Most end-of-life satellites simply burn up in the atmosphere. But a few drop to earth. What is the chance that we get hit by pieces of falling satellites?
(Photo: NASA / Unsplash)

Most end-of-life satellites simply burn up in the atmosphere. But a few drop to earth. What is the chance that we get hit by pieces of falling satellites?

Lees in het Nederlands

At the end of August, eye witnesses saw how an old NASA satellite fell back to earth between the Cook Islands and Tahiti. They filmed how the 1964 OGO-1 was burning up and broke into ever smaller pieces.

The return of OGO-1 was a rare occurrence as most space debris drops into remote parts of the ocean unseen.

One in billions
Remote parts of the ocean. That sounds relatively safe and far away. But what is the chance that a satellite crashes onto land? And what is the chance that a human will be hit? Engineer Ron Noomen who teaches Satellite Systems (Aerospace Engineering), knows all about this.

He says that the chance that space debris falls on earth is “very small. I don’t want to quote figures, but undoubtedly they can be found on the internet. It’s rare otherwise it would be in the news more often.”

That’s true. Only a handful of incidents are reported online. In 2001 the Star-48 rocket fell onto the desert in Saudi Arabia and large chunks of the Space Shuttle Orbiter fell onto the American state of Texas in the Columbia accident in 2003. In 2007, an Airbus A340 pilot saw pieces of a Russian spy satellite while he flew over the Atlantic Ocean.

‘The weight of an empty drink can’

In 2011, NASA scientist Mark Matney calculated that the chance of one of the billions of people on earth being hit by a crashing satellite was one in 3,200. The chance that it is you that would be hit is even one in several billion.

The chance may be miniscule, but it happened to the American Lottie Williams in January 1997 when a piece of the Delta-II rocket fell on her shoulder. Williams compared the impact to the weight of an empty drink can.

But we should not worry as Noomen says that it would be “really bad luck” if you would be hit. “We try to steer satellites with the last bits of propulsion in their end-of-life phase to drop into the ocean.”

Safe circuits
Without propellant, a satellite would be uncontrollable. It would be impossible to manoeuvre it into safe orbits or take avoiding action to avoid crashing into other objects in space.

This is a highly undesirable situation so satellite operators try to anticipate problems. ‘Old’ satellites are thus directed into a safe and stable final trajectory or into a rapid return to the atmosphere. “This is also in compliance with international guidelines, set by various bodies including the United Nations.”

If the propellants are used up, the satellite can only leave space naturally, through air resistance. “In this case the satellite would gradually lose energy and be pushed into lower altitudes and, ideally, will ultimately burn up.” How long this process lasts depends largely on the original orbit. “The natural lifespan at 800 kilometres could be a couple of hundred years, at 400 kilometres a few years and at 300 kilometres a couple of months.”

Space debris
Now that more and more satellites are being launched, the amount of space debris is increasing rapidly. According to Noomen, there are more than one million pieces of debris floating around the earth. These are from old satellites, broken spacecraft and broken pieces of rockets. Of these, about 40,000 objects are catalogued. “These are objects that are larger than eight centimetres that are visible from earth with a telescope,” explains Noomen. But there is also space debris that is not visible. “Statistical models show that there are about 900,000 objects.”

All that space debris can ultimately cause a lot of problems. Noomen says that this is “not only because of chunks that may drop to earth and cause immediate damage, but also because of the implications on entry into space for new launches.”

As early as 1978, NASA engineer Donald Kessler sketched a situation in which there is so much space debris that collisions were unavoidable and that the amount of debris would only increase because of the uncontrollable collisions. This would be the case even if we would not launch any new satellites.

Some researchers believe that this scenario, called the Kessler syndrome, has been reached. “But,” says Noomen, “most experts believe that this scenario is still to come. Studies suggest that if we remove five large satellites from space every year, the risk level would be stable.”