NASA opened Discovery's payload bay doors Saturday to remove the Hubble Space Telescope's batteries for recharging and to replace a faulty part in preparation for an April 25 launch. Technicians took out a faulty auxiliary power unit responsible for grounding the shuttle last week. A new unit was being installed to be tested Sunday and fired up briefly later this week. ``We haven't run into any problem,'' said Lisa Malone, a spokeswoman for the National Aeronautics and Space Administration. ``We're looking good for the 25th.'' Hubble's six nickel-hydrogen batteries will be recharged for 5{ days in a laboratory at the Vehicle Assembly Building. NASA decided to take the batteries there because they can be recharged at a cooler temperature and thus retain more power. ``We will have a higher capacity in them than we would have had we been recharging at the pad,'' said Fred Wojtalik, a NASA project manager. ``This will make the opportunities for additional launch days a lot easier for us.'' The batteries will power Hubble from the time it is disconnected from the shuttle's power system during deployment until its energy-collecting solar panels are unfurled. Discovery was poised for launch Tuesday, but the mission was scrubbed four minutes before liftoff when the auxiliary power unit failed. NASA was not sure how long it would take to replace the 88-pound unit because the procedure had never been performed before at the launch pad. The work went smoothly, Malone said. The defective power unit was returned to the manufacturer, Sunstrand Corp. of Rockford, Ill., for testing. The shuttle's three auxiliary power units pressurize the hydraulic system, which is used to move wing and rudder surfaces and in the ship's braking and steering systems. NASA planned to close Discovery's payload bay doors Sunday as soon as the lengthy process of removing the batteries was completed. The chamber will remain sealed to reduce the risk of getting too much dust on the telescope's 94-inch mirror, the finest ever made, until the batteries are returned next weekend. The $1.5 billion telescope is NASA's most expensive and complex payload. Once it is in orbit above Earth's distorting atmosphere, it will be able to detect objects 50 times fainter and with 10 times greater clarity than the best observatory on the ground. Astronomers expect to study stars and galaxies so distant that their light has been traveling toward Earth for 14 billion years, close to the beginning of the universe.
