Galileo Nears Epic Jupiter Rendezvous : Science: Key maneuver is set for Thursday. Two decades of scientific hopes ride on the distant spacecraft.
It’s white-knuckle time once again at the Jet Propulsion Laboratory in Pasadena, 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 in 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 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 around 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 look 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--about 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 about 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 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.”
Galileo’s most excellent adventure started when it was chosen to be the first spacecraft launched from the space shuttle in 1982. A series of delays--plus the Challenger explosion--postponed its launch until 1989, and forced it to take an out-of-the-way route through the solar system that stretched its travel time from 2 1/2 to six years.
Disaster threatened again in April, 1991, when Galileo’s 16-foot antenna got stuck while unfurling, rendering the craft’s main mouthpiece for sending data to Earth mute. Luckily, the long launch delays allowed engineers time to upgrade its on-board computers with sophisticated software that did not exist when the mission was planned--performing what Johnson called “brain surgery” on the craft. Meanwhile, receiving technology on Earth has improved enough for NASA’s three Deep Space Network antennas to work as a team to capture a much weaker radio signal.
With luck, Galileo may still accomplish about 70% of its scientific goals. Most of what will be lost are global color images of the planet, although project scientists still expect to send back about 1,500 images, as well as continuous data on Jupiter’s magnetic field.
But success is hardly in the bag. The rendezvous will require delicate maneuvering. Already, Jupiter “has put its lasso” around both the probe and the mother ship, said project manager William O’Neil. At the rate that Jupiter’s gravity sucks them up, both will be traveling well over 100,000 miles per hour when they reach the planet.
Ever since the probe separated from the mother ship July 13, Galileo’s offspring has been on its own. As JPL scientists await its first signal Thursday, Johnson said, “that will definitely be white-knuckle time.”
Six hours before entry, the probe, about three feet by four, 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 the 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,” Young said.
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.
Shortly before it slows to 100 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 trickle down to 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 an artificial moon. Looking like a giant insect with its mechanical arms akimbo, the orbiter carries 10 scientific instruments on two segments.
Over 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 will sling the spacecraft onto the next moon in a heavenly series of perfectly timed do-si-dos.
The Jupiter system boasts a freakish family of moons: Volcanoes on hotblooded Io spew sulfurous plumes hundreds of miles high. Icy Europa may well harbor a huge liquid ocean under its cracked crust--a water world that could be a likely environment for life forms like those found in Earth’s own deep ocean floors. Ganymede--the largest moon--has enormous parallel ridgelines, like glaciers on Earth, and deep trenches--signs of geological activity. Yet Callisto is heavily cratered, like the Earth’s moon, and equally dead.
Scientists would love to know why such differences appear in bodies influenced by roughly the same forces.
It is not surprising that Jupiter was named after the head honcho of the ancient gods. It has everything other planets have, and then some. Sitting on the borderline between planet and star, Jupiter “is about as big a planet as you can make,” Johnson said. Yet its fierce gravitational contraction causes it to “shine,” radiating twice the energy it receives from the Sun.
Just why Jupiter got to be so immense--1,300 times the size of Earth--remains something of a mystery, although Johnson explains that the giant simply may have started gobbling up matter before the other planets began to condense out of the cloud; before long, it would have had a strong gravitational grip on all matter in its area.
The composition of Jupiter is essentially the same as that of the sun--mostly hydrogen and helium. But because Jupiter isn’t quite big enough to ignite a nuclear fire in its belly and become a star, it has not altered the composition of the matter it was formed with. At the same time, its powerful gravity has kept matter from boiling off or blowing away.
That makes it a perfect, pristine laboratory to study the stuff that went into making our solar system. Its gravity has allowed nothing to escape its grasp, and the matter it started with would not have changed its form.
Jupiter’s weather also offers lessons in stellar and planetary atmospheric science. In a sense, the entire planet is an atmosphere, since it is gas all the way down until pressure turns hydrogen into a metallic liquid thousands of miles down (although some scientists think the core may be rocky).
Swirling orange-brown ammonia clouds paint stripes on the rapidly spinning sphere, which rotates in a 10-hour day. At the equator, winds get up to 250 mph, then reverse direction, then reverse again, causing bands of colors and eddies near the borders. “It’s a major puzzle what drives these wind systems,” Young said. Another major puzzle is why lightning appears to strike in some latitudes but not others.
The cloud patterns also create semipermanent features such as Jupiter’s great red spot--a 300-year-old storm as big as several Earths. Just how this organized structure can survive in such chaos is beyond current understanding.
To many scientists, however, 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 Io’s volcanoes and captured in the magnetic clutches of its mother planet.
It is a hazardous environment for satellites carrying delicate electronic equipment, so Galileo will spend as little time there as possible.
