- Par Fabian Zander
- BBC Future
Photo credit, Getty Images
Earlier this year, two separate incidents saw space junk 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, in Australia – now confirmed to have come 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 can present a risk. But what risk exactly?
Space debris is the remnants of components of a space system that are no longer needed. It might 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 uncontrolled orbit. This means there was no way of knowing where they were going to land.
As for the SpaceX debris found in the Snowy Mountains, Australia, SpaceX deorbits parts of its rockets in a controlled manner and designs the other components to burn up upon 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 regarding the size of her hand and is believed to have come from a Delta II rocket. She picked it up, took it home, and reported it to authorities the next day.
However, as more and more objects go out into space and back, the chances of someone or something getting hit increase. This is especially true for large, uncontrolled objects such as the Long March 5B rocket.
Photo credit, Getty Images
Of the three times this rocket model has been launched: the first returned on May 11, 2020, with components landing on two villages in Côte d’Ivoire, the second returned on May 9, 2021, near the Maldives, and the third re-entered 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 anyone being hit, anywhere in the world. world. On the other hand, the probability that a given person will be affected (like you or me) is of 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.
Specifically, in these orbits, the vehicle spends regarding 20% of its time above the earth. A general estimate is that 20% of the earth is inhabited, which means that there is a 4% chance that the Long March 5B rocket’s atmospheric re-entry will occur over an inhabited area.
This figure may seem high. But if you consider the amount of “inhabited land” actually covered by people, the likelihood of injury or death decreases significantly.
The probability of property damage, on the other hand, is higher. It might 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 by 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 ton – with plans to expand its space segment ( with Japan, Russia, India and the United Arab Emirates).
Chances are that the chances of being hit will only increase (while still hopefully being very low).
Two questions come to mind: can we predict debris re-entry, 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 a margin of uncertainty of approximately one hour. So if an object orbits Earth every 60-90 minutes, it can enter just regarding anywhere.
Improving this margin of uncertainty is a major challenge that will require significant research. Even then, we are unlikely to be able to predict an object’s re-entry location more accurately 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 launched 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 requiring space debris risk reduction, such as the UN guidelines on the long-term sustainability of space activities, but the mechanisms of these guidelines are not specified.
Also, how do these guidelines apply internationally, and who can enforce them? These questions remain unanswered.
In summary, should you worry regarding being hit by space debris? For the moment no. Is continued research on space debris important for the future? Absolutely.