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.

Putting a spacecraft into a retrograde orbit about the Sun using little or no fuel is a pretty neat trick by anyone's standards.. The Chinese team's calculations demonstrate the point. They show that a 10 kg sail in retrograde orbit, that hits Apophis a year before 2029, would deflect it enough to miss the keyhole, thereby eliminating the chance that the asteroid will return in 2036.

And such a mission ought to be relatively cheap and relatively easy to deploy.

That sounds easy enough. In practice, however, threading this camel through the eye of a needle will be extremely tricky. There are all kinds of variations in the solar wind that could send such a spacecraft wildly off course.

It also requires a huge sail that will be difficult to unfurl and also liable to damage during the course of the journey, which will itself take years.

Then there's the structure and make up of Apophis, which is a complete mystery. Without knowing the material properties of the asteroid, it's impossible to determine how the impact will affect it.

So there's a little more work to be done in Beijing before this plan can get off the ground. Perhaps they should team up with the Europeans we talked about the other week.