Orbital space is dangerous for humans and their machines. There is no atmosphere and it is very cold and full of risks from radiation and small meteorites and space dust. Then there are custom built space satellites of all shapes and sizes. These have proliferated since the 1960d when only a few per year were sent into orbit. These had short lifetimes, though some dead satellites continue in orbit for decades. In addition to the satellites there are many other objects put into orbit by the last stage of the SLV (Satellite Launch Vehicle) which transported the satellite to orbital space. The final insertion of the satellite into a useful orbit usually involves several items bits of equipment that remain in orbit along with the satellite. Over the decades, as more dead satellites and space junk has accumulated up there have been more collisions that create more debris. By the 21st century the accumulation of debris was so great that it had to be tracked and operational satellites had to avoid it. In the last decade the debris problem has become critical and increasingly unavoidable for nations that are sending thousands of new satellites into orbit. Most of the satellites are American but every major nation now regularly puts satellites into orbit. The debris crisis is a problem for everyone using orbital space. The key problem is that there is currently no known technology that can eliminate space debris.
At the end of 2022 there were 8,261 space satellites orbiting the earth but only 58 percent of them are still active. In the last few years, the number of satellites put into orbit each year has gone from one or two hundred a year to over a thousand a year and growing.
Since the 1960s there have been over 6,000 rocket launches that put more than 50,000 satellites or satellite size objects into orbit. There are half a million larger (at least 100 mm, or four inches) pieces of debris in orbit that can be tracked. There are over 120 million untracked objects smaller than one mm in orbit. The 100mm debris is capable of damaging or disabling active satellites. The smaller debris that cannot be tracked causes less damage that can eventually cause major damage. The growth in the number of individual pieces of debris is already forcing countries that put expensive satellites into LEO (Low Earth Orbit) to place their satellites in an orbit that is less likely to encounter a lot of debris.
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. 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 where it burns up. 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.
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 100mm or four inches) can destroy satellites and seriously damage the ISS (International Space Station). While there are over 130 million fragments in orbit, most of the pieces are tiny. At least a thousand bits of debris are truly dangerous and these are the ones that are at least 10omm 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.
No one has yet put a system into orbit that can destroy this debris. There have been several specialized satellites put into orbit that can grab and move a satellite to a less dangerous location or move a defunct satellite into a less troublesome location. China put the latest of these satellite movers into space. This is the SJ-21 which was launched in Oct0ber 2021. SJ-21 is large enough to grab large defunct satellites and move them to less valuable geostationary orbits. These orbits are 35,786 kilometers from earth and any satellite placed in one will remain above the same area on earth. In other words, the satellite and earth will rotate together. For some types of satellites these orbits are very useful and when a satellite occupying one of these orbits ceases to function you have to move it away in order for a working satellite to take its place. SJ-21 does the moving.
More mobile satellites with robotic arms are a welcome new development because they can be used to inspect satellites in trouble and help diagnose the problem and perhaps fix it. Chinese and Russian satellites similar to SJ-21 have already been spotted getting close to satellites belonging to other nations and, in some cases, apparently practicing disabling them. That was not unexpected. While these movement and inspection satellites have a wartime use to disable enemy satellites, the more immediate problem is managing and eventually eliminating a lot of the man-made debris in orbit. The only way to destroy this stuff is to use another satellite to push it close enough to earth for gravity pulls to the surface and moving through the atmosphere at high speeds causes the debris, and in most cases, be complete destroyed. This does not work with the majority of space debris, which is too small to track from earth and too numerous to collect and send into the atmosphere. As this debris crisis becomes more acute, more effort will be put into solutions and those solutions will appear, or there will be fewer operational satellites in orbit. No precise date when the orbit debris crisis becomes critical but when it does show up it shouldn’t be a surprise.