The U.S. Air Force has successfully completed testing of its new, higher resolution Space Fence Surveillance System Radar. The new radar is based on Kwajalein Atoll, in the Marshall Islands, 3,900 kilometers southwest of Hawaii. Construction began in 2015 for the solid-state S-band phased array radar which is able to detect and track objects as small as 10 cm (four inches) long. The older (1960s) Space Surveillance System Radar could only detect objects at least 75 cm (30 inches) long. As a result, the number of orbital objects/debris that can be tracked goes from 23,000 to over 200,000.
This space debris is moving at high velocity meaning that objects as small as one cm (0.4 inch) can damage satellites and larger stuff (at least 10cm) can destroy satellites and seriously damage the ISS (International Space Station). The 1960s era Space Surveillance System consisted of three radars (in Texas, Alabama and Arizona) and six receiving stations. The cost of maintaining the older system led to it being shut down in 2013. The Space Fence will work in conjunction with a Space-Based Space Surveillance satellite in a 620 kilometer high orbit as well as a large (85 ton) telescope in Australia that specializes in getting a more detailed look at space debris.
A second, and possibly third, Space Fence radar is planned to provide more persistent and accurate tracking of debris. Two or more radars make it possible to determine which debris is a danger to existing satellites and larger orbital objects like the ISS. Orbital debris is a growing danger in orbital space and the major satellite launching nations cooperate in identifying and tracking the debris swarms.
There are millions of fragments in orbit. Most of the pieces are tiny, but at least a thousand are truly dangerous (at least 10 cm/four inches long, wide, or in diameter). There are many such debris swarms up there that have to be watched and avoided. Not all the debris swarms are the result of accidents. For example, in 2007 a Chinese KillSat test put a huge debris swarm in orbit followed by another new swarm created by the accidental explosion of a Russian rocket that put over 1,100 dangerous fragments in orbit. Those two incidents increased the dangerous debris in orbit by about fifteen percent.
While the debris is a danger, it should be put into perspective. Orbital space is actually quite large. Each “layer” of orbital space is over 600 million square kilometers. A layer is anything you want it to be (say a kilometer) between orbits. Thus even in low orbit (500-2,000 kilometers) you have 1,500 such layers. Orbits lower than 500 kilometers will rapidly drag debris back into the atmosphere. While this amounts to two billion square kilometers of orbital space for half a million bits of dangerous debris, most satellites occupy a small portion of these orbits and move through an orbit every 90 minutes. Most of the debris is concentrated in a small number of debris swarms but these swarms tend to be in the most heavily used orbits. Bottom line is that current chances of any live satellites getting hit by debris are low but as more debris accumulates the chances of getting hit increase. It has reached the point where satellite operators take precautions, like equipping their satellites with the ability to move (until the fuel runs out) and paying people to constantly monitor the debris collision situation. For a satellite costing several hundred million dollars to build and put into orbit, this is considered a prudent way to operate.
Currently, about a thousand active satellites are in orbit, and nearly half of them are American. About 75 percent of all satellites are non-military, most of them commercial, the rest government non-military birds. The most common time for orbital debris to be created occurs as a satellite is put into orbit. Thus in late 2012 the third stage of a Russian satellite launcher unexpectedly exploded after it failed to put two satellites into orbit. Launched via a Proton rocket, there was some kind of problem in the final stage and apparently the remaining fuel in that stage caused an explosion. This created a debris field of several hundred new bits of space junk, mainly pieces of the third stage and the two satellites. This prompted satellite (and space station) operators to check their orbits and make adjustments if there might be a collision with this new cloud of deadly (at high speed coming from the opposite direction) debris. On the bright side, many of these new bits of junk are large and in a low orbit, meaning that they will soon fall towards earth and burn up. That also has to be tracked, so you know what debris is gone and what is not.
A less common cause of orbital debris is stable satellites that explode in orbit. In early 2015 an American space satellite did this. This one was a twenty year old weather satellite that experienced an equipment failure that showed up to ground monitors as a rapid increase in internal temperature followed by the satellite exploding into 43 pieces. This was not catastrophic for weather monitoring since this satellite, because of its age, was relegated to backup duty in 2006. Eventually, when it failed, it would have been maneuvered into a lower orbit where it would eventually burn up in the atmosphere and leave no debris in orbit. As soon as signs of malfunctioning in the satellite were detected plans were made to maneuver the satellite into the lower orbit, but the satellite soon became uncontrollable and exploded. While rare, satellite unexpectedly exploding is not unknown.
The space junk situation has been getting a lot worse in the 21st century. In 2007 the United States became the first nation that had to change the orbit of one of their satellites to avoid the cloud of debris created when China tested an anti-satellite weapon earlier in 2007. This call began when China launched an anti-satellite system (a KillSat or Killer Satellite) on January 11th that destroyed an old Chinese weather satellite, about 850 kilometers up. That's at the upper range of where most reconnaissance satellites hang out. The KillSat hit the weather bird, and the result was several million fragments.
The IADC (Inter-Agency Space Debris Coordination Committee) is an international organization that coordinates the exchange of information, and space operations, as they relate manmade and natural debris in orbit around the earth. Every year some of this stuff falls into the atmosphere and burns up but there are always new accidents or deliberate operations that add more junk to the spaceways. There are many government and amateur groups monitoring low orbit space and most of these observers share their information for the safety of everyone going up there.