QUESTION:
What is the "time delay" in communications betwen Earth and Galileo and 
what effect does this have on flight control, ie. "critical burn" timing.

ANSWER from Steven Tyler on December 20, 1995:
 The one-way light time between the Earth and Galileo varies.  During 
Jupiter Orbit Insertion, it was about 52 minutes (I think the actual 
value was 51 minutes and 59 seconds).

Right now, Jupiter and the Earth are just about on opposite sides of the 
Sun.  Sometime in the next couple of days, Galileo and Earth should be at 
our maximum distance apart for the mission ( a one-way light time of 
about 52 minutes and 21 seconds).  The distance will slowly decrease as 
we approach summer.  Our minimum distance in 1996 will be near the time 
of our first encounter with Ganymede (June 27), when the one-way light 
time will be about 34 minutes and 45 seconds.  In 1997, early in August 
we should be at a minimum time of aobut 34 minutes a means at least an 
hour and 44 minutes.

Of course, we can not reply instantly from the ground.  If we are going 
to look at what the spacecraft is doing, it will take a few minutes to
recieve any data, a few more minutes to be sure the data look proper, and 
a few more minutes to approve the simplest possible command.  This means 
a wait of 2 hours just to send a "Go" command to the spacecraft (and it 
could be much longer if there ae problems with a ground transmitting 
station).

In some cases, the wait time seems significant.  For example, supppose 
there were a rapid leak of Helium preesurant from the spacecraft.  If the 
one-way light time were 2 seconds and we could easily read the Helium 
pressure reliably at once, we might have considered monitoring the Helium 
Pressure from the ground and pushing a button that would seal off the 
pressurant in case of a fast leak.  With a respnse delayed by over an 
hour and a half, that strategy won't work.  All the helium could leak out 
in a few minutes, so the response would come too late.  As a result, we 
put in autonomous monitoring of helium pressure.  If the pressure 
dropped, the spacecraft would take action automatically.

But reducing the wait time might not have made much of a difference in 
terms of controlling our critical Jupiter Orbit Insertion burn.  This 
burn had to be accomplished on schedule for us to achieve orbit.  We put 
in some very complex software to guarantee that the brun would continue 
even in the presence of a variety of faults.  It would be difficult to 
rely on the ground to provide this sort of control even with perfect and 
immediate visibiltiy.  We might react slowly on the ground, and it might 
be hard to be sure that the commands we sent would be recieved on the 
craft.  In addition, telemetry can take a while to reach our screens 
(even after it reaches Earth), and thre may be mistakes in it.

The biggest effect of the 52-minute one-way light time involved a few 
commands that we hoped would be recieved on the spacecraft at specific 
times.  We simply sent them 52 minutes before they were supposed to be 
recieved.  But there aren't any critical commands that have to be sent in 
this manner.  Any truly critical command is aslready on board the craft 
well before the critical sequence begins.

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ANSWER from Dennis Byrnes on December 28, 1995:
Information from Galileo comes back by radio. Since radio waves
are the same as light, but with a much longer wavelength, they
travel at the speed of light. Light travels very quickly, about
300,000 kilometers per second (that's about 186,000 miles per
second). Since Jupiter is very far away, right now (Dec. 28, 1995)
it takes light (and radio) waves 52 minutes and 20 seconds to
get from Galileo to the Earth! Since we are on the opposite side
of the sun from Jupiter we are about as far away as we will ever be.
Next summer, about the time that Galileo goes by the moon Ganymede
for the first time, we will be as close as we ever get. This is
because then the Earth will be on the same side of the sun as
Jupiter. Then it will take about 35 minutes and 40 seconds for light
and radio to reach us from Galileo.

With the information I have given you, you can make a scale drawing
and determine how far both the Earth and Jupiter are from the sun.
(Hint: draw two circles around the sun, one big and one small. The
distance from the edge of the small circle through the sun to the
edge of the big circle is the longer time times the speed of light.
The shorter time times the speed of light is the distance from the
small circle straight out to the big circle.)

Unfortunately, since our big radio antenna on Galileo is broken, we
can only get information from Galileo with the small antenna. This
means that it can come back only a little bit at a time. It's kind
of like your teacher reading you a story, but only reading one letter
of each word every few seconds! You would have to write it all down
very carefully and then after many hours you could read the story.
You have to be very patient to read a story this way don't you?