CAPE CANAVERAL, Fla. -- For the second time in a month, NASA has delayed the planned launch of a Hubble Space Telescope servicing mission to give engineers more time to ready a replacement for a balky pointing device on the observatory.
Shuttle Columbia and crew of seven astronauts now are slated to blast off Feb. 28 on a mission to outfit the flagship telescope with new solar arrays, a critical power switching unit and an advanced planetary camera.
Once targeted for launch on Feb. 14, the flight initially was pushed back to Feb. 21 so that engineers could ready a spare for one of four actuators that play a key role in precisely pointing the telescope at celestial targets.
The spare, however, won't be delivered to Kennedy Space Center until early next month, a fact that prompted mission managers on Thursday to slip the launch until Feb. 28.
"The (Hubble and shuttle) program managers got their stories together, and it was decided that we needed another week," said KSC spokesman Bruce Buckingham.
Launched aboard shuttle Discovery in April 1990, the Hubble is equipped with four Reaction Wheel Assemblies, each of which house large flywheels that are used to move the telescope into position for scientific observations.
One of those devices suffered a seven-minute glitch late last year, prompting concern among project managers. Data beamed back from the telescope showed that the suspect wheel was spinning, but an associated tachometer indicated that the device was not rotating.The balky Reaction Wheel Assembly since then has been operating properly, but mission managers decided it would be prudent to replace the device during the upcoming servicing mission.
The telescope can operate with only three of the assemblies working, but astronauts aren't scheduled to service the observatory again until 2004.
The suspect assembly is to be replaced on the second of five, back-to-back spacewalks to be carried out by the Columbia crew, which is headed up by veteran shuttle astronaut Scott Altman.
Mission specialists Jim Newman and Michael Massimino will carry out the repair work, and two other astronauts -- John Grunsfeld and Richard Linnehan -- also will conduct servicing jobs on the telescope during the 11-day flight.
In what promises to be one of NASA's most complex servicing missions ever, the spacewalkers aim to equip the telescope with next-generation solar arrays that will produce 20 to 25 percent more power than the arrays now on the telescope.
An advanced planetary camera also will be installed on the observatory, providing scientists with an instrument that can deliver images that are 10 to 20 times sharper than those now churned out by the telescope.
A power control unit that distributes electricity from the solar wings to telescope batteries and other parts of the observatory also will be changed out, and the spacewalkers will retrofit a dormant infrared camera with a new experimental cooling system.
The mission will be the fourth of five planned servicing trips to the observatory, which is expected to operate in orbit through 2010.
Made by Lockheed Martin, the observatory has no onboard steering thrusters, exhaust from which could foul sensitive telescope instruments. Consequently, plans now call for a shuttle crew to return the 13.5-ton observatory to Earth to avoid an uncontrolled atmospheric reentry.
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