The Discovery astronauts sailed through an experiment-packed second day in space Tuesday with engineers on the ground working to resolve a potential problem with ship's power system.

The main goal of the 28th shuttle mission, the deployment of a $100 million communications satellite, was accomplished Monday, six hours and 13 minutes after blastoff from the Kennedy Space Center, giving NASA a major success in the first of seven flights planned for 1989.With the deployment behind them, the crew - commander Michael Coats, 43, co-pilot John Blaha, 46, James Bagian, 37, Robert Springer, 46, and James Buchli, 43 - was busy with an array of medical, biological and engineering experiments as well as Earth observation photography.

Engineers on the ground asked the astronauts to conserve electricity by turning off computer gear and other items until they can figure out what caused pressure surges, or "spikes," in one of three hydrogen tanks on board the orbiter that feed the ship's electricity-producing fuel cells.

The tank was isolated from the fuel cell system overnight, and NASA spokesman Brian Welch said by conserving electricity, the crew will be assured of at least a four-day mission with two additional days for contingency, a standard shuttle precaution in case bad weather delays a landing.

"Since the performance of that tank was not understood, the decision was made to isolate it," he said. "The fuel that's in that tank is not necessary at the moment, we've got plenty of juice in the other two hydrogen tanks."

The crew was not in any danger and it was business as usual aboard the orbiter. Coats at one point joked that thanks to Bagian's somewhat balding head, "We only need one light on up here."

NASA spokesman Ed Campion said if the tank cannot be brought back "on line," one option would be to shorten the five-day mission to four days, bringing Discovery back to Earth Friday instead of Saturday.

After a meeting of top NASA managers, however, one official said privately "the chances of coming back early are extremely remote.

Blaha used a large format IMAX camera Tuesday to shoot scenes of the planet below for a documentary titled "The Fragile Earth." Later in the day, Discovery was to serve as a target for optical sensors in Hawaii that may be useful to the "Star Wars" missile defense program.

The crew also planned to activate an experimental 51-foot space station heat radiator in the payload bay and to document the growth of plant seeds in an experiment to understand how weightlessness affects plant growth.

Flight director Ron Dittemore said the crew was asked to be on the lookout for increased auroral activity because of recent solar flares.

Along with his camera chores, Blaha is responsible for an experiment designed by Purdue University engineering student John Vellinger to study the effects of weightlessness on 32 chicken embryos.

Blaha checked on the spring-cushioned eggs late Monday and asked mission control to tell Vellinger that "everything really looks great."

A second student experiment involves four rats with deliberately broken legs that are housed in a special cage in Discovery's crew cabin. Brown University medical student Andrew Fras designed the experiment to find out how weightlessness affects bone healing.

The launch of a Tracking and Data Relay Satellite, or TDRS, Monday completes a globe-spanning communications network, providing near-constant communication between shuttle crews and mission control.

It gives NASA a fully operational two-satellite TDRS system and allows the agency to shut down five ground stations at a savings of $27 million a year.