A Bruise the Size of the Pacific Ocean
Without warning, a mystery object struck Jupiter on July 19, 2009, leaving a dark bruise the
size of the Pacific Ocean. The spot first caught the eye of an amateur astronomer in Australia,
and soon, observatories around the world, including NASA's Hubble Space Telescope, were zeroing in on the unexpected blemish.
Astronomers had witnessed this kind of cosmic event before. Similar scars had been left behind during
the course of a week in July 1994, when more than 20 pieces of Comet P/Shoemaker-Levy 9 (SL9) plunged
into Jupiter's atmosphere. The 2009 impact occurred during the same week, 15 years later.
Was This Impact a 500 Meter Wide Asteroid?
Astronomers who compared Hubble images of both collisions say the culprit may have been an asteroid about
1,600 feet (500 meters) wide. The images, therefore, may show for the first time the immediate aftermath of
an asteroid, rather than a comet, striking another planet.
The Jupiter bombardments reveal that the solar system is a rambunctious place, where unpredictable
events may occur more frequently than first thought.
The Impact Caught Astronomers by Surprise
"This solitary event caught us by surprise, and we can only see the aftermath of the impact, but
fortunately we do have the 1994 Hubble observations that captured the full range of impact phenomena,
including the nature of the objects from pre-impact observations" says astronomer Heidi Hammel of the
Space Science Institute in Boulder, Colo., leader of the Jupiter impact study.
"The object that hit Jupiter this time would have been small, dark, and cold-in other words, hard for us
to see before the impact, regardless of which wavelength we used for observations," adds Amy Simon-Miller, a
co-investigator at NASA's Goddard Space Flight Center in Greenbelt, Md.
In 2009 Hammel's team snapped images of the debris field with Hubble's recently installed Wide Field
Camera 3 and newly repaired Advanced Camera for Surveys.
Comparing Asteroid and Comet Impacts
The analysis revealed key differences between the two collisions (in 1994 and 2009), providing clues to
the 2009 event. Astronomers saw a distinct halo around the 1994 impact sites in Hubble ultraviolet (UV)
images, evidence of fine dust arising from a comet-fragment strike. The UV images also showed a strong
contrast between impact-generated debris and Jupiter's clouds.
Hubble UV images of the 2009 impact showed no halo and also revealed that the site's contrast faded rapidly.
Both clues suggest a lack of lightweight particles, providing circumstantial evidence for an impact by a
solid asteroid rather than a dusty comet.
A Shallow-Angle Impact
The elongated shape of the recent impact site also differs from the 1994 strike, indicating that the
2009 object descended from a shallower angle than the SL9 fragments. The 2009 body also came from a
different direction than the SL9 pieces.
Studying the Impact Debris
The visible spectrum, however, was "nearly identical in this case to what we saw for SL9," says Simon-Miller,
who recalculated the SL9 spectrum for this analysis. "This isn't surprising, because most of the debris
we're looking at is actually burnt-up atmosphere: hydrogen, hydrocarbons, and soot. It's very black,
just like the soot we're familiar with, and has a very flat spectrum."
By analyzing the temperatures and the spread of debris around the impact site, Simon-Miller also determined
that much of the debris was located high in Jupiter's stratosphere. "Based on the temperature, we figured out
how the wind changed with height," she explains. "And by looking at how the debris was moving, we figured
out how high in altitude it must be."
Source of the Impactor
Team member Agustín Sanchez-Lavéga of the University of the Basque Country in Bilbao, Spain, and
colleagues performed an analysis of possible orbits that the 2009 impacting body could have taken to
collide with Jupiter. Their work indicates the object probably came from the Hilda family of bodies, a
secondary asteroid belt consisting of more than 1,100 asteroids orbiting near Jupiter.
How Powerful Was the Impact Blast?
The 2009 strike was equal to a few thousand standard nuclear bombs exploding, comparable to the blasts from
the medium-sized fragments of SL9. The largest of those fragments created explosions that were many times
more powerful than the world's entire nuclear arsenal blowing up at once.
The Contributions of Amateur Astronomers
The recent impact underscores the important work performed by amateur astronomers. "This event beautifully
illustrates how amateur and professional astronomers can work together," notes Hammel.
The study by Hammel's team appeared in the June 1 issue of The Astrophysical Journal Letters.
About the Hubble Space Telescope
The Hubble Space Telescope is a project of international cooperation between NASA and the European Space Agency.
NASA's Goddard Space Flight Center manages the telescope. The Space Telescope Science Institute (STScI) conducts
Hubble science operations. STScI is operated for NASA by the Association of Universities for Research in Astronomy,
Inc. in Washington, D.C.
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| Hubble Space Telescope image of the Jupiter 2009 Impact. NASA, ESA, M. Wong (University of California, Berkeley), H.B. Hammel (Space Science Institute, Boulder, Colorado), I. de Pater (University of California, Berkeley), and the Jupiter Impact Team. Enlarge image. |
| Hubble Space Telescope images of the Jupiter 2009 Impact and its change over time. NASA, ESA, M. Wong (University of California, Berkeley), H.B. Hammel (Space Science Institute, Boulder, Colorado), I. de Pater (University of California, Berkeley), and the Jupiter Impact Team. Enlarge image |
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