President Bush revived a White House space tradition Thursday and placed a long-distance call to the shuttle Discovery, calling its high-flying crew "pioneers" and promising support for a "strong, active space program."

With Discovery's freshly combed five-man crew floating together in the orbiter's lower-deck area, Bush, with Vice President Dan Quayle at his side, told the astronauts on closed-circuit satellite television that they serve as role models for the nation's youth."The space program, especially space station Freedom, is an investment in our future. We're living in tough budgetary times, but I am determined to go forward with a strong, active space program, and I want to congratulate you for this wonderful, wonderful mission. We look forward to your safe return."

Discovery's crew - commander Michael Coats, 43, co-pilot John Blaha, 46, James Bagian, 37, Robert Springer, 46, and James Buchli, 43 - have been carrying out a series of experiments since blastoff Monday and the launch of a new NASA communications satellite. All five took turns answering Bush's questions.

"It's been an exciting time," Springer told Bush. "Of course, three of us on the flight are rookies in space, and I think the cleanliness and the smiles you see are just a reflection of the fun that goes along with the hard work."

If all goes well, the astronauts will end the 28th shuttle mission, the first of seven planned for 1989, with a landing Saturday at Edwards Air Force Base, Calif.

Thanks to an analysis by the mission control team at the Johnson Space Center in Houston, earlier problems with an electrical system hydrogen tank were corrected Wednesday, clearing the way for Discovery to stay in orbit a full five days as originally planned.

In general, the experiments have worked well, but the crew has had problems with a 51-foot-long heat radiator for the space station Freedom project stowed in the shuttle's payload bay. NASA blamed the problem on a gas bubble.

Blaha continued using a high-format IMAX camera for a film about Earth's environment that will be shown at the National Air and Space Museum in Washington along with other outlets. Early Thursday, he filmed Sri Lanka and Teague Crater in Western Australia.

Coats and Blaha also planned to dump waste water overboard during a pass over Maui, Hawaii, to give detectors on the ground in Maui, Hawaii, a chance to characterize the resulting "plume" of ice crystals.

Such plumes can contaminate sensitive shuttle instruments, and engineers wanted to learn more about how they behave in weightlessnes.