Rocket Power

     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
     http://www.jpl.nasa.gov/galileo.