Page last updated at 13:37 GMT, Monday, 12 January 2009

Hunt for space rocks intensifies

By Laurence Peter
BBC News

Mount Lemmon telescope/astronomer Andrea Boattini
The Mount Lemmon telescope in Arizona has a 1.5m diameter

The discovery of six comets made 2008 a truly stellar year for Italian astronomer Andrea Boattini.

He spends solitary nights keeping vigil on the heavens at Mount Lemmon Observatory in Arizona - looking for cosmic debris hurtling through the inner Solar System.

Two telescopes in Arizona and one in Australia are conducting the Catalina Sky Survey - part of a Nasa project to find and track near-Earth objects (NEOs).

Six comets in one year is the most found by an Italian astronomer since the mid-19th Century, even if it is not a world record.

Asteroids are far more common in the Solar System and potentially more hazardous to Earth. Mostly they are rockier, smaller and less spectacular than comets, so usually they are harder to find. There is a vast belt of asteroids between Mars and Jupiter.

'More predictable'

In 1998, Nasa started compiling an inventory of NEOs larger than one kilometre (0.62 miles) in diameter.

Don Yeomans, head of the NEO programme at the Jet Propulsion Laboratory (JPL) in Pasadena, California, says his team has found 763 asteroids and 82 comets in that category. That is 81% of the estimated total of near-Earth asteroids wider than 1km.

Comet C/2007 W1 (Boattini) - pic by J. Zakrajsek and H. Mikuz, Crni Vrh Observatory, Slovenia, 20 July 08
This comet was discovered by Andrea Boattini in November 2007

"It's getting much more predictable. Before 1998 we hadn't found many and didn't do much follow-up," he told BBC News.

But he admits that the new task set for the US space agency in 2005 - to log NEOs as small as 140m (460ft) in diameter - is much harder. The target is to find 90% of them by 2020.

An object is classed as an NEO if it comes within 45 million km (28 million miles) of Earth's orbit. Astronomers call that within 0.3 astronomical units (AU) - one unit being the distance of the Earth from the Sun. If the distance is only 0.05AU or less it is a potentially hazardous object.

The rationale for the project is that, while the chances of a NEO smashing into Earth are very slim, it would only take one to cause a global catastrophe and perhaps wipe out mankind. Even one smaller than 140m in diameter could cause a continental disaster - a giant fireball or a tsunami.

A rock about 50m wide which exploded over the Tunguska region of Siberia in 1908 levelled a vast area of forest and the impact energy was comparable to a modern nuclear weapon.

The impact of comet Shoemaker-Levy 9 in 1994 inflicted giant bruises on Jupiter, even though the comet was tiny in comparison with the giant gaseous planet.

Impact alert

Yet the NEO team's observations are paying dividends, in terms of early warning of a hazard.

Jupiter impact site of comet Shoemaker-Levy 9 in 1994
The Hubble space telescope revealed the comet damage on Jupiter in 1994

A major success was the first ever accurate prediction of an Earth impact, last October. The Arizona observatory spotted a two-metre space rock heading for Earth and predicted exactly when and where it would land. The rock exploded in an empty part of northern Sudan, within a day of discovery.

"We have to react very fast, especially when objects are very close to Earth," Mr Boattini told BBC News.

He said the 10-member Catalina team - one of three doing the NEO survey - had found 70% of the NEOs in the past year.

His digital cameras take a 30-second exposure of a specific area of sky, then keep going until there are 12 images of 12 different fields. Then the whole process is repeated, so finally there are four images for each field, and each image is separated from the next one by a 10-minute interval. A computer then picks up and displays the motion of a NEO.

Composed of ice, dust and small rocks, with vast tail caused by solar radiation
Diameter of nucleus usually several km or tens of km
Fly in mostly from outer Solar System or beyond - may have been flung out during planet creation
Orbits range from less than 200 years to 100,000 years or more
Rocky or metallic objects that orbit the Sun
Diameter ranges from a few metres to more than 500km
Believed to be debris from creation of Solar System
Orbits usually more predictable than comets - many lie between Mars and Jupiter, but some closer to Earth, others in outer Solar System

Mr Yeomans says new wide-field telescopes specially designed for the sky survey will be operational in the next few years and "should improve the discovery rate by a factor of about 40".

One of the new telescopes, called PanSTARRS, will be installed in Hawaii.

"The idea is to have four telescopes co-located, all pointing at the same region of sky," Mr Yeomans said.

The NEO team also uses two radar telescopes - one in California's Mojave desert, the other at Arecibo, in Puerto Rico. "Radar gives you range - the distance between the observer and the asteroid, so it's a more powerful data set," he explained. But the radar facilities are much in demand for other space projects.

The aim is to find NEOs 20 or 30 years before they come close to Earth - and that is already achievable, Mr Yeomans says. If necessary, a craft could be sent up to deflect it - and a big space rock might require a nuclear explosion to push it away from Earth.

In April 2029, an asteroid called Apophis will come within five Earth radii - below the orbits of geosynchronous satellites, "but we can already rule out an Earth impact," Mr Yeomans told BBC News.

Andrea Boattini with 0.7m telescope
Andrea Boattini also uses this 0.7m telescope on Mount Bigelow, Arizona

Space probes are increasingly sending back a rich harvest of data about asteroids and comets - again making their behaviour more predictable.

Comets streak towards the Sun from the outer Solar System or beyond, usually leaving a tail millions of kilometres long as solar radiation reacts with their ice particles.

Mr Yeomans said the Stardust mission (1999-2006) indicated that comets "are surprisingly fragile". The comet has a density of less than 1g per cubic cm - less than water's density - and may have "interior voids", he said.

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