QUESTION:
I read about the difficulty with the deployement of Galileo's antenna.
Why did NASA worry about the spacecraft getting too cold?  What
happens when the spacecraft goes below the allowed temperture?

ANSWER from Erik Nilsen on March 6, 1997:
The spacecraft is designed to operate within a fairly narrow range 
of temperature.  When things get cold, electronic circuits quit 
working reliably, and if they get too cold, they could be damaged.  
Propellant for the thrusters can freeze in the propellant lines, 
causing them to burst.  Instrument optics and sensors can be damaged 
by excessively cold environments.  On the flip side, things can get 
too hot, which can cause a lot of problems too.
     
In general, the environment of space is very unforgiving.  The sun 
is a huge radiator, and any part of the spacecraft pointed towards the 
sun can get very hot and the parts of the spacecraft that are pointed 
away from the sun can get very cold (close to absolute zero, 
approximately -460 degrees F).  When designing the spacecraft, the 
engineers take into account the expected operations (this end of the 
spacecraft will be pointed to the sun, etc) and design the systems to 
accommodate the thermal environment.  In addition, in all of the parts 
of the spacecraft which have specific temperature requirements (where 
the computers and electronics are) the designers include heaters to 
keep the equipment warm.
     
When we were trying to get the antenna free, we didn't have much 
that we could do.  We could turn on the motor again, but that didn't 
work the first time and we wanted to save the motor for later.  So to 
try and spring the antenna free, we decided to try and heat up the 
antenna (make it expand) then cool it down (make it contract).  By 
heating and cooling, we hoped to work loose whatever was holding the 
antenna shut.  We did this by turning the antenna sideways to the sun 
(so the sunlight could hit the antenna and warm it) then turn the 
antenna away from the sun (so no sunlight would hit it, to cool it).  
This also had the effect of turning parts of the spacecraft towards 
the sun that would normally not be in the sun, and vice versa.
     
So now, the problem was, we were operating the spacecraft in a 
manner the original designers did not anticipate.  If we turned away 
from the sun (to cool the antenna) for too long, some critical pieces 
of equipment could get too cold (or too hot, if they were now in the 
sun).  We had to calculate the length of time we could stay pointing 
away from the sun to allow the maximum cooling of the antenna, without 
damaging other equipment in the process.  This timing was critical, 
and the maximum time we stayed at the cooling attitude (with the 
antenna pointed away from the sun) was 52 hours.  By the same token, 
when trying to warm the antenna, other parts of the spacecraft became 
critical limitations on the length of time at the warming attitude.
     
We were able to do the activities and we did not permanently damage 
any other parts of the spacecraft.  We didn't get the spacecraft 
antenna open, but we tried everything we could that wouldn't endanger 
other parts of the spacecraft or the mission.

Can you guess why the Galileo spacecraft is covered in black 
material, and the Magellan spacecraft is covered in white?  Well, 
Galileo was designed to go to Jupiter, which is farther from the sun 
than the Earth.  The effects of sunlight decrease the farther from the 
sun that you go.  For Jupiter, the effect is about 1/25 as much as on 
Earth.  So the designers used black covering material to absorb the 
sunlight, and help keep the spacecraft warm.  However, Magellan was 
designed to go to Venus, which is closer to the sun.  The designers 
needed to keep the spacecraft cool, so they covered it in white 
material to reflect the sunlight.  You will also see gold material 
covering sensitive equipment, which reflects the sunlight.