- By Fabian Zander
- BBC Future
image credit, Getty Images
Earlier this year, two separate incidents saw space debris return to Earth in unexpected places.
The uncontrolled re-entry of a Chinese Long March 5B rocket over Malaysia was quickly followed by reports in July that spacecraft parts had been found in the New South Wales area of Australia – now confirmed to be from a SpaceX Crew-1 mission.
As the space industry develops, it can be said that such incidents will become more frequent and that they may pose a risk. But what risk exactly?
Space debris is the remains of components of a space system that are no longer needed. It can be an end-of-life satellite (like the International Space Station when it reaches the end of its operational life in 2031) or parts of a rocket system that have served their purpose and are discarded.
To date, China has launched three Long March 5B rockets, and each was deliberately left in an uncontrolled orbit. This means there was no way of knowing where they would land.
Regarding the SpaceX debris found in the Snowy Mountains, Australia, SpaceX moves parts of its rockets in a controlled manner and designs the other components to burn up on re-entry into Earth’s atmosphere. But as you can see from the latest news, things don’t always go as planned.
Is space debris really dangerous?
As far as we know, only one person has been hit by space debris. Lottie Williams, a resident of Tulsa, Oklahoma (USA), was hit in the shoulder by a piece of debris in 1997. It was about the size of her hand and believed to have come from a Delta II rocket. She picked it up, took it home and reported it to the authorities the next day.
But as more and more objects go into space and back, the chances of someone or something getting hit increases. This is especially true for large, uncontrolled objects like the Long March 5B rocket.
image credit, Getty Images
Of the three times this rocket model has been launched: the first was returned on 11 May 2020 with components landing on two villages in Côte d’Ivoire, the second was returned on 9 May 2021 near the Maldives, and the third re- into this year over Indonesia and Malaysia, with debris landing around those islands.
Should we be worried?
There are many estimates of the risk of collision with space debris, but most are on the order of a 1 in 10,000 chance. This corresponds to the probability of someone being hit, anywhere in the world. On the other hand, the probability of any given person being affected (like you or me) is on the order of one in a trillion.
Several factors drive these estimates, but let’s focus on one key factor for now. The image below shows the orbital path of the recent Long March 5B-Y3 rocket during its last 24 hours (different objects follow different orbital paths), along with its re-entry location marked in red.
As you can see, the rocket is in orbit above the earth for a long time.
In these orbits specifically, the vehicle spends about 20% of its time above the ground. A general estimate is that 20% of the earth is inhabited, meaning there is a 4% chance that the Long March 5B rocket’s atmospheric re-entry will occur over a populated area.
This number may seem high. But if you consider the amount of “inhabited land” actually covered by people, the likelihood of injury or death drops significantly.
The probability of damage to property, on the other hand, is higher. It can reach 1% for any atmospheric re-entry of the Long March 5B rocket.
Furthermore, the overall risk posed by space debris will increase with the number of objects launched and re-entering the atmosphere. Current plans from companies and space agencies around the world call for many, many more launches.
China’s Tiangong space station is expected to be completed by the end of 2022. And South Korea recently became the seventh country to launch a satellite payload of more than one tonne – with plans to expand its space segment (joining Japan, Russia, India and the United Arab Emirates).
Chances are, the chances of getting hit will only increase (while hopefully still very low).
Two questions arise: can we predict that waste will return and what can we do to reduce the risk?
Let’s start with the predictions. It can be extremely difficult to predict where an object in uncontrolled orbit will re-enter the Earth’s atmosphere. The general rule is that the uncertainty of the estimated re-entry time is between 10 and 20% of the remaining orbital time.
This means that an object whose time of re-entry is predicted in 10 hours will have an uncertainty margin of approximately one hour. So if an object orbits the Earth every 60-90 minute, it can enter virtually anywhere.
Improving this margin of uncertainty is a major challenge that will require significant research. Even then, it is unlikely that we will be able to predict an object’s re-entry location more precisely than within 1,000 km (621 miles).
Ways to reduce risk
Reducing the risk is a challenge, but there are a few options.
First, all objects sent into Earth orbit must have a plan for safe deorbiting to an uninhabited area. This is usually the SPOUA (South Pacific Ocean Uninhabited Area), also known as the “spacecraft graveyard”.
It is also possible to design the components in such a way that they completely disintegrate upon re-entry into the atmosphere. If everything burns up when it reaches the upper atmosphere, there will be no significant risk.
There are already guidelines that require reducing the risk of space debris, such as the UN Guidelines for the Long-Term Sustainability of Space Activities, but the mechanisms in these guidelines are not specified.
And how do these guidelines apply internationally and who can enforce them? These questions remain unanswered.
In short, should you worry about being hit by space debris? At the moment no. Is continued research into space debris important for the future? Absolutely.