While most news outlets were freaking out about killer pythons, the Japanese helo carrier, or the 68th anniversary of the bombing of Hiroshima, the US Air Force was quietly preparing to shut down Space Fence, one of the oldest but most productive parts of its space surveillance network.
It’s also one of the few facilities that can provide 24/7 “uncued detection” capability… which is nerd-speak for the ability to find things in orbit that nobody expected to be there (ie. space junk). Contrary to what Hollywood likes to show you, it’s impossible to track every single object of space debris. There are over half a million distinct objects, some smaller than a penny, and no one nation (or even group of nations) has enough radar or visual coverage to follow them all. In fact, the US Space Surveillance Network has only 6 facilities worldwide dedicated to on-orbit detection and tracking; Space Fence was the largest and most effective of them, spanning the continent at the 33rd parallel.
Why is the closure of this capability important, you ask? It might be easier to have some visual reinforcement.
This is what a mere fleck of paint did to a cockpit window on OV-099 Challenger during the STS-7 mission, back in June of 1983. The pit you see there is only 1mm in diameter, but remember that paint is probably the most innocuous bit of debris one can run into when travelling 17,580 miles an hour. During STS-59, OV-105 Endeavour experienced similar window damage, and the object actually penetrated half of the window’s total depth. That’s kind of a big deal. Other STS missions have seen micro-collisions where objects have punched right through the orbiter’s silvery radiator panels. Objects like this paint fleck are too small for Space Fence to detect and track, but there are literally hundreds of thousands of larger objects.
Despite being several decades old, Space Fence is still relevant today because it has a longer reach than most other sensors, and because it can do its work un-cued. In other words, it detects without being specifically tasked to look for a known or unknown object in a given region of orbital space. It is responsible for around 40% of the observations on the 23,000 objects the Department of Defense currently tracks.
Though part of a broader surveillance network, the VHF Space Fence is crucial because it can track objects up to 24,000 kilometers away. Other sensors in the network generally track objects at altitudes lower than a few thousand kilometers, [Brian Weeden, technical adviser at the Secure World Foundation, a nonprofit organization dedicated to space sustainability] said.
“The Space Fence is very important as it gives an ‘uncued tracking’ capability,” Weeden said. “Because it’s constantly transmitting, it can detect objects without being tasked to do so. There are some other sensors in the network that can do uncued tracking to some degree, but the Space Fence is rather unique in the sheer size of the detection coverage it has.”
— Gruss, Mike. Shelton Orders Shutdown of Space Fence. SpaceNews, 6 August 2013.
Granted, the current Space Fence is a little long in the tooth and suffers the additional handicap of being based on 1960s technology. Thanks to sequestration, not only is the current capability being forced to close, but its future replacement is also stuck in limbo—waiting for the Pentagon to decide what projects it can reasonably afford. Meanwhile, there are about 500,000 debris objects too small (from 1-10cm in diameter) for the current space surveillance network to keep tabs on. And their numbers will only increase.