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 here there 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), space crafts 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.
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
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.
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.
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