A computer-generated image of objects in Earth's orbit that are currently being tracked. NASA

Mike Coletta was sitting in the basement of his home outside Pueblo, Colo., on Aug. 31 listening to echoes of space objects. Every minute or so, a ping would sound in his headphones as a satellite or piece of orbital debris passed overhead and bounced back a signal from a series of radar transmitters known as the Space Fence. The International Space Station was set to arrive in a few minutes, but time passed and there was no sound. "Maybe they did an orbit change or something, I thought, because there’s nothing and there should be something," Coletta recalled. "I looked at the time and saw that it was right around 0:00:00 UTC (Coordinated Universal Time) on Sept. 1, and it hit me: Those sons of guns — I bet they turned it off." Coletta was right. The Space Fence — which, despite its name, consisted of operational facilities on the ground, across the southern United States — had been shut down. In more than 50 years of operation, it had played a key role in the Space Surveillance Network, set up by the U.S. military to track man-made debris, and help keep valuable satellites and spaceships from smashing into it. According to the Air Force Space Command, which ran the network, the shutdown was made necessary by the 2013 budget sequester and will save $14 million per year in operating expenses. But some argue that the shutdown has reduced the capability of an already imperfect surveillance system, potentially increasing the risk of a costly collision.

Normally it wouldn’t be just a few hours between the cascades; it would be tens of years. Donald Kessler Former NASA scientist for orbital debris research

The 1,000 or so currently active satellites provide essential infrastructure for the modern world. GPS, television transmission, weather prediction, scientific exploration, search-and-rescue and international financial transactions are key functions facilitated by satellites. Functioning satellites are vastly outnumbered by orbital debris — a smorgasbord of defunct satellites, spent rockets, exploded satellite and rocket bits, paint flecks and liquid leaked from nuclear reactors that once powered Soviet spy satellites. The Space Surveillance Network maintains a catalog of more than 23,000 orbiting objects larger than a grapefruit, and there are estimated to be tens of millions of pieces of debris too small for the network to detect. Hurtling around Earth at up to 20,000 miles per hour, even a small piece of debris can rip through a satellite or spaceship. In Alfonso Cuaron’s blockbuster film "Gravity," a Russian missile blows up a satellite, whose shards collide with other satellites, producing a storm of debris that disables the space shuttle. "The movie was extremely realistic in the sense that this is what collisional cascading is all about," said Donald Kessler, who was NASA’s senior scientist for orbital debris research before retiring in 1996. In 1978 he predicted that when the mass of orbital debris reached a certain point, a "collisional cascade" would begin, in which collisions would create more debris, in turn leading to more collisions. "It’s just that normally it wouldn’t be just a few hours between the cascades; it would be tens of years." The trigger for the "Gravity" cascade is based on a real event. In January 2007, China fired a missile at one of its satellites as part of an anti-satellite test, creating more than 3,000 trackable pieces of debris. Six months later, NASA had to move its Terra satellite, which is responsible for collecting massive quantities of weather, environmental and Earth-imaging data, to avoid a potential collision with one of these pieces. Then, in February 2009, an Iridium communications satellite and a defunct Russian spy satellite smashed into each other, creating another 2,000 pieces of trackable debris. The ideal solution would involve removing debris from orbit. Scientists have proposed an array of removal technologies — space sweepers and tugs, fishing nets and harpoons, tethers, laser blasters — but no practical application has been developed. Even if one were available, using it could stoke international tensions, said Dave Baiocchi, a senior engineer at the RAND Corp. who worked on a military project that aimed to "figure out what the garbage truck of space should look like." "If we were to develop one, how do you convince other countries that it’s not an anti-satellite weapon?" Baiocchi asked. "How do you convince Russia that your garbage truck is truly a garbage truck and not out to disarm other countries’ satellites?"

It may be that we don't have any major collisions over the next five years, and therefore it's not a big deal. It may be that we have a bunch of them, and it's going to be a really big deal. Brian Weeden Technical adviser, Secure World Foundation

Lacking a removal solution, the U.S. military tracks orbiting objects and takes steps to prevent collisions. Data collected by the Space Surveillance Network — which includes a range of radars and telescopes in addition to the Space Fence — is used by the Joint Space Operations Center (JSpOC), at Vandenberg Air Force Base in California, to predict when a satellite or spaceship faces a collision risk. JSpOC then sends warnings to the satellite or spaceship operator, which decides whether to perform an evasive maneuver. Even before the Space Fence was shut down, this system had serious shortcomings. To start with, it was blind to the vast number of pieces of debris too small for its sensors to detect. A planned new network of radar stations would greatly improve its sensitivity, but the first will not be online for several years at the earliest. In the network, the Space Fence was the only sensor that performed "uncued" detection. The other sensors "are told, 'OK, over the next day, we want you to go track these objects and collect observations and send them to us,'" said Brian Weeden, technical adviser for the Secure World Foundation. "That doesn’t happen to the fence, because it’s just there. Things fly through the fence; it collects observations and sends them off." To make up for the shutdown, the Air Force Space Command directed two other radars — in North Dakota and Florida — to pick up the slack. However, because these stations are each based in a single location rather than spread out across the country, they cover less of the sky than the Space Fence did. This reduced coverage has led to "a loss when it comes to detecting and characterizing events like breakups," said Weeden. "You can’t predict when those kind of events are going to happen. It may be that we don’t have any major collisions over the next five years, and therefore it’s not a big deal. It may be that we have a bunch of them, and it’s going to be a really big deal."

The worst-case scenario is that it gets a lot more risky and a lot more expensive to operate in some of the most important regions in space. Brian Weeden Technical adviser, Secure World Foundation