- measured specific wavelengths of solar radiation to determine their absolute values very accurately. These measurements are used in models of the middle and upper atmospheric regions. The data will help determine not only how solar output varies by wavelength but will also be useful in inferring the response of Earth's atmosphere to such variations.

*The preliminary analysis of SOLSPEC data indicates that the solar output magnitude values for the wavelengths of interest (ultraviolet, visible, and infrared) are close to the expected values. The results of subsequent and more detailed analysis will be helpful in validating models of the interaction of sunlight with the atmosphere.

*The SUSIM team collected over 100 spectra. These data are also being compared to those from the SUSIM instrument on the Upper Atmosphere Research Satellite, a comparison that will establish a calibration factor for the satellite instrument.

Three investigations explored the space plasma environment: the Atmospheric Emissions Photometric Imaging (AEPI), the Space Experiments with Particle Accelerators (SEPAC), and the Energetic Neutral Atom Precipitation (ENAP) investigations.

*The AEPI recorded optical emissions from the SEPAC artificial auroras, natural auroras in the Southern Hemisphere, and airflow. Investigators are studying AEPI's images of auroral arcs to learn more about the chemical processes occurring in the arcs. The AEPI also produced clear images of atmospheric gravity wave structures from the emissions of atmospheric oxygen. (Gravity waves are disturbances that often originate from lower atmospheric activity and produce structured features in the airflow.) At least one of these structures was ~100 km (62 mi) long.

*SEPAC used an electron beam accelerator to create 60 artificial auroras over the southern Pacific Ocean. These were the largest and brightest artificial auroras ever made: the optical emissions were brighter than expected and occurred over a much larger portion of the image.

*The ENAP investigation used the ISO instrument to measure faint nighttime glows that occur above Earth's middle latitudes when neutral atoms from Earth's ring current rain down on the atmosphere. Although the brighter glows were observed south of Australia when Atlantis' orbit was nearest the southern auroral zone, the ENAP investigation detected faint glows all the way to the equator. Another region of brighter emissions was detected over the south Atlantic Ocean where trapped particles from the ring current penetrate to lower altitudes than elsewhere at these latitudes.

The Far Ultraviolet Space Telescope (FAUST) obtained over 20 wide-field images of the far ultraviolet sky. These images contain information on galactic stars and dust, extragalactic objects, and even auroral and atmospheric emissions and reveal a wealth of newly discovered sources of ultraviolet radiation. Because they were produced by a new technique of counting photons electronically, the images show sources that are up to 10 times fainter than those from a previous ultraviolet survey of the same regions of the sky.

THE ATLAS 2 CREW

Commander

Kenneth D. Cameron received a B.S. and an M.S. in aeronautics and astronautics from the Massachusetts Institute of Technology. He enlisted in the Marine Corps and then earned a commission. He was assigned to the Republic of Vietnam as a platoon commander and later served with the Marine Guard at the U.S. Embassy, Saigon. After receiving his Naval Aviator wings and graduating from the U.S. Navy Test Pilot School, Cameron became a project officer and test pilot. He is currently a colonel in the U.S. Marine Corps. Cameron became an astronaut in June 1985. His technical assignments have included work on the Tethered Satellite System Payload, flight software testing in the Shuttle Avionics Integration Laboratory, and launch support activities at Kennedy Space Center. He has also served as capsule communicator for five Shuttle missions. Cameron flew as pilot on STS-37, during which the Gamma Ray Observatory was deployed. ATLAS 2 will be his second Shuttle mission.

Flying the Space Shuttle on orbit offers the crew a unique vantage point that reinforces the idea that our planet is really a spacecraft itself, with a crew of 5 billion people. Just as the astronauts study the Shuttle's systems to learn to fly it safely, we on Earth all need to learn more about our planet, its atmosphere, and our Sun's influence on it, in order to pilot it safely and successfully for a long voyage. I am very pleased and honored to be a part of this mission to study spacecraft Earth.

Pilot

Stephen S. Oswald received a B.S. in aerospace engineering from the U.S. Naval Academy. He was designated a Naval Aviator and flew the Corsair II aboard the U.S.S. Midway in the Western Pacific and Indian Oceans. After graduating from the U.S. Naval Test Pilot School, Oswald conducted flight tests at the Naval Air Test Center. Upon leaving active naval service, he became an aerospace engineer and instructor pilot for NASA and subsequently was selected as an astronaut in 1985. His NASA assignments have included serving as flight crew representative to the Kennedy Space Center, testing flight software, and working with the Marshall Space Flight Center on the redesigned solid rocket booster. Oswald served as pilot on STS-42, the First International Microgravity Laboratory. ATLAS 2 will be his second Shuttle mission. Any opportunity to fly aboard the Space Shuttle is an extraordinary privilege, but it is especially gratifying to be participating in the ATLAS 2 mission, which promises to benefit all of humanity. Gathering data on the interrelationship between the Sun and our fragile atmosphere is precisely the kind of job that NASA should be doing and to which the Shuttle is best suited. ATLAS 2 could prove to be of tremendous importance to the long- term health of our planet, and I'm glad to be a part of the team.

