SEVEN astronauts and their space shuttle Endeavour are ready to head for the heavens to repair the Hubble Space Telescope.
Officials of the National Aeronautics and Space Administration (NASA) consider this the most ambitious orbital service call their agency has made. They also acknowledge that the agency's future is riding on the mission. As Hubble project scientist Edward Weiler has observed, ``This project is going to be in the history books ... as a national disgrace or as a triumph.''
If it is successful, astronomers will gain essentially the full designed power of a superb instrument that has been crippled by manufacturing faults and component failures. If this high-profile mission falls substantially short of its goals - let alone fails completely - NASA may well feel disgraced. That would compound the original embarrassment over the flawed mirror. It would pair with the recent loss of the billion-dollar Mars Observer craft and strengthen the criticism of congressmen who doubt the wisdom of proceeding with a space station that depends on shuttle-borne astronauts for its construction and maintenance.
At press time, controllers were aiming for a one-hour, 11-minute launch window that opens at 4:57 a.m. EST tomorrow. Mission commander Richard Covey and pilot Kenneth Bowersox are to take Endeavour to a circular orbit 587 kilometers (365 miles) high. There, on the third day of this 10-day, 22-hour, 56-minute mission, European Space Agency astronaut Claude Nicollier is to grab the 14-meter-long, 13-ton telescope with the shuttle's manipulator arm and stow it in the shuttle cargo bay.
Mission specialists Story Musgrave, Kathryn Thornton, Tom Akers, and Jeffrey Hoffman will work in teams of two to carry out the five six-hour spacewalks now planned. Much of the work - such as replacing an instrument - is what Mr. Hoffman calls ``EVA (extravehicular activity) friendly'' because Hubble was designed for on-orbit servicing. But some is what he calls ``EVA unfriendly.'' That includes such things as disconnecting and reconnecting seven electrical connectors that weren't designed for hands made clumsy by spacesuit gloves.
Hubble suffers from an optical fault called spherical aberration. The 2.4-meter diameter main mirror it carried into orbit April 24, 1990, has a curvature that is about 2 millionths of a meter too flat at the edge. This causes light reflected from the edge to focus 38 millimeters behind light reflected from near the mirror center.
Hubble was designed to concentrate 80 percent of a star's light into a core image only 0.1 arc-seconds in diameter. But only 15 percent of the light makes it into the image circle. The rest is smeared into a surrounding halo. Computer processing can use that 15 percent of well-focused light to recover many images. That is why Hubble has made important astronomical discoveries in spite of its handicap. But many desirable targets are too faint for computer enhancement. After the planned repair, more than 60 percent of a star's light should fall within the core image circle, giving Hubble the sharp-eyed vision its designers intended.
One instrument - the Wide Field Planetary Camera - was planned to be replaced anyway. Astronauts will install a replacement that contains its own corrective optics.
The astronauts will give three other instruments a set of ``eye glasses.'' This is a two-meter-long device with five pairs of mirrors. These will be used in various combinations to sharpen up the blurred images coming from the main mirror.
Astronauts will also replace Hubble's two 12-meter-long solar-cell arrays. The current pair flex as they heat or cool in passing from sunshine into Earth's shadow and out again. This jiggles the telescope. The new arrays will be stiffer. It will be easier to maintain the telescope's exquisite pointing accuracy of 0.005 arc-seconds. That's the width of a dime seen 800 kilometers (500 miles) away.
Hubble has lost three of the six gyroscopes that are part of its pointing system. It needs at least three for the system to work. The astronauts will replace the failed units. They also are to give the telescope's main computer a coprocessor to beef up its power.
It will cost an estimated $721.3 million to repair the $1.5-billion telescope.