Discovery starting drawing fuel Wednesday from a hydrogen tank that had been closed because of a sticky valve, and Mission Control expected full electricity to be restored to the spacecraft.

A single heater in the tank, which is part of the shuttle electrical system, was activated to see if this would coax the valve to work properly, and Mission Control reported to the astronauts, "the tank heaters are looking real good.""OK," said Discovery commander Michael L. Coats. "That's good news."

Officials said indications were encouraging that Discovery could draw enough hydrogen from the tank to restore the power needed to complete its five-day mission.

"All of the numbers look good," Mission Control announced shortly after the astronauts punched switches to turn on a heater in the tank.

The heater warmed the hydrogen from a liquid state and started it flowing through the valve and on to the shuttle's chemical generator, called a fuel cell.

"We'll watch it a little more before we declare that the tank is operational, but everything looks good at this point," said Mission Control commentator Brian Welch.

The valve, which started jamming shortly after Discovery was launched on Monday, forced the five-man crew to dim lights and shut down unneeded computers in order to conserve electricity.

The astronauts were in no danger, but officials said the problem could force the spacecraft to return a day early by limiting the electricity available.

Despite a dim cabin and a careful use of electrical power, the astronauts kept to their schedule of conducting experiments and photographing environmentally damaged places on Earth during their second morning in space.

They focused a powerful 70mm camera on the Tanzanian plains in Africa and then looked for evidence of pollution near Zanzibar, where a coral reef is dying. Other targets included the scars from a recent fire in the Everglades and an erupting volcano in Guatemala.

Shuttle's daily schedule

Thursday:

1:57 a.m. - Crew wakeup.

Day of experiments.

5:57 p.m. - Begin eight-hour sleep period.

Friday:

1:57 a.m. - Crew wakeup.

7:42 a.m. - Crew will be interviewed on NBC's "Today" show.

Astronauts begin shutting down experiments, stowing gear and checking flight control systems in preparation for landing the next day.

5:57 p.m. - Begin eight-hour sleep period.

Saturday:

1:57 a.m. - Crew wakeup.

7:27 a.m. - Close payload bay doors.

10:03 a.m. - Deorbit burn.

11:04 a.m. (8:04 a.m. PST) - Land at Edwards AFB, Calif., after flight of five days, one hour, seven minutes.