Mission Specialists

C. Michael Foale received a B.A. in physics from Queens College, the University of Cambridge, and a Ph.D. in laboratory astrophysics from Cambridge University. There, his research interests included vacuum ultraviolet spectroscopy and the study of natural laser emissions from stars. In June 1983, Foale joined the NASA Johnson Space Center Mission Operations Directorate and was responsible for payload operations on four Shuttle missions. He was selected as an astronaut in 1987 and was later assigned to support and fly the Shuttle Avionics Integration Laboratory simulator, which provides verification and testing of the Shuttle flight software. More recently, he helped develop extravehicular activity assembly and rescue operations plans for Space Station Freedom. He flew as a mission specialist on STS-45, the first of the ATLAS series of missions. ATLAS 2 will be his second spaceflight.

Atmospheric and solar physics have been major interests of mine since I studied physics at Cambridge. I am very lucky to have the chance to fly in space as well as work with scientists on atmospheric problems, all at the same time! Everyone who works the ATLAS missions will have contributed to understanding the development and changes that are happening to our planet's climate and atmosphere.

Ellen Ochoa received a B.S. in physics from San Diego State University and an M.S. and Ph.D in electrical engineering from Stanford University. Her research in optics for her doctoral dissertation resulted in a patent for a system to detect defects in manufactured objects with repetitive structure. Her work as a research staff member at Sandia National Laboratories led to two additional patents in the area of optical information processing. Ochoa joined NASA Ames Research Center in 1988 to lead a research group working primarily on optical recognition systems for space automation. She later served as Chief of the Intelligent Systems Technology Branch, acting as technical and administrative leader of 35 engineers and scientists in research and development of high-performance computational systems for aerospace missions. Ochoa became an astronaut in 1991, and she has worked on Space Shuttle flight software verification in the Shuttle Avionics Integration Laboratory. ATLAS 2 will be her first spaceflight.

ATLAS 2 plays an important role in NASA's Mission to Planet Earth program. Not only will we return data on the atmosphere and the Sun taken throughout the mission, but by safely bringing back and recalibrating the sensitive instruments used to make measurements, we will also be able to provide an accurate standard of comparison for other orbiting instruments. In addition, the atmospheric studies of this mission will focus on the problem of ozone depletion in the northern hemisphere. More personally, ATLAS 2 is a wonderful opportunity for me to participate in an ambitious research program that is vital to the future of our planet.

Kenneth D. Cockrell received a B.S. in mechanical engineering from the University of Texas and an M.S. in aeronautical systems from the University of West Florida. He was designated a Naval Aviator and flew A-7 Corsair IIs aboard the U.S.S Midway in the Western Pacific and Indian Oceans. After graduating from the U.S. Naval Test Pilot School, Cockrell conducted flight tests at the Naval Air Test Center. He served as a staff officer for the commanders of the U.S.S. Ranger and U.S.S. Kitty Hawk battle groups. He was then assigned as a pilot in an operational F/A-18 squadron and made two cruises on the U.S.S. Constellation. After finishing his tour of duty, Cockrell accepted a position at the Aircraft Operations Division of the Johnson Space Center and in 1991 became an astronaut. His technical assignments include duties in the Astronaut Office Operations Development Branch, working on landing, rollout, and mechanical issues. ATLAS 2 will be his first spaceflight.

I'm pleased and honored to be flying a mission that plays such an important role in the understanding of the health of our planet's atmosphere. I hope the ATLAS series of missions will give us better insight into the problems the atmosphere is facing and will help steer us in the right direction as we look for the solutions.

ATMOSPHERIC TRACE MOLECULE SPECTROSCOPY -- ATMOS

Principal Investigator: Dr. Michael R. Gunson NASA Jet Propulsion Laboratory Pasadena, California, USA

PURPOSE

The Atmospheric Trace Molecule Spectroscopy experiment is designed to make global measurements of the composition of the troposphere, stratosphere, and mesosphere. The spectrometer simultaneously measures the concentrations of gases that are involved in the complex chemical and radiative interactions occurring at altitudes between 10 and 150 km (6 to 93 mi). With the instrument's ability to detect these trace gases in concentrations lower than 1 part per billion, its data are used to make critical tests of theoretical models that describe the physics and chemistry of the stratosphere.

Trace gas molecules absorb specific wavelengths of incoming solar radiation. By determining which wavelengths are absorbed by the atmosphere, ATMOS scientists can develop a more detailed picture of the molecular composition of the atmosphere, thereby providing a baseline of data against which to detect future changes in atmospheric composition. These data (with information collected from similar experiments carried on balloons, rockets, satellites, and aircraft) will provide a reference source for atmospheric scientists in the future.

