Will Discovery Help Repel Asteroids Headed for Earth?

Sarah Ives
for National Geographic News

December 11, 2003

NASA scientists have detected a small but significant force that has the power to change an asteroid's orbital path. The discovery may help scientists predict asteroids' travel routes with greater accuracy and even provide crucial data to help deflect asteroids headed for Earth, researchers say.

Scientists detected the force, called the Yarkovsky Effect, on a near-Earth asteroid (NEA) known as asteroid 6489, or Golevka, that has been tracked for a dozen years.

Analyzing data from radar observations, an international team of researchers led by scientists at NASA's Jet Propulsion Laboratory in Pasadena, California, discovered that the asteroid's orbital route has shifted about 10 miles (16 kilometers) over the past 12 years.

While the distance is small compared to the billions of miles the asteroid traveled during that time, Steven Chesley, leader of the study, believes it is significant. "Over tens of millions of years that [change in orbit] can have a big effect," he said, noting that understanding the effect of the force can help scientists predict how asteroids move.

The researchers described their findings in the December 5 issue of Science.

The Yarkovsky Effect

The Yarkovsky Effect is produced by an asteroid's absorption of solar energy. During the day, the sun heats the asteroid. At night, the asteroid cools off. Because of this heating and cooling, an asteroid tends to emit more heat on its "afternoon" side—the same phenomenon that makes the Earth's evening twilight warmer than its morning twilight. The result is unbalanced thermal radiation that produces a small acceleration.

For years, scientists have applied the theory of the Yarkovsky Effect to explain aspects of asteroids, from rates of travel to the age of meteorite samples. But until now, the force had never been directly observed.

"We measured a force of about one ounce [28 grams] acting on an asteroid that weighs 460 billion pounds [208 billion kilograms]," Chesley said. By his calculations, that figure amounts to the weight of a strawberry moving an asteroid that is longer than five football fields. Using the Yarkovsky Effect, scientists also determined, for the first time, an asteroid's mass using ground-based observation.

"This is a unique method to determine a mass of a single…asteroid without sending a spacecraft there," said Vokrouhlicky.

Determining the mass of an asteroid could have enormous implications, said Steven Ostro, a study co-author also based at NASA's Jet Propulsion Laboratory. Knowing an asteroid's mass and volume from radar imaging enables scientists to determine the celestial body's density, a crucial piece of information.

Ostro said: "If we ever have to deflect an asteroid, in order to design a procedure to do the deflecting, we have to know a lot about the interior."

Asteroid 6489

Most asteroids in the solar system travel along an asteroid highway that lies between the orbits of Mars and Jupiter known as the main belt.

Occasionally, an asteroid will break free of the belt and move into the inner solar system. If the asteroid comes within 121 million miles (195 million kilometers) of the sun, the asteroid is known as a near-Earth asteroid.

Compared to other near-Earth asteroids, Golevka is relatively unremarkable, measuring only half a kilometer (0.3 mile) in diameter.

Scientists believe that asteroids that collide with Earth must be larger than a kilometer (0.6 mile) in diameter to create massive climate change and greater than 10 kilometers (6.2 miles) wide to cause mass extinction, Chesley said.

None of the large near-Earth asteroids astronomers are currently observing appear on track to hit Earth any time soon. Data from Golevka, however, will help make predictions more accurate.

David Vokrouhlicky, a study co-author based at Charles University in Prague, Czech Republic, said he can imagine cases where the discovery of the Yarkovsky Effect will help scientists "when we would try to decide whether [an] asteroid will hit the Earth or not."