Galileo @ Jupiter

Planetary Science hits a high note

Galileo is to robotic planetary science missions what Apollo 13 was to manned exploration, only better. After all the near disasters and heroic improvisation, the spacecraft and its probe have accomplished their primary goal and made it to Jupiter.

by Larry Lowe

NOTE: This piece, originally written upon the arrival of the Galileo spacecraft at Jupiter in 1995, was republished on the web as supplemental material to the collection of photographs compiled and annotated by Tony Reichhardt in the Aug/Sept 2002 issue of AIR&SPACE, entitled "Galileo's Last Look"

The Beckman Auditorium on campus at Cal Tech was jammed to capacity, all eyes focused on the video projector image of a solid purple graph with a single yellow line traced horizontally part way across the top. A muted voice over from an open mike somewhere in the bowels of JPL provided an irrelevant monologue as the line proceeded at a snail's pace across the big screen. Gathered in the auditorium was a loose conglomeration of science teachers, media types, Planetary Society members and others with a common interest in planetary science, enough to sell out the hall weeks in advance.
The video cut abruptly to a long shot through a glass window of a number of aging rocket scientists in the "Tele-Com" room staring intently at a computer monitor and nervously mouthing unheard comments. This momentous event in planetary science history was represented by what appeared to be a group of insurance salesmen clustered around a thawing aquarium betting on when the last icicle would fall.

Suddenly smiles broke out on the faces of the group and a few hands were raised in the air. Soon the room was silently clapping and thumbs-upping one another. The video in the Beckman auditorium cut back to the purple graph. The single yellow line had changed direction and was inching downward at a 45 degree angle. This slim evidence was all there was to indicate the moment Earth first learned that the Galileo spacecraft had begun to slow enough to be captured into orbit by Jupiter's enormous gravity.

The line represented the frequency of a carrier wave transmitted by the Galileo spacecraft, the units on the vertical axis calibrated to represent the Doppler shift induced by the 106 thousand mile per hour velocity of the spacecraft away from Earth. The fact that it was now inching downward meant that the spacecraft was slowing down. And that implied that the German-made main spacecraft engine, cold soaked over the course of a 2 1/2 billion mile journey from Earth to Jupiter, must have ignited as planned.

The Jupiter Orbital Insertion (JOI) burn had begun, and there was not a dry eye in the house.

Arrival At Jupiter: Although they parted company in July of 1995, the Orbiter and the Probe arrived at Jupiter the same day. Until six hours before entry the Probe had been in a dormant state since its separation from the Galileo Orbiter on July 13, 1995 when both were at a distance of 81,520,000 km (50,660,000 miles) from Jupiter. Due to a last minute concern over the condition of the onboard tape recorder, the data of Io was not captured on the entry pass, to be certain of recording the data from the probe as it drifted down through the Jovian atmosphere.

For the next 49 minutes, the attendees were treated to a round-robin video tour of the JPL mission control facilities and a talk show set while various dignitaries and anonymous engineers alternately stared into their computer monitors, waited for main engine shut off and explained how exciting this all was. Tension built slowly again until finally the Yellow line leveled off precisely where the blip in the graphic indicated it should, and the mission director announced to the grand satisfaction of all concerned that Galileo was now officially in orbit around Jupiter.

After a break for dinner, the invited guests returned to Beckman to hear NASA administrator Dan Goldin proclaim the event a great day for planetary science. This alone signaled the real importance of the event, for it represented the first time in the history of the Space Program that the NASA administrator had been present at a significant unmanned planetary science fly-by or orbital insertion. Goldin's presence - and his remarks - underscored the new administrator's commitment to unmanned planetary science. Goldin spoke of the dawn of a new age of planetary science, of the commitment for two missions to Mars every launch window, of a fleet of cheap, small, fast and smart robotic exploration craft fanning out across the solar system to take stock of our resources and map our local space.

"All I asked for," Goldin said "is for projects that would be 10 times more capable, 10 times faster, do 10 times more science, and be 10 times cheaper." He thinks he's getting it. Goldin's description of the next generation Mars probes is filled with pride at what he calls a new can-do spirit at NASA.

But the day belonged to Galileo, one of the last of the expensive, big mission projects of the old school. Goldin praised the team on its resourcefulness in shepherding the spacecraft through what he termed the "Perils of Pauline" to see it successfully entered into orbit around Jupiter.

Galileo as a project is now almost 20 years old. The spacecraft was mission specified, designed, build and tested and nearly ready to launch by 1986. Then Challenger was lost. In the aftermath, the plan to put an Atlas Centaur upper stage along with Galileo inside the cargo bay of the shuttle, which would deliver the package out of the gravity well of Earth for launch was scrapped, and the mission had no effective means of propelling the spacecraft to its intended target.

