Astronomers have made an unprecedented long-range forecast of the orbit of a kilometre-size asteroid, stretching nearly 900 years in the future.
The bad news is that it has a chance of up to one in 300 of crashing into the Earth during 20 critical minutes on 16 March 2880. The good news is that it could be redirected onto a harmless course by changing how it radiates heat into space.
Asteroid 1950 DA is an exception to the rule that asteroid orbits can be predicted for only about 100 years. The limit comes from observational uncertainties and the complex gravitational interactions of asteroids with other bodies.
Although 1950 AD was lost from view just 17 days after its initial discovery in 1950, its rediscovery in 2000 provided positional data spanning half a century. Radar measurements of its passage 7.8 million km from the Earth on 7 March 2001 gave uncommonly precise data on its distance, velocity, and speed.
An unusual orbit helps, says Jon Giorgini of the Jet Propulsion Laboratory, California, who led the new research. The asteroid orbits 12 degrees from the plane of the Earth's orbit, so it encounters few other objects that could disturb it.
1950 DA is also in a gravitational resonance with the Earth, circling the Sun five times for every 11 Earth orbits. Giorgini found the resonance prevents the orbital uncertainty from growing, a previously unrecognised effect.
Passing near the Earth on 14 May 2641 changes the orbit, weakening the resonance, he says, and that leads to the chance of a 2880 impact.
"It's nicely done," says Brian Marsden of the Harvard-Smithsonian Center for Astrophysics. "They tried to allow for everything, including uncertainties in the masses of planets."
"The spin direction of the asteroid ... is the biggest remaining unknown" affecting the asteroid's path, Giorgini told New Scientist. Spin is important because the asteroid radiates heat into space after the Sun warms its surface, generating a weak force that pushes on the asteroid, slightly shifting its orbit.
The direction of the push depends on surface energy transfer - and on the asteroid's spin. This "Yarkovsky effect" is small, but builds up over time. Just changing heat transfer in the surface layer could shift a one-km asteroid 1400 km in a century, Joseph Spitale of the University of Arizona reports in the same issue of Science.
"The offset goes with the square of the time," so in 300 years it could shift the asteroid more than 10,000 kilometers, enough to miss the Earth, he told New Scientist.
Giorgini says 1950 DA "could be redirected by spreading chalk or charcoal across the surface, or by sending a solar sail spacecraft that collapses around the asteroid and sort of shrink-wraps it".
However, the priority is to check how the asteroid is spinning. Giorgini's calculations show an impact is possible if it is spinning in one direction, but not if it is spinning in the other.
Journal reference: Science (vol 296, p 77, 132)
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18:53 05 September 2008
