The space shuttle era is finished, its vehicles museum-bound. The deep-space forays of Apollo astronauts are long gone, their final moon voyage nearly 40 years in the past. And still, space today is more crowded than ever.

Since the space age began, the orbital realm has become increasingly littered with the detritus of skyward human striving—spent rocket boosters, dead satellites, stray pieces of hardware. And in the past decade that debris has piled up with such speed that it has become an inescapable threat to the space-faring endeavors that spawned it in the first place.

Millions of pieces of debris five millimeters and up circle Earth in a high-velocity swarm, each packing enough kinetic energy to disable a satellite. But far more sobering is the threat to human life. In June the six astronauts aboard the International Space Station (ISS) were alerted to take shelter in escape capsules when a piece of debris came within a few hundred meters of the station. In at least five other instances in the past few years when close debris passages have been predicted well in advance, the station has moved out of harm's way.

The recently retired space shuttle program managed to draw to a close last month without a fatal debris strike, but the risks shuttle astronauts faced on all 135 of their missions were nontrivial. A typical shuttle flight carried a roughly 1-in-250 chance of catastrophic collision with a micrometeoroid or, more likely, with orbital debris. (Those odds may not sound overly troubling, but over 10 missions the cumulative risk grows to 4 percent; over 100 missions the risk of disaster reaches 33 percent.) Whatever rocket and crew capsule NASA chooses to succeed the shuttle in the coming years will face the debris threat as well.

The U.S. is now taking preliminary steps to manage the threat of space junk by implementing better tracking systems, but a solution to the root problem is nowhere in sight. There are no feasible approaches to actually removing debris from orbit, and simulations show that, even if all spaceflight activity ceased today, the number of pieces of debris would actually increase over time as collisions between objects in orbit produce huge fragmentation clouds. (Sometimes space junk does fall out of orbit on its own, but not rapidly enough to deplete the debris pool. In March a 75-centimeter metal sphere, apparently a pressurant tank from a Russian rocket that had launched in January, turned up in Colorado. That same month a 1.2-meter piece of a Delta 2 rocket motor that in 2003 had lofted a GPS satellite into orbit fell in Uruguay. Earlier GPS launches had caused three similar debris falls since 2001, raining 50-kilogram titanium ovoids on Saudi Arabia, Argentina and Thailand.)

A dramatic orbital collision has already happened. In 2009 a defunct Russian Cosmos satellite smashed into an active Iridium communications satellite; what had been two intact spacecraft became a cloud of debris, including roughly 2,000 fragments of at least 10 centimeters, large enough for the U.S. military to track and catalogue. One of those larger fragments drew too close to the ISS in April, forcing station operators to fire thrusters in a collision-avoidance maneuver.

The Cosmos–Iridium crash produced more catalogued orbital debris than any single event in space-age history other than a 2007 missile test in which China intentionally destroyed one of its satellites. Earth's orbital environs, especially the region between 700 and 1,000 kilometers above the planet, are now so crowded that collisions involving pieces of debris are projected to occur every four or five years, according to a recent analysis by orbital debris researcher Jer-Chyi Liou of NASA's Johnson Space Center.

Some collisions might be avoided if space agencies and satellite operators had a better accounting of what is in orbit and where it is headed. The U.S. Air Force is planning a new radar system called Space Fence that could dramatically increase the number of orbital objects under surveillance after it comes online around 2017. The U.S. military has already catalogued more than 16,000 satellites and large pieces of debris, but the Space Fence system will allow for monitoring of much smaller objects. "We are pretty confident that the catalogue will grow tenfold within months of Space Fence's initial operating capability," says John Morse, the Space Fence program director for Lockheed Martin. Lockheed and Raytheon are vying for the project; each firm received a $107-million Air Force contract in January to develop demonstration technologies. The Air Force will select a contractor for the project, which could cost as much as $6.1 billion, next year.

As planned, Space Fence would comprise two radar stations in the southern hemisphere, which will take over for a 1960s-era radar system now involving nine sites in the U.S. Whereas the present radar system operates in the VHF band, Space Fence will use shorter wavelength S-band radar, which affords better resolution for tracking debris. "The smaller the wavelength, the smaller the objects," says Scott Spence, director of Raytheon's Space Fence program. Whereas the current debris catalogue goes down to roughly softball-size objects, Spence says that Space Fence planners are "looking at tracking down to the order of golf-ball or marble size at lower altitudes, or softball size at higher altitudes."

Space Fence and a fleet of related smaller-scale projects aim to increase what the military calls "space situational awareness." But how that awareness might someday progress to remedial action—the removal of orbital debris—remains unclear. NASA's Liou notes that there has never been so much as a demonstration of a prototype system for active debris removal (ADR). Some proposals that have been floated for ADR include targeted laser systems to nudge debris into atmospheric burn-up or spacecraft equipped with tethers or nets to drag pieces of debris out of orbit. Unfortunately those ideas are still essentially science fiction. "Currently there is no economically viable and technically feasible solution for ADR," Liou says. The 2010 U.S. National Space Policy calls for NASA and the military to pursue research and development of ADR approaches but includes no timetable for its deployment. The National Academy of Sciences plans to release a report on the problem September 1, including an assessment of steps that could be taken to limit the risk to satellites and space missions.

The size and complexity of the problem will very likely require significant intergovernmental collaboration, perhaps on the scale of constructing the International Space Station. Such cooperation would only be fitting, considering that space junk itself is global—of the catalogued debris now in orbit, 37 percent is Russian or Soviet in origin, 29 percent is American and 28 percent is Chinese. The 2009 Cosmos–Iridium collision, involving a Russian satellite and a satellite owned by a U.S. corporation, demonstrated that the consequences of a polluted orbital environment will be international as well.

Fortunately orbital debris has yet to cause any loss of life. But if human space exploration continues apace in the coming years and decades, it is only a matter of time. Whether any space-faring nations can muster the resources to tackle the growing problem in orbit before tragedy happens remains to be seen.