Sarah Thiele, The University of British Columbia; Aaron C. Boley, The University of British Columbia
Keywords: ASAT tests, megaconstellations, orbital debris, modeling, collisional probability, collisional risk
Abstract:
On March 27th 2019, India conducted its first successful anti-satellite (ASAT) test. In this operation, code-named “Mission Shakti”, a modified anti-ballistic missile interceptor was used to destroy a 740 kg Microsat-R satellite by India’s Defence Research and Development Organization (DRDO) (1). This event placed India among the three other countries with demonstrated direct ascent anti-satellite capabilities – United States, Russia and China (2).
The mission was implemented with intentions of minimizing the amount of debris generated by the impact and thus avoid creating a massive debris field like the 2007 Chinese ASAT test. In this vein, the DRDO conducted the test when the target satellite was at a low altitude of 282 km. The satellite was also on the smaller end of Indian communication satellites, with a surface area of about two square metres (3).
Despite these efforts, more than four months later, there were still 57 of the initial 400 pieces of trackable debris in orbit, as catalogued by USSPACECOM (4). Many of those remaining debris pieces had high altitude apogees. NASA strongly criticized this ASAT test for the long-term risks it posed. Ten of these 57 fragments had apogee altitudes greater than 1000 km, and some as high as 1725.7 km, spanning the majority of LEO’s altitude range and thus endangering most LEO spacecraft. High altitude fragments further crossed the orbital path of the International Space Station, and the risk of collision between this debris and the ISS had increased by 44% between the time of the event and a study conducted in August 2020. It is important to recall that this is only the trackable fragments. Calculations by Jiang (2020) suggest that the number of debris fragments created by Mission Shakti with a size larger than 0.001 metres may be of order 105, which have the possibility to create significant damage (5).
In addition to the direct danger to spacecraft posed by debris-generating ASAT tests, there is also the well-known possibility of a domino effect, in that collisions with debris can cause further fragmentation events and increase the overall collisional cross-section (6). Megaconstellations make the risks of fragmentation events and conducting explosive ASAT tests much more acute, creating the need to rethink how we determine acceptable risk thresholds for space-based missions and our tolerance for debris-generating ASAT tests. SpaceX has now delivered 1,015 satellites for its megaconstellation Starlink as of 2021 (7), with anywhere from 11,000-41,000 on the way. Another system, OneWeb, has 144 of its proposed 648 satellites in orbit as of February 2021 (7). These massive deposits of spacecraft into LEO create a need for risk evaluation from a cumulative perspective. While the collision probability between a debris fragment and a single satellite may be low, the integrated probability of impact over an entire system could be significant.
In this study, we explore the collisional risk posed by an ASAT test, similar to the 2019 India test, in a full megaconstellation environment. We use a modified version of the REBOUND code (8) to integrate debris generated by a fragmentation event, using a range of explosion locations and energies. We use the distribution of megaconstellation satellites, based on FCC filings, to evaluate debris impact risks. We assume that satellites have perfect collision avoidance with each other and thus collisions are only due to ASAT fragments. Each fragment is followed until it deorbits due to gas drag.
So that we can explore a wide range of ASAT scenarios, the megaconstellation is time-averaged and the effective number densities of satellites are determined at evenly distributed grid nodes. Those densities are then used with the integrated orbits of the fragments to determine the cumulative collisional probability for any fragment to impact any single megaconstellation satellite over the lifetime of the fragments.
Through this work we hope to shed light on any additional risk that is posed by adding satellite megaconstellations to the space environment and the consequences of disruptive behaviours.
References:
Langbroek, The Diplomat, April (2019). Available at https://thediplomat.com/2019/05/why-indias-asat-test-was-reckless/.
Roy Chaundry, The Economic Times, March (2019). Available at https://economictimes.indiatimes.com/news/politics-and-nation/explained-whats-mission-shakti-and-how-was-it-executed/articleshow/68607473.cms.
Tellis, Carnegie Endowment for International Peace, April (2019). Available at https://carnegieendowment.org/2019/04/15/india-s-asat-test-incomplete-success-pub-78884.
Grush, The Verge, August (2019). Available at https://www.theverge.com/2019/8/8/20754816/india-asat-test-mission-shakti-space-debris-tracking-air-force.
Jiang, Science Direct, Vol. 6, Issue 8 (2020).
Kessler & Cour-Palais, JGR Space Physics, Vol. 83, 2637-2646 (1978).
USSPACECOM SSA data, Space-Track.Org, updated (2021). Available at https://www.space-track.org.
Rein & Spiegel, MNRAS, Vol. 446, 1424-1437 (2015).
Date of Conference: September 14-17, 2021
Track: Conjunction/RPO