By Gwyneth Dickey Zakaib of Nature magazine

A ground-based telescope that can scan the skies faster than any other of its size could help to protect satellites from collisions with space debris and each other. The Space Surveillance Telescope (SST), developed by the U.S. Defense Advanced Research Projects Agency (DARPA), is to be used to protect U.S. and international assets and commercial and international satellites in orbit around Earth.

"We've got a lot of high-value missions up there, and if you're trying to do those missions with a blindfold on, you just don't know what's going to run into you at any time," says Chuck Laing, deputy division chief of the Architecture and Integration branch of Air Force Space Command at Peterson Air Force Base in Colorado. "It's important to know where everything is, how fast it's moving, and in what direction."

Researchers are currently tracking an estimated 22,000 artificial objects that are orbiting Earth, from small bits of debris to large satellites. That number is expected to triple in the next 20 years, says Laing. Even a centimeter-sized piece of debris can cause considerable damage to crucial weather, communication or missile-warning systems.

The Air Force keeps a catalogue of all known orbiting objects through its Space Surveillance Network, an integrated system of ground- and space-based telescopes and radar. The network tracks debris to anticipate possible impacts, but better surveillance is needed to cope with the increasing number of objects, says Laing. The SST would focus mostly on the region in which objects in geosynchronous orbit reside, about 35,000 kilometers from Earth.

The telescope, which took nine years and $110 million to build, has a wide field of view, is very sensitive and can scan the sky several times in one night. It can collect data faster for dimmer objects than existing telescopes in the Space Surveillance Network. With the increased information that it provides, officials will be able to better predict the path of debris and warn satellite operators of potential collisions.

"You can make a better decision if you have more data to feed into the model," says Laing.

Eye on the sky

The telescope's superior data-collection capacity comes from its 3.5-meter aperture, which is more than three times the size of ground-based telescopes already in use. It also has a three-mirror system to bring images into sharp focus over a wide field. But the engineering advances brought problems: whereas traditional two-mirrored telescopes focus light onto a flat surface, the three-mirrored type focuses onto a curved one, which makes it difficult to manufacture matching detectors.

"We had to do pretty hard work on the optics," says Grant Stokes, head of the aerospace division at the Massachusetts Institute of Technology's Lincoln Laboratory in Lexington. His lab developed curved charge-coupled devices that capture photons and turn them into electrons for digital processing in the telescope.

The SST took its first images in February 2011 and is currently being tested and aligned. It still has to pass an evaluation before joining the Space Surveillance Network.

If it passes the tests, the SST could begin collecting data in about six months. And if it's a big enough improvement on current technology, the Air Force might place similar telescopes at key sites around the globe for 360-degree surveillance. At the moment, the SST sees only the night sky visible from its base at the White Sands Missile Range near Socorro, N.M.

Researchers won't be able to use the telescope for their experiments, says Joseph Gambrell, chief of space situational awareness at the Air Force Space Command. "If we're going to try and get the most out of it, we really do need to maintain it as a Space Surveillance Network resource," he says.

But a subset of the data might be made available on a Web site operated by US Strategic Command, and researchers might be able to get further data on request, says Gambrell.

This article is reproduced with permission from the magazine Nature. The article was first published on April 22, 2011.