Investigating the Risks of Debris-Generating ASAT Tests in the Presence of Megaconstellations

Abstract

The development of large constellations of satellites (i.e., so-called megaconstellations or satcons) is poised to increase the number of LEO satellites by more than an order of magnitude in the coming decades. Such a rapid growth of satellite numbers makes the consequences of major fragmentation events ever more problematic. In this study, we investigate the collisional risk to on-orbit infrastructure from kinetic anti-satellite (ASAT) weapon tests, using the 2019 Indian test as a model. We find that the probability of one or more collisions occurring over the lifetime of ASAT fragments increases significantly in a satcon environment compared with the orbital environment in 2019. For the case of 65,000 satellites in LEO, we find that the chance of one or more satellites being struck by ASAT fragments of size 1 cm or larger is more than 25% for a single test. Including sizes down to 3 mm in our models suggests that impacts will occur for any such event. Finally, we apply our methods to examine the November 2021 Russian ASAT test, also finding a significant collision probability over the lifetime of the fragments. The heavy commercialization of LEO demands a commitment to avoiding debris-generating ASAT tests.

Appendix A: Russian 2021 DA-ASAT Test

On 15 November 2021, the Russian Federation conducted a direct-ascent ASAT weapon test. The target was Cosmos-1408, a 2200 kg defunct Soviet satellite (about three times as massive as that used for the Indian ASAT test), previously used as part of an electronic intelligence system. The impact occurred at an altitude of about 480 km, directly endangering China’s Tiangong space station and the International Space Station, forcing crew members to undertake shelter procedures while the ISS passed through the debris cloud. Due to the high altitude of the test where air drag is less influential, the fragment lifetimes will be much longer than those seen in the Indian test.

Because the Russian test was recent, an analysis of the event is not included in the main text of this article. However, due to the event’s impact on the space environment, we have conducted a preliminary analysis based on the information known at this time, including the distribution of approximately 1500 Cosmos-1408 debris fragments tracked on space-track.org. Here, we describe the results of several simulations that we run to assess debris collision probabilities, using the same methods employed for the Indian-like ASAT test.

As done before, satellite density fields are created for the simulations. One density field is based on the satellite catalogue as of November 17th, 2021, for which there are approximately 5700 satellites (active and defunct) in LEO. The second field is the same as the 65,000-satellite satcon field used in the Indian ASAT test simulations.

Table 5 shows the orbital parameters for Cosmos-1408 just prior to the impact. Its high mass along with the assumption of catastrophic fragmentation ensures that a significant amount of debris was created from the test. For example, the NSBM predicts that 1168 pieces of debris with $L_c \ge 10$ cm would be generated for an Indian ASAT-like event. In contrast, the Russian ASAT test generates 2876 of such debris, again based on the NSBM. The potential collision risks posed by this higher number of fragments is compounded by the higher altitude at which the test took place, inducing significantly longer orbital lifetimes for the debris fragments.

Table 5 Orbital parameters of Cosmos-1408, destroyed during the Russia ASAT test

The Russia ASAT test took place during a solar phase $\phi \approx 1.17 \pi$ (as a reminder, zero and $\pi$ are minima for a full 22-yr cycle, i.e., two solar cycles); this is included in the modelling of the atmospheric gas density and is expected to help in reducing the lifetime of the fragments.

Simulations are initialized with the NSBM. A kill energy of 150 MJ is assumed, consistent with a catastrophic collision in the NSBM modelling. For comparison, we also run a simulation using a Rayleigh velocity distribution and fixed A/M ratio, similar to that done for the Indian ASAT test. We use A/M values of 0.05 m$^2$ kg$^{-1}$ and 0.04 m$^2$ kg$^{-1}$, which show the best matches between the actual de-orbit times of the tracked Indian ASAT debris and the simulation results. Fragments are integrated until they de-orbit or reach a maximum integration time of 10 years. Again, the integrations are done using a satellite density distribution for LEO that reflects the time of the event (see also Sect. 2.2). In addition, a second set of simulations are run using a full 65,000-satellite satcon environment, so that the affects of a Russian ASAT-like test on a future LEO environment could be assessed.

