New space radars track small but deadly space junk

PANICKY news headlines in recent months have warned of the perils of the fiery debris that might rain down on us as three derelict spacecraft re-entered the atmosphere. In the end, NASA's UARS satellite, Germany's ROSAT X-ray telescope and Russia's crippled Mars probe, Phobos-Grunt, all burned up or ditched harmlessly in the oceans. But at least mission controllers knew where they were headed - the craft were around the size of an SUV, and so were easy to track via radar.

The same cannot be said for the estimated 200,000 pieces of space junk between 1 and 10 centimetres across that we cannot see. Existing radars simply can't track objects of that size in orbit. That's a problem since they can damage crewed spacecraft like the International Space Station and the 24 GPS satellites that we depend on back on Earth.

Light is about to be shed in this darkness. Independent tests undertaken in late February by two US aerospace firms - Lockheed Martin and Raytheon - showed that a new ground-based radar technology can detect those small bits of orbital debris.

Both Lockheed and Raytheon are competing to win a US air force contract to build what's called the Space Fence, which will detect, track and identify debris, rather than fence it off as the name suggests. The Space Fence is designed to overcome a fundamental limit in the Air Force Space Surveillance System, which was built in 1961 to spot Russian Sputniks during the cold war. Its three very high-frequency band radar sites in Texas, Arizona and Alabama ping the heavens with radio waves at wavelengths between 1 and 10 metres and their reflections enable us to detect objects down to the size of a basketball.

To detect smaller objects, radar scanners must operate at wavelengths between 1 and 10 centimetres, in the so called S-band of the electromagnetic spectrum. It is far from easy to make sensitive radars that operate in that band because the smaller the object being detected, the bigger the problem created by radio noise from aberrations in the atmosphere. Such noise competes with reflected signals, making it hard to distinguish a radar reflection from a random blip.

Enter gallium nitride, (GaN) a semiconductor that can amplify very precise wavelengths in the S-band, allowing a reflected signal to be more easily sifted from noise. Raytheon has described GaN as "an essential discriminator for new radar programs like Space Fence", and it's thought that Lockheed has harnessed this material for their system as well.

In last month's tests, both firms say their prototypes successfully tracked a known "resident space object" of the relevant size (see picture, for what the completed system might be able to see). The air force is expected to choose a winner later this year, who will then build a full Space Fence network by 2017.

"The Space Fence is an important upgrade," says Richard Crowther, a space debris specialist at the UK Space Agency.

"You can divide the orbital population into those objects big enough to track, small enough not to pose a catastrophic hazard, and too small to track but large enough to kill your satellite," Crowther says. "The fence will address the latter population against which we have had no real protection until now."

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