Recently, the six man crew of the ISS (International Space Station) had to take shelter in their escape pods because radar had detected debris (from a collision between two communications satellites three years ago) approaching. The fragments came no closer than 14 kilometers and the crew were able to get back to work. These collision alerts are becoming more frequent. There was one last November and one last June, where orbital debris came within 260 meters of the ISS. Before that, there were alerts in 2009 and 2008.
The threat is growing. In the last 16 years eight space satellites have been destroyed by collisions with one of the 300,000 lethal (the size of a marble or larger) bits of space junk that are in orbit. As more satellites are launched more bits of space junk are left in orbit. Based on that, and past experience, it's predicted that ten more satellites will be destroyed by space junk in the next five years. Manned space missions are at risk as well. Three years ago a U.S. Space Shuttle mission to fix the Hubble space telescope faced a one in 229 chance of getting hit with space junk (that would have likely damaged the shuttle and required a backup shuttle be sent up to rescue the crew). Smaller, more numerous, bits of space junk are more of a danger to astronauts (in space suits) working outside. The shuttle crew working outside to repair the Hubble satellite had a much lower chance of being killed by space junk because a man in a space suit is much smaller and the space suits are designed to help the person inside survive a strike by a microscopic piece of space junk.
The U.S. is spending nearly a billion dollars a year in an attempt to better identify, and track, the larger, more lethal bits of space junk. Two years ago the U.S. Air Force put a special Space Based Space Surveillance system (SBSS) satellite into orbit. This $830 million system uses a satellite that contains a digital camera to take pictures of space debris and make it easier to count and track the growing quantity of space junk. Getting a better and timelier look at space junk has become a priority.
The U.S. has proposed using a space based laser to destroy much of the space junk. The laser either vaporizes debris or damages the larger bits so that its orbit "decays" and the junk moves down into the atmosphere and burns up. Many nations object to this proposal, as such a laser system could also be used as an anti-satellite weapon. However, if the growing swarm of space junk destroys a lot more satellites, that attitude may change.
After over half a century of humans putting objects into orbit there is a lot of junk circling the planet. Currently, over 300,000 dangerous objects 10 mm (.4 inch) in size have been identified. The smallest of these is capable of disabling a satellite or damaging a spacecraft. The damage is severe because these objects can hit at very high speed (9-10 times faster than a bullet) if they and their target are coming from different directions. There are nearly 18,000 objects 10 centimeters (4 inches) or larger. These can do some catastrophic damage to satellites or spacecraft. There are millions of objects smaller than 10mm, and these are responsible for many satellites failing early because of cumulative damage from getting hit by a lot of these micro objects.
The U.S. Air Force Space Surveillance Network tracks over 18,000 objects 10mm and larger but cut back on information sharing eight years ago, for national security reasons. The United States eased up on this policy once the SBBS went into operation. With some 900 active satellites in orbit (nearly half of them American) there is a need to provide better tracking of dangerous space junk. About 75 percent of all satellites are non-military (most of them commercial, the rest government non-military birds). With SBBS the U.S. will be much better able to protect its satellites from the growing debris menace. Other nations, particularly American allies, will want the same degree of safety.
There are other organizations keeping an eye on the debris. The Russian Space Surveillance System is known to use radar to track over 5,000 objects in low orbit. But the Russians have never shared this data completely, or regularly. Filling in the gaps are two international organizations: IADC (Inter-Agency Space Debris Coordination Committee) and ISON (International Space Observation Network). IADC is a government operation, whose members include the U.S., NASA and the equivalents in Russia, China, and several other major nations. Like most government organizations not all data is shared.
ISON is a non-government organization, and they come up with some of the most interesting stuff. ISON comprises 18 scientific institutions, 18 observatories, 25 telescopes, and over a hundred professionals. ISON does not, as far as anyone knows, withhold data because of any national security concerns. This is fairly certain because ISON work is monitored, and complemented, by the efforts of thousands of amateur astronomers and orbital addicts who connect via the Internet and constantly scour the orbital space for new objects and dangerous movements by existing ones.
ISON already has spotted nearly 200 larger (over 10mm) objects that have never been reported by any of the government organizations. The Internet based amateurs are often the first to spot a lot of this new activity, mainly because they have more eyeballs and, in some cases, impressive optical equipment, searching the skies.
When someone spots an object headed for a maneuverable satellite the owner is alerted and the bird is moved. This has happened several times in the last few years. The number of dangerous objects up there increases 10-20 percent a year. That's even with many of them falling into the atmosphere and burning up each year. Even when you spot a potential collision between debris and an active satellite, the high speed of these objects, and slight instability of their orbits, can turn an expected collision into a near miss. This is not an exact science but the more information you have, the more accurate your predictions will be.
SBBS has a military purpose, to spot and track hostile KillSats sent up to destroy American satellites. If the initial SBBS continues to be successful more will be launched, to provide real time surveillance of orbital space. But most of the time SBBS will serve to make space safer from catastrophic accidental collisions.