A hefty chunk of space rock that's small enough to go almost unnoticed yet big enough to wreak havoc if it collided with Earth is whizzing our way at more than 14 kilometres a second.
The asteroid, roughly 30 metres across and about the weight of Sydney's Harbour Bridge, follows an elliptical, egg-shaped route as it orbits the sun, routinely crossing Earth's path.
During the last lap two years ago, it flew by at the reassuringly safe distance of more than 2 million kilometres.
Detected as a faint moving spot in the sky, 2013 TX68, as the rock is known, will not hit our planet when it sails by about 11am, Eastern Standard Time, on March 8, NASA scientists predict.
But its trajectory could take the cosmic body as close as 30,000 kilometres or as far out as more than 16 million kilometres.
Odds are one in 250 million that the near-Earth asteroid could hit next time round – namely on September 28 next year, the experts say. Fly bys in 2046 and 2097 have a lower probability of impact.
"Were TX68 ever to hit Earth in some future orbit, it would probably explode in the atmosphere – creating an enormous air blast with twice the energy of Chelyabinsk, damaging buildings and injury [to] people."
As two-thirds of Earth's surface is covered by water, most asteroids of this kind tend to disintegrate harmlessly above the ocean.
"If TX68 flies by at the most likely distance of about 5 million kilometres, it will be beyond the reach of backyard telescopes," said Perth astrophysicist Dr Rob Soria, of ICRAR-Curtin University. "If it passes as close as 30,000 kilometres, it could be observable with binoculars."
Although NASA's Centre for Near-Earth Object Studies keeps track of more than 90 per cent of asteroids of one kilometre or more across, there are many smaller objects that could potentially collide with Earth in the future.
TX68 is likely three times the mass of the asteroid that exploded above Chelyabinsk in Russia in February, 2013.Dr Alan Duffy, astrophysicist
If an asteroid similar in size to TX68 happened to hit land – as opposed to burning up in the atmosphere – it would carve a crater several hundred meters in diameter, Dr Soria explained.
"This could happen once a century or so," he said. "It would be more likely to hit the water, or Antarctica, Greenland or the middle of a desert – so the chance of it affecting people is small."
Detecting elusive objects of this size requires dedicated telescopes. "Unfortunately, the world lost a key sentry in this search when the search led by Dr Robert McNaught at Australia's Siding Spring Observatory was cancelled," Dr Duffy said.
Keeping track of asteroids is essential if something is to be done about a possible rogue. One idea under consideration is to land a spacecraft on a threatening asteroid and to gently push it over time, changing the orbit.
"Another possibility is to fly a spacecraft near the asteroid as a 'gravity tractor'," Dr Duffy said. "The asteroid's gravity would pull the spacecraft, with the spacecraft in turn pulling the asteroid a tiny bit. Then we'd move the spacecraft away and repeat the process. Over time, the asteroid would be moved to a safe orbit."
The worst idea, he added, would be to brazenly "nuke" a slab of space rock. "This would likely turn a single asteroid 'bullet' into a broken-up 'shotgun' blast, still travelling in approximately the same direction."
TX68 is zipping along too fast for scientists to capture and place in a convenient Earth orbit, in order to mine its resources. "But NASA plans to capture a small piece of another asteroid to tow back to Earth as part of the Asteroid Redirect Mission in the mid-2020s," Dr Duffy explained.
The possibilities for space mining loom large, with billionaires such as Google's Larry Page and Eric Schmidt investing in plans to find and then mine asteroids in coming decades.
"Australia has extensive experience in the resources sector and asteroid mining is the final frontier," Dr Duffy said. "Remote mining and advanced processing are key technologies that Australia has already developed and so these should be on the radar for our industries."
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