It’s white knuckle time once again at the Jet Propulsion Laboratory in Pasadena, Calif., as a determined spacecraft named Galileo nears its long-awaited rendezvous with Jupiter on Thursday.
After an odyssey of six years and 2.3 billion miles, a 745-pound probe that separated from the mother ship last July will blaze through the ammonia cloud cover of the planet like a shooting star to enter a realm never visited by human or robot - the inner world of a giant gas planet.
Galileo itself will cruise 120,000 miles overhead for about 75 minutes to gather signals from the probe; then it will kick itself into orbit to begin a two-year tour of the 16-moon Jovian system.
The maneuver is fraught with risk: If the probe pierces the Jovian sky at an angle even a degree and a half wrong, or if the orbiter engines fail to push Galileo into proper orbit, almost two decades of scientific hopes could be destroyed.
Anxious scientists at the Jet Propulsion Laboratory already have sprouted a lot of gray hair waiting for this moment. The $1.5 billion Galileo mission was approved by Congress almost 20 years ago, and its attempt to reach the planet named for the king of the gods has been like one long good news/bad news joke.
Time after time, JPL’s engineers have been called in to save the mission from what seemed like certain disaster. And for each bit of bad luck, Nature rewarded the mission with another piece of serendipitous good fortune; the spunky craft was in the right place at the right time to discover the first moon orbiting an asteroid, and it had a front row seat for the crash landing of comet Shoemaker-Levy on Jupiter last year.
Now that Galileo has arrived, researchers are looking forward to their first chance to study what amounts to a miniature solar system. As much a failed star as a giant planet, Jupiter rules a system of at least 16 moons. Its enormous bulk - some 318 times the mass of Earth - makes it a major player in the solar system, with more mass than all the other planets and moons combined. Its gravitational pull is so great that it distorts the orbits of all other bodies and flings countless stray comets out of the solar system - in a sense, sweeping a clear lane for Earth, and the evolution of life.
Most intriguing, scientists think that Jupiter remains a pristine glob of the original cloud that formed our solar system some 5 billion years ago. Learning about its inner nature should reveal much about the process of turning interstellar dust into stars and planets.
“We think we know pretty much what the composition of the material was,” said Galileo project scientist Torrence Johnson. “But then a miracle occurs and planets form … the details are sketchy.”
Never before, said probe scientist Rich Young, has an instrument from Earth peered “under the surface of a giant planet.”
Six hours before entry, the roughly 3-by-4 foot probe will get a wake-up call, telling it to turn on its batteries and get ready to collect data. The entry has to be as precise as a hypodermic needle slipping through skin - too shallow, and probe will skip out; too steep and it will turn to cinder before it gets below the clouds. “It’s the most difficult atmospheric entry we’ve ever done,” said Young.
When the probe slams into the ammonia crystal clouds, it will be twice as hot as the sun, and traveling twice as fast as a rifle bullet.
As soon as it’s slowed to about 1,000 mph, two parachutes will open, and explosive bolts will blow off the protective cocoon to bare the miniature chemical laboratory inside. Six instruments will measure temperature, pressure, density, sunlight and chemistry. They will look for lightning, which is expected to be frequent and fierce, and 200 mph winds.
Among other things, scientists hope to learn what causes the colors in the candy-striped planet’s atmosphere. The ammonia cloud cover is just the icing. The probe should penetrate 125 miles through many layers before it vaporizes minutes later, and its molecules disperse to become part of the giant gas planet itself.
While it takes radio signals only 52 minutes to get to Earth from Jupiter, scientists will have to wait until at least mid-December to see the data; most will not arrive at JPL until next year. Meanwhile, the information will be stored on Galileo’s computer and on the tape.
The next tense moments come about an hour after the probe mission ends, when Galileo revs its main rocket engine for a 49-minute burn. If all goes well, that push will nudge the craft into Jovian orbit - making it into an artificial moon. Looking like a giant insect with its mechanical arms akimbo, the orbiter carries 10 scientific instruments on two segments.
For the next two years, Galileo will orbit Jupiter 11 times, each time swinging back for a look at another one of the large “Galilean” moons - like the spacecraft, named after the 16th-century scholar who discovered them. Each encounter slings the spacecraft onto the next moon in a heavenly series of do-si-dos.
To many people, nothing will be more interesting than getting a close look at Jupiter’s magnetic field, which extends farther out than any in the solar system. Its tangle of field lines stick out like porcupine quills, then fold back into interlocking loops. In the center sits a highly charged halo of electrified particles swirling around the planet’s equator - probably material spewed out of volcanoes on the moon Io and captured in the magnetic clutches of its mother planet.