Hubble's repair: a second mission needed?

For two years, NASA has planned a single repair mission to correct the Hubble Space Telescope's flawed optics and replace its ailing hardware. In May, an independent task force urged the agency to schedule a second space shuttle visit in case one doesn't suffice. And at a press briefing last week, NASA seemed to agree. While agency scientists said they remained confident it will take but one visit to fix the telescope's major equipment and optics problems, they announced the addition of a back-up mission as part of the blueprint for Hubble's repair.

The first repair mission, now scheduled for December, will cost $630 million. NASA estimates that a second mission, if needed, would fly six months to a year later and cost at least the price of a shuttle flight, about $300 million.

The December mission, which will require a record five spacewalks, has an ambitious list of 11 items to fix or replace. Edward J. Weiler, Hubble program scientist at NASA headquarters, gives only 50-50 odds that the crew could address all 11 items, but says he is virtually certain astronauts can accomplish the key repairs, which involve six tasks.

To compensate for Hubble's flawed primary mirror, which makes faint stars and galaxies appear fuzzier than researchers had hoped, astronauts will apply two fixes. After snaring the telescope with a grappling arm, they will install corrective mirrors for three Hubble instruments-two spectrographs and a camera. The crew will also replace the telescope's Wide-Field/Planetary Camera with a newer, more sensitive version that has a built-in corrective mirror.

But even without the telescope's highly publicized optical problems, other Hubble troubles now require repairs, Weile. notes. The crew must also replace Hubble's solar arrays, which power the craft but vibrate unacceptably due to temperature fluctuations caused when the telescope passes in and out of Earth's shadow. Researchers fear the jittery arrays will eventually snap off.

In addition, NASA wants to replace at least two of Hubble's three failed gyroscopes; if another goes awry, engineers can't point the telescope accurately enough to do scientific observations, Weiler says. Other priority tasks include replacing at least one of two troublesome magnetometers and adding a coprocessor to improve the degraded memory of its onboard computer. Weiler notes that the telescope was designed to undergo maintenance every three years.

Given the complexity of the key repairs, "1 wouldn't be surprised if a second mission were necessary," says aerospace engineer Eugene E. Covert of the Massachusetts Institute of Technology, Covert served on the task force that urged NASA to consider a back-up mission.

Robert C. Bless, a Hubble researcher at the University of Wisconsin-Madison, feels encouraged by the extensive planning for the December flight. But he expresses concern that in underwater simulations of the space environment, the diving gear worn by the crew allowed them to stay immersed for only four hours, compared with the six-hour spacewalks required for the repair mission. Bless adds that more of Hubble's aging equipment could fail by the time of the rescue flight. Weiler says that by October, after further crew training, NASA should have a better idea if Hubble will need a second mission.