Hurdles on the way to Jupiter
It's easy to find Jupiter this month. The planet blazes throughout the night. Getting there is something else again. Two major spacecraft are lined up for the single ``planetary mission opportunity'' that NASA's new shuttle manifest offers for the fall of 1989. Both were to have gone by shuttle last spring. One of them will have to wait yet again, at least until late 1990.Skip to next paragraph
Subscribe Today to the Monitor
Galileo, the mission to orbit Jupiter and probe its atmosphere, has already waited a long time. Conceived a decade ago, it has been repeatedly delayed as NASA shifted its launch from unmanned rocket to the shuttle and then postponed it following the Challenger accident last January.
Ulysses, the joint US/European mission to orbit over the poles of the sun, also has to go by Jupiter. It needs to swing around that planet to twist its course out of the sun's equatorial plane, which is roughly the plane of Earth's orbit, and over the solar poles. Available rockets can't send it there directly.
The fact that the projects' managers here at the NASA Jet Propulsion Laboratory (JPL) can even contemplate a 1989 launch is a tribute to the skill of mission engineers who have redrawn flight plans and rethought hardware to fit NASA's new circumstances.
It's not just a matter of rescheduling shuttle flights. Centaur liquid hydrogen/oxygen rockets were to boost the craft out of the orbit in which the shuttle placed them. NASA judged the Centaur to be too dangerous for shuttle-based operation. So mission engineers turned to a rocket originally designed to boost shuttle-released satellites to higher Earth orbits. It goes by the esoteric name of Inertial Upper Stage or, more simply, IUS. Two of those, used as a double stage booster, can send substantial payloads into interplanetary space.
When it comes to sending craft to Jupiter, however, the IUS doesn't have the Centaur's power. Galileo project manager Allan E. Wolfe says his mission can only make it by clever navigation. In effect, the new trajectory his team has worked out taps fuel caches along the way. It takes energy out of the orbital motion of Earth and Venus.
It's the same technique, called gravity assist, that has allowed Voyager 2 to go from Jupiter to Saturn to Uranus and on toward Neptune. A spacecraft swings by a planet in such a way that the planet's gravity puts the craft on a new course and kicks it along. The planet slows down imperceptibly while the spacecraft gains new energy.
For a Nov. 4, 1989, launch, Galileo would pass some 20,000 kilometers from Venus Feb. 19, 1990, and swing back to within 3,600 km from Earth Dec. 11 of that year. Then it would head out through the asteroid belt, tweak its orbit with a slight rocket burn Dec. 20, 1991, and return to Earth Dec. 6, 1992. That second flyby would be close, within 300 km of the planet. There's little margin for error between setting Galileo on course to arrive at Jupiter Nov. 29, 1995, and burning it up in the atmosphere. Mr. Wolfe calls this celestial ballet VEEGA (Venus, Earth, Earth, Gravity Assist).
For now, November 1989 remains the target date for the American Galileo team and its German partners, who supplied the craft's propulsion system. They'll have to wait another 13 months for the planets again to be in a favorable arrangement if their rival Ulysses gets NASA's nod for the 1989 launch opportunity.