The six astronauts were awoken early and scrambled into escape capsules. Nasa ground control had spotted a piece of space debris hurtling towards their temporary home aboard the International Space Station 244 miles (390 km) above the Earth. The fast-moving junk was spotted just one day before its potential impact, making it too late to manoeuvre the station to a safer orbit. The only course of action for the three Russians, two Americans and one Dutch astronaut crew was to take shelter and prepare to evacuate if required.
In the event, the debris, a chunk of a defunct military communications satellite, missed by the ISS by approximately 7.5 miles (12 km). The incident, in March, was the latest close shave for the space station. Six other crew members had to take shelter in June last year when another piece of junk whistled just 1,100 feet (335 metres) past the station. Again, it was a precautionary measure, however Nasa estimates that anything larger than a baseball poses a potentially catastrophic threat to the ISS. It's a problem that is getting rapidly worse. US Air Force Space Command is tracking around 22,000 pieces of man-made space debris, mostly debris bigger than 10 cm across, and there are estimated to be hundreds of thousands more smaller bits.
"It would totally wipe out the space station if it were hit by a catalogued object," says Donald Kessler, a former Nasa scientist, who is widely regarded as the leading expert on the issue.
Kessler first started thinking about space debris in 1970 when there was concern about pieces of asteroids and comets, known as meteoroids, and the risk of them striking an Apollo spacecraft. Kessler was studying asteroid collisions, which are responsible for much of the meteoroid population, when it suddenly occurred to him that sooner or later satellites in Earth’s orbit would also collide, producing growing numbers of fragments. “I was just curious as to how long it would be before that would happen,” recalls Kessler, who is now retired and living in North Carolina.
Before he could come up with solid estimates, Nasa ended the Apollo program and put him to work in another area. But Kessler continued to think about satellite collisions, and he got another shot at pursuing the problem in the late-1970s when Nasa created an office to look at the environmental impacts of the space shuttle program. Kessler was asked to look at solar power stations in Earth’s orbit. “I used that as an excuse to then work on orbital debris and did the calculations,” he says.
At the time Nasa scientists generally believed that North American Aerospace Defense Command (Norad) was cataloguing all man-made objects in space, but Kessler suspected there might be small debris that was slipping past the military’s radar network. And if the debris wasn’t there yet, it would be eventually, he reasoned. In a landmark paper published in 1978, he calculated that the number of trackable man-made objects orbiting Earth had increased by 13% per year from 1966 to 1976. He predicted that by about the year 2000 fragments from satellite collisions would pose a serious threat to other satellites, creating a "debris belt". “I was shocked at the answer,” he says.
That’s not all, however. Because each collision in space would result in hundreds of pieces of new debris, the threat of future collisions would grow exponentially, until low Earth orbit, the region between 100-1,240 miles (160-2,000 km) above our planet, would become a no-go zone for spacecraft. “That ended up getting a lot of people’s attention," he said.