WASHINGTON — The National Aeronautics and Space Administration, drastically revising its most ambitious and costly planetary probe for the 1990s, announced Wednesday a plan to send the much-delayed Galileo spacecraft on a six-year odyssey to Jupiter that will begin in the wrong direction.
Space agency officials said that after its launch in 1989, the $890-million Galileo craft will fly initially past Venus--in the opposite direction from its main destination--then skim twice back around the Earth to gain a triple "gravity assist" that will fling the three-ton spacecraft toward Jupiter.
The route, mission design manager William J. O'Neil said, will be the most complex trajectory ever devised for a planetary probe. By playing pinball among the planets, mission planners intend to let nature do what technology cannot.
Galileo's complex new mission is a case of making virtue of necessity. Developed by the Jet Propulsion Laboratory in Pasadena for launch from a space shuttle in 1982, it has been delayed five times, most recently by the Challenger space shuttle disaster in January, 1986. In the aftermath of the disaster, NASA decided not to use the heavy booster rocket that would have propelled the spacecraft on a more direct path to Jupiter.
Saddled with a smaller, substitute booster, mission planners devised the new route that would use the gravitational force of Venus and the Earth to compensate for the smaller booster.
"We've been busy the past two years trying to find a way to get there," project manager John Casani told a news conference.
On its journey, the spacecraft is now scheduled to perform mankind's first close encounter with an asteroid, flying about 620 miles from two fragments of planetary material 10 and 20 miles across known to astronomers respectively as Gaspra and Ida.
In July, 1995, five months before its scheduled arrival at Jupiter, Galileo is to release a 737-pound ballistic probe that is to sample the atmosphere of the solar system's largest planet for the first time. The main body of the spacecraft is to embark then on a two-year orbital tour of four of Jupiter's largest moons, Io, Europa, Callisto and Ganymede.
Booked to Fly Oct. 8, 1989
With shuttle flights scheduled to resume next June, Galileo is booked for the ninth flight on Oct. 8, 1989. The following February, the spacecraft would fly within 9,300 miles of Venus, photographing the planet and receiving its first of three gravitational boosts.
In December of 1990, Galileo would become the first interplanetary probe to return to the vicinity of Earth. As it skims by the Earth at an altitude of 620 miles, it will pick up enough additional momentum to make a two-year loop back into interplanetary space, carrying it for 10 months through the asteroid belt that circles the sun between the orbits of Mars and Jupiter.
Here, on Oct. 29, 1991, according to NASA's current schedule, Galileo will aim its cameras and scientific instruments at Gaspra before heading back toward its last and closest encounter with Earth, skimming by at the slender range of 200 miles.
By "falling" toward earth a second time, Galileo will gain enough momentum to reach Jupiter on Dec. 7, 1995. Along the way, on Aug. 28, 1993, it will pass 620 miles from Ida, the second asteroid on its itinerary.
Space agency officials acknowledge that the need to carom between the gravitational fields of Venus and Earth has added years to Galileo's journey and tens of millions of dollars to the cost of the project. Project manager Casani said that redesigning the spacecraft for a longer, more arduous mission would raise its price to $890 million from $675 million, while the cost of tracking and operating it would more than double to about $450 million when future inflation is taken into account.
Officials insist, however, that a wealth of additional scientific information will more than compensate for the mission's added complexity and cost. In addition to the first close-up look at asteroids, Galileo's new itinerary would provide fresh information from cloud-wreathed Venus and the first surveys with modern, remote-sensing technology of the far side of the moon, which is never visible from earth.
Focal Point of Interest
Its most important task, however, is to launch the sampler probe into Jupiter's atmosphere, the composition of which is believed to have changed little from the early stages of the solar system's development. This, and the fact that Jupiter makes up two-thirds of the mass of the entire solar system apart from the sun, makes it a focal point of interest among scientists seeking to understand how planetary systems evolve.
Plunging into the Jovian atmosphere at 100,000 m.p.h., the probe is to descend on a parachute, radioing back data for relay by Galileo for up to 75 minutes before its batteries die or atmospheric pressure crushes the probe.
Galileo is a joint U.S. and West German project, with the Germans providing several of its instruments and the spacecraft's on-board propulsion system.