Space Debris

Space Debris

By: Saanjali Maharaj

SPACE DEBRIS IS DEFINED AS any non-functional human-made object, or any part thereof, in the Earth’s orbit or re-entering its atmosphere [1]. While leaving a small defunct satellite in orbit in the vastness of space might not seem harmful at first glance, the consequences can indeed be detrimental. The speed at which these non-functional objects travel reaches approximately 15,700 mph in low Earth orbit (LEO). Due to this high velocity, even the smallest fragments of space debris, such as paint flecks, can cause damage to functional spacecraft. Moreover, as it re-enters the atmosphere, the debris can also pose a potential threat to people and property on the ground. According to the U.S. Department of Defense’s Space Surveillance Network (SSN) sensors, approximately 27,000 pieces of debris as small as 2 inches (5 cm) in diameter, are currently in space [1]. As the Space Age progresses, mitigation strategies become an increasingly important concern following the escalating probability of damage caused by the debris.

Figure 1: Debris Velocity Analysis for 2009 Satellite Collision (Iridium 33 in blue, Kosmos-2251 in orange)

Firstly, a discussion of the sources of space debris is warranted. Debris may be released in the regular operation of spacecraft, for example during the abandonment of launch vehicle stages. Another cause of debris generation is breakup, whether intentional or accidental. A break-up is defined as an event that generates fragments, including ruptures due to internal pressure, explosions caused by chemical or thermal energy from propellants, or collisions with other objects. Derelict satellites and dead spacecraft also account for a major source of debris. Many satellites are boosted into medium altitude ‘graveyard’ orbits at the end of their functional lives, but this does not eliminate the risk of debris generation. Alternative decommissioning activities may involve intentional destruction of the space object thereby generating debris. Furthermore, derelict satellites left in orbit may also lead to collisions. For instance, in 2009, a deactivated Russian satellite, Kosmos-2251, was involved in a hypervelocity collision with an active US communications satellite, Iridium 33. This event resulted in 2300 cataloged pieces of debris, at least half of which are expected to remain in orbit for over a hundred years [2]. The testing of anti-satellite weapons (ASATs) has similarly contributed to space debris. In 2007, China’s ASAT test created over 3500 pieces of debris when a ballistic missile with a kinetic kill vehicle payload destroyed its target, Fengyun-1C (FY-1C), a defunct weather satellite. It is estimated that 79% of this debris, or almost 2800 pieces, will still be in orbit 2LEO altitude: 160 - 2000km above the Earth’s surface. a century after the ASAT test [3]. Not included in this definition is the generation of fragments as a result of the ageing and degradation of a spacecraft.

It is clear from the longevity analyses discussed above that space debris poses a long-term threat. In fact, each collision that produces space debris contributes to an increased likelihood of future collisions and debris generation. This collisional cascading effect is known as Kessler syndrome, put forward in 1978 by Donald J. Kessler, a NASA astrophysicist [4].

Space debris mitigation strategies can be categorized as those limiting debris generation in the short-term and in the long-term. In 2022, the Inter-Agency Space Debris Coordination Committee (IADC) published Space Debris Mitigation Guidelines [5], which were most recently revised in March 2020. The guidelines are further enforced by the United Nations Office for Outer Space Affairs (UNOOSA) in its 2010 publication, Space Debris Mitigation Guidelines of the Committee on the Peaceful Uses of Outer Space [6]. Member States are strongly encouraged to adhere to these mitigation strategies in the planning and operation of future missions and, if feasible, in existing missions. However, it should be noted that these guidelines are not legally binding under international law.

Preventative space debris mitigation procedures include limiting mission-related debris during normal operations and avoiding the planning and implementation of intentional break-ups. Precautions should also be taken to reduce the likelihood of accidental break-ups. This can be achieved by passivation: the elimination of all stored energy on spacecraft or orbital stages. Examples of passivation measures include venting or burning excess propellant, deactivating battery charging lines, relieving pressure vessels, and terminating power to flywheels and momentum wheels.

Collision avoidance procedures are another recommended strategy to mitigate space debris. NASA uses SSN to track debris and identify collision courses that require debris avoidance maneuvers. For the ISS, the surrounding 4 x 50 x 50 km region is closely monitored [1]. If a tracked object poses a significant threat, Mission Control centres in Houston and Moscow collaborate to determine the best course of action. If the probability of collision is sufficiently high and the risk it poses to mission objectives is significantly detrimental, the debris avoidance maneuver is conducted once it does not cause additional risk to the crew. If there is significant uncertainty in the tracking data, or if there is not enough time to safely implement the avoidance maneuver, the crew must isolate in a spacecraft used to transport humans to and from the ISS, which acts as a ‘lifeboat’ in case of emergency. In 2007, NASA extended its collision tracking to all NASA maneuverable satellites within LEO, and within 200 km of geostationary orbit (GEO)5 . The U.S. Space Force performs these collision risk assessments every 8 hours for human spaceflight and daily Monday through Friday for robotic spacecraft and satellites. If a debris avoidance maneuver is warranted, the updated trajectory is sent to the Space Force for iteration until the final trajectory does not pose a risk of collision with either the same or another space object.