RESULTS FROM PREVIOUS MISSIONS

During Spacelab 3 (1985), ATMOS made the first simultaneous inventory of the majority of important trace gases, including most of the molecules involved in ozone photochemistry. Scientists have now identified the atmospheric concentrations of over 30 different gases and their vertical distribution, including several chlorofluorocarbons, most of the nitrogen-oxygen compounds, ozone, carbon monoxide, carbon dioxide, water, and methane. The instrument has detected several previously unobserved molecules, confirming the presence of dinitrogen pentoxide (N2O5) and chlorine nitrate (ClONO2), which are important temporary reservoirs of more reactive molecules involved in photochemical processes leading to ozone loss.

Some of the most dramatic data obtained during the 94 atmospheric observations on the ATLAS 1 mission were related to the volcanic eruption of Mt. Pinatubo in June of 1991. This eruption (the largest this century) deposited a large amount of crustal material and gases into the atmosphere at heights coinciding with the ozone layer. With time, the gases have converted into an aerosol layer made up of sulfuric acid and water droplets (H2SO4/H2O) located at altitudes between 20 and 28 km (12 and 17 mi); the layer is so thick that it is absorbing almost all of the infrared sunlight coming into it. Thus, scientists are presented with the challenge of determining how to probe this opaque area. By the time ATLAS 2 launches, the aerosol layer will have diminished, and investigators will look for long-term effects as well as continue to look for other trends in atmospheric composition.

INSTRUMENT OPERATIONS

The ATMOS instrument views the stratosphere at orbital sunrise and sunset and gathers information in the infrared portion of the electromagnetic spectrum. It contains two mirrors that lock onto the Sun automatically, directing sunlight that passes through the atmosphere into the instrument. There, the sunlight is collected by a telescope before being split into two beams that travel different paths through the instrument. The two beams are recombined and directed onto a detector. By changing the distance traveled by one beam with respect to the other, different wavelengths of light in the beam recombine in or out of phase. When light at a particular wavelength is in phase, the two beams simply add together, producing a bright spot. Other wavelengths are out of phase, meaning they cancel each other, producing a dark region. This process creates a pattern of light and dark bands called an interferogram. A mathematical procedure is used to convert the interferogram thus formed into an absorption spectrum that gives scientists the intensity of the light at each of these wavelengths. Losses of intensity at specific wavelengths can be equated with the absorption of light by molecules in the atmosphere, and in this fashion the identities and concentrations of such molecules can be determined.

To provide a backup for the Shuttle High Data Rate Recorder (HDRR), a flight tape recorder dedicated to ATMOS data was added as a separate package for the ATLAS 2 mission. It is capable of recording up to 6.5 hours of ATMOS high-rate data and has a higher capacity and operates at higher speeds than the HDRR.

Spectral coverage: 600 to 4,800 cm-1 Maximum resolution: 0.013 cm-1 Detector: HgCdTe @ 77 K Field of view: 1, 2, or 4 mrad Aperture: 7.5 cm (diameter) Operating temperature: -5 C to +45 C Scan time: 2.2 sec Data rate: 16 Mb/sec Mass: 250 kg Power: 360 W

Co-Investigators:

Dr. Reinhard Beer Dr. Linda R. Brown Mr. Odell F. Raper Dr. Geoffrey C. Toon Dr. Robert A. Toth Jet Propulsion Laboratory USA

Dr. Crofton B. Farmer Distinguished Visiting Scientist Jet Propulsion Laboratory USA

Dr. Curtis Rinsland NASA Langley Research Center USA

Dr. Rodolphe Zander Institut d'Astrophysique Universite de Liege Belgium

MILLIMETER WAVE ATMOSPHERIC SOUNDER - MAS

Principal Investigator: Dr. Gerd K. Hartmann Max-Planck-Institut fur Aeronomie (Max Planck Institute for Aeronomy) Katlenburg-Lindau, Germany

Co-Principal Investigators: Dr. Nicklaus Kampfer Institut fur Angewandte Physik (Institute of Applied Physics) University of Bern Bern, Switzerland

Prof. Dr. Klaus F. Kunzi Institut fur Fernerkundung (Institute for Remote Sensing) University of Bremen Bremen, Germany

Dr. Phillip R. Schwartz Naval Research Laboratory Washington, D.C., USA

PURPOSE

The MAS instrument studies the photochemistry of ozone in Earth's middle atmosphere. The experiment will provide a complete set of simultaneous measurements that are particularly relevant to the study of catalytic cycles related to ozone loss. Chlorine monoxide, formed predominantly by the photodissociation of chlorofluorocarbons in the middle atmosphere and subsequent reactions of the breakdown products with ozone, is the key player in chlorine-catalyzed ozone photochemical loss. Evidence suggests that dramatically increased chlorine monoxide concentrations result in high ozone loss rates during the Antarctic spring and play an important role in the formation of the ozone hole. Concentrations of chlorine monoxide, however, are extremely difficult to determine. The MAS will supply important global measurements of this molecule, which is present in measurable quantities in the sunlit atmosphere. An important objective for ATLAS 2 is to obtain these measurements (as a function of latitude and altitude) in both the Southern and Northern Hemispheres.

