Mission Commander Dick Richards thinks Discovery is able to make her short window on the Sun. He calls the orbiter a “tight” ship, meaning that it has displayed no tendencies during her last flight, STS-31 in April 1990, to the kinds of leaks that have plagued Columbia and Atlantis all summer. “Discovery had a very tight flight on STS-31, came back in great shape,” he says. “We have every reason to expect we’re going to have an October flight.
Yet Richards also calls the 19-day launch window tight. “It it was an ideal world, we would have liked to have had about a three-month window to get Ulysses off. This is pretty tight, so we don’t have a lot of room to maneuver as far as handling malfunctions.”
If the window is missed, Ulysses will have to wait 13 months until the planets are again in proper position. “If we can’t do it, we’ll just have to make that painful decision to wait until the Earth spins around thirteen months from now,” Richards says.
The small 807-pound probe, in order to lift itself above the ecliptic plane upon which the planets orbit and gain a view of the Sun’s poles, has to fly directly to Jupiter and gain a gravity assist sling-shot from the giant planet. To achieve enough kick to reach Jupiter, a modified IUS will be used.
The two-stage IUS will be topped with a Payload Assist Module (PAM) upper stage in a combination never before flown together. Ulysses is attached to the IUS at a maximum of eight points. They provide substantial load-carrying capability while minimizing thermal transfer across the interface. Power and data transmission to the spacecraft are provided by several IUS interface connectors.
Built by Boeing Aerospace Co. for the U.S, Air Force, the IUS has been used alone to boost the Magellan and Galileo interplanetary explorers. The PAM booster, built by Mc Donnell Douglas Space Systems Co., has been the first upper stage to be flown on the shuttle and was used on many pre-Challenger flights to boost communications satellites into higher orbits. Having been used on the shuttle as well as on expendable launch vehicles, PAMs have successfully placed over 40 satellites in orbit. A mission-specific modified version, the PAM-S, will be attached to Ulysses. Spacecraft and upper stages altogether weigh about 38,600 pounds.
Ulysses will be the third interplanetary probe launched on the shuttle, preceded by the Magellan Venus radar mapper on STS-30 in May 1989 and the Galileo spacecraft launched aboard STS-34 in October 1989, at this time headed toward Earth on a roundabout route to Jupiter.