NASA/IRTF/JPL-Caltech/University of Oxford
NASA's Infrared Telescope Facility in Mauna Kea, Hawaii, shows particle debris in Jupiter's atmosphere after the July 19, 2009, impact. It was originally believed that a comet did the damage.
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updated 1/27/2011 1:11:24 PM ET 2011-01-27T18:11:24

On July 19, 2009, 15 years to the day after the famous comet Shoemaker-Levy 9 peppered Jupiter with huge chunks of ice, the gas giant was hit once again by a cosmic bullet.

Australian amateur astronomer Anthony Wesley was the first to notice a scar in the planet's atmosphere after the fact, leading us to believe that Jupiter had been hit by another comet.

However, in this cosmic game of whodunit, jumping to the conclusion that "another" comet was to blame for the 2009 impact turns out to have been a red herring. Just because we know a comet slammed into the planet in 1994, it doesn't necessarily mean the 2009 event used the same weapon.

After some detective work, two papers recently published in the journal Icarus have concluded that Jupiter was actually shot by an asteroid, not a comet.

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What's more, the asteroid that did the damage was probably about the size of the Titanic.

But how do astronomer detectives know if an impact is caused by a comet or asteroid? In this cosmic crime scene, it's all about heat and chemistry. And when an asteroid takes a nosedive into a gas giant's atmosphere, it leaves different thermal and chemical fingerprints to a comet impact.

Using infrared data from ground-based telescopes that observed Jupiter's scar for the days after Wesley's discovery, an international team of scientists were able to piece together all the evidence. But the fact that the data suggest an asteroid was to blame is a huge surprise.

"Both the fact that the impact itself happened at all and the implication that it may well have been an asteroid rather than a comet shows us that the outer solar system is a complex, violent and dynamic place, and that many surprises may be out there waiting for us," said Glenn Orton, an astronomer at NASA's Jet Propulsion Laboratory in Pasadena, Calif. "There is still a lot to sort out in the outer solar system."

Leigh Fletcher, a researcher at Oxford University, summarized why all the evidence is pointing at an asteroid impactor:

"Comparisons between the 2009 images and the Shoemaker-Levy 9 results are beginning to show intriguing differences between the kinds of objects that hit Jupiter. The dark debris, the heated atmosphere and upwelling of ammonia were similar for this impact and Shoemaker-Levy, but the debris plume in this case didn't reach such high altitudes, didn't heat the high stratosphere and contained signatures for hydrocarbons, silicates and silicas that weren't seen before. The presence of hydrocarbons, and the absence of carbon monoxide, provide strong evidence for a water-depleted impactor in 2009."

Until now, it was thought that comets were the only objects that still slammed into Jupiter. The majority of asteroids near Jupiter's orbit should have been sucked into the gas giant's enormous gravitational well by now — it's not known as the "solar system's vacuum cleaner" for nothing — so to witness an asteroid impact is either extremely lucky or our understanding of the outer solar system underestimates how many asteroids are really out there.

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Interactive: Remembering the Challenger crew

Photos: The Challenger tragedy in pictures

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  1. From joy to tragedy

    The shuttle Challenger's mission in 1986 was meant to mark a milestone in spaceflight: the first orbital voyage of an American teacher. NASA's choice for the honor was Christa McAuliffe, a social-studies teacher at Concord High School in New Hampshire. Here, McAuliffe rides past the New Hampshire State House in Concord with her daughter Caroline and son Scott, during a Lions Club parade on July 21, 1985. (Jim Cole / AP) Back to slideshow navigation
  2. Packing for Houston

    High-school teacher Christa McAuliffe folds her training uniform as she packs for the trip to Johnson Space Center in Houston on Sept. 8, 1985. (Jim Cole / AP) Back to slideshow navigation
  3. Preparing Challenger

    The space shuttle Challenger is transferred to the high bay of the Vehicle Assembly Building at NASA's Kennedy Space Center in Florida on Dec. 17, 1985. Inside the cavernous VAB, the Challenger orbiter was mated with its solid rocket boosters and external tank in preparation for its launch a month later. (Terry Renna / AP) Back to slideshow navigation
  4. Zero-G and she feels fine

    Christa McAuliffe gets a preview of microgravity on NASA's specially equipped KC-135 "zero gravity" aircraft on Jan. 13, 1986. The plane flies in a parabolic pattern that provides short periods of weightlessness. For some people, those bouts of zero-G can induce nausea - which is why the airplane was nicknamed the "Vomit Comet." (Science & Society Picture Library / Getty Images) Back to slideshow navigation
  5. To the launch pad

