January 12, 2009
NASA Program Protects Earth From Space Debris
Italian astronomer Andrea Boattini discovered six comets in 2008 while tracking cosmic debris throughout the solar system at Mount Lemmon Observatory in Arizona. And though it isn't a world record, the discovery of six comets in one year is the most found by an Italian astronomer since the mid-19th Century.
The Catalina Sky Survey is part of a NASA project to find and track near-Earth objects (NEOs) using two telescopes in Arizona and one in Australia.
In 1998, NASA started compiling an inventory of NEOs larger than 0.62 miles in diameter.
But 763 asteroids and 82 comets in that category were discovered by Don Yeomans, head of the NEO program at the Jet Propulsion Laboratory (JPL) in Pasadena, California. His team has found 81% of the estimated total of near-Earth asteroids wider than 0.62 miles.
"It's getting much more predictable. Before 1998 we hadn't found many and didn't do much follow-up," he said.
In 2005, the U.S. space agency started a program to log NEOs as small as 460ft in diameter, a task that is much more difficult. The target is to find 90% of them by 2020.
NASA classifies an asteroid as NEO if it comes within 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.
They say that while the chances of a NEO smashing into Earth aren't likely, it would only take one incident to cause a global catastrophe and perhaps wipe out mankind. Even one smaller than 140m in diameter could cause a continental disaster such as a massive fireball or a tsunami.
A large area of forest in the Tunguska region of Siberia was leveled after a rock about 50m wide detonated in 1908. Astronomers liken the impact energy to a modern nuclear weapon.
And giant bruises were inflicted on Jupiter after the impact of comet Shoemaker-Levy 9 in 1994, even though the comet was tiny in comparison with the giant gaseous planet.
However, the NEO team's observations are paying dividends, in terms of early warning of a hazard.
Last October, the team celebrated a major success with the first ever accurate prediction of an Earth impact. A two-meter space rock was spotted heading for Earth and the team predicted exactly when and where it would land. Within a day of discovery the rock exploded in an empty part of northern Sudan.
Boattini said his team has to react fast, especially when objects are very close to Earth. The 10-member team in Catalina, one of three doing the NEO survey, had found 70% of the NEOs in the past year.
He explained how the digital cameras take a 30-second exposure of a specific area of sky, and then keeps going until there are 12 images of 12 different fields.
"After the whole process is repeated, 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."
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, according to Yeomans.
Hawaii will be home to one of the new telescopes, called PanSTARRS.
"The idea is to have four telescopes co-located, all pointing at the same region of sky," Yeomans said.
Two radar telescopes are also used by the NEO team, one in California's Mojave Desert, and 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," Yeomans said. "But the radar facilities are much in demand for other space projects."
The team hopes to be able to discover NEOs a good 20 or 30 years before they come close to Earth, which Yeomans said is already achievable.
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.
An asteroid called Apophis will come within five Earth radii in April 2029, well below the orbits of geosynchronous satellites. Yeomans said, however, they can already rule out an Earth impact.
Asteroid and comet behaviors are becoming more and more predictable due to space probes that are increasingly sending back a rich harvest of data.
When comets sail towards the Sun from the outer Solar System or beyond, they typically leave a tail millions of miles long as solar radiation reacts with their ice particles.
"Comets are surprisingly fragile," Yeomans said, based on data collected from the Stardust mission between 1999-2006.
"The comet has a density of less than 1g per cubic cm - less than water's density - and may have interior voids," he said.
On the Net:
- NASA Near-Earth Object Program
- Catalina Sky Survey
- Mount Lemmon Observatory
- NASA Jet Propulsion Laboratory