NASA PREPARES FOR A SERVICE CALL ON HUBBLE

* The National Aeronautics and Space Administration (NASA) is preparing for what it calls "the most aggressive" space-shuttle mission yet. It's the mission to repair the Hubble Space Telescope. The launch is presently set for Dec. 7, 1993.

The mission is "aggressive" because it will take four astronauts, working in two-person shifts, several days to make the repairs. They will need three - possibly four - space walks to do the work.

Their main task is to install a new camera and a set of corrective mirrors to compensate for spherical aberration in the telescope's main 2.4-meter light-gathering mirror. This flaw has prevented that mirror from sharply focusing all but 15 percent of the light it gathers.

The astronauts also are to install stiffer solar panels. The present panels flex as the observatory moves in and out of Earth's shadow. This jiggles the telescope. Astronauts will also replace two failed stabilizing gyroscopes and two failed computer memory banks. They may try to repair an instrument power supply and a magnetometer if they have the time.

These repairs should give the $1.4-billion telescope 90 to 95 percent of its design capabilities.

Although the main mirror fault makes this a rescue mission, the telescope is designed for in-orbit servicing. Service calls to replace failed components and install new instruments are part of the regular mission for the Hubble facility, which is expected to have a working life of 15 years or longer.

The orbiting observatory is a joint project of NASA and the European Space Agency.