Sky survey predicts lesser risk of asteroid collision with Earth

A recently published study that sought to more accurately pinpoint size, location and number of asteroids asserts there is a decreased probability of an asteroid colliding catastrophically with Earth in the next 100 years.

Using data provided by the Sloan Digital Sky Survey — of which the University is a member — researchers have found that there are approximately 700,000 asteroids large enough to destroy Earth, as compared with a previous estimate of two million. The Sloan Survey data has also shown the chances of collision in the next century to be much less than previously thought.

Data for the study was collected over a continuous four-day observation period at the Apache Point Observatory in New Mexico. During the four days of the study, approximately 10,000 asteroids were observed. The results of the study were published this month in the Astronomical Journal.

The University's main contribution to the project was developing the camera used to record images of the sky and the computer program cataloging the data from those images. University researcher Robert Lupton played a major role in creating this technology.

The basic technology behind the new camera is analogous to the technology found in regular digital cameras, though "much more complicated and sensitive," according to astrophysics professor Gillian Knapp, the University's representative on the Collaborative Committee of the Sloan Survey.

This new technology has allowed the researchers to folow through with their groundbreaking new study. Lead University researcher Zeljko Ivezic said that "all old maps of the sky were based on photographic, rather than digital film and therefore were not as accurate." Measurements made by the Sloan Survey have an accuracy rate of 90 percent. Prior to the Sloan Survey, researchers were only able to detect three to four out of every 10 asteroids, Ivezic said.

"The new thing that is being introduced here," Ivezic noted, "is the direct measurement of asteroids that measure one kilometer or larger." Such a measurement is significant because one kilometer is the minimum size an asteroid must reach in order to pose a catastrophic threat in case of collision with Earth.

Without the new digital technology of the Sloan Survey, studies could only detect asteroids of five kilometers or larger. Based on the figure they observed for these larger asteroids, researchers would estimate the number of smaller asteroids. Upon actual observation using the new technology, the small asteroids were found to be less abundant than had been previously thought.

The ultimate goal of the Sloan Survey is to map one-fourth of the sky, an ambitious project that Knapp estimates will take a total of three years to complete.

Once the mapping is finished, researchers will have determined the position and absolute brightness of more than 100 million celestial objects.

While the recently published study seeks to give answers, its results also pose some important questions. Serge Tabachnik, a University researcher and team member of the recent study, said that an explanation of why the estimates of smaller asteroids had been high could provide important information about the origins of the solar system.

The completed catalogue of data for the study records the existence of main-belt asteroids, asteroids found directly in the asteroid belt. The asteroid belt is the source for near-Earth objects, asteroids that have broken away from the main belt and that have the potential to collide with the Earth. The recently published study did not directly observe near-Earth objects, a move which drew criticism from some quarters.

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"Some people disagreed with the approach we used," Ivezic said, explaining that critics felt the researchers should look directly at near-Earth objects. However, Ivezic pointed out that the research team was able to obtain accurate and thorough results by looking at asteroids directly in the asteroid belt. He also noted that it is important to have "a complementary way to answer the same question."

The approach used by the researchers was validated by the fact that a study of near-Earth objects, soon to be released by a team at the University of Arizona, produced the same results. Ivezic cited the correlation between the results of the two studies as "very encouraging, because the two studies were based on entirely different methods."

Ivezic credits the success of the recent study to the great improvements in digital technology for observing the asteroids, which — because of their reputation as being difficult to detect — had once been labeled as "the vermin of the universe."

In addition to the University, the Sloan Survey is a collaboration of the University of Chicago, Fermilab, the Institute for Advanced Study, the Japan Participation Group, Johns Hopkins University, the Max-Planck Institute for Astronomy, the Max-Planck Institute for Astrophysics, New Mexico State University, the U.S. Naval Observatory and the University of Washington.

Funding for the Sloan Survey came from the Alfred E. Sloan Foundation and its partner institutions, including NASA, the National Science Foundation, the U.S. Department of Energy, the Japanese Monbukagusho and the Max Planck Society.