    The shuttle Challenger is delivered to its launch pad at NASA's Kennedy Space Center atop a mobile crawler-transporter. (NASA) Back to slideshow navigation
  6. Practicing for an escape

    Challenger's crew members practice the procedure for escaping from the launch pad at Kennedy Space Center using slide wire baskets. From left are Ronald McNair, Gregory Jarvis and Christa McAuliffe. Directly behind them are astronauts Judy Resnik and Ellison Onizuka. The basket system was designed to take the astronauts off the pad quickly if an emergency arose just before launch. (NASA Headquarters) Back to slideshow navigation
  7. Ready for flight

    Challenger's crew members stand in the White Room at Launch Pad 39B after a dress rehearsal for launch. From left are Christa McAuliffe, Gregory Jarvis, Judy Resnik, commander Dick Scobee, Ronald McNair, pilot Michael Smith and Ellison Onizuka. (NASA Headquarters) Back to slideshow navigation
  8. Heading for the pad

    Challenger's crew members leave their quarters at Kennedy Space Center for the launch pad on Jan. 27, 1986. Commander Dick Scobee is at the front of the line, followed by Judy Resnick, Ronald McNair, Gregory Jarvis, Ellison Onizuka, Christa McAuliffe and pilot Michael Smith. NASA had to scrub the launch attempt on Jan. 27, due to high winds at the pad, and liftoff was rescheduled for Jan. 28. (Steve Helber / AP) Back to slideshow navigation
  9. The first sign of trouble

    A launch-pad camera captures a close-up view of the shuttle Challenger's liftoff on Jan. 28, 1986. From this camera position, a cloud of gray-brown smoke can be seen on the right side of the solid rocket booster, directly across from the letter "U" in "United States" on the orbiter. This was the first visible sign that a breach in the booster's joint may have occurred. Investigators determined that frigid overnight temperatures caused the booster joints' normally pliable rubber O-ring seals to become hard and non-flexible. The failure of the seals caused hot exhaust gases to blow through the joints, cutting into the external fuel tank. (NASA Headquarters) Back to slideshow navigation
  10. Liftoff!

    A wide-angle view shows the ascent of the shuttle Challenger on Jan. 28, 1986. In the seconds after ignition, the rocket engines' hot blast began the process of destruction. (NASA Headquarters) Back to slideshow navigation
  11. Ice at the pad

    Why did the O-rings fail? On the day of the shuttle Challenger's launch, icicles draped structures at NASA's Kennedy Space Center in Florida. The unusually cold weather, beyond the tolerances for which the rubber seals were approved, most likely caused the O-ring failure. (NASA Headquarters) Back to slideshow navigation
  12. Watching the launch

    Classmates of the son of America's first teacher-astronaut cheer as the space shuttle Challenger lifts skyward from Launch Pad 39B on Jan. 28, 1986. Their delight turned to horror as the shuttle exploded 73 seconds into flight. The boy in the white hat and glasses at center is Peter Billingsley, the star of "A Christmas Story" and a spokesman for the young astronaut program. (Jim Cole / AP) Back to slideshow navigation
  13. The final seconds

    The right solid rocket booster on the shuttle Challenger begins to explode, just a little more than a minute into the shuttle's ascent from NASA's Kennedy Space Center on Jan. 28, 1986. (NASA via AFP-Getty Images) Back to slideshow navigation
  14. Moment of tragedy

    An orange fireball marks the explosion of the space shuttle Challenger on Jan. 28, 1986. (Bruce Weaver / AP) Back to slideshow navigation
  15. Remains of the rockets

    At about 76 seconds, fragments of the orbiter can be seen tumbling against a background of fire, smoke and vaporized propellants from Challenger's external fuel tank. The left solid rocket booster is still shooting skyward. A reddish-brown cloud envelops the disintegrating orbiter. The color is indicative of the nitrogen tetroxide oxidizer propellant in the orbiter's reaction control system. (NASA Headquarters) Back to slideshow navigation
  16. Flying fragments

