ATLAS 3 Public Affairs Status Report #20
6:00 a.m. CST, Nov. 13, 1994
MET 9/19:00
Spacelab Mission Operations Control
Marshall Space Flight Center
Huntsville, Ala.

The Atlantis crew maneuvered the orbiter's cargo bay to face 
the sun last night for the last of four ATLAS 3 solar 
observation periods.  All four solar experiments -- the 
Active Cavity Radiometer Irradiance Monitor (ACRIM), the 
Solar Constant (SOLCON) and Solar Spectrum (SOLSPEC) 
experiments, and the Solar Ultraviolet Spectral Irradiance 
Monitor (SUSIM) -- are adding nine orbits of observations to 
the excellent data they acquired during previous solar 
periods.  The Shuttle Solar Backscatter Ultraviolet (SSBUV) 
instrument is operating in its solar-viewing mode during one 
of the sun orbits.  Solar observations are a vital part of 
this atmospheric mission, because energy coming from the sun 
drives the Earth's climate system and the photochemistry of 
ozone in the stratosphere.

ACRIM Co-Investigator Dr. Roger Helizon, of NASA's Jet 
Propulsion Laboratory, is especially pleased with comparative 
sunspot activity measurements his team has made with similar 
instruments aboard the Upper Atmosphere Research Satellite 
(UARS).  "We learned more about the sensitivity of this 
device than ever before," he commented, adding that the sun 
during ATLAS 3 has "just the right amount of sunspot 
activity" to facilitate sunspot measurements.

SOLCON's unattended monitoring of solar irradiance began at 
around 11:00 p.m. CST with instrument activation, and the 
instrument continues to make solar observations.  Data from 
these observations will help determine the total amount of 
energy reaching Earth from the sun and how this energy 
changes over time.

The solar constant experiments, designed to take measurements 
to an accuracy of one-tenth of one percent, are part of a 
long-term plan during which scientists hope to collect 100 
years of data on solar variations.  The total energy output 
of the sun varies only slightly, but variations of just one-
half percent over a time scale of decades are thought to be 
capable of creating major climate changes.  Observations of 
the sun made from space, above the distorting influence of 
the atmosphere, can be much more precise than ground-based 
readings.  The first observations with this level of accuracy 
were made by NASA's Solar Maximum Mission in the 1980's.

Before it powered down and entered a deep space cooling 
period in preparation for its solar observations, SSBUV took 
measurements in a single wavelength, just short of the 
visible light wavelengths.  This data will help the 
scientists better understand ultraviolet scattering in the 
atmosphere, which is important in understanding the accuracy 
of ozone measurements.  After one orbit of pointing at deep 
space for its cooling period, SSBUV began its solar 
observations, and is in the process of planning unprecedented 
lunar observations in conjunction with SUSIM and SOLSPEC to 
measure the moon's albedo, or the ratio of reflected to 
incoming sunlight, in various wavelengths.  According to 
SSBUV Principal Investigator Ernest Hilsenrath, this will be 
the "most accurately calibrated set of instruments ever to 
look at the moon."

The Atmospheric Trace Molecule Spectroscopy (ATMOS) 
experiment, having completed all of its atmospheric 
observations for the mission, is gathering solar spectral 
data by looking directly at the sun with no atmospheric 
interference during sunsets.  This will provide solar 
scientists with a high quality solar infrared spectrum.  
ATMOS has already taken more solar data than on all of its 
previous missions and is hoping for a solar spectrum with a 
high ratio of signal to noise, to "capitalize on the 
opportunity to maximize the amount of data we have," 
according to Principal Investigator Dr. Mike Gunson.

Following an instrument calibration, SOLSPEC performed three 
sets of direct Earth measurements in the ultraviolet ranges 
to assist in the determination of the chemical composition of 
the atmosphere.  The instrument then began unattended solar 
observations.  SOLSPEC is currently gathering data on solar 
irradiance in the ultraviolet, visible and infrared 
wavelengths.

After performing a pre-observation calibration to help 
monitor the instrument's high-resolution performance, SUSIM 
resumed performing solar observations.  The instrument is 
currently obtaining a complete set of spectral scans to 
determine the ultraviolet solar irradiance.

Currently, crew members are scheduled to deactivate the 
payload later this evening.  Science teams and payload 
controllers at Marshall are still awaiting word as to whether 
the Shuttle landing will be delayed due to weather.  ATLAS 
instruments might have an opportunity for four bonus 
observations, two atmospheric and two solar, if a decision is 
made to postpone landing.

After the mission, analysis of these data will begin, 
producing results which will eventually be publicly archived.  
"The data belong to everybody," observed NASA Headquarters 
Program Scientist Dr. Jack Kaye.  "Following post-flight data 
analysis, the data ultimately will be deposited in Earth 
Observing System Data Information System archives at NASA's 
Goddard Space Flight Center, where it will be made available 
to atmospheric scientists around the world."