Loading the cannon: how do asteroids from the belt between Jupiter and Mars get into near-Earth orbits?

Natural History, May, 2005 by Charles Liu

Sixty-five million years ago the Mesozoic era ended with a bang. An asteroid several miles across slammed into our planet, just off what is now the northern coast of Mexico's Yucatan Peninsula. The energy released by the collision--far greater than the combined explosive power of all the nuclear warheads on Earth--caused a global environmental catastrophe: firestorms, choking clouds, acid rain, and more. The impact coincided with a mass extinction that wiped out the nonbird dinosaurs and led, eventually, to the rise of mammals as the dominant large fauna.

Before that asteroid swung into "death star" mode, it was the kind of body planetary astronomers would call a near-Earth object, or NEO--a chunk of metal, rock, or ice orbiting the Sun, whose orbit happens to intersect the orbit of Earth. Astronomers have already discovered more than 3,200 NEOs, 700 of which are more than a kilometer across--large enough to threaten humanity if just one made a direct hit. Perhaps thousands more NEOs of that size remain undiscovered.

None of the NEOs discovered so far is predicted to hit Earth in the foreseeable future. Long-term, though, even if astronomers plotted all the orbits NEOs are tracing today, they couldn't predict all the potential large impacts. That's because the orbits of the vast majority of NEOs are unstable; after a few million years the objects generally swoop in too close to a planet or the Sun. When they do, the larger body's gravity acts either like a slingshot, swinging them around so fast that they get flung out of their Earth-crossing orbits, or like a vacuum cleaner, dragging them to oblivion on the surface of the larger body. All of the NEOs around when the dinosaurs met their demise are probably long gone.

But NEOs are still abundant, so how does their population get replenished? According to the most widely accepted hypothesis, most NEOs arrive in our local neighborhood from the main asteroid belt, a zone of rocky bodies between the orbits of Mars and Jupiter. Now a research team led by Simone Marchi, an astronomer at the University of Padua, Italy, has found supporting evidence for that hypothesis. Vesta, the third largest asteroid in the solar system, lies in the main asteroid belt, nearly 130 million miles farther from the Sun than Earth does. Yet fragments of Vesta have been found scattered throughout the solar system. Marchi's analysis shows that among those fragments of Vesta are four NEOs, discovered in 2003.

Vesta itself was discovered 198 years ago by the German astronomer Heinrich W. M. Olbers. Perhaps its most remarkable feature is a gargantuan impact crater on its surface, stretching some 280 miles across--more than three-quarters of the 330-mile diameter of Vesta itself. Long ago, a massive collision with another large asteroid must have gouged out a huge part of Vesta. The liberated fragments, launched into solar orbit, are called Vestoids.

Planetary astronomers can infer an asteroid's origin by measuring the intensity of its reflected sunlight; all objects that come from a common parent body tend to have the same mineral makeup, and so their reflectivity is the same. Marchi and his associates measured the reflectivity of a number of NEOs across a range of visible and infrared wavelengths, and they identified four NEOs in their sample that matched Vesta's visible-light reflectivity remarkably well--Vestoids.

If they remain in their current orbits, the four Vestoid NEOs will never collide with our planet. As NEOs, however, they are only one unpredictable gravitational slingshot away from a collision course. What would happen if one of them hit Earth? H. Jay Melosh, a planetary scientist at the University of Arizona in Tucson, and Ross A. Beyer of the NASA Ames Research Center at Moffett Field, California, have created a user-friendly Web page (www.lpl.arizona.edu/tekton/ crater.html) that can estimate the results.

The largest of the four NEOs is about 0.7 mile (1.1 kilometer) across and made of solid rock. Type in 1,100 meters on the Web site. In a head-on collision the NEO would hit Earth at around 38,000 miles an hour (17 kilometers a second). Type that in as well. The Web site then calculates that the energy released on impact would equal some 5 million Hiroshima-power atomic bombs, and leave an impact crater one and a half times the size of New York City. For comparison, the earthquake that caused the horrific tsunami of December 26, 2004, released only 0.5 percent the energy of such a devastating strike.

So what made the four Vestoids leave the asteroid belt and swing in close enough to earn the name "near-Earth object"? One possible mechanism is another variety of gravitational slingshot: a random close encounter with another asteroid. Such random events, though, aren't very common; in fact, among asteroids they are so unlikely that even in the billions of years since the beginning of the solar system they could not have given rise to the NEO population we observe today.

To account for the observations, planetary astronomers have proposed a mechanism based on so-called orbital resonances. When large numbers of objects orbit together, they can create complex patterns of motion; the structure within Saturn's rings is a good example. According to Marchi, at least two orbital resonances funnel asteroids from the main asteroid belt into the inner solar system. So, more NEOs will probably be coming down the resonance pike, keeping the threat of a catastrophic collision alive for the foreseeable future.