Fig. 5

A comparison of distributions between the known fragments from the Russian ASAT test and our implementation of the NSBM and Rayleigh models. The top row from left to right shows Gabbard plots of the initial state of the Rayleigh model and after three months of evolution. The bottom row is similar, but for the NSBM. In addition, the catalogued debris from the Russian ASAT test as of February 10th, 2022, are also shown. A large amount of orbital evolution has already occurred. The altitude and period of the Cosmos-1408 satellite is marked with a blue star. The altitudes of the ISS and Tiangong space station are also indicated. Although the de-orbit timescales of the Rayleigh model is a better fit to the catalogued debris from the Indian ASAT test, the NSBM shows a closer correspondence to the evolution of the Russian ASAT test debris thus far

Table 6 Collision Probabilities: Russia ASAT-like test

Fig. 6

De-orbit timescales for models of the Russian ASAT test debris evolution. Only fragments that last longer than five days are included. Two NSBM simulations are implemented with our chosen minimum characteristic lengths, shown as dark and light blue lines. Rayleigh models A/M0.05 and A/M0.04 are also shown as the dot-dashed royal blue and purple lines. The NSBM leads to short fragment orbital lifetimes compared with the Rayleigh model. The differences in de-orbit outcomes are interpreted as representing the range of possibilities we might reasonably anticipate

Figure 5 shows Gabbard plots for two different simulations, as well as the actual distribution of the Cosmos-1408 debris. The top row of two panels corresponds to the Rayleigh A/M0.05 model at the initial moment of fragmentation and after three months of integration time, respectively, while the bottom row shows the NSBM model for the initial state and after three months of integration as well. The main right panel is the catalogued Cosmos-1408 debris based on Space-track.org on February 10th, 2022, about three months since the ASAT test took place. A considerable amount of debris evolution has already occurred, which is most obvious in the morphology at orbital periods less than 1.6 hours. This first three-months of evolution is better captured by the NSBM than the Rayleigh model in this instance. However, we also keep in mind that the Rayleigh model showed higher fidelity to the long-term evolution of the debris for the Indian ASAT test. There are biases in the catalogued data, and using the NSBM with $L_{{\mathrm{c,min}}}=10$ cm does not correct for this behaviour. There seems to be value in predictions made by both the NSBM and Rayleigh methods, potentially reflecting the initial orbital evolution and the long-term behaviour, respectively.

The range of evolution scenarios for the debris is emphasized in Fig. 6, which shows the de-orbit timescales for simulations of the Russian ASAT test debris using two NSBM initializations and two with the Rayleigh A/M0.05 and A/M0.04 models. The Rayleigh simulations, shown as a dash-dotted purple and royal blue curves, yields a de-orbit timescale that is more consistent with what might be expected based on the Indian ASAT test. Regardless, the figure shows that after roughly a full year of evolution, we should be able distinguish between models.

Table 6 lists the unscaled and scaled collision probabilities for the NSBM and Rayleigh modeling of a Russian ASAT-like test in the current satellite environment. We also show probabilities for a satcon environment of 65,000 satellites for comparison. Between the 2021 LEO satellite distribution and the 65,000 satcon environment, there is an increase by a factor of 3-5 in the cumulative collision probability, consistent across all models. As expected, the collision probabilities associated with a Russian ASAT-like test are much higher than for a test similar to the India 2019 event, due to the high altitude of the test and longer de-orbit times. Similar to that seen in our main discussion, the probability of a 10+ cm-sized fragment hitting any one satellite is relatively small, but is still non-negligible at about $10\%$ for a 2021 distribution for A/M0.04; nonetheless, the risk associated with such large collisions should not be dismissed. Of course, if most of these large debris pieces are successfully tracked, then in principle, they can be avoided. However, we again see an effective guarantee that at least one mm-sized fragment will experience a collision with a satellite. Such small fragments are generally non-trackable, so the collision risk is a serious concern. This is alleviated somewhat by such collisions not necessarily disabling a satellite. The number of fragments larger than 3 mm in size is nearly $7\times 10^5$ under the NSBM.

The Russian ASAT test was a large and acute deposit of debris into orbit, with fragments spanning many different LEO altitudes. Figure 7 shows the percent increase in debris due to the Russian ASAT test. Catalogue data are taken as of January 27th, 2022 and compared with the catalogued data on November 17th, 2021. We specifically filter for debris and ignore other catalogued objects like payloads or rocket bodies. We assume that any significant increases to the debris field over this time span is due solely to the Russian ASAT test. The purple histogram shows the 10 km altitude-binned percentage increase, and the grey shading in the background shows the number density of satellites for each contour, again based on 10 km bins. There is a significant, multi-fold increase in the amount of debris, specifically for altitude bands below 500 km, where there is still a non-negligible satellite number density. This image depicts that despite some beliefs that debris-generating events are only minor contributions to Earth’s existing debris field, there is a large LEO impact associated with acute events like ASAT tests.

Fig. 7

Percent increase of debris density in LEO due to the Russian ASAT weapon test (purple curve). Only fragments that remain in orbit after five days are included. The grey contoured background shows, using a log scale, the number density of satellites. Ten kilometre bins are used for determining the debris densities, calculated using the same method described in sect. 2.2 for the satellite density field