Water vapor too, plays a central role in ozone photochemistry because it is the source of active hydrogen compounds that dominate ozone photochemical loss above 50 km (31 mi). It is also a primary tracer of motion in the middle atmosphere; therefore, MAS water vapor measurements will provide information pertinent to understanding the distribution of ozone and the mechanics of chemical transport.

RESULTS FROM PREVIOUS MISSIONS

Investigators consider the ATLAS 1 mission a tremendous success for the MAS experiment. In general, preliminary results agree with theoretical expectations and with photochemical model calculations. For example, a large diurnal ozone variation is observed at altitudes above 70 km (43 mi), with much larger abundances on the night side of the orbit. Also, peak mixing ratios of chlorine monoxide (between 0.5 and 1 part per billion) do occur at 40 km (25 mi). In addition, preliminary analysis of the data indicates a large latitudinal gradient in mesospheric water vapor with higher abundances in the tropics than at mid-latitudes. This again is qualitatively consistent with the expected circulation pattern in the middle atmosphere and current theories on its role in establishing the global distribution of water vapor in the middle atmosphere.

INSTRUMENT OPERATIONS

Many important atmospheric molecules emit millimeter-wave radiation at well-defined frequencies. MAS measures the strength of these emissions at specific frequencies. The investigation uses a parabolic antenna that scans Earth's limb to collect spectral information at different altitudes in the middle atmosphere. The MAS spectral measurements will help scientists determine the vertical distribution of ozone and several atmospheric constituents and conditions important in ozone photochemistry, namely, chlorine monoxide, water vapor, temperature, and pressure.

During the ATLAS 2 mission, investigators plan to test a new method of pointing the MAS antenna based on the instrument pressure channels, rather than the Shuttle horizon sensor data. They have also developed a new pointing mode for measuring chlorine monoxide in which they successively direct the antenna toward altitude points from 20 to 50 km (12 to 31 mi) in 5-km (3-mi) increments. Since chlorine monoxide has a very large diurnal variation with exceedingly small abundances at night, the instrument will automatically revert to the improved scan mode (to measure water vapor and ozone over the full MAS measurement range) on the night side of each orbit.

Frequencies recorded: 61, 62, 63, 183, 184, 204 GHz Spectrometer: 50 channels each for temperature, water vapor, ozone; 30 channels for chlorine monoxide; 30 channels each for 2 pressure lines (total of 240 channels) Vertical resolution: 3 km Highest spectral resolution: 200 kHz Antenna diameter: 1 m Data rate: 86.4 kb/sec Mass: 203 kg

Co-Investigators:

Dr. Christoph Aellig Institut fur Angewandte Physik Universitat Bern Switzerland

Dr. Richard M. Bevilacqua Dr. Thomas Pauls Naval Research Laboratory USA

Dr. Charles L. Croskey Dr. John J. Olivero Dr. Enrique Puliafito Pennsylvania State University USA

Dr. Werner Degenhardt Dr. Paul Hartogh Dr. Alfred Loidl Dr. Michael Richards Dipl. Phys. Gerhard Umlauft Dipl. Eng. Richard Zwick Max-Planck-Institut fur Aeronomie Germany

Dr. Jorg Langen Universitat Bremen Germany

Approximately 20 Collaborative Investigators from various institutions, as listed in "MAS Characteristics," produced by the Max Planck Institute for Aeronomy

SHUTTLE SOLAR BACKSCATTER ULTRAVIOLET EXPERIMENT --SSBUV

Principal Investigator: Mr. Ernest Hilsenrath Goddard Space Flight Center Greenbelt, Maryland, USA

PURPOSE

The SSBUV instrument measures solar ultraviolet irradiance and radiation backscattered from Earth's atmosphere. Since ozone absorbs radiation in the ultraviolet wavelengths, the amount of ozone present can be determined by calculating the ratio of backscattered radiation to the amount of solar radiation reaching the top of Earth's atmosphere. This ratio is called the ultraviolet albedo. Variations in the ultraviolet albedo indicate the amount and vertical distribution of ozone in the atmosphere.

A primary objective is to provide highly accurate ozone measurements that will be used to verify the accuracy of atmospheric ozone and solar ultraviolet irradiance data obtained by the SBUV/2 instruments on National Oceanic and Atmospheric Administration (NOAA) and NASA satellites, flown as part of the National Plan for Stratospheric Monitoring. The SSBUV will take readings of backscattered radiances and the solar ultraviolet irradiance, data also measured by these satellite instruments. In addition, the SSBUV will gather calibration data for the Total Ozone Mapping Spectrometer instruments on the Nimbus-7 (U.S.) and Meteor-3 (Russian) satellites. SSBUV ozone data will be compared with the nearly coincident measurements taken by other types of ozone-sensing instruments on the Upper Atmosphere Research Satellite.