The solution was a slingshot roller coaster ride through the inner solar system that resulted in enough gravitational whiplash after two trips past Earth and one past Venus to fling the Galileo spacecraft and its probe out towards Jupiter. On October 18, 1989 the mission was finally launched from the cargo bay of the Shuttle Atlantis during STS-34.

Last Look Back: On December 16, 1992, Galileo took this picture of Earth and the Moon from 6.2 million kilometers as it headed out towards Jupiter.

The next problem presented itself when the big high gain main antenna failed to open, depriving the spacecraft of an effective means to transmit the data gathered by its senors and the probe. By resorting to data compression and refitting the receivers on Earth, the Galileo project engineers managed to recover 70% of the original data rate using one ten thousandth of the original bandwidth.

The problem after that evidenced itself when the onboard tape recorder, required to capture the burst of data from the probe, appeared to be stretching its tape, and had to be shut off while engineers devised a means to insure it would not pull the tape off one spool and be lost altogether. A work-around was devised and the errant tape recorder was rendered useful, if only carefully, by never letting the tape get wound to its end.

Probe Mission Profile: Protected by its aeroshell, the probe entered Jupiter's atmosphere at 170,700 kilometers per hour, making it the fastest projectile ever created by humans. Once the aeroshell burnt away, slowing the probe, first a drogue chute and then a main parachute deployed to allow the probe to make its measurements before it was crushed.

Earlier in the day the spacecraft had signaled that it had managed a lock-on to a telemetry stream - presumably from the probe - and was storing it on the useful portion of the tape. It will be weeks before the full amount of data from the probe's entry into Jupiter's atmosphere will be recovered on Earth. All anybody knew at the moment was that after 2 1/2 billion miles of space travel and a series of mission-threatening cliffhangers worthy of a Saturday matinee, everything had appeared to work and the first ever probe of a gas giant had sent its data to the waiting orbiter for safekeeping and relay to Earth.

Atoms to Atoms: The probe concluded its existence as such by being absorbed into the Jovian atmosphere after reaching depths where the pressure and temperature evaporated its various parts.

Galileo Chief Scientist Torrence Johnson spoke next, too happy with the momentary outcome of the mission to go into elaborate detail of the science planned. There were only two new facts he could report with confidence, he said. "The moon count of Jupiter has just gone up by one, and the planet itself has gained a small amount of weight." Johnson emphasized that this moment, the Orbital Insertion, was really only the beginning of the actual science mission. If Voyager was a 5 bank pool shot, then Galileo can be thought of as a 14 bank shot. Now that Galileo was in orbit around Jupiter, it is an observatory that will monitor the planet and make periodic close passes on the various moons for another two years. The schedule laid out shows the interlocking variables of the close passes on the planet and its moons when the Galileo spacecraft will be in full data capture mode and the long orbital periods when the spacecraft will be out of the intense radiation of the planet and transmitting the data stored on the fragile tape recorder through the back up navigation antenna back to Earth.

11 Bank Shot: Over the next 23 months Galileo will have 10 very close encounters with the large moons of Jupiter, using the gravity of each moon to tweak its orbit enough to retarget for the following encounter. The next encounter will be with Ganymede on July 4, 1996.

Three of the four major moons are scheduled for a close look. One of the intriguing possibilities is that of a room temperature water ocean under the ice pack of Europa. Aside from some sunlight, most of what you would need for life may be down there. But it is too early for much speculation, after all, the whole point of the mission is discovery. "Exactly what we target on each moon and what we study most closely on later encounters will be determined by what we find on the first looks."

The finale of the evening came when Cornell Astronomer and Planetary Science icon Carl Sagan strolled onto the stage and into the spotlight. He began by dispensing with his prepared speech - the moment, he said, demanded more. Choosing his words carefully, placing the trademark emphasis on the occasional adjective, bobbing his head gently like a bird pecking at a tray of seed corn, Sagan described the social circumstances of the spacecraft's namesake and poked a sharp edged needle at the institutions who would presume to dictate man's place in the universe.

"Only in 1992 has the Catholic Church come to admit that the Earth revolves around the sun and that mankind is not the center of the universe. This is a step in the right direction." Sagan paused to deliver the stinger. " A little late, perhaps, but in the right direction."

Sagan is the master of the long view perspective. He managed to educate the audience on why the Galileo mission was a milestone in planetary exploration and not just another probe to another planet. "We are in exciting times. The encounter of Galileo is an important historical event. I feel confident that 500 years from now, the entry of the Galileo spacecraft into orbit around Jupiter and the first actual contact of a gas giant outer planet by our probe, will be remembered in the history books long after the media - and the public - have forgotten the debates over the budget of the country, the sordid tales told on the afternoon talk shows, the campaign promises of presidential candidates or the testimony given in murder trials."