In March of 2880, a kilometer-wide asteroid known as 1950 DA could collide with Earth, scientists say. But with nearly 900 years to go before the potential impact, scientists should have ample time to come up with preventive measures. First discovered when it flew by our planet in 1950, the asteroid returned on New Year's Eve 2000, enabling researchers to gather additional information about the space rock and to predict its path far into the future. New research based on these observations appears in the current issue of the journal Science.

J. D. Giorgini of the California Institute of Technology and colleagues used radar and optical measurements made during 1950 DA's two appearances to determine the trajectory of its orbit, as well as the likelihood of an impact with Earth. They considered a number of additional factors that might influence its path, such as the gravitational pull of the sun, moon, planets and several other large asteroids. As it turns out, a phenomenon known as the Yarkovsky effect generates the most uncertainty in their prediction. According to the Yarkovsky effect, when solar energy absorbed at the asteroid's surface exits as thermal radiation, the tiny thermal photons can generate a slight accelerating force. Over time, these forces could add up, altering 1950 DA's orbit and possibly ruling out a collision with Earth. Unfortunately, a precise prediction requires a better understanding of the asteroid itself. "How close 1950 DA will approach Earth turns out to depend on the asteroid's physical attributes--its size, shape and mass, and how it spins, reflects light and radiates heat into space," Giorgini explains.

The most recent opportunity to track the asteroid's journey will not be the last: 1950 DA will be observable again in 2032 and several times thereafter. But even those sightings may not suffice. "Depending on the results of such experiments," the authors write, "a satisfactory assessment of the collision probability of 1950 DA may require direct physical analysis with a spacecraft mission." If the outlook from such investigations were grim, there are several possible ways of averting disaster, writes Joseph Spitale of the University of Arizona in a related Science report. For example, Spitale proposes that controlled explosions on the surface of 1950 DA could change the absorption and radiation of energy, thus altering the Yarkovsky effect and shifting the asteroid's path.