RESULTS FROM PREVIOUS MISSIONS

SSBUV has flown on STS-34 (1989), STS-41 (1990), and STS-43 (1991). Its latest flight was on the ATLAS 1 (STS-45) mission in March of 1992. During this mission the instrument measured the solar ultraviolet irradiance on 7 orbits and measured ozone concentrations on 40 Earth-view orbits. Using the preflight calibrations, the irradiance values were very close to the measurements made on earlier SSBUV flights. This indicates the close precision of the instrument's measurements. After employing the postflight calibration to process the solar data, investigators will be able to derive solar ultraviolet irradiance variations related to changes in solar activity since 1989, when the SSBUV made its first observations.

INSTRUMENT OPERATIONS

The SSBUV spectrometer is housed in a Get-Away-Special canister equipped with a Motorized Door Assembly, which opens to allow Earth and solar viewing and closes to protect the instrument from contamination. An adjacent canister contains data, command, and power systems. An interconnecting cable provides the communication link between the two containers.

As light enters the instrument, it is directed through a system of mirrors and gratings, to a photomultiplier. The photomultiplier converts the light to a small electric current, which is then magnified by a three-range electrometer amplifier. The desired wavelength is selected by a grating controlled by a microprocessor, allowing the instrument to scan through 12 discrete channels in the ultraviolet range. The four longest channels are used to derive the total column ozone amount while the remaining channels determine the ozone vertical distribution between about 25 and 50 km (15.5 and 31 mi). The instrument also contains a narrow band filter radiometer to measure Earth's surface brightness at 370 nm where ozone is not absorbed. The grating scans from 160 to 405 nm during solar irradiance measurements.

The SSBUV will be turned on after it undergoes 10 hours of outgassing. After outgassing, it warms up for 10 to 12 hours before any on-orbit operation is performed. The instrument is activated by the crew and then controlled from the ground. Improvements have been made in SSBUV for the ATLAS 2 mission, giving investigators the ability to control the spectrometer grating drive from the ground, which provides more flexibility and allows the instrument to observe sulfur dioxide and nitric oxide as well as ozone. There are three operating modes: Earth view, solar view, and calibration. During Earth view operations, the instrument is aimed at the daylit Earth (during the daylit portion of an orbit) and measures backscattered radiance, while simultaneously making observations of the horizon. Several 30-minute solar viewings and 60-minute calibrations will occur early, midway, and late in the mission.

Spectral coverage: 160 to 405 nm Spectral resolution: 1.1 nm Field of view (spectrometer): 11.3 x 11.3 deg Field of view (solar aspect sensor): 64 x 64 deg Grating: holographic, 2,400 lines/nm Detector: photomultiplier, 5 stages, end on, quartz window, Bi-alkali (400S) Hamamatsu (R2477) Data rate: 1.6 kb/sec Mass: 450 kg

Co-Investigators:

Dr. Richard P. Cebula Dr. Donald F. Heath ST Systems Corporation USA

Dr. John E. Frederick University of Chicago USA

Dr. Bruce W. Guenther Dr. James E. Mentall Dr. Richard D. McPeters Goddard Space Flight Center USA

Mr. Alvin J. Miller National Oceanic and Atmospheric Administration (NOAA/National Meteorological Center USA

Mr. Walter G. Planet NOAA/National Environmental Satellite Data and Information Service USA

SOLAR SPECTRUM MEASUREMENT FROM 180 TO 3,200 NANOMETERS -- SOLSPEC

Principal Investigator: Dr. Girard O. Thuillier Service d'Aeronomie du Centre National de la Recherche Scientifique (Aeronomy Service of the National Center for Scientific Research) Verrieres-le-Buisson, France

PURPOSE

Since the Earth System is driven by energy received from the Sun, it is important to measure the spectral details of the solar energy available to the system. Different wavelengths of the Sun's rays penetrate the atmosphere to particular depths where they trigger chemical reactions. Although most solar energy is contained in the visible light that reaches Earth's surface, the energy present in X-ray and ultraviolet wavelengths can vary significantly during solar cycles and can change the amount of energy available for chemical reactions in the middle and upper atmospheres, affecting the chemical composition and thermal structure at those heights.

The SOLSPEC instrument measures the energy in the ultraviolet, visible, and infrared wavelengths between 180 and 3,200 nm with a 1- to 5-percent accuracy. This part of the solar spectral irradiance is being studied to determine accurately the amounts of these energies and how they change with time. It is important that scientists know which wavelength ranges of the solar spectrum are involved in the variability of the solar constant. Variability below 300 nm influences the ozone layer and chemical balance in Earth's upper atmosphere, while variability above 700 nm affects water vapor and carbon dioxide absorption at lower altitudes. Data gathered will enable scientists to identify the regions of the atmosphere that are likely to respond to particular variations in solar infrared, visible, and ultraviolet ranges, allowing investigators to better understand and anticipate atmospheric changes.

