Scientists Agree on Spacecraft’s Jupiter Itinerary

Scientists at the Jet Propulsion Laboratory in Pasadena have agreed on the itinerary of Galileo, a spacecraft that will be launched from the space shuttle next May on a voyage of three-quarters of a billion miles to Jupiter, the largest, and some say the most mysterious, planet in the solar system.

The agreement capped years of work for a team of “tour designers” who used computer simulations to custom-tailor the 10 orbits the craft will take on its exploration of the giant, gas-shrouded planet and its four major moons.

The challenge, according to team manager Roger Diehl, was to come up with a series of orbits that “squeezed the most science” into the mission. The process entailed juggling the often-competing interests of more than 100 scientists who will devour the data that will stream earthward from a variety of instruments aboard the spacecraft.

Jupiter, which can easily be seen after sunset as a steady, white light in the southeastern sky, has fascinated scientists since Galileo Galilei first discerned its four largest moons in 1610. Astronomer Fred Whipple has called it “the antithesis of everything earthlike.” The vast ball of liquid hydrogen, helium gas and traces of other elements more closely resembles a star that never ignited than it does the other planets.

One-Hour Life Span

As the 5,622-pound spacecraft approaches Jupiter in 1988, a probe crammed with instruments will be ejected from the main orbiter and parachute through the turbulent, many-layered atmosphere toward the planet’s molten core, transmitting millions of bits of data during its one-hour life span, until it is crushed by the quickly increasing pressure.

The orbiter will then be steered into an elliptical orbit around the planet. It will carry nine instruments that range from an extremely sensitive electronic camera to a dust detector that will measure micrometeorites and other particles that may form Jupiter’s ring.

The three main scientific objectives of the $864-million mission are analyses of Jupiter’s atmosphere, magnetic field and moons.

Jupiter crackles with energy, emitting radiation of all sorts and a powerful magnetic field that extends millions of miles into space. Margaret Kivelson, professor of space physics at UCLA, is a principal investigator on a team that will study the “magnetosphere,” the array of charged particles trapped in this field.

Andrew Ingersoll, a professor of planetary science at Caltech, will direct the study of the Jovian atmosphere, which seethes with storms including the Red Spot, a swirling oval cyclone as wide across as four Earths. Photographs and soundings using radio waves will plumb its depths, he said.

Each of the four Galilean moons--Io, Europa, Ganymede and Callisto--has its own mystery. Io, the “pizza moon,” as one JPL researcher calls it, is shaken by perpetual volcanoes. Europa, in contrast, is a smooth “cue ball” of ice and may contain a buried sea of liquid water. Ganymede, the largest known moon in the solar system, is covered with “claw marks” of unknown origin, and Callisto’s surface is more thoroughly pocked with craters than any other object in the solar system.

Galileo will pass close to Io only once, on its initial approach. That moon, being the closest to Jupiter, is in a belt of destructive radiation that can damage the craft. Each of the other moons will be visited up close at least twice.

The tour designers had to keep in mind the main targets--the atmosphere, the magnetosphere and the moons--in crafting the series of orbits, according to JPL’s Diehl. The five-man team used computers to estimate the continually changing positions of Jupiter and its moons from Galileo’s arrival on Dec. 10, 1988, through the 22-month tour. By exploiting the known gravitational pull of each of these objects and planning a few carefully timed bursts from the orbiter’s rockets, they devised a way to swing Galileo from orbit to orbit as if it were an acrobat flying from trapeze to trapeze.

Duration Limited

The length of the mission is limited by the supply of propellant that is used periodically to nudge the craft back on track and by progressive damage to Galileo’s microcircuits caused by Jupiter’s radiation, said Aron Wolf, one of the designers.

Louis D’Amario, who has worked on the Galileo project since 1977, used a computer program of his own design to fine-tune the tour, simultaneously accounting for the gravitational tug of the distant sun, each moon and Jupiter itself. “Every time you change something, that effect cascades downstream and gets amplified,” he said. “To keep track of this inside the program is a very difficult problem.”

More difficult than the purely mathematical task of charting the tour was the task of making it fit the “competing objectives” of diverse scientific interests, said Torrence Johnson, JPL project scientist.

Everyone prefers that his atmospheric measurement be taken first, or her moon be photographed first. The reason: as time goes by, the probability of equipment failure increases. Those who go first have the best shot at getting their data back to Earth.

Because of recent refinements in their computer programs and technique, the designers were able to add a number of extra, “nontargeted flybys” of moons to the mission. “The result has been a tour that gets about twice the science as the tours we were thinking about three or five years ago,” Johnson said.

All Groups Agree

The extra fiddling paid off. Diehl said he presented five options, winnowed down from dozens of possible tours, to a gathering of about 60 scientists last Wednesday. The meeting split into three groups, each concerned with a particular area of study. After separate, lunchtime huddles, they had all chosen the same tour.

“The designers responded very well to inputs from very different directions,” said Michael Belton, an astronomer at Kitt Peak National Observatory in Arizona and the leader of the photographic imaging team. But, he added, no scientist is ever truly satisfied. “We are somewhat insatiable creatures,” he said.