Ironically, Io’s volcanoes came as a complete surprise when Voyager visited the planet in 1979; so were the faint rings that surround Jupiter. In fact, Johnson said, theorists were so skeptical about the possibility of rings that the mission almost didn’t look for them. The lesson, he says, is “never believe a theoretician. . . . I would be amazed if we don’t find things we don’t expect.”
Whatever surprises are in store will have to wait, however. With its nonfunctioning main antenna and crippled tape recorder, Galileo has precious little room for storing information and less ability to send data to Earth. That means that it will have to be picky about what data it collects, and hundreds of scientists throughout the world waiting for the data will have to be patient.
One major disappointment already announced is that Galileo will not take pictures when it skims by Io on its closest approach to this most active moon.
“It’s an enormous loss,” said UCLA astrophysicist Margaret Kivelson, chief investigator on the project. “But we are also enormously grateful that we will get [magnetic field data].”
Kivelson has suggested that a magnetic field on Io might explain some of its most unusual properties. Mostly, the craft will suck up and store away only select parts of the data, then slowly dribble it back when Jupiter and Earth lie in favorable positions.
For now, Johnson said, the mission is “all in the laps of the gods. And if they fail us, there’s always the engineers.”
(BEGIN TEXT OF INFOBOX / INFOGRAPHIC)
Galileo / Probing Jupiter
GETTING TO JUPITER: Unable to use a rocket powerful enough to boost Galileo all the way to Jupiter, engineers figured out a way to use the gravity boosts from Earth (twice) and Venus (once) to sling the craft to its destination. On two trips through the asteroid belt, it discovered a new moonlet.
****
THE PROBE: Protected by heat shields, the probe--carrying six experiments--will slam into Jupiter’s atmosphere with a force 230 times Earth gravity as it slows from 106,000 mph to 100 mph in 2 minutes. A drogue parachute will pull off the rear heat shield, exposing the instruments for direct measurements of the giant planet.
****
THE ORBITER: The 2.5 ton spacecraft, designed at JPL in Pasadena, carries 10 scientific instruments. (It’s shown here after the probe was jettisoned.) Because the main umbrella-like antenna failed to unfurl properly early in the mission, thousands of images will be lost. Instead, engineers will rely on a tiny ancillary antenna--boosted by sophisticated new software--to relay data to Earth.
****
THURSDAY: Due to data transmission problems, the orbiter will not take close up pictures of Io, as hoped. It will take data about magnetic fields.
****
THURSDAY: The probe’s 75-minute descent into the planet’s gaseous atmosphere will measure temperature, pressure, density, chemical composition and the form--ice crystals or drops of liquid--of the clouds, and relay the data to the orbiter. Increasing heat and pressure will eventually vaporize the probe.
****
NEXT STEP: After receiving the probe’s data the orbiter will continue on for 22 months, orbiting Jupiter and its moons 11 times.
Sources: Jet Propulsion Lab; NASA; Hughes Aircraft; Scientific American.
Web site: www.jpl.nasa.gov/galileo/index.html
(BEGIN TEXT OF INFOBOX / INFOGRAPHIC)
Mission Chronology
1977: Galileo mission approved by Congress.
1982: Planned launch, delayed until May, 1986, because of problems with rocket design.
1986: Shuttle Challenger explodes, delaying all shuttle flights.
1989: Galileo launched from shuttle Atlantis, but less powerful rocket requires tricky trajectory, including two loops around Earth and one around Venus for gravity assists--stretching travel time from 2-1/2 years to 6.
1991: In April, main antenna fails to unfurl. Problem blamed on loss of lubricant during Galileo’s many long-distance truck rides between Cape Canaveral and Pasadena. JPL engineers figure out how to use an onboard magnetic tape recorder to store collected data until the small secondary antenna can trickle it back to Earth.
1991: In October, first-ever asteroid encounter with Gastra suggests odd-shaped object might have magnetic field.
1993: Asteroid Ida flyby, discovery of first moon, Dactyl, around an asteroid.
1994: In July, Galileo watches Comet Shoemaker-Levy 9 smash into Jupiter.
1995: In March, new software begins operating in Galileo computer; partially makes up for loss of main antenna.
1995: Several times during the summer, Galileo flies through largest planetary dust storms ever measured.
1995: July 13, successful separation of probe from mother ship.
1995: Oct. 11, tape recorder fails to rewind, threatening virtually total loss of data; JPL engineers figure out a fix that requires some of tape to be off limits--meaning no data can be taken during closest encounter with moon Io.
1995: Dec. 7, 2:56 pm PST. Probe enters Jovian atmosphere.
1995: Dec. 7, 5:19 p.m. Main engines begin burn to put Galileo into orbit.
1995: Dec. 19. First scientific “quick look” results from probe.
1996: March, second set of new software installed in main computer.
1996: Late spring. First images will arrive on Earth.
