It wasn't a trivial matter to get Atlantis and its launch vehicle- 2,056,277 kilograms
(4,523,810 pounds) at launch-into orbit. Each of the three main engines in tail of the
shuttle can provide almost a half-million pounds of thrust. The thrust to weight ratio
for these engines (about 70:1) is the best in the world- each engine weighs less than
3,200 kilograms (7,000 pounds) but puts out the power equivalent of seven Hoover
Dams! The shuttle experiences a maximum of 3 g's of gravity (that is, three times the
gravitational force that we feel here on Earth) during ascent; due to vibration, loads
on parts of the spacecraft may exceed 10 g's.
Since the shuttle needs to have a daylight landing opportunity at the trans- Atlantic
landing abort sites, and since there are performance constraints on Galileo's inertial
upper stage, spacecraft liftoff could only occur during certain periods of time. The
launch opportunity opened on Oct. 12, 1989 for a 10-minute period. The launch
window then grew each day, reaching a maximum of 47 minutes on Nov. 2. The
window then decreased each day through the remainder of the launch opportunity,
which ended on November 21, 1995.
Aiming at Jupiter
To get to Jupiter, Galileo had to be inserted into its interplanetary trajectory at the
correct time so that, when it arrives at Jupiter's orbit, Jupiter is right there (and not,
say, half an orbit away!). This task might be compared to throwing a water balloon
at someone running in front of you. You have to lead the aim point by just the right
amount to hit the target; if you aim at where the person is right when you throw the
balloon, you'll end up missing the target. If an interplanetary spacecraft misses its
launch opportunity, we have to wait for the orbital motion of the planets to realign
them into the "correct" geometry for a successful launch opportunity.
Because Galileo used a VEEGA (for Venus-Earth-Earth Gravity Assist) trajectory
to fly to Jupiter (rather than a direct trajectory), its launch opportunities did not
repeat at regular intervals the way direct trajectories do. This is because there are
many ways to combine a launch from Earth with one Venus and two Earth flybys to
get to Jupiter. The next three VEEGA opportunities to Jupiter after the one Galileo
used occurred in November 1989 and May/June 1991. There were actually two
different opportunities in the May/June 1991 time period.
The Jet Propulsion Laboratory, Pasadena, CA manages the mission for NASA'is
Office of Space Science, Washington , DC.
This image and other images and data received from Galileo are posted on the
World Wide Web, on the Galileo mission home page at URL