 | Asteroid Might Hit Earth in 2880, Unless it is Painted |
An asteroid first spotted in 1950 has a maximum 1-in-300 chance of striking the Earth in the year 2880 scientists will report Friday in the journal Science. The odds, which might turn out to be zero, are for now greater than for any other asteroid ever determined to be a threat.
However, scientists are not worried; a little chalk, some charcoal, or perhaps a giant bucket of paint and the asteroid known as 1950 DA could be flung harmlessly off course if need be.
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| Details of the asteroid's threat, and why you shouldn't worry. [READ MORE] |
The trick lies in a strange natural phenomenon: solar-powered orbital mechanics.
The idea goes back to a scientific paper written by Russian engineer I.O. Yarkovsky a century ago and since lost. Yarkovsky is said to have proposed that the Sun warms an asteroid more on the "day" side than on the "night" side. The warmer side of the rock emits more thermal radiation, creating a slight difference in momentum that gently nudges the object in a manner similar to how a rocket is propelled.
Over hundreds of years, the movement can be significant, scientists say.
So paint it
Joseph Spitale of the University of Arizona will report in the journal that knowledge of this Yarkovsky effect, along with a little dynamite or some paint, could be employed to alter the path of any asteroid known to be on a collision course with Earth.
"It would not be terribly costly to deliver an amount of TNT necessary to pulverize the top centimeter or so of the surface," Spitale told SPACE.com. "The debris generated in this way could be useful in changing the surface thermal conductivity. Although you might not retain the debris generated in this way, such an approach might alter the surface character in other useful ways."
A more costly approach, Spitale said, would be to paint the asteroid or cover it with dirt in order to change how it reflects sunlight.
Any of these methods would require a serious and expensive space mission.
"In terms of mobility per unit mass launched, I think the TNT may win," Spitale said, "but I suspect that the maximum effect you could attain that way may be smaller than by using the other more expensive approaches."
Spitale's ideas, wild as they might seem, are no more outlandish than the long-running suggestion that an incoming asteroid could be deflected or destroyed with nuclear explosions. Others have even suggesting attaching a giant solar sail to a space rock in order to nudge it off course.
Details of any of these methods, including the science of how they would work, still need to be investigated. It would not be good, for example, to blow up an asteroid with a nuclear warhead only to have the pieces slam into Earth.
"I think the biggest problem currently is developing the ability to compute the consequences of such an action accurately enough to be confident that you're fixing the problem and not making it worse," Spitale said. "We're getting there with the Yarkovsky calculations."
Spitale is not the only scientist thinking along these lines.
Impact in 2880?
Suggestions for how to employ the Yarkovsky effect to save Earth stem from work done by several leading asteroid experts and led by Jon Giorgini of NASA's Jet Propulsion Laboratory. The team has been studying an asteroid discovered in 1950, later lost, then rediscovered in 2000.
The orbit of the asteroid, called 1950 DA, has been observed over a time frame spanning five decades. This allowed the researchers to project its approximate path farther into the future than is possible with most asteroids. The result: 1950 DA currently has at most a 1-in-300 chance of hitting Earth on March 16, 2880.
Because 1950 DA is large -- more than 1 kilometer (0.6 miles) across -- the consequences would be grave and global. Clouds of debris would create a multi-year winter that would kill off many species and might even threaten civilization.
In another study in the April 4 issue of Science, Giorgini and his colleagues map out the projected path of 1950 DA and discuss the chance that it might hit Earth.
The asteroid swings inside Earth's orbit on its closest approach to the Sun, then travels out to the main asteroid belt between Mars and Jupiter. Over the next eight centuries, it will approach Earth 13 times and pass near Mars twice.
Giorgini said in an e-mail interview that the Yarkovsky effect, amplified by close encounters with the planets and their gravity, have the potential to advance or delay arrival at the orbit intersection point in 2880 by several days. Which is to say that it may well miss Earth. Other factors, including direct pressure from solar radiation and the gravity of thousands of other asteroids, could cancel out some or all of the Yarkovsky effect.
Figuring out the asteroid's exact encounter circumstance, as astronomers call it, is difficult for now. Because the researchers know so little about the rock's surface and shape, they cannot accurately account for the Yarkovsky effect. They can only map out a range of outcomes the effect might produce.
"We need to know about the physical characteristics of these objects to do precise long-term hazard predictions," Giorgini said.
In fact, the odds of impact in 2880 may be practically zero, depending on the asteroid's axis of rotation, which greatly affects how it absorbs and radiates solar energy. Radar data indicates two general possibilities.
"The situation is similar to a biased coin that is known to give one side 80 percent the time, but it is not known which," Giorgini explained. "All you can say is the chance of heads is either 20 percent or 80 percent. So we say the collision probability of 1950 DA is either near the maximum of 1-in-300 or near zero. There's a range. It may take years or decades to determine which."
If the collision is eventually determined to be imminent, Giorgini and his colleagues agree with Spitale: Mitigation is possible. But they suggest slightly different methods.
"The study shows how the asteroid could be diverted, if need be, by covering its surface with chalk or charcoal, or by collapsing a solar sail spacecraft mission around the asteroid," Giorgini said. "These things would change the way it reflects light and emits heat, thus allow sunlight to do the work."
Yarkovsky's dark side
Ironically, the Yarkovsky effect has natural consequences that seem to herd space rocks toward Earth.
Smaller asteroids, especially, are more vulnerable to being propelled by the uneven solar heating. A study released last November found that bits and pieces created when two asteroids collide undergo changes in their orbits do to their new surface properties.
Computer simulations led by the Southwest Research Institute's William F. Bottke, Jr. found that some of these smaller asteroids can be propelled to the edges of gaps in the main asteroid belt -- regions of the belt that have been swept clean by the gravitational effects of Jupiter.
Unfortunately, when asteroids enter these gaps, Jupiter's gravity tends to fling them into near-Earth orbits.
An earlier study by European researchers, released in 2000, also showed that the Yarkovsky effect feeds these portals with ancient chunks of stone and iron, which can then be hurled Earthward.
When researchers improve their understanding of the Yarkovsky effect, as well as their knowledge of the surface properties of asteroids, they will learn whether certain asteroids actually target Earth. Perhaps equally important, they'll have figured out how to change that fate.
