CAPE CANAVERAL, Fla. - NASA managers yesterday ordered the Discovery astronauts to mount a fifth, unplanned spacewalk Monday to shore up cracked and peeling insulation on the Hubble Space Telescope.
The observatory, one of the premier instruments of 20th-century science, is not in any immediate danger of overheating or suffering any other ill effects from the surprisingly tattered insulation.
But with Discovery's crew already on hand for a $350-million overhaul, managers decided to order additional repairs to help keep the observatory cool and healthy until the next shuttle servicing mission in 1999.
More extensive repairs likely will be carried out then.
"We're doing the prudent thing," said program scientist Edward Weiler. "We have the (extra spacewalk) time available. We're not going to take chances. We've got a $2-billion investment here. Why take chances?"
And that's just for the telescope and its instruments. Throw in mission operations and other costs, and the nation's investment to date exceeds $3 billion.
Putting first things first, astronauts planned to float into Discovery's cargo bay shortly before midnight yesterday night to complete the telescope's long-awaited overhaul by installing a new solar-array control system.
During three previous spacewalks, Harbaugh, Tanner, Mark Lee and Steven Smith installed two new science instruments, two data recorders, a new guidance system, a data-relay computer and other equipment to give the space telescope a new lease on life.
The work went smoothly, and tests confirmed the new equipment was working properly, at least during the initial stages of checkout.
But shortly after Hubble was hauled aboard Discovery last week, engineers discovered extensive cracking in the multi-layer insulation protecting the telescope's upper-tube assembly, and several equipment bays where sensitive electronic systems are housed.
The damage was limited to one side of the telescope: the "hot side" that always faces the ultraviolet glare of the sun and is bombarded with atomic oxygen as it travels. This combination of UV radiation and erosion by atomic oxygen in the extreme upper atmosphere is believed to have caused the unexpected insulation cracks and widespread peeling.
"Basically, this thing is just falling apart; it's cracking all over the place," Lee observed Sunday at one point. "To repair one particular spot would just totally open up a few more."
The insulation is needed to maintain a room-temperature environment inside the space telescope despite constant trips into and out of Earth's shadow and the fiery embrace of the sun.
Because the telescope was working as expected, with no signs of high temperatures before its capture by Discovery, engineers believe the insulation degradation is a slow process and not an immediate threat.
"I would be very comfortable flying away right now without having to make these repairs," Weiler said. "Because right now, we know when we fly away, we'd see no difference (in performance).
"I can't say for sure, but we might go all the way to '99 without seeing a difference," he added.
While NASA managers played down the significance of the issue, flight controllers told the astronauts in an overnight message the insulation problem "has many folks concerned. We are currently investigating potential repair options but concede that it will be a difficult task."
But Weiler said the decision to order an extra spacewalk was similar to the decision a homeowner faces after finding a few cracked shingles on a seven-year-old roof that is guaranteed for 15 years.
"Do you rush right out and put a new roof on? No," Weiler said. "Will the next rain storm cause Niagara Falls to come into the living room? Of course not.
"The prudent thing would be to go up there, replace a few shingles, perhaps put some tar on and then perhaps three or four years later, replace the roof. ... So I'm very comfortable. This is not an urgent matter."