QUESTION:
When you flew the ATLAS package, were the telescopes mounted to the orbiter,
or did you have to have an extra gyro stabilized mounting to damp out
orbiter jitter?  I have read about a tail toward earth stability, but does
this help?

ANSWER from Tracy Gill on March 24, 1997:
Well, this is a good question.  The ATLAS science package was flown on one
Spacelab pallet for the ATLAS-2 and -3 missions and on two pallets for the
ATLAS-1 mission.  ATLAS was an Earth observation mission primarily and
really didn't have any telescopes for astronomical observation.  It was
mainly a suite of spectrometers to study chemical composition of the Earth's
atmosphere.  Actually ATLAS-1 did have a telescope for far ultraviolet
observations which operated briefly on that mission.  For ATLAS, there was a
package on board called the horizon sensor.  One of its primary functions
was to provide more accurate pointing information than was provided by the
orbiter navigational system.  And the orbiter was manueuvered for
observations using that additional data.

What you may be referring to were the ASTRO missions.  These two missions
also flew on two Spacelab pallets and had three ultraviolet telescopes
mounted on a platform pointing system.  ASTRO-1 also had an X-ray telescope. 
The ASTRO package did have a gyro system on it called the Image Compensation
System.  It did keep the pointing system's platform more stable than the
rest of the orbiter.  In the cases of both ASTRO, ATLAS, and even other
microgravity missions where crystals are being grown or fluid samples are
being studied, there are critical time periods for science gathering.  At
such times, the orbiter can be put in a "free drift" mode.  This is the most
stable mode the orbiter can provide.  This means that even small orbiter
thrusters will not fire at all for a short period.  These periods have to be
fairly brief however because the thrusters are required to keep the orbiter
Ku-band antenna pointed correctly at a satellite for data downlink to Earth. 
In especially criitcal times, the astronauts will even be asked to remain
still for a short period, and activities such as crew exercise on a
treadmill are not scheduled in conjunction with critical science
observations.

The tail toward Earth orientation that you descibed is called the gravity
gradient attitude.  Extra stability is provided in the orientation because
the heaviest portion of the orbiter, the aft or tail, is closer to the
gravitational pull of the Earth. This is the preferred orientation for
science missions where you are growing crystals or studying fluids over long
periods because orbiter thruster firings are minimal.  The International
Space Station, once operational, will be maintained in a gravity gradient
attitude for science operations like these that will be conducted on board. 
For ATLAS and ASTRO however, too many manuevers have to be made to point at
different targets to maintain a gravity gradient attitude.

I hope I was able to answer your question and provide you with a little bit
more insight into our operations.