Rogue Asteroid, Not Comet, Smacked Into Jupiter
This Hubble picture, taken on 23 July, is the sharpest visible-light picture taken of the atmospheric debris from a comet or asteroid that collided with Jupiter on 19 July. This is Hubble's first science observation following its repair and upgrade in May. The image was taken with the new Wide Field Camera 3.
Credit: NASA, ESA, and H. Hammel (Space Science Institute, Boulder, Colorado) and the Jupiter Comet Impact Team

The culprit in a cosmic collision on Jupiter last year was most likely a rogue asteroid and not a comet as scientists originally suspected, a new study suggests.

A close analysis of the July 2009 impact on Jupiter in photos from NASA's Hubble Space Telescope led forensic astronomers to suspect that the mystery object was actually an asteroid approximately 1,600 feet (500 meters) wide.

Evidence of the collision was first seen by Australian amateur astronomer Anthony Wesley, who spotted a large, black spot close to the polar region of Jupiter ? the scar from a mystery object that struck the gas giant on July 19, 2009.

The massive bruise was the size of the Pacific Ocean. Astronomers observed four days after the impact using the Hubble's recently-installed Wide-Field Camera 3 and the newly-repaired Advanced Camera for Surveys, following its last servicing mission a month before the Jupiter crash. [Gallery: Jupiter gets smacked.]

"The observation itself was really difficult to do because it was after the servicing mission, and Hubble was not ready to observe anything," lead researcher Heidi Hammel of the Space Science Institute in Boulder, Colo. told "Just the fact that we got the data at all was a story in itself."

The results of the study led by Hammel appeared in the June 1 issue of The Astrophysical Journal Letters.

Jupiter gets smacked?again

When pieces of the comet Shoemaker-Levy 9 slammed into Jupiter in 1994, scientists were able to examine the trajectory and debris to better understand planetary impacts.

For Jupiter, the largest planet in our solar system, researchers predicted that such an impact would occur approximately every 50 to 250 years. So, when the unknown object collided with Jupiter last year, only 15 years later, scientists were puzzled.

Hammel and a team of astronomers compared Hubble images of the 2009 impact with those from the Comet P/Shoemaker-Levy 9 (SL9) collisions from 1994, when more than 20 pieces of SL9 plunged into Jupiter's atmosphere.

The researchers noticed striking differences between the two incidents, which helped them piece together details about the object in the 2009 collision.?

"In 1994, the impact sites had a halo around them," Hammel said. "We didn't see that in the UV images for the 2009 impact. The shape of the 2009 impact was also a little different ? the site was elongated, so that was a clue that something was different here."

Comet vs. asteroid

The halo around the SL9 impact sites was evidence of fine dust arising from a comet-fragment strike. Hubble's UV images also showed a strong contrast between impact-generated debris and Jupiter's clouds.

For the 2009 impact, that distinct halo was missing, and the impact site's contrast also faded rapidly. These discrepancies both suggest a lack of lightweight particles, pointing to circumstantial evidence for an impact by a solid asteroid rather than a dusty comet.

"The physics that occurs when an object explodes in Jupiter's atmosphere is not different, so this leads us to expect that the body itself is different," Hammel explained. "The lack of halo surrounding the impact site suggests that the body in 2009 was not surrounded by debris."

The elongated shape of the 2009 impact site also suggests that the object likely had a different trajectory from SL9, and entered Jupiter's atmosphere at a shallower angle.

Fellow researcher Agustin Sanchez-Lav?ga of the University of the Basque Country in Bilbao, Spain, and his colleagues performed an analysis of possible orbits that the 2009 impacting body could have taken in colliding with Jupiter.

"The trajectory the object followed was in an opposite direction to that of the previous one, and backward calculation in time of the orbit suggests a 50 percent change for the impactor being an asteroid or a comet-like object," Sanchez-Lav?ga told in an e-mail interview.

The study indicates that the object likely came from the Hilda family of bodies, a secondary asteroid belt consisting of over 1,100 asteroids with elliptical orbits near Jupiter.

Lone space gunman

Future studies that examine the chemistry of the impact site will likely provide a more precise identification of the object.

"Right now, it's looking suggestive from different lines of evidence from Hubble and early ground-based observations that this was a single body acting alone," Hammel said. "There are also detailed studies of the chemistry of the impact site that are not published yet. Those might be the smoking gun that can nail this down."

The force of the 2009 collision was equal to a few thousand standard nuclear bombs exploding, which is comparable to the blasts from the medium-sized SL9 fragments. The largest of the SL9 fragments, which measured over a kilometer (0.62 miles) across, created explosions that were many times more powerful than the world's entire nuclear arsenal if they all blew up at once. ?

Still, the violent explosions and visible impact sites are not thought to cause any serious lasting effects on Jupiter.

"They're very transient," Hammel explained. "The impacts trigger localized effects, but then a month or two later, they're gone. The 2009 impact happened in July. We took images periodically all the way through November. By November, we couldn't see the impact site anymore. So, the visible wavelength and ultraviolet traces were gone. That's similar to what we saw with SL9 in 1994."

This recent impact underscores the important synergy that can happen between amateur and professional astronomers.

"It's a great opportunity for amateurs to contribute to scientific study," Hammel said. "Now that an amateur community has terrific cameras and vast amounts of time to study Jupiter, it's possible we may find out that these things happen more frequently than suspected."

Indeed, statistical calculations based on available imaging surveys of Jupiter from Earth-based telescopes indicate that the impact rate of 500-meter objects with Jupiter could, in fact, be higher than expected, said Sanchez-Lav?ga.

If so, "their detection could be easy with medium-sized amateur telescopes equipped with CCD (cameras) and webcams," he added.

Furthermore, impact studies will help researchers understand the statistics and overall nature of these events, and will be particularly helpful in answering questions about how planets' atmospheres respond to impact.

"These are all important questions to find the answers to, in case we ever find an object that could hit the Earth's atmosphere," Hammel said. "We can take these findings from Jupiter and we'll be able to fold them back into our predictive models for potential Earth impact."