At midnight on Dec. 31, thousands watched the ball drop in New York City's Times Square, marking the arrival of 2001. But out in space, after traveling several billion miles, two satellites were sending home snapshots of another famous orb -- the mysterious planet Jupiter and its four largest moons.
As the intrepid probe Galileo -- which has been orbiting Jupiter since late 1995 -- completed its 29th circuit of the giant planet on Dec. 28, it was joined by another spacecraft, Cassini, which swooped by to test its instruments and pick up a gravitational shove on its way to distant Saturn. In what NASA dubbed the Millennium Flyby, Cassini made its closest encounter -- a mere 6 million miles away from Jupiter -- a bit early on Dec. 30.
The $3.4 billion Cassini, which was launched on an 11-year mission in 1997, will work in tandem with Galileo until it departs the Jovian environs in March. Scientists hope that the simultaneous collection of data by the various instruments on the two probes will help solve some of the riddles of the fifth -- and largest -- planet in the solar system.
"Now we can make observations with a spacecraft that has more capabilities than any that has visited Jupiter," says Bob Mitchell, Cassini program manager at the Jet Propulsion Laboratory in Pasadena, Calif., which manages the operations of both spacecraft.
PERFECT STORM? Indeed, Jupiter is more than twice as massive as all the other planets combined. It consists mainly of hydrogen and helium surrounding a solid core about 14 times the mass of Earth. The Galileo spacecraft photographed immense lightning storms that churn across its surface, where winds reach 400 miles an hour. The largest storm, the Great Red Spot, is a vortex wide enough to hold two Earths and has been swirling away for at least 300 years.
Its four largest moons, discovered by the astronomer Galileo in 1610, are just as strange. Ganymede, the largest moon in the solar system, is bigger than the planet Mercury and is the only moon known to generate a magnetic field like that of Earth. The innermost, Io, is the most volcanically active body in the solar system. About the size of Earth's moon, Io has more than 100 volcanoes spewing very hot lava and giant plumes of gas and dust.
The smallest Galilean moon, Europa, is an icy world that may contain more water than all of Earth's oceans. Its surface is marked by "rafts" of ice that appear to have formed when liquid water welled up from below and froze. Callisto, its surface marred by ancient craters, is calm by comparison. But instruments on Galileo lead scientists to suspect that this moon, which is roughly the size of Saturn's satellite Titan, may also have a thick layer of melted, salty water under its ice-rich surface.
CHANGES IN THE WIND. The two spacecraft -- the first to visit Jupiter since Pioneer and Voyager spacecraft in the 1970s -- have already swung into action observing Jupiter's magnetic environment, atmosphere, rings, and moons. As Galileo's orbit brings it closer to Jupiter, its instruments are gathering 14 weeks of continuous data about the vast bubble of magnetic force that surrounds the planet and contains its dangerous radiation belts.
"The data we're collecting are part of a collaboration with Cassini to understand how the magnetosphere of Jupiter responds to changes in the solar wind," says Duane Bindschadler, manager of Galileo science operations at JPL.
Galileo zipped past Ganymede on Dec. 28. Because the moon was in the shadow of Jupiter, the spacecraft was able to examine the faint glows of aurora -- like Earth's Northern Lights -- that would be overwhelmed by sunlight at other times.
"It looks like a nice, calm flyby," says Jim Erickson, Galileo project manager. "Ganymede is one of the most interesting places in the solar system, and we're looking forward to see what new surprises Galileo may tell us." The data will help scientists determine the chemical makeup of the gases in Ganymede's atmosphere and the structure of its magnetic field. They hope to compare this data to Titan's when Cassini reaches Saturn in 2004.
SOLAR SOUNDS. Meanwhile, Cassini, which is a joint project of NASA, the European Space Agency, and the Italian Space Agency, has sent back spectacular images of Jupiter, its Great Red Spot, rings, and four major moons. Researchers have pieced together images to make a video that shows Jupiter's mysterious orange and white bands sliding against each other, and the gyrations of giant oval storms. And, as Cassini approached Jupiter, it recorded the eerie sound waves created as charged particles from the solar wind are deflected by the planet's magnetic field.
Protestors feared the plutonium on Cassini would contaminate Earth during a "slingshot" manuever
Both spacecraft have had their share of troubles. Cassini was launched amid protests in October, 1997. The reason: The power source for its instruments is provided by three radioisotope thermoelectric generators, which produce electricity from the heat emitted by the radioactive decay of plutonium. RTGs have powered about two dozen spacecraft -- including Galileo -- and are useful in the outer reaches of the solar system where sunlight is too weak for solar panels to be practical.
But Cassini carries a record 72 pounds of plutonium, and protestors feared that a mishap could've showered Earth with the deadly element when the spacecraft roared past the planet in August, 1999. To perform this "slingshot" maneuver, which was necessary to increase the craft's speed and put it on a course to Jupiter, Cassini had to descend to just 700 miles over the Pacific at a blistering 42,000 miles an hour.
BROKEN COGS. Although the Earth encounter didn't end in disaster, Cassini's controllers are now unable to communicate with a probe that was intended to be parachuted to the surface of Titan. And just days before its closest approach to Saturn, the systems that control the pointing of Cassini's instruments began malfunctioning and were shut down. The problem, however, was solved several days later.
Two years after it was launched in 1989, Galileo's main communications antenna failed to unfurl, limiting communication with the spacecraft to a small back-up antenna. Then, Galileo's data-storage tape recorder jammed.
Even so, Galileo slid into orbit for a planned two-year mission on schedule in 1995. And it has proved to be far tougher than its tenders at JPL thought possible. When its initial work was completed, its mission was extended, and it was sent on increasingly hazardous missions, swooping deep into Saturn's radiation belts.
"BATTLE FATIGUE." After enduring more than twice the time in orbit and three times the radiation dosage that it was originally planned to withstand, it continues to send home valuable information. By the time the last of the data collected from Ganymede are transmitted to Earth in April, Galileo will have logged 2.8 billion miles. "Galileo is showing some signs of battle fatigue, but it's still a capable spacecraft," says JPL's Erickson.
Indeed, the venerable Galileo and its younger sibling, Cassini, offer powerful proof that the U.S. space program is still on track, despite the depressing loss of two Mars probes in 1999. And Cassini, along with the recent occupation of the International Space Station, are an affirmation that space exploration is no longer a race between superpowers but an international effort. Don't be surprised if some human leaves a footprint in the red Martian dust in the not-too-distant future.
Photos of Jupiter and its moons
of Jupiter's atmosphere
Recording of solar wind By Alan Hall in New York