STS-56 ATLAS 2 FACT SHEET ATLAS 2, the second in NASA's series of Atmospheric Laboratory for Applications and Science Spacelab missions, is the primary payload for the STS-56 flight. The Space Shuttle-borne remote sensing laboratory studies the sun's energy output and the middle atmosphere's chemical makeup, and how these factors affect global ozone levels. A thin layer of ozone in the stratosphere shields life on Earth from most of the sun's harmful ultraviolet radiation. However, a "hole" in that protective layer occurs each year over the Antarctic, and other ozone depletion has been observed in both the Southern and Northern Hemispheres. Is ozone depletion over populated areas in the Northern mid latitudes a concern? How do we determine and understand its causes? Are they reversible? What are their implications for humans? Ozone is both created and destroyed by complex reactions among ultraviolet radiation from the sun and gases in the middle atmosphere. While some of those gases occur naturally, concentrations of destructive chemicals are increasing due to human activity. To fully understand the many factors which drive atmospheric chemical reactions and to predict changes, scientists must have a comprehensive knowledge of the gases which make up the atmosphere. In addition, they must have precise data on the sun's energy output as it fluctuates from maximum to minimum activity and back again over an 11-year solar cycle. Measurements taken by the ATLAS missions, along with those made by free-flying satellites, will help scientists determine the causes and extent of damage to the ozone layer over time. The highly successful ATLAS 1 mission, which flew in March 1992, established a voluminous baseline of atmospheric data against which to measure future global change. ATLAS 2 and subsequent missions will track subtle, year-to-year variations in solar activity and in atmospheric composition. ATLAS instruments are precisely calibrated before and after flight, so they also provide a valuable cross-check for data being gathered on a continuous basis by similar instruments aboard free-flying satellites. The ATLAS series is a vital part of "Mission to Planet Earth," NASA's long-term effort to study the Earth as a global environmental system. Mission to Planet Earth will observe and monitor the interaction of large environmental components (land, oceans/water/ice, atmosphere and biosphere.) Data gathered will be distributed to global change researchers worldwide, allowing them to better understand natural changes in the Earth and to differentiate natural change from human-induced change. Mission to Planet Earth research is essential to humans making informed decisions about protecting their environment. Scientists from six nations are participating directly 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. The ATLAS 2 Instruments The Space Shuttle Discovery will carry the ATLAS 2 Spacelab into orbit for eight days of remote sensing experiments. Six instruments are mounted in the orbiter's cargo bay on a Spacelab pallet. The open, U-shaped platform is a component of the reusable Spacelab equipment provided by the European Space Agency in 1981 as its contribution to the Space Shuttle program. The instruments' power supply, command and data-handling system and temperature control system are housed in a pressurized container called an igloo (also standard Spacelab equipment) located in front of the pallet. The Shuttle Solar Backscatter Ultraviolet spectrometer, co-manifested with all ATLAS flights, is housed in two Getaway-Special canisters mounted on the side of the cargo bay. These seven instruments form the core ATLAS payload which will fly aboard ATLAS 2 as well as ATLAS 3, now scheduled for late 1994. * The Millimeter Wave Atmospheric Sounder (MAS) measures water vapor, ozone and chlorine monoxide (a key compound in the photochemical cycle which contributes to ozone loss), as well as temperature and pressure, in the mesosphere and stratosphere. * The Atmospheric Trace Molecule Spectroscospy (ATMOS) experiment identifies the distribution by altitude of 30 to 40 different gases in the stratosphere. * The Shuttle Solar Backscatter Ultraviolet (SSBUV) spectrometer measures ozone concentrations by comparing solar ultraviolet radiation with that scattered back from the Earth's atmosphere. * The Solar Spectrum Measurement (SOLSPEC) instrument studies the distribution of solar energy by wavelength, from infrared through ultraviolet. * The Solar Ultraviolet Irradiance Monitor (SUSIM) concentrates on the sun's ultraviolet radiation, which undergoes wider variations than other wavelengths. * The Active Cavity Radiometer (ACR) and the Solar