RESULTS FROM PREVIOUS MISSIONS

The SOLSPEC instrument flew on Spacelab 1 in 1983 and was part of the ATLAS 1 payload in 1992. The Shuttle is an excellent vehicle for the SOLSPEC investigations because of its operational flexibility and maneuverability, which allow the SOLSPEC instrument to perform special, perhaps unscheduled, observations at opportune times. For instance, during the added ninth day of the ATLAS 1 mission, the instrument was able to make observations of the light backscattered by the ozone layer. These measurements can be compared to those taken by SSBUV and SUSIM.

INSTRUMENT OPERATIONS

The SOLSPEC instrument is a double-monochromator that uses two holographic gratings as a dispersive element. The instrument has three spectrometers (one each for the ultraviolet, visible, and near-infrared portions of the spectrum), scanning at 650 different positions. Each position corresponds to a 1-nm bandpass in the ultraviolet and visible ranges and to a 20-nm bandpass in the infrared, producing a total of 1,950 bands. A hollow cathode lamp measures the wavelength scale of the spectrometers.

Four calibration lamps (two deuterium and two tungsten ribbon lamps) are included to assure accuracy in flight. Their light follows the same optical path as the Sun's light. During normal operations, observations of the Sun will alternate with observation of the calibration lamps at 15-minute intervals during orbital day. During orbital night, only the calibration lamps are observed. The instrument is calibrated once a day during the ATLAS 2 mission. Also, the entire system is calibrated on the ground against a blackbody at 3,300 K (3,027 C/5,481 F) and a set of deuterium and tungsten ribbon lamps.

In addition to the SOLSPEC scientists stationed at Marshall Space Flight Center during the ATLAS 2 mission, some members of the instrument team will be on duty in Europe at the Royal Meteorological Institute of Belgium. From this center, they will be able to communicate with the SOLSPEC instrument and monitor its data.

Spectral coverage: 180 to 3,200 nm Bandpass in ultraviolet and visible: 1 nm Bandpass in infrared: 20 nm Total number of bandpasses: 1,950 Precision of individual bandpass: 0.01 nm Photometric accuracy: 5% in ultraviolet 1% in infrared and visible Time to record solar spectrum: 13 min Number of spectra per orbit: 3 Data rate: 500 b/sec Mass: 32 kg

Co-Investigators:

Dr. Michel Herse Service d'Aeronomie du Centre National de la Recherche Scientifique France

Pr. Dr. Dietrich Labs Heidelberg Landessternwarte Germany

Mr. William Peetermans, Dr. Paul Simon Institut d'Aeronomie Spatiale de Belgique Belgium

SOLAR ULTRAVIOLET SPECTRAL IRRADIANCE MONITOR -- SUSIM

Principal Investigator: Dr. Guenter E. Brueckner Naval Research Laboratory Washington, D.C., USA

PURPOSE

Solar ultraviolet radiation in the wavelength range of 120 to 400 nm is absorbed by Earth's atmosphere at altitudes between 20 and 120 km (10 and 75 mi). Although this radiation constitutes only a small percentage of the total solar output, it is the main source of energy for the middle atmosphere. Also, the ultraviolet component of sunlight varies considerably more than the visible radiation, which makes up the greatest percentage of the Sun's total solar irradiance. During an 11-year cycle of the Sun's activity, changes in ultraviolet radiation bring about corresponding changes in a number of atmospheric conditions, including the concentration of the fragile ozone layer in the stratosphere.

Accurate measurements are crucial if scientists are to determine how the fluctuation of the ultraviolet part of the Sun's spectrum varies over a given period and if they are to understand the role this fluctuation plays in orchestrating changes in atmospheric physics. The SUSIM instrument operates during the Shuttle's solar-pointing periods and is designed to establish a new and more accurate base of solar ultraviolet irradiance measurement over the ultraviolet wavelength region.

RESULTS FROM PREVIOUS MISSIONS

ATLAS 2 will be SUSIM's fourth spaceflight. It flew during the Office of Space Science-1 mission in 1982 and was part of the Spacelab 2 payload in 1985 and the ATLAS 1 payload in 1992. During the ATLAS 1 mission, the instrument collected more than 100 solar spectra with various optical configurations and was able to make coincident measurements on 4 different days with the SUSIM instrument flying on the Upper Atmosphere Research Satellite.

