Satellites in orbit around the Earth are at risk of collision with space-based objects, which have either been discarded by space missions or created in satellite collisions. But now a team of researchers at NASA believes it may have found a relatively cheap solution for dealing with this "space junk" – aim a medium-powered laser into space and nudge any objects on a collision course out of harm's way.

Earth is surrounded by thousands of discarded man-made objects, from spent rocket stages to flecks of paint. Even very small items pose a danger to working satellites because they travel at tens of thousands of kilometres an hour. In 2009 the threat to satellites hit the headlines when a defunct Russian satellite destroyed an Iridium telecommunications satellite – creating even more junk.

More and more rubbish

Computer models suggest that in the most popular low-Earth orbits (altitudes of about 800–1000 km) the rate at which new debris is created through collisions exceeds the rate at which debris drops back down to Earth. This means that even without any new missions launched the amount of space junk is destined to keep on increasing. This is the "Kessler syndrome", first identified in 1978 by NASA scientist Don Kessler.

Proposals to remove individual items of space junk involve moving them into progressively lower orbits. Ideas include robots that push objects closer to the Earth and ground-based laser beams that impart recoil to an object by vaporizing its surface material. Each proposal could cost tens of millions of dollars or more to implement and given the still relatively low risk that space junk poses to satellites, none currently deliver value for money.

Giving junk a nudge

Now, a team headed by James Mason of the NASA Ames Research Center and the Universities Space Research Association in California has come up with an alternative approach. It points out that knocking an object out of orbit requires a laser with a power output of millions of watts. Instead, they propose using a laser of a few kilowatts to change the velocity of an orbiting object just enough to move it out of the path of a second, oncoming object. The change would occur simply by the momentum of the photons in the beam.

The scheme would involve continually evaluating the chances of a collision between any two items of space junk with a diameter of 5 cm or more, using radar data provided by the US Space Surveillance Network. Objects on a collision course would then be tracked by an optical telescope and, with one of the objects locked on, the laser beam would be sent along the telescope's main optical path into space.

Using a computer model, Mason and colleagues calculate that the risk of slightly more than half of all potential collisions in low-Earth orbit could be significantly reduced by using just one 5 kW commercially available laser mounted on a 1.5 m telescope somewhere close to the poles (since most of the objects are in roughly polar orbits). Likely to cost no more than about $10m, including instrumentation for adaptive optics, they say this approach could represent a much cheaper alternative to the direct removal of space debris, and they calculate that the far lower rate of collisions should lead to a reversal of the Kessler syndrome within a few decades

Testing needed

However, team member William Marshall, also of NASA Ames and the Universities Space Research Association, points out that there are a number of uncertainties in the model, one of which is the precise nature of the objects in orbit. Proving the validity of the scheme will involve rigging up a suitable laser and directing it through an existing telescope, as well as running space-debris models to see whether the scheme really can reverse the Kessler syndrome in the long term. The researchers say that it would be desirable to get international backing for the implementation of their scheme, given possible safety concerns about firing laser beams into space.

Kessler is now a private consultant on space debris and said of this latest work: "it is encouraging to see new ideas to prevent the growth in the orbital debris population". But he points out that a kilowatt-power laser could only alter the path of quite small objects and that it would therefore be of no use in preventing the most destructive collisions – those involving two large, intact items. "This system might reduce the number of required removals," he says, "but it would not be a total solution".

The work is described on arXiv.