Apophis is a 46 million tonne asteroid that will pass within a hair's breath of Earth in 2029. However, Apophis's trajectory is likely to take it through a region of space near Earth known as a keyhole that will ensure the asteroid returns in 2036.

Nobody knows how close Apophis will come on that pass. But if there's a chance of a collision, we'll have only 7 years to work out how to avoid catastrophe.

Today, Shengping Gong and pals at Tsinghua University in Beijing say they've come up with a plan that will ensure Apophis never returns to Earth on this timescale .

They point out that keyholes are tiny, in this case just 600 metres wide. So deflecting Apophis by only a small amount in the near future will ensure it misses the keyhole and so cannot return to Earth.

There are various ways to deflect an asteroid. Gong and pals say their preference is to use a solar sail to place a small spacecraft into a retrograde orbit and on collision course with Apophis. The retrograde orbit will give it an impact velocity of 90km/s which, if they do this well enough in advance, should lead to a collision large enough to do the trick.