INSTRUMENT OPERATIONS

The instrument is essentially a light meter, composed of two precision ultraviolet spectrometers with two sets of optics and an inflight calibration deuterium lamp. This assembly provides an accurate recording of solar spectral irradiance from 120 to 400 nm and a technique for tracking any change in ultraviolet sensitivity. Also, there are seven detectors that allow cross-checks of possible detector changes. One spectrometer operates as the primary unit and makes the solar spectral measurements. The other spectrometer gathers data from a deuterium lamp used to calibrate both this unit and the primary unit. Because the spectrometer is not exposed to solar radiation, its readings can be used as reference information to track any degradation in the first spectrometer.

The basic difficulty in measuring the Sun's irradiance is that the solar ultraviolet radiation being measured is also responsible for the rapid degradation of any instrument making the measurement. As a result, the accuracy of photometers on satellites is quickly degraded, making it impossible to distinguish between long-term solar effects and changes in the instrument itself. SUSIM has overcome this problem by using multiple measuring systems and a self-calibration system to determine whether instrument changes or solar flux variations are being recorded. An on-board deuterium lamp tracks any change in sensitivity caused by vibration during launch or by contamination during flight. This ultraviolet lamp is turned on and placed in front of the spectrometer, which then scans the known ultraviolet spectrum to measure the instrument's sensitivity. Any of the seven detectors (two photon counters and five photodiodes) can then be used to compare the intensity of solar radiation with the intensity of the standard deuterium lamp. Also, since the instrument is being flown on the Shuttle, it can be calibrated preflight and postflight, and its data can be used to recalibrate similar instruments aboard free-flying satellites.

Spectral coverage: 110 to 410 nm Spectral resolution: 5 nm, 0.15 nm Accuracy: 5% absolute Inflight calibration source: deuterium lamp Data rate: 156 b/sec Mass: 69 kg Power consumption: 53 W

Co-investigators:

Mr. Michael E. VanHoosier Dr. Dianne K. Prinz Naval Research Laboratory Washington, D.C., USA

ACTIVE CAVITY RADIOMETER IRRADIANCE MONITOR -- ACRIM

Principal Investigator: Dr. Richard C. Willson NASA Jet Propulsion Laboratory Pasadena, California, USA

PURPOSE

Interaction of the radiation of the Sun with the atmosphere, oceans, and land regions of Earth determine weather and climate. Climatic models show that small but persistent variations in total solar irradiance may explain a wide range of past climatic changes. To understand the effects of variations of solar irradiance on the climate, it is essential to monitor total solar irradiance over long intervals and to understand the physical origin of its variability. As part of a long-term program to study the physical behavior of the Sun and its effect on Earth's climate, NASA is compiling a high-precision solar irradiance database from the irradiance observations of several ACRIM instruments aboard satellites, rockets, and other Shuttle missions, one of which will fly on the ATLAS 2 mission. The primary objective is to determine the degree and direction of possible fluctuations in the Sun's total output of optical energy (X-ray to microwave frequencies) by measuring the total solar optical irradiance outside Earth's atmosphere. The ACRIM measures the total solar irradiance from ultraviolet through infrared wavelengths to within 0.1 percent accuracy.

RESULTS FROM PREVIOUS MISSIONS

The ACRIM experiment is also part of an ongoing program that involves identical instruments. The instrument flew on Spacelab 1 in 1983, on ATLAS 1 in 1992, and aboard the Solar Maximum Mission Satellite from 1980 to 1989. Another ACRIM instrument is currently aboard the Upper Atmosphere Research Satellite, which was launched in 1991. Data from the ATLAS 2 ACRIM experiment will be compared to those made at the same time by the Measurement of Solar Constant investigation and by instruments aboard other satellites. Through successive comparisons, the precision of the satellite measurements can be sustained. Also, the Shuttle ACRIM can be recalibrated so that it provides a benchmark of accuracy for satellite data.

INSTRUMENT OPERATIONS

The ACRIM contains four cylindrical bays. Three of the bays house independent heat detectors, called pyrheliometers, which are independently shuttered, self-calibrating, and automatically controlled. Each pyrheliometer consists of two cavities, and temperature differences between the two are used to determine the total solar flux. One cavity is maintained at a constant reference temperature, while the other is heated 0.5 K (1.6 C/0.9 F) higher than the reference cavity and is exposed to the Sun periodically. When the shutter covering the second cavity is open, sunlight enters, creating an even greater difference in cavity temperatures. The power supplied to the second cavity by the ACRIM electronics decreases automatically to maintain the 0.5-K temperature difference between the two cavities. This decrease in the amount of electricity is proportional to the solar irradiance entering the cavity. The fourth bay holds a sensor that measures the relative angle between the instrument and the Sun.

To guarantee precision, the ACRIM cavities have mirror-like black surfaces that reflect light toward the apex of the cavity, where 99.99998 percent of the Sun's incoming energy in the 180- to 3,000-nm wavelength range is absorbed. Also, periodic comparisons between the detectors will be made to monitor the stability of the detector exposed to the Sun or to calibrate any apparent degradation.

