Space, as the name suggests, is mostly empty. However, since the first satellite launch in 1957, mankind began to populate the Earth orbit with all kinds of spacecraft. On the downside, space also became more and more cluttered with trash from defunct or broken up rocket stages and satellites. Moving at speeds of nearly 30,000 km/h, even the tiniest object can pierce a hole through your spacecraft. Therefore, space junk poses a real threat for both manned and unmanned spacecraft and that is why space agencies are increasing their efforts into tracking, avoiding, and getting rid of it.
Earth Orbit is Getting Crowded
According to NASA’s Orbital Debris Program Office (ODPO) Earth orbit currently hosts an estimated number of 500,000 marble-sized debris objects and a whopping 100,000,000 objects of 1 mm or smaller. As shown in the picture there are two main debris fields. While most of the debris is located in low-Earth orbit (LEO) at altitudes of <2000 km, there is also a ring of space junk in geosynchronous orbit (GEO) at an altitude of ~36,000 km.
Space trash includes derelict spacecraft like the Vanguard I satellite which has been in orbit for over 60 years and thus holds the record for the oldest man-made object in space. Other culprits are upper stages of rockets that broke up or exploded, which is why nowadays they are usually “passivated” by venting their unburned fuel.
In 2007, China earned a lot of criticism for blowing up their Fengyun-1C weather satellite as part of a missile test. Together with the accidental collision of the US communications satellite Iridium-33 and the defunct Russian Kosmos 2251 satellite in 2009, these events are responsible for much of the large debris currently located in orbit.
To protect themselves from micrometeorites and orbital debris (MMOD), spacecraft use so-called Whipple shields consisting of several thin layers that are spaced apart. Upon impact, the outermost layer shatters the projectile thereby spreading its kinetic energy upon a large area as it passes through. To avoid collisions with known larger objects, spacecraft sometimes have to perform evasive maneuvers.
For the ISS, such a maneuver is ordered if the chance of impact is greater than 1/10,000 which happens on average once per year. In 2012, a record number of four of such moves had to be performed which are always costly because of the large amount of fuel that needs to be spent. NASA’s space shuttles have frequently been pierced by MMODs, but luckily all of the catastrophic collisions so far have been limited to unmanned spacecraft. One example is that of the French satellite Cerise which was hit in 1996 by part of an Ariane rocket booster. And we’ve already mentioned the Iridum-Kosmos crash.
Keeping Track of All the Junk
It is vital to catalog and track all the junk floating around in orbit to prevent future crashes, and to prevent future crashes from further contributing to the space junk problem. The most comprehensive catalog of space is junk is held by the US Space Surveillance Network (SSN). Currently, they keep track of more than 22,000 man-made objects orbiting Earth that are 10 centimeters or larger.
Depending on their altitude, objects with sufficient size can be detected by ground-based radar and optical telescopes. Optical telescopes measure the sunlight reflected by debris, while the distance can be accurately determined by laser ranging. The method is based on measuring the round trip time of a short laser pulse shot from the ground and reflected by the object. The technique has long been used to track satellites that are equipped with a retroreflector. Since the diffuse reflection from space debris is much fainter, the measurement is significantly harder. So far the technique could only be used during twilight when the laser ranging station on Earth is in darkness but debris objects are still lit up by the Sun. With improved imaging techniques, Austrian scientists just recently succeeded to use space debris laser ranging during the daytime, doubling the viewing window.
NASA’s Long Duration Exposure Facility (LDEF) taught us a lot about space debris. It was essentially a target that was left in space for about six years before it was retrieved by the Columbia Space Shuttle in 1990. The LDEF hosted 57 individual scientific experiments designed to study the long-term effects of outer space environment on different materials, electronics and biological samples. Because of its large surface area and long exposure, much statistical information was gained from studying the Swiss cheese pattern that had formed on its surface as shown in the picture.
Cleaning up the Orbit
Since 2002, all major space agencies are following some common guidelines to reduce the growth of space debris. Spacecraft in GEO are required to move to a graveyard orbit at a higher altitude after they finished their mission. Objects passing through the LEO region should be de-orbited or at least put into an orbit with a reduced lifetime.
Due to atmospheric drag, all orbital debris will eventually fall back to Earth. However, at altitudes of 800 km, this may take decades, while above 1,000 km orbital debris normally will continue circling Earth for a century or more. We’re adding space junk faster than it’s raining down. Therefore, in the long run, we not only need to stop the ongoing pollution of space but also actively get rid of some of the space trash already in orbit. Otherwise, the density of debris may become large enough to create a cascading effect where the fragments created in a collision trigger new collisions. This scenario is known as the Kessler syndrome and nicely explained by Donald Kessler himself in this video.
Plans for the active removal of space debris include the ClearSpace-1 mission of the Swiss startup ClearSpace which was funded by ESA and is planned to launch in 2025. ClearSpace-1 will use robotic arms to capture part of a Vespa (Vega Secondary Payload Adapter) upper stage left in orbit from a previous ESA mission. Both spacecraft will then be deorbited to burn up in the atmosphere. Eventually, the goal is to have a “tow truck” in space that can capture and remove multiple objects with a single mission.
The same lasers that are used to track space junk might also be used to remove it. There are several concepts, to use high-power ground- or space-based laser systems to remove debris of 1 – 10 cm size from LEO space. The laser evaporates material from the object which forms a jet that slows down the target so that it will re-enter the atmosphere faster.
After we have already polluted Earth to a devastating degree it would be nice to not see the same thing happening in space. It would be a shame if the scientific progress and communication technologies enabled by space missions were put to a halt by the Kessler syndrome. With future new mega-constellations of satellites like the Starlink project, this risk is quite imminent.