Image: Asteroid collision
Don Davis  /  NASA
An artist's conception shows a massive asteroid slamming into Earth. Any asteroid threatening Earth is likely to be smaller, calling for subtle methods to deflect rather than destroy the space rock.
updated 11/28/2007 1:21:52 PM ET 2007-11-28T18:21:52

When it comes to 22-million-ton asteroids, the small stuff, it turns out, can make a huge difference in a potentially disastrous path toward Earth.

Using limited observations and lots of high-end computer modeling, astronomers have gotten a better handle on the limitations of asteroid-track forecasting in a new study of a potentially threatening asteroid called Apophis. In this high-stakes game of Whack-a-Cosmic-Mole, just knowing exactly what it is you don't know can be useful.

Apophis' chance of hitting our planet in 2029 is now slim to none, but astronomers will have to wait four to six years before they can predict what it might do during a second pass in 2036. A team of scientists arrived at the conclusion after accounting for small influences like the solar wind, gravitational drag of smaller asteroids and human error.

The rock is between 690 and 1,080 feet (210 and 330 meters) wide. Until better measurements lower the uncertainties, its predicted path could be severely off, potentially thwarting proactive efforts to change its course.

Uncertain future
In 2004, astronomers gave Apophis a frightening 1-in-37 chance of striking the Earth in 2029, which later measurements from Puerto Rico's giant Arecibo radio observatory knocked down to a 1-in-45,000 likelihood.

Apophis is expected to make a comfortably distant encounter of about 30 million miles (47.9 million kilometers) with Earth in 2036. But its first path-bending pass in 2029 — about three Earth diameters of our home — and other unknowns could greatly alter the asteroid's second approach seven years later.

In their study, to be detailed in an upcoming edition of the journal Icarus, astronomers examined hundreds of potential courses with computer simulations, then hashed out course-altering uncertainties after the first encounter. The largest such source of error is the sun's ability to push small asteroids around with solar radiation; in Apophis' case, up to 18.6 million miles (30 million kilometers) or about 2,350 Earth diameters off-course.

Other sources of course prediction error include:

  • Small uncertainties in planetary masses and position, up to 11.5 Earth diameters of error.
  • Earth's imperfect spherical shape, up to 1.5 Earth diameters of error.
  • Gravitational influence of small asteroids, up to 1.7 Earth diameters of error.

Astronomers said looking at the rock via telescope is necessary to cut out most of the error, but won't be possible until 2011 or 2013, when Apophis comes out of hiding from behind the sun. Noting the shape, colors and rotation, not just current position, should cut out up to 97 percent of the uncertainty about the asteroid's future track.

A little goes a long way
Until then, scientists are dreaming up of cost-effective ways to redirect Apophis before 2029, should the need arise.

One simple solution calls for strapping on a small reflective patch to the rogue body. By adding a 130-by-130 foot (40-by-40 meter) section of material that acts like a solar sail, Apophis could be pushed away by at least one Earth radius, or about 4,000 miles (6,400 kilometers) — more than enough to avoid cataclysm.

Another solution calls for distributing just 550 pounds (250 kilograms) of weight around the asteroid, perhaps as a mesh of carbon fibers. By adding weight to Apophis before its first Earthly encounter, its course could be altered enough to gravitationally sling it away from Earth's path for good.

But a lack of good observations, astronomers warn, could turn such proactive deflections into disaster.

"Without such performance margin," the authors said, "the deflection action would instead create an unpredicted outcome or a new hazard."

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