Constant (SOLCON) experiments each make extremely precise, independent measurements of the solar constant, or total energy from the sun received by the planet Earth. The steep, 57-degree inclination of the Shuttle's orbit during ATLAS 2 will take it over points as far north as Juneau, Alaska; and as far south as Tierra del Fuego, Argentina; allowing readings to be made over virtually the entire globe. On ATLAS 2, the Atmospheric Trace Molecule Spectroscopy experiment, which made most of its measurements in the Southern Hemisphere during ATLAS 1, will focus on the Northern Hemisphere. In order for it to view orbital "sunrises" at high latitudes, a night launch is required for the planned launch date. ATLAS Missions and the Shuttle The Space Shuttle is the ideal platform for NASA's remote-sensing atmospheric laboratory. The flight crew can maneuver the orbiter so the instruments in the bay point precisely toward the atmosphere, the sun or the Earth's surface as necessary for scheduled observations. The Shuttle's generous payload capacity and power supply allow a diverse assembly of large instruments to make simultaneous observations. The Shuttle-borne ATLAS instruments make more detailed measurements than similar ones now flying aboard satellites. Most important, because the Shuttle returns the laboratory to Earth after each flight, it has the advantage of assured calibration. ATLAS instruments are verified to a high level of accuracy prior to launch, and shortly after the Shuttle lands they are recalibrated to ensure their sensitive measurements were absolutely correct. ATLAS missions take a "snapshot" of the atmosphere for about a week at a time. However, atmospheric and solar measurements are being made continuously by instruments aboard free-flying satellites, such as the Upper Atmosphere Research Satellite, launched in September 1991, and various National Oceanic and Atmospheric Administration (NOAA) satellites. Extended exposure to the harsh environment of space, especially to ultraviolet radiation, can degrade the accuracy of those instruments. By comparing data from the ATLAS instruments to their sister experiments aboard the free-flyers, scientists can correct for any drift in the satellite instruments and have a high level of confidence in the accuracy of their measurements. Science Operations Plan The ATLAS 2 science operations plan calls for periods of atmospheric data gathering interspersed with orbits dedicated to solar observations. During their designated orbits, the instruments investigating middle atmospheric phenomena will operate almost continuously. ATMOS will take solar radiation absorption readings during each orbital sunrise and sunset. (An orbital "day," with a sunrise and sunset, occurs approximately every 90 minutes during flight.) 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 portion of these orbits. The ATMOS and MAS instruments will be inactive during solar observation periods. Solar observations are scheduled early in the flight, on two occasions in the middle of the mission, and during the last full day of science operations. 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. The Shuttle orbit has been planned to allow numerous correlative measurements with the Upper Atmosphere Research Satellite. Similar instruments aboard the two spacecraft will make independent measurements of the same regions of the atmosphere at about the same time. Data gathered during these opportunities will be compared to check the accuracy of readings by the satellite instruments. The ATLAS experiments will gather data from about four hours after launch until approximately 12 hours before landing. ATLAS operations will be suspended temporarily during deployment and retrieval of the Shuttle Pointed Autonomous Research Tool for Astronomy (SPARTAN) free flyer, since Shuttle maneuvers required for those activities will prevent proper pointing of the ATLAS instruments. The ATLAS-2 Team The ATLAS program is sponsored by NASA's Office of Space Science and Applications and is directed by the Earth Science and Applications Division and the Flight Systems Division, located in Washington, D.C. The management and control of each ATLAS mission is the responsibility of NASA's Marshall Space Flight Center in Huntsville, Ala. The mission manager directs a civil service and contractor team effort to match science objectives with Shuttle-Spacelab resources so each flight is fine-tuned to gather the maximum amount of science information. This effort includes preparing a minute-by-minute schedule, called a timeline, that combines crew activities, experiment requirements, Spacelab resources and Shuttle maneuvers into an efficient operating