Jupiter’s crash of ’09

The body that slammed into Jupiter last July almost certainly was an asteroid rather than a comet, and such impacts might happen as often as every 10 to 15 years, new calculations and an analysis of observations taken last summer suggest.

If researchers are correct in their analysis, images taken by the Hubble Space Telescope and other instruments may have captured for the first time the immediate aftermath of an asteroid striking a planet.

The circumstantial evidence in favor of an asteroid comes in part from comparing Hubble Space Telescope images of the 2009 impact site with Hubble images of Jupiter recorded in 1994, when another impactor, the comet Shoemaker-Levy 9, struck the planet.

Before that impact, each fragment of Shoemaker Levy-9, which had broken apart during an earlier passage by Jupiter, was surrounded by a cloud of dusty debris, called a coma, that clearly identified the fragments’ source as a comet. The impact of the tiny particles that made up each coma generated the extended dark halo seen in Hubble’s 1994 ultraviolet images, notes Heidi Hammel of the Space Science Institute in Boulder, Colo. In contrast, she notes, the single body that smacked into Jupiter in 2009 produced no such halo, which argues for an asteroid as the source.

Hammel and her colleagues, including Agustín Sánchez-Lavega of Universidad del País Vasco in Bilbao, Spain, describe their findings in the June 1 Astrophysical Journal Letters.

In a separate article in the same issue, Sánchez-Lavega, Hammel and collaborators suggest that an object about 500 meters across strikes Jupiter every 10 to 15 years. Previous estimates, based solely on the 1994 collision, had put the rate at about once a century.

Although the new estimate is still based on limited data — from the bodies that collided with Jupiter in 1994 and 2009 — the researchers considered in their analysis the frequency of other observations from space and from the ground, as well as the data’s quality.

Amateur astronomers, with their continual monitoring of Jupiter, will be a great help in understanding the actual impact rate, Hammel says.

The researchers conclude there’s a 50-50 chance the 2009 impactor was an asteroid, but that estimate is based only on tracing the body’s orbit back in time. “We cannot be more precise due to uncertainty in the impact time,” says Sánchez-Lavega. Astronomers didn’t see the actual impact, which occurred on July 19, but only its aftermath.

It’s the Hubble observations, along with images and spectra from other telescopes, that tip the balance in favor of an asteroid, comments Carey Lisse of the Johns Hopkins Applied Physics Laboratory in Laurel, Md.

According to the calculations of the impactor’s path, if the culprit was indeed an asteroid, it most likely came from the Hilda family of asteroids in the outer part of the main asteroid belt, which lies between the orbits of Mars and Jupiter.

Taken together, the two new studies indicate that the population of potential impactors in Jupiter’s neighborhood is much greater than previously thought and that many of these bodies are rocky, like asteroids, rather than icy, like comets, says Andy Cheng of the Applied Physics Laboratory.

Jupiter’s gravity wields enormous influence, sweeping objects out of its neighborhood and also trapping some of them, such as the Hilda asteroids, in particular regions of space, he notes.