Scientists Calculate the Risk of Someone Being Killed by Space Junk

The authors also calculated a “casualty expectation” — the risk to human life — over the next decade as a result of uncontrolled rocket re-entries. Assuming that each re-entry spreads lethal debris over an area of ten square meters, they found that there is a 10% chance of one or more casualties over the next decade, on average.

To date, the potential for debris from satellites and rockets to cause harm at the Earth’s surface (or in the atmosphere to air traffic) has been regarded as negligible. Most studies of such space debris have focused on the risk generated in orbit by defunct satellites which might obstruct the safe operation of functioning satellites. Unused fuel and batteries also lead to explosions in orbit which generate additional waste.

But as the number of entries into the rocket launch business increases - and moves from government to private enterprise - it is highly likely that the number of accidents, both in space and on Earth, such as that which followed the launch of the Chinese Long March 5b, will also increase. The new study warns that the 10% figure is therefore a conservative estimate.

What Can Be Done
There are a range of technologies that make it entirely possible to control the re-entry of debris, but they are expensive to implement. For example, spacecraft can be “passivated”, whereby unused energy (such as fuel or batteries) is expended rather than stored once the lifetime of the spacecraft has ended.

The choice of orbit for a satellite can also reduce the chance of producing debris. A defunct satellite can be programmed to move into low Earth orbit, where it will burn up.

There are also attempts to launch re-usable rockets which, for example, SpaceX has demonstrated and Blue Origin is developing. These create a lot less debris, though there will be some from paint and metal shavings, as they return to Earth in a controlled way.

Many agencies do take the risks seriously. The European Space Agency is planning a mission to attempt the capture and removal of space debris with a four-armed robot. The UN, through its Office of Outer Space Affairs, issued a set of Space Debris Mitigation Guidelines in 2010, which was reinforced in 2018. However, as the authors behind the new study point out, these are guidelines, not international law, and do not give specifics as to how mitigation activities should be implemented or controlled.

The study argues that advancing technologies and more thoughtful mission design would reduce the rate of uncontrolled re-entry of spacecraft debris, decreasing the hazard risk across the globe. It states that “uncontrolled rocket body reentries constitute a collective action problem; solutions exist, but every launching state must adopt them.”

A requirement for governments to act together is not unprecedented, as shown by the agreement to ban ozone layer-destroying chlorofluorcarbon chemicals. But, rather sadly, this kind of action usually requires a major event with significant consequences for the northern hemisphere before action is taken. And changes to international protocols and conventions take time.

In five years, it will be 70 years since the launch of the first satellite into space. It would be a fitting celebration of that event if it could be marked by a strengthened and mandatory international treaty on space debris, ratified by all UN states. Ultimately, all nations would benefit from such an agreement.

Monica Grady is Professor of Planetary and Space Sciences, The Open University. This article is published courtesy of The Conversation.