Falling space stations are not the biggest hazard. It's the small stuff that matters

When it comes to space junk, large objects such as the Chinese Space Station, Tiangong-1 falling from the skypose little threat to those of us on the ground. But the tens of thousands of other bits orbiting the Earth pose an increasing threat in space. With more stuff on the way, it's time for a cleanup policy.

After 60 years of spaceflight, we think that there are likely fewer than 5,000 artificial satellites orbiting the Earth today the numbers are fuzzy because of secret military launches and precise definitions of a "satellite."

Only about a third of them are still functioning. The rest are derelictordeactivated.But there are also hundreds of thousands of other smaller pieces, such as spent rocket boosters and bits of debris from satellite collisions, or fragments created by anti-satellite weapons tests by the military, which have created a cloud around the planet that is agrowing hazard.

While all these pieces of space debris are scattered over an extremely wide area, orbital velocities are so high that a collision with a piece the size of your finger can have the explosive force of a grenade.

So tiny collisions, let alone large ones, have the potential to completely destroy a multi-million dollar satellite, or worse, threaten the lives of astronauts in space.

At the moment, there is no international agreement for dealing with satellites, or most recently, a space station, when they come to the end of their lives.

And ironically, as far as safety in space is concerned, we actually want old satellites and space stations like Tiangong-1, to fall out of orbit.

It makes news that debris from the Chinese space station may hit the ground, but for space junk, that's kind of what we want to happen.

Objects in low earth orbit like space stations tend to take care of disposing of themselves because they are not entirely out of the Earth's atmosphere. Friction with the tiny amount of air up there causes their orbits to naturally decay until they sink into ever thicker atmosphere.

Even the International Space Station, at 400 km altitude, experiences a tiny bit of atmospheric drag which causes its orbit to decay, so the station has to be boosted back up on a regular basis.

For the most part smaller satellites will burn to nothing on re-entry.It's only with larger things like space stations that fragments of debris surviveto hit the ground.

The problem comes when trying to predict exactly when and where the debris will fall. These objects are moving around 28,000 km/hr, so they skip across the top of the atmosphere like a stone across a pond.

And like a stone, how far they goand when they stop depends on a lot of factors.

The number of skips a stone will make depends on the shape of the stone, the angle it hits the water, and any waves that could get in the way.

Some stones dig in immediately, others seem to go on forever before coming to a stop and dropping straight down.

Derelict satellites are odd shaped and often tumbling end over end, so aerodynamics are uncertain. The upper atmosphere of the Earth rises and falls every day depending on activity on the Sun. So the final impact point cannot be predicted with any accuracy until the last few orbits and even then there is uncertainty.

We saw this in 1979 with the fall of Skylab, the first American space station, and the Russian space station Salut 7 in 1991. Scientists tried to reassure the public by pointing out that the odds of being hit by debris falling from the sky were extremely low because very little of it actually reaches the ground, and the fact that most of the planet is ocean, so it is more likely to fall in water.

But both Skylab and Salut 7 overshot their predicted impact points with bits of the American station falling in Australia and the Russian station in Argentina.No one was hurt in either case, but it shows how difficult it is to pin down the final resting place of an uncontrolled large object.

The best case scenario was the intentional de-orbit of the Russian space station Mir in 2001. The 140-tonne complex isthe largest object to re-enter the Earth's atmosphere and it was done while still under control from the ground.

A rocket engine guided it to an unoccupied area in the south Pacific Ocean. That is what should have happened with Tiangong-1 and will likely be the ultimate fate of the International Space Station.

The rest of space junk in higher orbits is a much bigger problem, as it orbits at heights where it will take thousands of years to drop to Earth if at all.

It's scattered over a huge volume that reaches all the way out to geostationary orbit at 36,000 kilometres. Some of it circles around the equator, some flies over the poles. And with an ever-increasing number of satellite launches in the future, as time goes on the amount of debris and derelict satellites will only increase.

There are a range of plans for cleaning up space junk. The European Space Agency has a plan to send a robotup to capture derelict satellites either with a robot arm or a net and drag them into the atmosphere.

Other plans call for sending up refuelling robots to keep satellites operating longer, and the Australians have an idea of using a high powered laserto blast the very small pieces out of orbit. The Chinese have proposed a similar plan.

All of these ideas are very expensive,even though the cost of getting to space is coming down. The sheer volume of material and vast area to be covered means a cleanup by these methods will take decades.

A better idea is to ensure that rocket boosters shed as little material as possible on the way up (reusable boosters like SpaceX's could be a big step in that direction) and that satellites carry contingency fuel specifically for disposing of themselves at the end of their useful lifetimes.

There was a time in the 1950's when people didn't think twice about throwing garbage out a car window. Now littering is against the law. It is time for a similar law to be put into place for the entire international space community so we can clean up the dangerous litter that surrounds the planet.