plan. Principal investigators of the individual 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 management and science teams control the ATLAS instruments around the clock from NASA's Spacelab Mission Operations Control facility at the Marshall Center. The facility contains banks of computers, monitors and communication consoles which enable the ground team to monitor the payload, collect data, send direct commands to the experiments and communicate with the Shuttle crew. The science teams meet twice daily as a Science Operations Planning Group to evaluate science activities, solve problems and recommend ways to take full advantage of any unplanned opportunities. The two European solar experiments, SOLCON and SOLSPEC, will be jointly operated from the NASA control center in Huntsville and from a control center in Europe during portions of the ATLAS 2 mission. Every ATLAS flight crew is divided into two teams, each of which works a 12-hour shift so science operations can continue around the clock. At least one member of each team has 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 or by the science teams in Huntsville. However, 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 maneuevering the Shuttle when an instrument requires precise pointing or must be operated in a specific attitude. Mission Manager: Program Manager: Ms. Teresa Vanhooser Mr. Earl Montoya Marshall Space Flight Center NASA Headquarters Assistant Mission Manager: Experiment Program Manager: Mr. Gerald Maxwell Mr. George Esenwein Marshall Space Flight Center NASA Headquarters Mission Scientist: Program Scientist: Dr. Timothy Miller Dr. Jack Kaye Marshall Space Flight Center NASA Headquarters Alternate Mission Scientist: Dr. Steve Smith Marshall Space Flight Center Chief Engineer: Ms. Angie Jackman Marshall Space Flight Center Atmospheric Science To understand how the atmosphere evolved to support life on Earth, as well as how it is maintained and continues to change, we must understand more about how it works. Earth's atmosphere, composed mainly of nitrogen and oxygen, with traces of carbon dioxide, water vapor and other gases, acts as a buffer between our planet and the sun. Over the years, human activities and natural occurances have affected the complex components of the atmosphere, but to what degree, we do not know yet. In order to establish a refined point of reference for future observations in the ATLAS series of missions and add to the data gathered on previous flights, three ATLAS 2 investigations will map the characteristics of this protective blanket we call our atmosphere. Earth's atmosphere is composed of five layers: the troposphere, stratosphere, mesosphere, thermosphere and exosphere. These are classified by temperature, pressure and chemical composition. The boundaries of these layers are not exact; they interact and form a chain from Earth's surface to interplanetary space. Since they are interconnected, what happens at levels above the clouds affects life on the ground below. The troposphere is the dense, well-mixed region adjacent to the ground which contains essentially all of Earth's weather. Above it is the stratosphere, home of the life-protecting ozone layer. Next is the mesosphere, the coldest layer of the atmosphere. Chemical activity in the mesosphere is much simpler than in the more complicated stratosphere. By studying the mesosphere, scientists can improve their understanding of what is happening in the stratosphere below, including the photochemical reactions that affect ozone levels. In contrast to the cold mesosphere under it, the thermosphere's temperature varies from 932 to 3,362 degrees Fahrenheit (500 to 1850 degrees Celsius) depending on the sun's activity. The exosphere extends to the undefined region where the Earth's atmosphere gradually merges with interplanetary gases. ATLAS 2 atmospheric instruments concentrate on the middle atmosphere: the stratosphere and mesosphere. Atmospheric Trace Molecule Spectroscopy (ATMOS) Principal Investigator: Dr. Michael R. Gunson NASA Jet Propulsion Laboratory Pasadena, California Changes in the atmosphere have been observed over the years, but the causes and effects of those changes are not fully understood. For this reason, the Atmospheric Trace Molecule Spectroscopy (ATMOS) ethe alterations are man-made, natural or a combination of both. As the Shuttle's orbit carries the spacecraft in or out of Earth's shadow (orbital night), the ATMOS instrument views thethe atmosphere. The spectrometer measures changes in the infrared component of sunlight as the sun's rays pass through this segment of atmosphere, called the Earth's limb. Because trace gases absorb at very specific