Scientists Calculated The Probability of Falling Space Junk Killing Somebody

It may seem absurdly unlikely that somebody might be killed by space debris that fell from the sky. After all, although there have been incidents of injury and property damage, no one has yet died in such an accident.

But do we need to start taking the danger more seriously now that we are sending more satellites, rockets, and spacecraft into orbit?

The likelihood of fatalities from falling rocket pieces over the next 10 years has been calculated by a recent study that was published in Nature Astronomy.

We are exposed to the danger of debris falling from space on us every minute of every day. Around 40,000 tonnes of dust are produced annually by the minuscule fragments from asteroids and comets that patter through the atmosphere and land undetected on the Earth's surface.

Although we are not affected by this, spacecraft can be harmed by this debris, as was recently revealed with the James Webb space telescope. A body tens of meters wide occasionally manages to drive through the atmosphere to create a crater, and occasionally a bigger sample enters the atmosphere as a meteorite.

The absence of dinosaurs from today's Earth is evidence that kilometer-sized objects can occasionally reach the surface and cause death and destruction. These are a few instances of natural space debris, whose unplanned arrival is unpredictable and dispersed quite equally over the world.

However, the current study focused on the unplanned influx of manmade space debris from satellites and rocket launches, including discarded rocket stages.

The scientists calculated the locations of rocket debris and other space trash when they fall down to Earth using mathematical modeling of rocket part inclinations and orbits in space and population density below them, as well as 30 years' worth of prior satellite data.

They discovered that there is a little but considerable chance of pieces returning in the next ten years. However, across southern latitudes rather than northern ones, this is more likely to occur.

In fact, the study calculated that the latitudes of Jakarta in Indonesia, Dhaka in Bangladesh, or Lagos in Nigeria are around three times more likely to have rocket corpses land than those of New York in the US, Beijing in China, or Moscow in Russia.

As a result of uncontrolled rocket re-entries, the scientists also estimated a "casualty expectation"—the danger to human life—over the following ten years. They discovered that there is an average 10% risk of one or more casualties during the following ten years, assuming that each re-entry disperses fatal debris across an area of ten square meters.

To date, it has been believed that there is very little chance that satellite and rocket debris will affect the surface of the Earth or aviation traffic in the atmosphere.

The majority of research on this type of space debris has been on the danger posed in orbit by abandoned spacecraft that might interfere with the safe operation of active satellites. Explosions in orbit caused by unused fuel and batteries also produce more trash.

However, it is quite expected that the number of mishaps, both in space and on Earth, like the one that occurred after the launch of the Chinese Long March 5b, will also climb as the number of entrants into the rocket launch sector rises and shifts from government to private company.

The 10 percent number is thus a modest estimate, according to the current study.

How to proceed

It is certainly feasible to regulate the re-entry of debris thanks to a variety of technologies, but doing so is costly. For instance, spacecraft can be "passivated," in which case any remaining energy (such as fuel or batteries) is used up rather than saved after the mission's completion.

A satellite's chosen orbit can potentially lessen the likelihood of junk formation. It is possible to instruct a retired satellite to fly into low Earth orbit, where it will burn up.

Reusable rocket launches are also an effort, as shown by SpaceX and Blue Origin, among others. As they return to Earth in a controlled manner, these produce a lot less debris, but there will be some from paint and metal shavings.

The hazards are taken seriously by several agencies. The European Space Agency is preparing a mission to use a four-armed robot to try and gather and remove space junk. The UN published a set of Space Debris Mitigation Guidelines through its Office of Outer Space Affairs in 2010, and they were updated in 2018.

These are recommendations, not international law, and do not provide instructions as to how mitigation actions should be carried out or managed, as the authors of the current study note out.

The paper makes the case that improving technology and more careful mission design will lower the rate of uncontrolled re-entry of spacecraft debris, reducing the danger of hazards across the world. According to the article, "uncontrolled rocket body reentries form a collective action problem; solutions are available, but each launching state must embrace them."

Government cooperation is not unusual, as seen by the agreement to outlaw ozone-depleting chlorofluorocarbon compounds.

But unhappily, before taking such action, there typically has to be a large incident with ramifications for the northern hemisphere. Additionally, international agreements and protocol modifications take time.

It has been seventy years since the first satellite was sent into orbit in five years. If a stronger and binding international convention against space debris could be signed by all UN member states to commemorate that day, it would be appropriate. In the end, such an agreement would be advantageous to all countries.

Monica Grady, Professor of Planetary and Space Sciences, The Open University.