Spaced-Out Telescope

Following the Hubble Space Telescope is a bit like following Friends. In a nutshell, good luck. After a bumpy start, the Hubble proved enormously successful, giving astronomers an unprecedented look at the solar system. But as Hubble's days near an end, NASA is gearing up for a second act with the James Webb Space Telescope (JWST).

Expected to be launched in 2011, the JWST will be stationed far further out in space than the Hubble, in an orbit 940,000 miles from Earth. It will also be able to see much fainter objects than the Hubble can. Astronomers hope it will help them explain how galaxies formed, determine the shape of the universe, and better understand the birth of stars.

Building the Webb telescope presents some formidable challenges. Its far-off orbit puts rescue missions out of the question, so it will have to work from the moment it is deployed and for the next five to ten years. Advanced computer simulations let engineers test the telescope's systems before deployment and calculate the exposure times needed to conduct observations.

Like the Hubble, the JWST needs to maintain a perfect shape to its mirror to capture sharp images. The Hubble does this by using a thick, solid glass mirror. That approach, however, won't work for the JWST; its primary mirror is more than twice the size of the Hubble's.

Instead, the JWST's mirror is composed of 18 lightweight segments that are kept in perfect alignment by computer- controlled actuators that adjust the mirror's overall shape. NASA expects to use the technology in future space missions.