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.
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.
In making their case for launch priority, the Ulysses team may feel it has justice on its side. The European Space Agency (ESA) is providing the spacecraft while NASA supplies the launch. NASA was also to have provided a second spacecraft in what had been planned as a twin-probe mission. Then, without consulting ESA, NASA cancelled its spacecraft to ease budget pressure.
That was over half a decade ago. Project manager Willis G. Weeks doubts that any of the European scientists remain bitter over what they consider a breach of contract and of good faith. He says, ``The scientists are still excited about this mission. It's new science for us. Nobody's ever been over the [sun's] poles before.'' Nevertheless, he adds, ESA will use its grievance as a negotiating tool to argue for extra consideration. ``You can bet that ESA people will be pushing to get us out first,'' Mr. Weeks says. He's aiming for a Sept. 3, 1989, launch.
While Ulysses won't go through such tricky maneuvering as Galileo's VEEGA trajectory, its booster is a challenge. The probe needs an extra kick beyond the push of an IUS. That will come from a PAM-D engine. It's the same rocket that lofts communications satellites into their 22,300 mile-high working orbit after being released by the shuttle.
The IUS/PAM-D combination has never been carried by the shuttle. Weeks calls integrating that rocket combination with the shuttle -- that is, finding a safe and efficient configuration -- ``a formidable task.'' He's confident his team can do it in time for a September 1989 launch. As with Galileo, the next launch window is 13 months later in October 1990.
The longer any such mission is delayed, the better are the prospects that an unmanned rocket will be available as an alternative to the shuttle. It would be a commercial vehicle -- a CELV (Commercial Expendable Launch Vehicle) in space jargon. It would probably use a Centaur upper stage. So once again, project managers would have to redesign their flight plans.
However, managers such as Wolfe and Weeks can only speculate about using CELVs. JPL deputy director Robert S. Parks says that ``right now, we're not even allowed to consider unmanned rockets.'' He points out that such rockets cost $200-$250 million. That being the case, he explains: ``[NASA's administrator says] Where's the extra money going to come from? Whereas he looks at the shuttle launches as more or less for free. You're going to be launching them anyway. . . . You can debate that bookkeeping. But that's the way it looks to him.''
Even with an available shuttle date, there's no guarantee the upper stage boosters will be available. Neither the necessary IUS stages nor the PAM-D for Ulysses are likely to be produced in time for a 1989 launch. So the project managers are hoping to borrow such rockets from an Air Force mission or another NASA project that has them in stock but doesn't need them right away. JPL would then replace the borrowed hardware. Wolfe notes that this strategy can work only if the rockets' owners don't have an unexpected need for the boosters themselves. Any launch program JPL puts together now, says Wolfe, ``is not quite a house of cards. But it takes a lot of careful planning and understanding.'
Second of three articles. Next: Voyager's triumph.