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© Joe Tucciarone/SPL
Where did that spring from?
A tiny asteroid that buzzed Earth last week highlighted our planet's vulnerability to objects whose peculiar orbits put them in a game of hide-and-seek with us.

An Earth-based telescope spotted the 10-metre space rock hurtling our way just three days before a near miss on 13 January, when it flew by at just one-third of the distance to the moon (see Object headed towards Earth an asteroid, not junk). The asteroid is never expected to hit Earth and would burn up before hitting the ground in any case. But its unusual orbit (see diagram) seems ingeniously designed to evade our surveys. It is likely that a handful of objects large enough to cause harm are hiding under similar circumstances.

Large asteroids are relatively easy to spot because they reflect the most sunlight. But smaller asteroids - which can still damage Earth if they span at least 30 to 50 metres - are usually too dim for telescopes to detect except during brief close approaches to Earth. For a typical near-Earth asteroid, these occurrences are a few years or decades apart.

However, last week's unexpected visitor, called 2010 AL30, kept far enough from Earth to be invisible for more than a century. The prolonged avoidance occurred because the period of its solar orbit was 366 days - very close to Earth's year (though the close pass shifted the space rock into a 390-day orbit). Like a slightly slower race car that is periodically lapped by its competitor on a circular track, it stays far from Earth for long stretches.

"2010 AL30 may become a sort of 'poster child' for hiding asteroids," says Alan Harris of the Space Science Institute in Boulder, Colorado.

Similar "synchronised" asteroids may be hiding with periods of very close to two, three, four years and so on, Harris says. Those with periods of about four years pose the greatest risk to Earth, because they would be in sync with both Earth and Jupiter, says Timothy Spahr of the Minor Planet Center in Cambridge, Massachusetts. Such asteroids would be particularly influenced by Jupiter's gravity, which could nudge them onto a collision course with Earth.

Asteroids with non-synchronous orbits can also hide. Those with orbits mostly interior to Earth's - called Aten asteroids - spend most of their time in the glare of the sun as seen from Earth, so telescopes have trouble spotting them.

But Atens would be easier to spot if a telescope were positioned closer to the sun - in an orbit near Venus's, say. Such a telescope would also make it harder for asteroids to hide in synchronised orbits, says Harris. He admits that the cost of such a mission would be high, given the small fraction of asteroids likely to be in synchronised orbits. "On the other hand, I suppose I'd rather spend money on that than on strip-search scanners at airports to detect crotch-bombers, which constitute a similar level of cost and risk to society," he says. A US National Research Council report evaluating asteroid-hunting strategies, including the use of space telescopes, is due soon.

But even if no observatories are placed near Venus, the newly launched Wide Field Infrared Survey Explorer (WISE) could spot any big Earth-synchronised asteroids - those larger than 1 kilometre across. Large asteroids in orbits like 2010 AL30's, at about Earth's distance from the sun or closer, would be warm and bright in the infrared, making them "a piece of cake" for WISE to spot, Spahr says. However, even WISE will only be able to spot a small fraction of any mid-sized asteroids in these orbits, he says.

Despite our best efforts, the majority of hidden asteroids are too small and dim to be detected until they are practically on top of us - regardless of their orbits. "The sad part is, the bulk of the population is invisible to us most of the time," Spahr says.