BOSTON — THE Jupiter survey spacecraft Galileo is coming home, briefly. It will speed past Earth Tuesday morning - flying over the South Atlantic Ocean - to get the final slingshot-like boost it needs to reach its target.
Project scientist Torrence Johnson says it is coming in right on time and right on course to make the tricky maneuver succeed. Pleased that the wandering spacecraft is at last on its way to the giant planet, he exclaimed, "everything's looking great. We're going to Jupiter. There's no doubt about it." Galileo is scheduled to reach Jupiter on Dec. 7, 1995.
This is a triumph for Dr. Johnson's navigational colleagues at the California Institute of Technology's Jet Propulsion Laboratory (JPL), which manages the Galileo program for the National Aeronautics and Space Administration. In addition to guiding Galileo during 1991 on the first asteroid inspection in history, they have already guided the craft through two of these "gravity assist" maneuvers since it first left Earth in October 1989. The first of the two 1990 trips sent the craft around Venus, and the second sent it traveling past Earth.
Johnson credits the controllers with the skill and precision needed to handle the trips. "All of the navigational things strike me like black magic. [The controllers] do a terrific job."
Galileo will race past Earth's moon Monday evening, coming within 68,350 miles of the satellite at 10:58 p.m. Eastern time. A little over 11 hours later, it will pass within 189 miles above the South Atlantic Ocean. It will skim the outer atmosphere at 32,400 miles per hour. That's a much faster moon-Earth passage than the old three-day journeys of the Apollo astronauts.
This is an opportunity for mission scientists to utilize Galileo's planet-scanning skills. It will take images of Australia, Indonesia, and the Andes. It will scan Antarctic ice clouds, whose crystals help activate ozone-destroying chemicals. Johnson says that it will not have as sharp a view as Earth-resource satellites now provide.
But he explains that it will demonstrate a planet-scanning technology - a different kind of technology that will be useful in future Earth monitoring.
When it comes to completely new scientific knowledge, Johnson sees the most payoff in Galileo's inspection of the moon. It will look directly down on the north polar lunar region. This has not been studied since the lunar orbiter missions of the 1960s.
Johnson notes that Galileo has more powerful instruments than those spacecraft. "Lunar scientists are excited," he says.
Mission engineers are also looking forward to Galileo's return as their last - and best - opportunity to release its stuck main antenna. Johnson explains that this high-gain antenna is needed to return the large data flows from Jupiter swiftly and efficiently.
The antenna is constructed to open like an umbrella. Galileo project manager William O'Neil has said that the equipment is performing as though three of the ribs, or spokes, are stuck. Dr. O'Neil has explained that a thermal adjustment should give it "the dimensions it had when manufactured." He anticipates this adjustment through a general warming of the antenna as the Galileo craft reaches the perihelion of its orbit - the point closest to the sun - on Dec. 13.
At that point, JPL controllers plan to take advantage of the adjustment to hammer the deployment mechanism open by turning its drive machinery on and off in rapid bursts. If the antenna still does not open to its full 16-foot-wide dish shape, the mission will not be lost. According to Johnson, mission planners now know how to use the craft's working low-gain antennas to recover much of the data. It just will be a more tedious and slower process.