More than 140 experts and dignitaries have signed an open letter published by the Outer Space Institute (OSI) calling for both national and multilateral efforts to restrict uncontrolled re-entries — the phenomenon of rocket parts falling back to earth in unguided fashion once their missions are complete. Among others, the letter is addressed to S. Somanath, chairman of the Indian Space Research Organisation (ISRO).

What are the stages of a rocket launch?

  • The Soviet Union launched the first artificial satellite in 1957. Today, there are more than 6,000 satellites in orbit, most of them in low-earth (100-2,000 km) and geostationary (35,786 km) orbits, placed there in more than 5,000 launches. The number of rocket launches have been surging with the advent of reusable rocket stages.
  • Rockets have multiple stages. Once a stage has increased the rocket’s altitude and velocity by a certain amount, the rocket sheds it. Some rockets jettison all their larger stages before reaching the destination orbit; a smaller engine then moves the payload to its final orbit. Others carry the payload to the orbit, then perform a deorbit manoeuvre to begin their descent. In both cases, rocket stages come back down — in controlled or uncontrolled ways.

What is an uncontrolled re-entry?

  • In an uncontrolled re-entry, the rocket stage simply falls. Its path down is determined by its shape, angle of descent, air currents and other characteristics. It will also disintegrate as it falls. As the smaller pieces fan out, the potential radius of impact will increase on the ground.
  • Some pieces burn up entirely while others don’t. But because of the speed at which they’re travelling, debris can be deadly.
  • A 2021 report of the International Space Safety Foundation said, “an impact anywhere on an airliner with debris of mass above 300 grams would produce a catastrophic failure, meaning all people on board would be killed”.
  • Most rocket parts have landed in oceans principally because earth’s surface has more water than land. But many have dropped on land as well.

Why are scientists worried about the re-entries?

  • The OSI letter cited examples of parts of a Russian rocket in 2018 and China’s Long March 5B rockets in 2020 and 2022 striking parts of Indonesia, Peru, India and Ivory Coast, among others. Many news reports have focused on Chinese transgressions of late, but historically, the U.S. has been the worst offender.
  • Parts of a SpaceX Falcon 9 that fell down in Indonesia in 2016 included two “refrigerator-sized fuel tanks”. If re-entering stages still hold fuel, atmospheric and terrestrial chemical contamination is another risk.
  • As per the letter, “Conservative estimates place the casualty risk from uncontrolled rocket body re-entries as being on the order of 10% in the next decade” and that countries in the ‘Global South’ face a “disproportionately higher” risk of casualties.
  • The U.S. Orbital Debris Mitigation Standard Practices (ODMSP) require all launches to keep the chance of a casualty from a re-entering body to be below 0.01%. But the U.S. Air Force and the NASA have waived this requirement on multiple occasions.
  • A July 2022 study by researchers in Canada found that this threshold, which some other countries have also adopted, is “arbitrary and makes little sense in an era when new technologies and mission profiles enable controlled re-entries,” and because many places have become more densely populated.
  • There is no international binding agreement to ensure rocket stages always perform controlled re-entries nor on the technologies with which to do so. The Liability Convention 1972 requires countries to pay for damages, not prevent them. These technologies include wing-like attachments, de-orbiting brakes, and extra fuel on the re-entering body, and design changes that minimise debris formation.

What can make minimum damage?

  • While the OSI letter admits that any kind of re-entry will inevitably damage some ecosystem, it recommends that bodies aim for an ocean in order to avoid human casualties.
  • The letter concludes by asking that future solutions be extended to re-entering satellites as well. Advances in electronics and fabrication have made way for smaller satellites, which are easier to build and launch in large numbers. These satellites experience more atmospheric drag than if they had been bigger, but they are also likelier to burn up during re-entry.
  • India’s 300-kg RISAT-2 satellite re-entered earth’s atmosphere in October after 13 years in low-earth orbit. The ISRO tracked it with its system for safe and sustainable space operations management from a month beforehand. It plotted its predicted paths using models in-house. The RISAT-2 eventually fell into the Indian Ocean on October 30.


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