infrared wavelengths, the science team can determine what gases are present, in what concentrations, and at what altitudes by identifying the wavelengths that are "missing" from their data. The ATMOS instrument is a single assembly mounted on a pallet in the Shuttle's cargo bay. Within the assembly are the sun tracker, detector, thermal control and electronics subassemblies. Commanding for this investigation is controlled by the onboard equipment computer, using what is known as timeline commands. During observation periods, precise pointing for ATMOS is controlled by the sun tracker, capable of tracking its target at two degrees per second. Two mirrors lock onto the sun automatically and direct sunlight into the instrument, where it is collected by a telescope before being split into two beams that travel different paths. These two beams are recombined and directed into a detector, which is cooled to -320 degrees Fahrenheit (-196 degrees Celsius/77 degrees Kelvin). By changing the distance traveled by one beam with respect to the other, different wavelengths of light in the beam come back together. At this time, the beam will either be "in phase" or "out of phase." When light at a particular wavelength is in phase, the two beams simply join together, producing a bright spot. Wavelengths that are out of phase cancel each other out, producing a dark region. This creates a pattern of light and dark bands called an interferogram. Using a mathematical procedure, the interferogram is converted into an absorption spectrum that lets scientists know the intensity of the light at each of these wavelengths. The loss of intensity indicates that sunlight was absorbed by molecules in the atmosphere. This process helps scientists identify the molecules which allow solar radiation to pass through the atmosphere. Since they know that molecules absorb specific wavelengths of radiation from the sun, they can then label which molecules are present in the atmosphere. Most of the total volume of ATLAS data created during this mission (about 1,000 billion bits of information) will be from the ATMOS instrument. Information gathered by ATMOS is returned to Earth using the Shuttle's high-data-rate recorder (HDRR). A new flight tape recorder has been added to the ATMOS package and will be tested during ATLAS 2. This additional flight tape recorder has a higher storage capacity and operates at higher speeds than the HDRR. During the ATLAS 1 mission, ATMOS measured distributions of a greater variety of gases in the stratosphere than has any single space-based instrument to date. Recent analysis of ATLAS 1 data has revealed an increase in levels of chlorine and fluorine since the instrument flew on Spacelab 3 in 1985. Both gases are products of the breakdown of chlorofluorocarbons and are involved in chemical reactions which destroy ozone. The ATLAS 1 instrument also 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 the Philippines in 1991. The ATMOS science team is currently studying how concentrations of ozone and other gases are affected by the presence of this aerosol layer, which is located between 12 to 17 miles (20 and 28 km) altitude and coincides with the ozone layer. Millimeter-Wave Atmospheric Sounder (MAS) Principal Investigator: Dr. Gerard K. Hartmann Max Planck Institute for Aeronomy Katlenburg-Lindau, Germany The Millimeter-Wave Atmospheric Sounder (MAS) studies the chemistry of the ozone in Earth's middle atmosphere. By measuring the strength of millimeter waves radiating at the specific frequencies of water vapor, chlorine monoxide and ozone, MAS provides observations that enable scientists to better understand the distribution of these gases in the atmosphere. The main objective of the MAS is to study the composition of the middle atmosphere in the height range of 32 to 160 miles (20 to 100 km). Chlorine monoxide, formed mainly by the breakdown of chlorofluorocarbons (CFC's) in the middle atmosphere, plays an important part in ozone loss. This compound is produced when CFC's are met by ultraviolet radiation in the upper atmosphere approximately 15 years after their release into the air at the Earth's surface. CFC's come from sources such as freon in coolers and air conditioners, from foam containers and from fire extinguishers. The harmful effects of these products were not foreseen when the chemicals were first put into use. During the ATLAS missions, MAS scans for increases in chlorine monoxide and decreases in ozone, in order to shed light on the impact of chloroflurocarbons on the atmosphere. Thus, global information about ozone and chlorine monoxide helps provide answers to the problem of human influences on the ozone layer, and