Spectral coverage: 18 to 3,000 nm Cavity field of view: +/-5 deg (max), +/-2.5 deg (operational) Effective cavity absorptance: 0.999980 +/-0.000020 Single sample irradiance precision: +/-0.012% Length of single measurement cycle: about 2 min Uncertainty for single shutter cycle: less than +/-50 ppm Data rate: 256 b/sec Mass: 35 kg

Co-Investigator: Mr. Roger S. Helizon NASA Jet Propulsion Laboratory Pasadena, California, USA

MEASUREMENT OF THE SOLAR CONSTANT -- SOLCON

Principal Investigator: Dr. Dominique A. Crommelynck Institut Royal Meteorologique de Belgique (Belgian Royal Institute of Meteorology) Brussels, Belgium

PURPOSE

SOLCON's purposes are to measure the absolute value of the total solar irradiance with improved accuracy and to detect and measure long-term variations that may exist in its absolute value. The total solar irradiance is the total radiant energy of the Sun received by Earth at a distance of one astronomical unit. The absolute value of the solar irradiance is one of the critical factors that determines Earth's absorption and reflection of radiation, or the energy balance that governs atmospheric circulation. More accurate measurements of the value of the solar irradiance are needed and can be made only from space.

RESULTS FROM PREVIOUS MISSIONS

SOLCON was part of the payloads on Spacelab 1 in 1983 and ATLAS 1 in 1992 and is currently flying on the European Space Agency's EURECA platform, launched in July 1992. Preliminary results from ATLAS 1 indicate a correlation between the number of solar spots on the rotating solar disc and the fluctuation of the values of the solar irradiance. Investigators are now analyzing the 23 orbits of solar observations. These will be compared to the data obtained by instruments on the Nimbus-7 and Earth Radiation Budget Satellites, as well as to those gathered by the Active Cavity Radiometer Irradiance Monitors on ATLAS 1 and on the Upper Atmosphere Research Satellite. The measurements of SOLCON are accurate and precise enough to allow improved data analysis.

INSTRUMENT OPERATIONS

The high-resolution, self-calibrating SOLCON instrument is a true differential, absolute radiometer with a digital processing/converter unit. Because investigators know its precise electrical, optical, mechanical, and thermal characteristics, the instrument is absolute and does not require a radiative source to be calibrated. Two openings admit sunlight into two cavities, which are painted black. Each cavity has an independently controlled shutter at the front to block sunlight and a thermopile to measure the heat that is generated electrically as well as by absorbed sunlight. The accuracy of the radiometer system is checked at the beginning and end of each measurement sequence. During this checkout sequence, the shutters of both cavities are closed, and one cavity is heated (using a reference power source). A servosystem then adjusts the power applied to the other cavity until the heat flux balance is restored. The roles of the channels are then reversed, and the same procedure is repeated. The measurement of the electrical power applied to each cavity gives a value for the precision of the instrument, and because the power of the reference electrical source is already known, this also gives a check of the data processing system.

The instrument itself detects whether it is properly pointed at the Sun and when the Sun is undergoing orbital sunrise or orbital sunset. Its digital processing subsystem automatically controls the radiometer's mode and collects and digitizes the radiometer and photometer data. The only part of the experiment that is not automatic is the pointing operation, which requires that the investigators analyze values obtained from a Sun sensor and, if necessary, request minor attitude changes that will correctly position the orbiter to point the experiment directly at the Sun.

The actual measurements of the solar irradiance are made by pointing the radiometer to the Sun's center and opening the shutter of one cavity, while the other cavity remains closed. The closed cavity is heated electrically until its heatflux to the heatsink matches the heatflux of the open cavity. The energy this requires is proportional to the incoming sunlight, and the difference in the power applied with the shutter opened and closed is a classic measure of the solar radiation flux. An alternate mode of operation is to supply constant electrical power to the closed cavity and to reestablish the heatflux balance by heating the open cavity.

During the mission, the experiment will be repeated several times, using the same cavity as the experimental channel. For other sequences, the roles of the cavities will be reversed. This allows investigators to compare the two cavities and provides the basis for detecting and compensating for any degradation of the black paint that may have occurred as it was repeatedly exposed to the Sun.

During ATLAS 1, members of the SOLCON science team located in an operations control center in Europe had opportunity to command the instrument remotely and to monitor its data. For ATLAS 2, a remote control center with enhanced communications and data monitoring capabilities will be based at the Royal Meteorological Institute of Belgium. Both remote exercises demonstrate Earth-based command and control technologies that have important applications to Space Station Freedom operations.

Spectral coverage: integrated over the full wavelength range Absolute accuracy: better than 0.1% Precision: better than +/-0.01% Sensitivity: better than 0.05% Field of view: 9 deg Data rate: 46 b/sec Digital resolution: 22 b Mass: 13 kg

Co-Investigators:

Dr. Bruce R. Barkstrom NASA Langley Research Center USA Dr. Vicente Domingo European Space Agency The Netherlands