The chance of someone being killed by space junk falling from the sky may seem extremely tiny. After all, nobody has yet died from such an accident, though there has been injury and damage to property. But given that we're launching an increasing number of satellites and rockets into space, do we need to start taking the risk more seriously? A new study has given us an answer.
The study investigated the uncontrolled arrival of artificial space debris(碎片), such as spent rocket stages associated with rocket launches and satellites. Using mathematical modelling of the inclinations (倾斜度) and orbits of rocket parts in space, as well as the past 30 years' satellite data, the authors estimated where rocket debris and other pieces of space junk will land when they fall back to Earth.
The authors calculated a "Casualty(死亡) expectation" over the next decade as a result of uncontrolled rocket re-entries. Assuming that each re-entry spreads deadly debris over an area of ten square metres, they found that there is a 10% chance of one or more casualties over the next decade, on average.
But as the number of entries into the rocket launch business increases and the business moves from governments to private institutions, it's highly likely that the number of accidents, both in space and on Earth, will increase. The new study warns that the 10% figure is therefore a conservative(保守的) estimate.
The European Space Agency is planning a mission to remove 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 improved in 2018. However, they are just guidelines and do not give specifics as to how effective activities should be performed.
There are some technologies that can make it possible to control the re-entry of debris, but they are expensive to carry out. 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 the orbit for a satellite can also reduce the chance of producing debris. Therefore more efforts should be made to perfect technologies and improve mission design.