Figure 2: Debris Cloud Evolution from 2007 ASAT test

The IADC and UNOOSA have established protocols for spacecraft and satellite end-of-life that mitigate the harmful impact of space debris. One option involves increasing the orbit altitude out of the GEO-protected region for disposal. If the termination of the operational phase occurs in the LEO region, the space object should be deorbited or maneuvered into an orbit with an expected residual orbit lifetime of at most 25 years and with a probability of disposal success of at least 90%. If the spacecraft or orbital stage is being disposed of by direct re-entry into Earth’s atmosphere, the debris should be limited and ideally confined to uninhabited regions, and the relevant air and marine traffic authorities should be well-informed about the trajectory and time of re-entry. Furthermore, there should be considerations of the environmental impact of any remaining debris, in particular, its radioactivity, toxicity, or other pollutive characteristics.

Figure 3: NASA’s Debris Avoidance Maneuver Protocol

In conclusion, space debris can cause significant damage to human life (both for crew members in space and civilians on Earth), active spacecraft and satellites, and other property on the ground. This threat has the potential to cascade according to Kessler syndrome, by which each collision further exacerbates the risk of subsequent collisions. It is evident that there is a dire need for mitigation strategies. To this end, NASA, in conjunction with the U.S. Space Force, has implemented debris avoidance procedures for all NASA maneuverable satellites. Additionally, the IADC and UNOOSA have established several key guidelines for preventative measures in mission planning, collision avoidance, and space object end-of-life protocols. This year alone, debris from a SpaceX rocket landed on a farm in Washington state [7] and the trajectory of the Long March 5B rocket was observed with bated breath, but thankfully, it landed in the ocean [8]. Yet, as of 2021, there are no international laws in place to enforce the aforementioned measures. For the sake of safety and security, international laws which regulate the generation and disposal of space debris should be passed, ensuring that the legal framework progresses in tandem with space technology.

References

[1] “Space Debris and Human Spacecraft”, NASA, 2021. [Online]. Available: https://www.nasa.gov/mission_pages/ station/news/orbital_debris.html. [Accessed: 07- Jun- 2021].

[2] T. Kelso, “Analysis of the Iridium 33-Cosmos 2251 Collision”, Celestrak.com, 2009. [Online]. Available: https:// celestrak.com/publications/AMOS/2009/AMOS-2009. pdf. [Accessed: 07- Jun- 2021].

[3] T. Kelso, “Analysis of the Iridium 33-Cosmos 2251 Collision”, Celestrak.com, 2007. [Online]. Available: https:// celestrak.com/publications/AMOS/2007/AMOS-2007. pdf. [Accessed: 07- Jun- 2021].

[4] S. Olson, “The Danger of Space Junk”, The Atlantic, 1998. [Online]. Available: https://www.theatlantic. com/magazine/archive/1998/07/the-danger-of-spacejunk/306691/. [Accessed: 07- Jun- 2021].

[5] “IADC Space Debris Mitigation Guidelines”, Inter-Agency Space Debris Coordination Committee, 2020. [Online]. Available: https://orbitaldebris.jsc.nasa.gov/ library/iadc-space-debris-guidelines-revision-2.pdf. [Accessed: 07- Jun- 2021].

[6] “Space Debris Mitigation Guidelines of the Committee on the Peaceful Uses of Outer Space”, United Nations Office for Outer Space Affairs, 2010. [Online]. Available: https://www.unoosa.org/pdf/publications/st_space_49E. pdf. [Accessed: 07- Jun- 2021].

[7] H. Waitering, “Debris from SpaceX rocket launch falls on farm in central Washington”, Space.com, 2021. [Online]. Available: https://www.space.com/spacex-rocket-debris-found-washington-farm. [Accessed: 07- Jun- 2021].

[8] S. Myers and K. Chang, “China Says Debris From Its Rocket Landed Near Maldives”, Nytimes.com, 2021. [Online]. Available: https://www.nytimes.com/2021/05/08/ science/china-rocket-reentry-falling-long-march-5b.html. [Accessed: 07- Jun- 2021].