SCIENCE & ENGINEERING NEWS

Ithaca, N.Y. — What are the odds that an asteroid will come careening across the sky to wreak havoc on Earth? Is this just a movie writer’s fantasy or a real possibility? Cornell astronomer William Bottke heads up an international team of researchers who are grappling with this very question. By simulating the evolution of asteroid orbits with supercomputers at the Cornell Theory Center and comparing their results with the orbits and sizes of real asteroids discovered by telescopes at Kitt Peak, Arizona, they have produced a map showing the paths most likely to carry an errant asteroid to Earth.

We know that many regions of the inner solar system are cluttered with asteroids, leftover pieces of the original building blocks that formed planets like Earth. Some of these asteroids follow complicated, irregular paths that can intrude upon Earth’s orbit. Bottke’s team is concerned with the threat these rocks, the so-called near-Earth asteroids (NEAs), pose for us on Earth. In the June 23 edition of the journal Science, they report that as many as 900 NEAs, each at least a kilometer across, can potentially strike Earth in the future. According to Bottke, “One kilometer (about 0.6 of a mile) in size is thought to be a magic number, because it has been estimated that these asteroids are capable of wreaking global devastation if they hit the Earth.” Each year a few of these pass dangerously close to our planet, within a few moon distances.

The team of astronomers mapping NEAs includes Bottke from Cornell, Robert Jedicke at the University of Arizona’s Lunar and Planetary Laboratory, and Jean Marc Petit and Brett Gladman of the Observatoire de la Cote d’Azur, Nice, France. From the beginning, they balanced theory with observation. Bottke’s team used the supercomputing resources at the Cornell Theory Center to follow changes in the orbits of asteroids located in the main asteroid belt between the orbits of Mars and Jupiter. Tracking these objects for 100 million years or more in their computer models, they watched as the tiny gravitational kicks produced by nearby planets slowly moved many main belt asteroids into new orbits, potential collision paths with Earth.

Because the motion of these objects was strongly chaotic, it was difficult for Bottke’s team to predict where each asteroid would go. However, by simulating the paths of thousands of model asteroids through many possible journeys, they identified which orbital pathways have the highest probability of being taken by the new NEAs. Bottke likens this to a game of statistical pinball, “If you shoot lots and lots of balls into a pinball machine, one at a time, and you keep track of where the balls go, eventually you can make a map telling you not just where all the balls have gone, but also where they are most likely to go.” Bottke’s map shows the location of the orbits for all the NEAs, even the ones that have not yet been found.

To calibrate (or fine tune) their results, the researchers compared their map with asteroid discovery data from a 10-year search conducted by the Spacewatch Survey, a University of Arizona-based group that had detected about 100 kilometer-sized NEAs. Then they compared the predictions from their numerical model with the Spacewatch observations. Tuning the model until they came up with a satisfactory match, Bottke’s team predicted that 900 kilometer-sized or larger asteroids have orbits that cross or nearly cross Earth’s orbit. After reviewing a database of NEAs based on asteroid surveys from around the world, Bottke estimates that we have discovered only roughly 40% of these potentially hazardous projectiles so far. The rest of these dangerous projectiles, says Bottke, are too far from Earth to be easily detected or are located in regions of the sky that are challenging for astronomers to survey.

To show off their results, the team called on CTC scientific visualization specialist Chris Pelkie, who provided them with a color map of the NEA orbits that can be used to help astronomers focus their efforts as they keep an eye on the sky. Unfortunately, anyone wanting to use the map must first learn the astronomers’ coordinate system.

According to Bottke, astronomers can predict the paths of the known NEAs for the next few hundred years, picking out and tracking any likely to pose an imminent threat. As for the missing 60%, surveys from around the world are actively searching for these objects. “Though predicting the future is difficult, there is a strong possibility that 90% of the most threatening asteroids will be found within the next 20 years,” says Bottke. Fortunately, kilometer-sized asteroids only strike the Earth once every 0.5 to 1 million years on average. Focusing the search effort with Bottke’s statistical map will hopefully speed up discovery efforts and improve our odds of finding an errant and dangerous asteroid before it finds us.

CTC is a high-performance computing and interdisciplinary research center located at Cornell University. CTC receives funding from Cornell University, New York State, a number of federal agencies, and Corporate Program members. Bottke’s research was funded by NASA and the European Space Agency.

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