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Astronomy and space

Astronomy and space

Falling rockets pose increasing danger to human life, study reveals

02 Aug 2022
Atlas V rocket
Lift off A United Launch Alliance Atlas V rocket safely carried the Mars 2020 mission with the Perseverance rover. (Courtesy: ULA)

The risk to people on the ground of being killed by the uncontrolled descent of a rocket stage is increasing, with legislation urgently needed to prevent potential tragedies, says a team of political scientists and astronomers in Canada.

Most space launches still result in the uncontrolled return to Earth of at least parts of rockets – uncontrolled in this sense referring to the absence of an engine burn to put the rocket stage on a safe trajectory into the ocean. Although many pieces of space debris meet a fiery end in Earth’s atmosphere, parts of rockets can be large enough to reach the ground and cause damage.

The group behind this fresh assessment of the risks is led by Michael Byers, who is a professor of global politics and international law at the University of British Columbia in Vancouver. The team found that the greatest danger is to people living in the southern hemisphere, often in poorer nations that have no direct connection with space launches. In May 2020, for example, wreckage from the 18 tonne core stage of a Chinese Long March 5B rocket hit two villages in the Ivory Coast, damaging buildings. In July 2022, suspected wreckage from a SpaceX Crew-1 capsule impacted farmland in Australia, while another Long March 5B was allowed to fall uncontrolled towards waters just south of the Philippines at the end of that same month.

A matter of luck

This is not a new phenomenon. In 1979, NASA’s Skylab fell back to Earth after its orbit rapidly and severely degraded, scattering debris across Western Australia. In all of these cases it was a matter of luck that nobody was injured.

Yet the lessons of Skylab were not heeded, says Aaron Boley, an astronomer at the University of British Columbia who worked with Byers on the latest study. “Because Skylab was a special, single event, it remained easy moving forwards to ignore cumulative effects from launches that typically have much lower risks when viewed in isolation,” Boley tells Physics World.

The risks from individual re-entry events are low, partly because of regulations. In the US, the Orbital Debris Mitigation Standard Practices stipulates that all launches should have a risk of casualty from an uncontrolled re-entry of no less than 1 in 10,000. However, the US Air Force, and even NASA, have flouted this rule on numerous occasions according to the new study. Other nations may have different regulations, or none.

Cumulative danger

The danger, as Boley implies, is cumulative. In the past 30 years more than 1500 rockets that have fallen out of their orbits, and almost three-quarters of these have done so in an uncontrolled fashion. In their new study, Byers, Boley their University of British Columbia colleague Ewan Wright and Cameron Byers of the University of Victoria in British Columbia, calculate that during those three decades there was about a 14% chance that someone on the ground could have been killed.

As the number of launches is set to continue increasing as more nations and companies join the space race, the new study recommends several actions that could be taken to mitigate the risk. One is providing extra fuel so that a rocket can be re-ignited and directed towards a safe re-entry, perhaps crashing at “Point Nemo”. This is a “spaceship graveyard” that is located at the point in the Pacific Ocean that is farthest from land — and will be the final resting place of the International Space Station. However, launch providers have been reluctant to direct rockets using extra fuel because of the additional cost.

Another approach is to adopt reusable rocket stages, as SpaceX have successfully pioneered. However, “not all missions will be conducive to reusability of all parts, and in those cases we should still strive to bring back equipment controllably,” says Boley.

Official response

In response to the study, the European Space Agency’s Tim Flohrer, who is the Head of the Space Debris Office at the European Space Operations Centre in Darmstadt, Germany, gave the following statement to Physics World.

“On-ground risk is addressed in space debris mitigation guidelines and standards. ESA is very active in the further development and adoption of these together with its international and industrial partners. The re-entry risk affects people at geographical latitudes below launch inclination, i.e. the risk evolves over time with the population density and distribution. As suggested in those guidelines, the de-orbiting of rocket bodies immediately after satellites have been deployed is a good approach to minimizing the risk from re-entries – and we have observed a positive trend in getting to better compliance levels, e.g. visible from our space environment report.”

NASA was also contacted by Physics World for comment but has not replied.

Possible solutions

Should spacefaring nations not adopt measures to reduce the risks of falling debris, the study suggests that the only course of action remaining to southern hemisphere nations in the firing line is to make their case to the United Nations and to create a treaty. Even if they cannot convince space-faring nations to sign on, the British Columbia team suggest that it could generate enough press and public attention to improve the situation. They draw analogies with the 1997 Anti-Personnel Landmines convention, which although it was not ratified by the major powers, has led to a significant reduction in mines.

Precedence may also be sought from the aviation world, where material cannot be dropped recklessly from aircraft. “Dumping fuel for weight considerations, such as what might be done in an emergency landing with a plane over its landing weight, is a highly coordinated activity,” says Boley.

Other examples of collective action to protect against a global problem include mitigative policies combating CFCs and oil spills, although regarding the latter it took the Exxon Valdez disaster in 1989 to really spur action. The concern is that it will also take a disaster to prompt space-faring nations to take action against uncontrolled re-entries.

The research is described in Nature Astronomy.

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