On My Own--At Last!

The Galileo Atmospheric Probe Release

Probe Release. How sweet that sounds. Ever since the October of '89 launch from Space Shuttle Atlantis, I've been aboard the Galileo spacecraft, a journey of over 3.7 billion kilometers (2.3 billion miles). And now my solo experience is about to begin. Chief Pat Melia and his Probe Engineering Team (dedicated to my health and career) have checked me out thoroughly, all 338.9 kg (see story, Ready to Go--The Probe Checkout Results). I'm ready for those Earthlings to proceed with my release.

Everything is planned. The Probe Release Sequence Timeline shows the important milestones. On July 5, I will be powered up so I can send data to PET via the new Command and Data Subsystem software; the data will also be tape recorded, just in case. Then on July 11, after they're sure I'm working fine, my umbilical cable, my only communication path to the Orbiter, will be cut. No more instructions from Galileo. I'll be on my own, going ballistic.

Of course, it is very important that the Orbiter target me to the right trajectory. To do this, the Orbiter will establish my entry Flight Path Angle (FPA), the angle between the direction I'm headed and the local horizontal (as measured in Jupiter's atmosphere). Two Trajectory Correction Maneuvers (TCMs 23 and 24) will nudge the Orbiter just enough so that the separation push and all other forces (such as gravity) will place me on a slightly descending FPA of -8.6 deg (+/-1.4 deg) at 450 km altitude above the oblate Jupiter (the planet is flattened at both poles)--the "sea level" at Jupiter being at 1-bar or about one Earth atmosphere.

On July 13, with an action something like the ascent of a child's toy helicopter, the Orbiter will carefully turn to the appropriate attitude for Release, switch from dual-spin to all-spin mode and spin up to 10.5 rpm, and let go! At the "let go'' command, the three separation nuts will explode, and separation springs will give me a gentle 0.3 m/s push to begin my 5-month coast to Jupiter. My attitude at Release, about 10 deg from Earthline, will be parallel to the atmosphere relative entry velocity or zero angle of attack (+/-6 deg) at the point during my entry phase when I am experiencing maximum heating. I want to go in heat shield first so it will protect my instruments from a head-on encounter with the atmosphere.

At key points throughout, the sequence of commands to the Orbiter will stop, waiting for "go commands" to be received from Earth. You can imagine that I am depending heavily on the Flight Team to perform these steps with great care and accuracy.

On December 7, the coast timer will start the action 6 hours prior to entry. After entering the upper atmosphere, slowing down, removing my heat shield, and deploying my parachute, my science instruments will collect data on Jupiter's atmosphere. Weather permitting (heat, rain, snow, and air pressure), I'll relay the data to the Orbiter via radio before my batteries wear out. They'll only last about 75 minutes or so. It will be the end for me, but what a thrill to say, "I fulfilled my mission!''