Space junk roulette: What are the odds of being hit?

This past week,the fall from space of an out-of-control Russian spacecraft into the Pacific Ocean relied on fairly good odds that no one would be hurt by debris raining down from the sky. We got lucky this time; but is beating the odds the best way to ensure safety from space junk?

One estimate for the odds of you personally being hit by a piece of debris from falling space junk is about 70 trillion to one. That means you have a far better chance of winning the lottery or getting hit by lightning.

• Progress spacecraft burns up in Earth's atmosphere over Pacific Ocean
• Out-of-control Russian spacecraft declared a total loss

But this absurd number is a little misleading, because it was derived from a number of different factors.

When you look at the Earth as a target for falling spacecraft, most of the surface is covered in water, so the chances are debris will land where there are no people. The land area is also largely empty because people congregate in cities, which only take up a small percentage of the land; and finally, there are seven billion people on Earth and you are only one of them.

The actual odds that the satellite industry works with, what it calls "ground hazard," are 10,000 to one of anyone on Earth being hurt by falling debris.

In other words, the safety of people on the ground is a gamble the industry takesthat debris from derelict or out-of-control spacecraft will simply fall somewhere other than your head. And those odds are only going to get worse as more and more satellites, both commercial and military, are sent into space.

The doomed Progress supply ship that spun out of control right after launch is a model that has avery long and reliable service record. They have been delivering supplies to space stations since the 1970s. One of them did crash into the Russian space station Mir, causing considerable damage, but that was because the automatic docking system was turned off so a cosmonaut could practice manual control. That didn't turn out well.

The Progress ships are intended to burn up in the atmosphere after they deliver about three tons of supplies to the International Space Station. Once unloaded, the crew then fills the capsules with garbage and equipment that is no longer needed, and sendsthe whole package down over the Pacific ocean, using the atmosphere as an incinerator.

But not all of it burns up. Somewhere between 10and40per cent of the vehicle which, in this case, is 700 to 3000kilogramsof bits and pieces makes it to the surface. Usually the piecesfall harmlessly into the middle of the ocean. But they could come down anywhere if they are out of control.

Therecent failure of a Progress spacecraftmight have been caused by an explosion as soon as the capsule separated from its booster; but once that happened, there was no way to control it. How soon it would fall out of orbit and where it would hit was determined by multiple factors, including the conditions of the Earth's atmosphere, which inflates and deflates depending on solar activity, the temperature over different parts of the globe, and the unpredictable aerodynamics of an object tumbling end over end.

All anyone could do was hope that the odds would be in our favour and this time we won.

That seems like an unsafe way to run an industry. Despite more than a half century of spaceflight, there are still no international agreements on what to do about space junk. Countries that launch things into space should be responsible for disposing of them in a safe manner.

There are thousands of satellites orbiting the Earth, and tens of thousands more pieces of debris from satellites that have collided, exploded or been used as target practice from the ground. So space junk is not just a problem for people on Earth; there is a very real risk for satellites themselves, as well as the crew of the International Space Station.

When I spoke to Canadian astronaut Dave Williams about his space walk for my latest book, he said that while outside, he looked at the giant solar wings that provide power for the station and noticed that they had many pinholes in them from tiny bits of space debris that the station is always ploughing through. Then, he had the sobering thought that he was out there with nothing but the fabric of his space suit for protection.

The odds of getting hit are still small, but considering objects in space are moving at 29,000km/h, literally faster than speeding bullets, being in the path ofeven a small object can have huge consequences.

Space has always been an international endeavour. The time has come for the community to clean up its act.