Cleaning Up Space
When you look up into the night sky, you appear to have an unobstructed view of stars, galaxies, nebulas, the moon, and occasionally other objects like meteors and comets. But it's not totally unobstructed. According to the United Nations Office for Outer Space Affairs (UNOOSA), there are 11,330 satellites up there right now. According to the Union of Concerned Scientists (UCS), 6,718 of them are operational. Adjusting for the dates that UNOOSA and UCS monitored the skies, that leaves 3,266 pieces of debris clogging orbital space. But it gets worse. Those are the intentional things up there. There is also junk, amounting to about 34,000 pieces of bigger than 10 cm (4 inches) in size, plus 128 million additional smaller pieces 0.1-10 cm (0.04-4.0 inches) big. The United States Space Surveillance Network (SSN) runs >30 ground-based stations and 6 satellites of its own to track 27,000 pieces of space junk 10 cm or greater in size. But, who is going to clean this up, and how?
What is space junk anyway? Pieces of rockets break off as they fly upward and break free from their main spacecraft (like probes sent to Mars). These pieces can be innocuous things like paint flecks. Satellites occasionally collide, though, or get hit by space junk. In 2021, a Chinese military satellite Yunhai1-02 was hit by a piece of debris left over from a Russian rocket that carried a spy satellite in 1996. That event alone left 37 pieces that are still tracked and many that are not. Satellites can even hit each other, as evidenced by the 2009 encounter between the Russian military communications satellite Kosmos-2251 and the U.S. communications satellite Iridium 33. The former had been out of operations for 14 years and had no means to move out of the way. The impact of the 560 kg (1,200 pound) Iridium 33 with the 950 kg (2,100 pound) Kosmos-2251 produced a thousand trackable fragments.
Space debris can also be caused by purposely destroying a satellite. In 2021, Russia launched a missile from Earth to destroy a long-defunct Kosmos 1408 satellite of its own, which had been nonoperational for almost 30 years. Although Russia had given advance warning so that International Space Station astronauts could take shelter, and there was no apparent damage as the ISS passed through the debris, other space missions had to make course corrections to avoid some of the debris. The U.S. also deliberately destroyed a Russian satellite in 1985, the only event of its kind.
Low Earth Orbit (LEO) is 160-2,000 km (miles) high and accounts for 55% of all satellites. Another 35% are higher up, around 35,000 km (miles) in what is called Geosynchronous Earth Orbit (GEO). In between is called Medium Earth Orbit (MEO) space. But the LEO orbit is not concentric path like a bull's eye target. See the diagram below.
Major space stations have been (or currently are) at the following LEO altitudes:
- SkyLab (1973-1979): 435 km
- Salyut 1, and 3 to 7 (1971, 1974-75, 1974-77, ): 210 km, 245 km, 245 km
- Mir (1986-2001): 358 km
- International Space Station (1998 to present): 420 km
- Tiangong-2 space laboratory (2016-2019): 393 km
So isn't space pretty big, and aren't the odds of hitting something up there slim? I mean, we don't seem to have problems viewing space with things buzzing in front of our eyes and telescopes all the time. David Kessler was a NASA scientist who didn't think so and proposed a chain-reaction series of events that might happen if too much debris was in orbit. His self-named "Kessler effect" was proposed as far back as 1978 and has been upgraded since then as more junk accumulated.
Space junk is traveling fast, from 25,000 km/hr (17,500 mph) to 54,000 km/hr (32,400 mph). That latter figure is 10 times the speed of an average bullet! Junk may be small but its impact can be very strong at those speeds.
There are currently about four techniques that have been proposed, and some even tested, to collect space junk to make orbits safer. (There are five techniques if you also include making satellites out of wood so that they burn up more readily.)
An international consortium called the RemoveDEBRIS project was co-funded by the European Union. It sent up a platform called a CubeSAT (cube satellite) equipped with a "harpoon" and a "net". The harpoon was tested at its maximum distance of 1.5 meters (inches) in 2019 and successfully hit/grabbed a target held about 10 cm in diameter. See the actual video below.
You can see the other test when a RemDeb (remove debris) net is fired to capture an object further away and the harpoon.
Another cleaning process is a UK effort that uses an extendable robotic arm to grip the junk object. It then pulls it in and burns up as they enter the atmosphere together. Its name is COSMIC, which stands for Cleaning Outer Space Mission through Innovative Capture. Here's an computer-generated simulation video of how it is meant to work.
The COSMIC program is working with a Japanese startup company called Astroscale to accomplish its goals. Astroscale is developing the radar imaging technology to study orbiting objects up close as they roll and tumble; once the data is analyzed on board, the robotic arm will have a better chance to latch on.
NASA has pursued a slightly different course. It has paid $850,000 to an American startup company called TransAstra to work on a twofold process. One component detaches from the other and seeks out the target debris. Using an inflatable bag designed by NASA, the TransAstra-built capture device scoops up the junk, then returns it to the mother ship. Several payloads of various sizes can be handled this way. That's just step one. Rather than falling out of orbit and burning up to eliminate the debris, the aim is to safely land the mother ship so that the junk can be recycled on Earth.
If you are familiar with the TV show Star Trek and any of its successors, you will recognize that instead of these mechanical devices, science fiction used some sort of "tractor beam" to hook onto things. No wired or nets were needed to secure them and haul them around. Scientists today have actually made strides to duplicate that as you can see below.
The 1960s' prequel show Enterprise (debuting in 2001) actually did have a cable device called the grappler, though, which is much like the RemoveDebris harpoon.
Here's a cool FAQ list on space debris put out by the NASA Orbital Debris Program.
