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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, Belgi
um

SPEC     Infrared to   Solar spectrum           G. Thuillier, Aeronomy Service f
or
         Ultraviolet                            the National Center for Scientif
ic
                                                Research, France

IM       Ultraviolet   Solar spectrum           G. Brueckner, Naval Research
                                                Laboratory, United States