MAS therefore serves as part of an early warning system to determine how widespread the destruction of ozone really is. Evidence suggests that great increases in chlorine monoxide concentrations, which are very difficult to measure, cause high ozone loss rates during the Antarctic's spring season and participate in forming the ozone hole. During ATLAS 2, MAS will provide important measurements of these chlorine monoxide quantities in both the Southern and Northern Hemispheres. The investigation uses an antenna that scans Earth's limb to collect spectral information at different altitudes. Millimeter-wave radiation coming from the atmosphere enters the steerable antenna, which consists of a three-foot-diameter main reflector and a smaller sub-reflector. The radiation is focused into the MAS receiving electronics. Both temperature and ozone concentration will be measured in this way. By gathering temperature data, MAS can help determine the rates of chemical production and ozone loss in the upper atmosphere. During the ATLAS-1 mission, the MAS instrument acquired results that agreed with theoretical expectations and calculations. For example, a large daylight-to-dark ozone variation was observed at heights above 43 miles (70 km), with much greater quantities on the night side of the Shuttle's orbit. Also, the varied quantities of water vapor measured in the middle atmosphere are consistent with expected results. Investigators plan to test a new method of pointing the MAS antenna during ATLAS 2. They have also developed a new pointing mode for measuring chlorine monoxide. Since chlorine monoxide has a very large daily variation with extremely small quantities at night, the MAS instrument will automatically use the improved pointing mode (to measure water vapor and ozone) on the night side of each orbit. Solar Backscatter Ultraviolet Spectrometer (SSBUV) Principal Investigator: Mr. Ernest Hilsenrath NASA Goddard Space Flight Center Greenbelt, Maryland The Shuttle Solar Backscatter Ultraviolet Spectrometer (SSBUV) will help scientists determine the reliability of ozone information gathered by satellite instruments, which are in orbit for extended periods of time. To map the ozone accurately over the long term, measurements muspaceflight hardware, causing the calibration of the instrument to drift. This calibration drift decreases the reliability of the data collected. For this reason, it is important to identify any changes in an instrument's accuracy and distinguish a drift in the instrument from true ozone trends. SSBUV flies aboard the Space Shuttle and compares its data observations of several ozone-measuring instruments on the National O ceanic and Atmospheric Administrations's NOAA-9 and NOAA-11 satellites and the NIMBUS-7 satellites. During the ATLAS series of missions, concurrent measurements are also being taken with the Upper Atmosphere Research Satellite. The same location is mapped by the Upper Atmosphere Research SaBackscatter Ultraviolet Spectrometer and other ATLAS instruments within a 60-minute timeframe to verify the accuracy of the data collected. The SSBUV measures solar radiation in 12 ultraviolet wavelengths that scatter back from the atmosphere. Because ozone absorbs solar radiation in these 12 wavelengths, the concentration of ozone can be determined when the amount of ultraviolet radiation backscattered from the atmosphere is compared to the amount of ultraviolet light from the sun which reaches the Earth. Variations in the 12 wavelengths of backscattered radiation also indicate how the ozone is distributed by altitude. Ozone absorbs shorter wavelengths of ultraviolet radiation more strongly than it does longer ones. Shorter wavelengths of ultraviolet radiation are backscattered from higher altitudes, while longer wavelengths move deeper into the atmosphere and are scattered from lower levels. The SSBUV spectrometer is located in a Get-Away-Special canister, attached to the side of the Shuttle's cargo bay. A motorized door assembly, which opens up to allow the SSBUV to view the Earth and sun, closes to protect the instrument from contamination when it is not in use. Data, command and power systems are housed in an adjacent canister and connected to the spectrometer by a communications-link cable. The SSBUV instrument will view the Earth and sun, and be calibrated periodically throughout ATLAS 2. During Earth-view operations, the instrument is pointed toward daylit Earth and measures backscattered radiance. Several 30-minute solar viewings and 60-minute calibrations will occur early, midway and late in the flight. During SSBUV operation, light enters the instrument and travels through a system of mirrors and gratings