Graphic: Tracking an asteroid: A brush with Earth in 2880?
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Science

In an extraordinary case of looking in the right place at the right time, astronomers recently spotted an asteroid previously detected 50 years ago. Observing the asteroid again gave the researchers enough information to predict the asteroid’s path over the next eight centuries — and to discover a chance that the kilometer-sized space rock may collide with Earth on March 16, 2880.

The odds of an actual collision are small, less than one in 300, the scientists estimate in Friday’s issue of the journal Science, published by the American Association for the Advancement of Science.

The way light bounces off the asteroid should have a major effect on how close it will be to Earth in 878 years, the scientists found. It should be possible, says lead author Jon Giorgini of NASA’s Jet Propulsion Laboratory, for humans to alter the asteroid’s surface to reflect light differently.

“The sun could eventually push it out of the way,” Giorgini said. “You wouldn’t have to send Bruce Willis or anything.”

A one-of-a-kinda asteroid
While the chances of an impact are slim to even slimmer, the asteroid 1950 DA could still be called “groundbreaking” for other reasons. Usually, researchers can only predict asteroids’ orbital paths for several decades. Giorgini’s team was able to stretch their predictions out for centuries, because of three lucky coincidences.

First, knowing the asteroid’s location both now and 50 years ago allowed the researchers to significantly narrow the range of possible paths the asteroid may be following in space.

The asteroid itself is also a special case. Its orbit around the sun sits in a plane that is tilted at an angle to the plane containing the orbiting planets. It’s therefore less vulnerable to the planets’ gravitational pull.

1950 DA also returns to the same position relative to Earth every few years, subjecting itself to Earth’s gravity at the same point in its orbit. This keeps the asteroid in check to a certain degree, limiting the potential courses it may take.

The usual and not-so-usual suspects
Based on their record of the asteroid’s orbit thus far, and the likely gravitational effects of a handful of other large objects in the solar system, Giorgini and his colleagues projected 1950 DA’s orbital path out to 2880, where they found a 20-minute window during which a collision with Earth may be possible.

The researchers wanted to be sure they were accounting for all the possible influences upon the asteroid’s orbital path.

“We just dug deep, and tried to put in every single factor we could think of,” Giorgini said.

They considered a number of variables, from the way the sun constantly loses mass, to possible perturbations from more than 7,000 different asteroids. In each case, the primary effect was to cause 1950 DA to either speed up or slow down along its orbital path, as if it were a train on a track.

There was still a large chunk of information missing however, involving something called the Yarkovsky effect. According to this phenomenon, the reflection of sunlight can cause an orbiting object to accelerate slightly. When these effects accumulate, they can significantly alter the object’s orbit.

How the light is reflected depends on several characteristics of the asteroid, including its precise shape, size, spin direction, and the types of material on its surface.

These characteristics aren’t known — or known precisely enough — for 1950 DA, so the researchers’ predictions have relatively large uncertainties in how the Yarkovsky effect would influence the asteroid’s orbit. Making a better estimate of how likely a collision is may require a spacecraft mission to the asteroid, to investigate its surface directly.

Between now and 2880
In a second Science paper this Friday, Joseph Spitale of the University of Arizona has calculated the possible results of using the Yarkovsky effect to deflect an asteroid headed for Earth. For a certain type of kilometer-sized asteroid, he estimated that modifying the way its surface conducts heat could help move the object about 1,400 kilometers in 100 years. Likewise, changing how much light a similar asteroid reflects could cause it to move as much as 15,000 kilometers in 100 years.

It might be possible to do this by blanketing the asteroid with a layer of dirt, Spitale proposed, or by shattering the surface with conventional explosives.

Other possibilities, according to Giorgini, may be to dust the asteroid with chalk or soot. His favorite idea is a solar sail mission, in which a device with a mirror-like sheet would sail out to the comet, propelled by radiation pressure from the sun. Ideally, the sail would collapse around the asteroid, covering it with a shiny surface.

For 1950 DA, we have several centuries to figure out whether such ideas could work. In the meantime, studies like Giorgini’s also expose a philosophical side of science, as researchers confront the limits of what they can know.

“The fact that there are actually ways of knowing and characterizing the extent of one’s ignorance, while still remaining ignorant, may ultimately be more interesting and useful to people than Yarkovsky,” Giorgini said.

© 2013 American Association for the Advancement of Science

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