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Two shuttle crewmen take a historic walk

United States astronauts have opened a new dimension in space flight. Floating untethered from their mother ship Challenger, like scuba divers no longer tied to the surface by air hoses, they maneuvered freely in three dimensions.

From now on, astronauts will begin to work as independent units. They will literally be satellites in their own right. This will give them a working freedom that will be needed when the next shuttle mission, scheduled for April, tries to retrieve and repair the Solar Maximum Mission satellite. It will be essential to the eventual construction of a space station.

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At this writing, mission specialists Bruce McCandless II and Robert L. Stewart were finishing their extravehicular activity (EVA) Tuesday. For them, testing the jet-powered backpacks has been a personal adventure. Mr. McCandless, who has spent a decade preparing for such independent space flight, first tried out one of the two units within the shuttle payload bay. Propelled by 24 nitrogen-firing jets, the manned maneuvering unit (MMU), proved easy to handle. Soon he was floating free of the shuttle.

Recalling Neil Armstrong's famous remark when he first stepped onto the Moon, McCandless quipped: ''That may have been one small step for Neil, but it's a heck of a big leap for me.''

Relative to the shuttle, the MMU-riding astronauts moved slowly. Maneuvering out to distances of 150 and 300 feet, they often moved at less than a mile an hour. At the same time, though, they, like the shuttle, were zipping along at orbital speed. Just to keep this in perspective, mission commander Vance D. Brand reminded McCandless that ''you may get the name of the world's fastest human being, going along there at 4 miles a second.''

This is not a trivial fact. An astronaut moving on his or her own with the MMU is subject to the laws of orbital flight just as the shuttle is. This means that every time the MMU jets are fired to move from one point to another the human satellite is changing its orbit slightly.

For example, a thrust away from the shuttle or toward another satellite may, from the orbital viewpoint, be equivalent to firing a retrorocket. This is likely to be the case when astronaut George Nelson approaches the Solar Max satellite on the mission now scheduled for April. The astronaut's change in orbital speed would then tend to make him drop below his target if he did not fire jets to compensate.

Besides testing the MMU - which McCandless called ''a nice flying machine'' - the two astronauts also practiced docking with a Solar Max standin. On Thursday, both men will practice docking with a shuttle pallet satellite that will be rotating on the end of the mechanical arm, duplicating the rate of rotation of the ailing Solar Max satellite.

The astronauts themselves rode on a platform on the end of the arm. This tested the arm's maneuverability as a work platform for repairing satellites held within the payload bay. They also practiced the specific repairs that are to be made to the Solar Max satellite using a special mock up.

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At this writing, mission officials were calling the EVA a success. A repeat performance is scheduled for Thursday.

However, NASA officials are far from calling Mission 41-B, as a whole, a success. The failure of the Payload Assist Module (PAM) - a solid-fuel rocket motor - to complete its burn on two communications satellite launches defeated the mission's primary goal. As with most shuttle missions now, the main objective is to launch satellites and carry out other jobs for fee-paying customers.

Both Western Union's Westar and Indonesia's Palaba B-2 satellites are working. But they are in orbits that render them useless as communications satellites. There has been speculation that one or both of them might later be rescued by a space shuttle. But Shuttle Program Manager Glynn Lunney says that, at this time, the National Aeronautics and Space Adminstration does not know whether such a rescue would be feasible.

Dr. Lunney also says that it is too soon to know whether the double failure will affect futher missions. A number of scheduled satellite launches depend on the PAM. It has worked well on five previous shuttle satellite deliveries. But until engineers learn what went wrong this time and can ensure better performance in the future, the launch schedule will be in doubt.

NASA officials emphasize that neither the astronauts nor operation of the Challenger is to blame. This was not the case, however, with the lost 6.5-foot balloon that was to have been a target for rendezvous maneuvers. That failure appears to be due to faults in the balloon container.