to a photomultiplier. The photomultiplier converts sunlight into an 2lectric current, which isthree-range electrometer amplifier. The desired wavelength is selected by a grating, which is controlled by a microprocessor. The gratings allow the SSBUV instrument to scan through 12 discrete channels in the ultraviolet range. The four longest channels are used to calculate the total amount of ozone in the instrument's view, while the remaining channels determine how the ozone is distributed by height between 15.5 and 31 miles (25 and 50 km). The SSBUV equipment will be activated by the crew aboard the Shuttle Discovery, then controlled from a payload control center at Johnson Space Center. SSBUV science team managers, however, will be stationed at the Spacelab Mission Operations Control center at Marshall in order to participate in mission science planning. Improvements have been made to the SSBUV instrument for ATLAS 2 which give scientists on the ground the ability to control the spectrometer grating drive. This will provide more flexibility and allow the instrument to observe sulfur dioxide and nitric oxide as well as ozone. During seven of the 40 SSBUV Earth-view orbits of ATLAS 1, unique information was gathered about ozone in the upper atmosphere. Scientists were able to study small-scale features of ozone by looking at three ultraviolet wavelengths, four times in a row (rather than by looking at 12 wavelengths for a whole ozone profile). The SSBUV also measured solar ultraviolet radiation during four orbits. Data from ATLAS 1 is being analyzed to identify changes in radiation from the sun as it relates to changes in solar activity measured on SSBUV's previous Space Shuttle flights in 1989, 1990 and 1991. Solar Science Four ATLAS instruments study the sun as their primary science objective. The Solar Constant (SOLCON) experiment and the Active Cavity Radiometer Irradiance Monitor (ACRIM) measure the total amount of light and energy emitted by the sun, called the solar constant. The other two solar instruments, the Solar Spectrum Measurement (SOLSPEC) experiment and the Solar Ultraviolet Spectral Irradiance Monitor (SUSIM) measure absolute solar irradiance as a function of wavelength. Sunlight provides the energy for many atmospheric processes; yet, the sun's radiant output fluctuates over an 11-year cycle, from a maximum to a minimum and back again. Within this 11-year cycle are the short-term variations of the 27-day solar rotation period. Earth's atmosphere is influenced by both cycles, especially by variations in ultraviolet radiation. By gathering nearly simultaneous data on the sun and the atmosphere, scientists hope to identify and quantify the connections between variations in solar energy and changes in the atmosphere. The sun's energy arrives at the top of Earth's atmosphere in the form of gamma rays, X-rays, ultraviolet radiation, visible light (where the energy is most intense), infrared radiation, microwaves anwas named the "solar constant." As more sophisticated and sensitive equipment measured the solar constant with greater precisi, it became apparent that the term was a misnomer: solar energy does indeed fluctuate. Scientists theorize that systematic changes of only 0.5 percent per century could explain the entire range of past climates from tropical to ice age conditions. Therefore, the ATLAS solar instruments were designed to measure this "constant" to a long-term accuracy of plus or minus 0.1 percent or better. Continuous, more accurate measurements of the solar constant will allow future generations to identify solar and climatic trends over the centuries. 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 the circulation of the atmosphere. More accurate measurements of the value of the solar irradiance are needed and can be made only from above the most dense layers of Earth's atmosphere. Instruments can be flown on the Space Shuttle, brought down, calibrated in a laboratory, and then flown again, whereas satellite-mounted instruments can easily degrade in orbit without scientists knowing the precise extent of the change. The Active Cavity Radiometer Irradiance Monitor (ACRIM) Principal Investigator: Dr. Richard C. Willson NASA Jet Propulsion Laboratory Pasadena, Calif. As part of a long-term program to study the physical behavior of the sun and its effect on Earth's climate, NASA is putting together a highly precise collection of information from solar irradiance observations of several Active Cavity Radiometer Irradiance Monitor (ACRIM) instruments aboard satellites, rockets and Space Shuttle missions, one of which will fly on ATLAS 2. The primary objective is to determine the degree and direction of