A three-colour view of 99942 Apophis taken by the Herschel infrared space telescope on about Jan 6, 2013. Courtesy ESA.

Today, January 10 2013, at around 11:00am AEST the asteroid 99942 Apophis passed by planet Earth at about 14 million kilometres. That’s far away, in fact its no more dangerous than standing beside a highway while trucks drive by!

However in 2029 Apophis will pass very close to Earth, only about 30,000km overhead. We are quite certain it will not hit us then. But that close pass will change its orbit leaving us uncertain, at present, whether it will impact Earth in 2036. And that is why today’s pass is so important. It is close enough to allow us to make good radar measurements of its speed and position to refine its orbit.

What could we do if does turn out to be on a collision course? There are many ideas but none have yet been tested. Most rely on having many years advance notice to slowly nudge the rock out of the way so that it just misses us – after all, Earth is a very small target and we only need to deflect it by a few thousand kilometres. We could attach a small spacecraft to it and nudge it out of the way or we could park a heavy “space-tractor” beside it whose gravity would nudge the asteroid. Blowing up the asteroid or exploding a nuclear weapon beside it is inherently uncertain and may turn one problem into many! Sorry, but no thank you Bruce Willis!

A space tractor is one possible, but untested, way to nudge an asteroid out of Earth’s way. Courtesy APOD, copyright Dan Durda.

If these new measurements show there will be no impact in 2036 then all is well…for now. However, if the new measurements show an impact is more likely than we previously thought I hope that will spur studies, and funding, into testing just how we can deflect it. Asteroid impacts are a real, if rare, danger so we do need to have a genuine strategy for dealing with the risk.

Update: Almost as soon as I wrote the above post NASA released results of the radar measurements it has already been making for the last month or so. “…we can effectively rule out an Earth impact in 2036,” says Don Yeomans, manager of NASA’s Near-Earth Object Program Office at JPL. Phew!