Spacewalking astronauts performed the equivalent of a brain transplant on the Hubble Space Telescope today, replacing its outdated 1970s-era computer with a faster machine that's cheaper to operate and more scientifically productive.

"Most excellent!" said astronomer-astronaut John Grunsfeld from the shuttle Discovery's flight deck after the new computer was plugged in. "The brains of Hubble have been replaced."

Spacewalkers Michael Foale and Claude Nicollier, a European Space Agency astronaut, then installed a refurbished fine-guidance sensor, one of three high-tech optical devices that help the Hubble stay locked onto its astronomical targets with mind-boggling precision.

With that work behind them, Foale and Nicollier were prepared to extend the excursion to complete additional repairs. But at 8:40 p.m. EST, with the spacewalk running about an hour behind schedule, flight controllers told them to call it a day instead.

With the completion of today's spacewalk, Discovery's crew has accomplished its highest-priority objectives, restoring the $1.5 billion Hubble to good health and leaving only a bit of preventive maintenance for a third spacewalk Friday.

If all goes well, the seven-man crew will release the telescope back into open space around 6 p.m. EST Christmas Day and return to Earth Monday, before NASA's Dec. 29 Y2K landing deadline.

Mir-veteran Foale and Nicollier, who helped restore the Hubble's flawed vision during a 1993 repair flight, began this mission's second spacewalk at 2:06 p.m. EST today, floating into Discovery's open cargo bay, breaking out tools and setting up mobile work platforms.

The first item on the agenda was to replace the Hubble's aging flight computer and its attached coprocessor, an add-on installed in 1993 to give the telescope a faster Intel 80386 processor. The new computer, which cost about $7 million, is built around a radiation-hardened Intel 80486 DX2 processor running at 25 megahertz.

While slow compared to a home PC or an Apple Macintosh, which typically runs 10 to 15 times faster, the new computer is 20 times faster than the model it replaced and features two megabytes of memory.

"That doesn't sound like much," said program manager John Campbell. "But you should keep in mind that we don't do [processor-intensive tasks such as running] Windows, we don't have disks and we don't do Internet. The old computer only had a tenth of a megabyte. So this is a major increase in capability."

More important, the new computer, which features three processors for redundancy, will not require the labor-intensive, custom programming formerly needed, improving efficiency and lowering operating costs. It also requires less electrical power to operate.

While NASA engineers privately joke about the most expensive 386-to-486 computer upgrade in history, program scientists say the new computer will greatly increase Hubble's scientific output in the years ahead.

Much of that output would be impossible without some way to lock onto dim astronomical targets for long-duration, rock-steady observations. The Hubble is equipped with three fine guidance sensors that do just that, tracking the light from selected guide stars. When any drift in that light is detected, the flight computer sends commands to counteract the unwanted motion, keeping the guide star--and Hubble's target--locked in place.

The sensitivity of the 480-pound guidance sensors is equivalent to holding a laser beam focused on a dime 200 miles away.

Only two guidance sensors are required for scientific operations. The third can be used for astronomical observations, helping scientists measure the diameter of stars and galaxies and search for the subtle wobbles that indicate a planet tugging on its parent star.

The guidance sensor carried aloft by the Discovery crew was removed from the space telescope during a 1997 mission and refurbished to improve its sensitivity.

While each guidance sensor is a large device measuring 5 1/2 feet by 4 feet by 2 feet--about the size of a refrigerator--installation is relatively straightforward. But given the critical nature of the device, great care must be taken to avoid causing any damage.