possible variations 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. The main role of the ATLAS ACRIM observations will be in support of extended solar irradiance experiments on free-flying satellites. Periodic reflights of the ATLAS ACRIM are essential to ensure the long-term accuracy and precision of the data gathered by these instruments. An ACRIM instrument flew on Spacelab 1 in 1983, aboard the Solar Maximum Mission Satellite from 1980 to 1989 and the Upper Atmosphere Research Satellite since 1991, and on ATLAS 1 in 1992. Results from the latter mission are still being analyzed. Data from the ATLAS 2 ACRIM experiment will be compared to those made at the same time by the Measurement of Solar Constant (SOLCON) investigation. Through successive comparisons, the accuracy of the satellite measurements can be maintained. These instruments will help establish the total solar radiation scale for the International System of Units. By comparing measurements of the solar constant made during later ATLAS missions, The ACRIM contains four cylindrical bays. Three of the bays house independent heat sensors, called pyrheliometers, which are independently shuttered, self-calibrating and automatically controlled. Each sensor consists of two cavities. The power required to maintain constant temperature differences between the two cavities is used to determine the total solar flux in absolute units. One cavity is maintained at a constant reference temperature, while the other is heated 1.6 degrees Fahrenheit higher than the reference cavity and is regularly exposed to the sun. 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 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 relaMeasurement of the Solar Constant (SOLCON) Principal Investigator: D. Crommelynck Belgian Royal Institute of Meteorology Brussels, Belgium The purpose of the Measurement of the Solar Constant (SOLCON) instrument is to measure the absolute value of the total solar irradiance with improved accuracy and to detect and measure long-term variati SOLCON is a high-resolution, self-calibrating radiometer with a digital processing/converter unit. 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, or device to measure the heat generated electrically as well as by absorbed sunlight. The radiometer system will be checked for accuracy at the beginning and end of each measurement sequence. During the checkout, the shutters of both cavities are closed, and one cavity is heated (using a reference power source). A device 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 orbital sunrise or orbital sunset. The 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 changes in the orbiter's attitude that will correctly position the experiment to point directly at the sun. Actual measurements of solar irradiance are made by pointing the radiometer to the sun's center and opening the shutter ocavity remains closed. The closed cavity is heated electrically until its heat flow to the heat sink matches the heat flow of the open cavity. The energy this requires is in relation to the incoming sunlithe power applied with the shutter opened and closed is a measure of the solar radiation flux. A different mode of operation is to supply constant electrical power to the closed cavity and to reestablish the balance of heat flow by heating the open cavity. During the mission, the experiment will be repeated several times, using the same cavity each time as the experimental one. 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. SOLCON was part of the payloads on Spacelab 1 in 1983 and ATLAS 1 in 1992 and a copy is currently flying on the European es, preliminary results from ATLAS 1 indicate the number of solar spots on the rotating solar disc seem to influence the fluctuation of the values of solar irradiance. When analysis of the 23 orbits of daytime solar observations during ATLAS 1 is complete, these data will be compared to those obtained by instruments on the Nimbus-7 and Earth Radiation Budget Satellites, as well as to those gathered by the Active Cavity Radiometer Irradiance Monitor on ATLAS 1 and on the Upper Atmosphere Research Satellite. The electrical measurements ofprecise enough to allow an improved estimate of the sun's total energy output. Solar Spectrum Measurement from 180 to 3,200 Nanometers (SOLSPEC) Principal Investigator: Dr. Gerard O. Thuillier Aeronomy Service of the National Center for Scientific Research Verrieres-le-Buisson, France The Solar Spectrum Measurement (SOLSPEC) experiment measures ultraviolet, visible and infrared radiation from the sun to determine how solar energy is distributed from 180 to 3,200 nanometer (ultaviolet through