Asteroid set to whistle past Earth

This image shows a simulation of asteroid 2012 DA14 approaching Earth. Photo / AP/Nasa/JPL-Caltech

If it strikes, DA14 would probably explode in the planet's atmosphere with the power of a nuclear blast.

A 45m-wide chunk of rock is set to whistle by Earth at a distance closer to the planet than some satellites - but there's no need to prepare for the end of the world just yet.

The asteroid 2012 DA14 will bypass us just 27,700km from Earth's surface, entering New Zealand's skies early on Saturday morning, in an unusually close miss to be recorded by Auckland's Stardome Observatory and Planetarium.

Classed as a "Near Earth Object" (NEO), as it occasionally crosses the orbit of Earth and could collide with us in the future, the asteroid was discovered only a year ago by astronomers in Spain.

If it struck Earth, DA14 would probably explode in the atmosphere at an altitude of about 8.5km, creating a blast with the power of a nuclear explosion and capable of levelling an entire city.

But scientists who have been plotting its course say that won't happen - and instead it will silently zoom by at 8km a second. Its closest point of approach is set at 8.25am NZT, meaning Kiwis will miss out on seeing it.

The best vantage point would be in Indonesia, where it would still be night-time during the approach and the asteroid could be viewed with binoculars.

In New Zealand, even the best backyard telescopes couldn't sight DA14, which could be tracked only by a computerised research telescope at Stardome.

Even then, it would appear as only a faint dot moving among the stars.

"By dawn, it's still going to be 200,000km away and that's the closest we'll see it," Stardome astronomer Dr Grant Christie said.

The telescope will take a sequence of images of the asteroid, allowing astronomers to make precise fixes on its position. "As this thing whistles close by Earth, it will be changing orbit a great deal, so we need to make precise observations so we can predict its future path.

"Monitoring programmes are getting more and more efficient at finding these things, which have been passing by us for eternity, but we are unaware of most of them."

As of last week, 9688 NEOs had been discovered - but this is thought to be a tiny fraction of the total flying about in space.

"With some of them, you only get a warning of a day or two - it just depends what the orbit is - but many of them have relatively small bodies."

These presented a hazard - and nearly 1400 identified NEOs had been classified as such.

One asteroid the size of DA14 is thought responsible for a massive explosion over Siberia in 1908, known as the "Tanguska event", which flattened 2000sq km of forest and remains the largest impact on or near Earth in recorded history.

But Dr Christie said there was a minimal risk of an asteroid the size of DA14 striking Earth any time for centuries.

"Apophis" caused a scare when it was thought the huge asteroid would slam into the planet in 2036, but the chances of that happening have since been put at less than one in a million.

And fears that another monster asteroid, 2012 YQ1, would bring about doomsday in 2106 were also dispelled last month when it was kicked off the Near Earth Object Programme's risk table.

"Statisticians have estimated that an object the size of DA14 is likely to collide with Earth about every 1200 years," Dr Christie said. "We copped one in 1908 so, on the law of averages, we are not set for one of that magnitude for a while."