Parachutes for rockets
After the recent collision between an american satellite phone satellite and a defunct russian satellite the problem of space junk has become more obvious. One major source of space junk is spent upper rocket stages which are lifted high enough to reach orbit.
These rocket stages are particularly dangerous to satellites as they often still contain fuel and if this vents they can change direction unpredictably or even explode.
In a low earth orbit there are faint wisps of an atmosphere and this will eventually slow down the debris and cause it to deorbit, but this will take at least 100 years could take thousands.
Max Cerf and Brice Santerre at the European aerospace firm EADS Astrium may have a solution. This involves attaching a super-light sail arrangement onto the rocket which deploys once the rocket has released its satellite. This would be supported by an inflatable structure probably impregnated with epoxy so that it would become solid and last a long time. For an Ariane 5 rocket it would have to be about 350 square metres.
This sail or aerobrake would then increase the air resistance enough to cause the rocket to come back to earth in only a couple of decades, hugely reducing the buildup of space junk in low earth orbit.