infrared) wavelengths. Although most solar energy is contained in the visible and infrared light that reaches the Earth's surface, the energy present in ultraviolet and shorter wavelengths can vary significantly during an 11-year solar cycle. This variation can change the amount of energy, driving changes in the middle and upper atmospheres. Using data gathered by the SOLSPEC instrument, scientists will be better able to understand observed atmospheric changes. The SOLSPEC instrument has an onboard calibration device and three double spectrometers that record solar radiation. It consists of four calibration lamps (two deuterium and two tungsten ribbon lamps) to assure accuracy during flight. The light of these calibration lamps follows the same optical path as the sun's light. During normal operations, SOLSPEC will alternately observe the sun and its calibration lamps at 15-minute inthe Spacelab Mission Operations Control center in Huntsville, Ala., during the first calibration. Later calibrations and observations are controlled through the onboard equipment computer. SOLSPEC is not new to the Spacelab environment. It flew on Spacelab 1 in 1983 and was part of the ATLAS 1 payload in March of 1992. During the added ninth day in space for ATLAS 1, SOLSPEC was able to make observations of the light backscattered by the ozone layer. The preliminary analysis of SOLSPEC data indicates that the values for ultraviolet, visible and infrared light are close to the expected values, which will be helpful in validating scientists' models showing the interaction of sunlight with the atmosphere. These measurements are being compared to those taken by the Shuttle Solar Backscatter Ultraviolet instrument and the Solar Ultraviolet Spectral Irradiance Monitor. The total irradiance measured will be compared with that measured by Solar Constant experiment and the Active Cavity Radiometer Irradiance Monitor. Solar Ultraviolet Spectral Irradiance Monitor (SUSIM) Principal Investigator: Dr. Guenter Brueckner Naval Research Laboratory Washington, D.C. Solar Ultraviolet Spectral Irradiance Monitor (SUSIM) has two purposes. SUSIM measures the fluctuation of the sun's ultraviolet radiation, allowing researchers to understand more about variations inaccuracy of the measuring instrument. Unless the extent of degradation is known, it is impossible to distinguish real changes in solar radiation from the loss of accuracy in the instrument. During an 11-year solar cycle, changes in ultraviolet radiation bring about changes in atmospheric conditions, such as the amount of ozone in the stratosphere. SUSIM operates during the Shuttle's solar-pointing periods to establish a new and more accurate base of solar ultraviolet irradiance (that portion of ultraviolet energy that reaches the top of Earth's atmosphere) data over a wide range of wavelengths. Essential spectrometers with two sets of optics and an in-flight calibration deuterium lamp. Only one of the two spectrometers operates at a time and is designated as the primary unit, taking solar spectral measurements. The second spectrometer gathers data from the same deuterium lamp used to calibrate the primary unit. As the second spectrometer is not to track any degradation in the first spectrometer. An advantage of having the SUSIM instrument fly aboard the Space Shuttle is that it can be calibrated preflight and postflight. During ATLAS 1, SUSIM collected over 100 solar ultraviolet radiation measurements, and its data is being used to recalibrate a similar SUSIM instrument on the free-flying Upper Atmosphere Research Satellite. Crew members aboard the Space Shuttle Discovery will activate, verify alignment and deactivate SUSIM. All other commanding will be done from the Spacelab Mission Operations Control center at Marshall Space Flight Center. ATLAS-2 Investigations Spectral Ivestigation Range Selected Objectives Principal Investigator Atmospheric Science: OS Infrared Water vapor, ozone, methane, M. Gunson, NASA Jet Propulsion chlorine and nitrogen Laboratory, United States compounds, chlorofluorocarbons, others Microwave Temperature, pressure, ozone, G. Hartmann, Max Plank chlorine monoxide, water vapor Institute for Aeronomy, Germany UV Near Ozone E. Hilsenrath, NASA Goddard Space Ultraviolet Flight Center, United States Solar Science: IM Total Energy Solar constant R. Willson, NASA Jet Propulsion Laboratory, United States CON Total Energy Solar constant D. Crommelynck, Belgian Royal Institute for Meteorology, Belgium SPEC Infrared to Solar spectrum G. Thuillier, Aeronomy Service for Ultraviolet the National Center for Scientific Research, France IM Ultraviolet Solar spectrum G. Brueckner, Naval Research Laboratory, United States