By Bettina Mrusek
Orbital debris is a growing problem. While advances in technology have improved our ability to successfully launch satellites into orbit, the trial and error period required to get to us there, combined with the enormous growth of the satellite market, have unintentionally congested many of the orbits surrounding our planet.
Debris found in the Low Earth Orit (LEO), which is roughly 100 – 1500 km from the Earth’s surface, presents a majority of this debris. While there are approximately 5000 satellites in LEO, only 39% are in operation. The remaining 61% are not functioning and can thus be regarded as debris.
There are many different reasons satellites become inoperable; some reach scheduled service limitations, while others are forcibly retired by in-orbit collisions. Inoperable satellites are tracked by space surveillance networks, but are on uncertain trajectories which increases the probability of a collision. These collisions result with the creation of multiple, additional debris which pose risks to other satellites, operational or not.
This effect is compounded with every collision. As noted by prominent NASA scientist Donald Kessler, the increased density of objects in LEO could result with a cascading event in which collisions reach a point where it would no longer be feasible to launch a satellite into orbit. While Kessler’s work accurately demonstrated the potential for such an event, the time at which this will occur has not been accurately proven. There are many variables that impact rates of collision making it difficult to pinpoint an exact point in time.
However, one thing is for certain: the growing number of satellites in LEO is making it increasingly difficult to launch satellites. Today’s launch windows must account for orbital debris; it doesn’t make much sense to launch a new, functioning satellite directly into the path of debris.
Kessler’s work on orbital debris was ground-breaking. It fueled considerable research on the subject and also influenced the formation of space surveillance networks to track and monitor debris. As a result, three notable high-impact collisions were recorded; one in 1996, a second in 2007, and a third in 2009. These events added more than 6,000 pieces of trackable debris to the inventory of space junk. However, while the events in 1996 and 2009 were unintentional, the collision in 2007 resulted from an intentional Chinese anti-satellite test, which used a missile to destroy an old weather satellite. This particular event highlights the lack of international regulatory accountability for space-related operations.
Growth of the Satellite Market
Considering the potential for collisions, the projected growth of the satellite market is a cause for concern, especially in LEO. While Elon Musk’s plan for Starlink, a constellation of nearly 7,000 satellites, is ambitious and would undoubtedly bring faster and more reliable internet capabilities, the addition of such a large number of satellites increases the likelihood of collisions, especially if existing debris cannot be removed. However, Elon Musk is not the only one attempting to capitalise on the lucrative satellite market. Companies such as Samsung, Boeing, and OneWeb have similar projections totaling over 12,000 additional satellites by 2030.
How Many Need to be Removed?
If we continue to increase the number of objects in LEO without removing any, the threat of collisions grows. The clear answer is to remove enough satellites to balance out the debris, thus minimising the potential for the Kessler Effect. Initial projections predicted that by removing 5-7 satellites per year could stabilize the effect, but could rise to as many as 8 per year by the year 2040.
Orbital debris Mitigation Efforts
How exactly can we remove non-functioning satellites? The first step is to ensure that when a satellite reaches the end of its service life, it is properly disposed of. One way to accomplish this is with pre-programmed end of life procedures which essentially sends the satellite into a graveyard orbit that is far enough away to pose any significant threats to operational satellites. While many satellites are equipped with such procedures, if the satellite is damaged prior to its end of service, it may not be able to initiate the sequence. The space environment is exceptionally harsh; orbital debris moving at incredible speeds and must account for the effects of solar radiation. This can make it difficult for any satellite to survive.
Another debris mitigation effort is the natural process of orbital debris removal referred to as atmospheric drag. As debris moves in orbit, those that are closer to the Earth’s surface are affected by the pull of gravity. Pieces of debris are eventually pulled down towards the surface of the Earth and are disintegrated (hopefully) as they fall through the Earth’s atmosphere. Larger pieces may not break up completely and can pose risks if the debris falls onto the land, as opposed to the ocean. However, the effects of atmospheric drag can take several years or even decades to occur.
There are also other large-scale efforts to repair inoperable satellites in orbit such as NASA’s Restore-L or DARPA’s (Defense Advanced Research Projects Agency) robot servicing, but these are designed for larger satellites. Additional removal efforts aimed at catching smaller debris with canopy-like structures have been researched, but the testing and implementation of these efforts has proved challenging.
If the number of inoperable satellites could be repaired in orbit, making them operable, adherence to post-mission procedures could be improved. Satellite maintenance could provide such an opportunity. Satellites fail for a number of reasons. Some run out of fuel, some need to be re-positioned, while others may need components upgraded or replaced. While some are undoubtedly beyond capable repair, others are not. This presents a financial opportunity as well as the prospect for sustainable space operations. This responsibility could come from the manufacturers, such as in the aviation industry, or from government or private sector companies.
Overall, orbital debris and the resulting impact on the Kessler Effect will continue to threaten the use of the orbits surrounding Earth. In order to ensure the long-term use of this space, clear, long-term mitigation efforts are needed.
Bettina Mrusek is an Assistant Professor at the College of Aeronautics at Embry-Riddle Aeronautical University.