ASADENA, Calif., June 29 - The Cassini spacecraft is closing in fast on its destination, Saturn, and mission officials say it is all set to apply its rocket brakes on Wednesday evening to become the first robotic visitor to orbit the spectacularly ringed planet and its family of 31 known moons.
Flight controllers here at the Jet Propulsion Laboratory said on Tuesday that the spacecraft was on course and appeared to be in excellent shape after journeying nearly seven years from Earth to the solar system's second largest planet, after Jupiter.
A 96-minute rocket firing, considered the riskiest maneuver of the mission, is to begin at 10:36 p.m. Eastern time as the spacecraft comes up from underneath the planet on the sunlit side of its broad system of rings. It will be shortly after midnight before the fate of the mission is known.
Planned and supported by NASA and the European Space Agency, the $3.3 billion mission is considered the most complex interplanetary venture ever undertaken. If all goes well, Cassini, the American craft, is supposed to complete at least 74 orbits of Saturn over the next four years, and perhaps continue long afterward. The smaller European craft, Huygens, is to be released from Cassini on Dec. 24 to land three weeks later on Saturn's largest moon, Titan.
At a news conference here, where the mission is being directed, flight engineers said that no problems that could jeopardize the mission hung over their heads. They radioed the last command to Cassini on Saturday, leaving the driving through the entire approach and orbital maneuver to onboard computers.
Scientists have already had their appetites for discovery whetted by the spacecraft's close pass of Phoebe, the planet's outermost moon. Pictures and data of the 137-mile-wide moon's dark surface, deep craters and patches of water and carbon dioxide ice moved Dr. Dennis Matson, the project's chief scientist, to call Phoebe "a real darling."
And last Friday, scientists picked up growling sounds, like a thunderstorm heard through static, coming from Cassini. The spacecraft had crossed from interplanetary space into Saturn's magnetosphere, a region bounded by an invisible bubble enveloping the planet and most of its satellites. The bubble is created by the expanding strength of the planet's magnetic fields beating back the pressure of electrified gases, the plasma of the so-called solar wind. The sound of the crossing is like an announcement of the impending arrival at Saturn.
So if there are no more pre-orbit commands to be sent and no malfunctions to contend with, Robert T. Mitchell, the project manager at the laboratory, was asked what flight controllers would be left to do between now and the time Cassini is either in orbit or not. "Chew our nails," he said.
As soon as the spacecraft settles into orbit, it is to turn its cameras down for a long close look at the rings and also conduct observations of the planet's outer atmosphere. On this maneuver the spacecraft will be making its closest approach to Saturn of the entire mission: about 12,400 miles above the outer fringes of the planets' thick gaseous atmosphere.
The 4,700-pound Cassini and the attached 700-pound Huygens were launched in October 1997 from Cape Canaveral, Fla. The combined craft took the long way to Saturn, with close flybys of Venus and Earth for a gravitational boost to get to Jupiter, which in turn, supplied the final spurt of energy to reach Saturn. In all, the flight has covered more than 2.2 billion miles.
Both craft are named for astronomers who made important early discoveries about Saturn. Jean Dominique Cassini, director of the Paris Observatory in the 17th century, discovered several of Saturn's satellites and detected the first gaps in Saturn's distinctive rings. The spacecraft is aiming to fly through the 18,000-mile gap between the F and G rings. Christiaan Huygens, a Dutch astronomer in the same century, was the first to recognize that Saturn had rings.
In the mission's first four years, Dr. Jeremy Jones, leader of the navigation team, said the Cassini spacecraft was expected to execute 157 maneuvers to change course for 45 close encounters with Titan, the object of greatest scientific curiosity. It has a dense nitrogen-methane atmosphere and evidence of complex carbon chemistry, perhaps resembling conditions on early Earth.
Dr. Jean-Pierre Lebreton, chief scientist for the European Huygens operations, said, "Titan is like going back to Earth four billion years ago."