    This picture, released by the presidential commission that investigated the Challenger tragedy, shows fragments of the orbiter flying away from the explosion on Jan. 28, 1986, 78 seconds after liftoff. The top arrow shows the orbiter's left wing. The center arrow shows the orbiter's main engine; and the bottom arrow shows the orbiter's forward fuselage. Investigators suggested that some of Challenger's crew members may have survived the explosion itself but died in the fall down to Earth. (AP) Back to slideshow navigation
  17. The tragedy sinks in

    Flight director Jay Greene studies data at his console inside Johnson Space Center's Mission Control Center in Texas, just minutes after the announcement that Challenger's ascent was not nominal. (NASA) Back to slideshow navigation
  18. A family's sorrow

    Members of teacher-astronaut Christa McAuliffe's family react shortly after the failed liftoff of the space shuttle Challenger from NASA's Kennedy Space Center on Jan. 28, 1986. Christa's sister, Betsy, is in front, with parents Grace and Ed Corrigan behind. (Jim Cole / AP) Back to slideshow navigation
  19. Death of a celebration

    Carina Dolcino, senior class president at Concord High School, is stunned by the news that the space shuttle carrying Christa McAuliffe, one of the school's teachers, exploded after launch on Jan. 28, 1986. Students watched the launch on television sets scattered throughout the school in Concord, N.H., and a celebration had been planned for a successful liftoff. (Ken Williams / Concord Monitor via AP) Back to slideshow navigation
  20. White House watch

    President Ronald Reagan, center, is surrounded by members of his senior staff on Feb. 3, 1986, as he watches a TV replay of the Challenger shuttle explosion at the White House. From left are Larry Speakes, deputy White House press secretary; presidential assistant Dennis Thomas; special assistant Jim Kuhn; Reagan; White House communications director Patrick Buchanan; and chief of staff Donald Regan. (Peter Souza / AP) Back to slideshow navigation
  21. Sympathy at school

    Lisa Mitten of Concord, N.H., wipes tears from her eyes as her daughter Jessica reads some of the letters of sympathy that were on display at Concord High School on Feb. 1, 1986. Hundreds of Concord residents visited the school library to see the many telegrams and letters that were sent from all over the United States. (Toby Talbot / AP) Back to slideshow navigation
  22. Recovering debris

    Debris from the ill-fated shuttle Challenger is unloaded from the Coast Guard cutter Dallas during February 1986. (AP) Back to slideshow navigation
  23. A piece of Challenger

    For weeks after the accident, search and recovery teams went out to retrieve Challenger debris from the Atlantic Ocean, with the help of the U.S. Coast Guard and U.S. Navy. Vessels brought pieces of debris to the Trident Basin at Cape Canaveral Air Force Station, from which they were shipped to Kennedy Space Center for investigation. The Coast Guard cutter Dallas transported this fragment of exterior tiling. (NASA Headquarters) Back to slideshow navigation
  24. Retrieved from the ocean

    A piece of debris from the space shuttle Challenger is hoisted onto the deck of the Stena Workhorse off the coast of Florida during a recovery mission. (Time & Life Pictures via Getty Images) Back to slideshow navigation
  25. Farewell to the fallen

    The remains of the shuttle Challenger's seven crew members are transferred from seven hearses to a MAC C-141 transport plane at the Kennedy Space Center's Shuttle Landing Facility, for transport to Dover Air Force Base, Del. (NASA Headquarters) Back to slideshow navigation
  26. In memoriam

    President Ronald Reagan and his wife, Nancy, stand with the wife of astronaut Michael Smith and other family members at a memorial service for the victims of the Challenger disaster. (Diana Walker / Time & Life Pictures via Getty Image) Back to slideshow navigation
  27. Grim investigation

    Apollo 11 moonwalker Neil Armstrong, a member of the presidential panel investigating the Challenger explosion, listens to testimony before the commission in Washington on Feb. 11, 1986. Another commission member, David Acheson, listens in the background. A model of the space shuttle sits on the table. (Scott Stewart / AP) Back to slideshow navigation
  28. Solving the puzzle

    Search and recovery teams located pieces of both the left and right sidewall of the shuttle Challenger during the months-long retrieval effort that followed the explosion on Jan. 28, 1986. Heat and fire damage scarred the right sidewall. But the left sidewall, depicted here, escaped the flames and suffered only from overload fractures and deep gouge marks. The largest intact piece formed part of the payload bay sidewall and measured approximately 30 by 12 feet. (NASA Headquarters) Back to slideshow navigation
  29. Washed ashore

    Some pieces of the shuttle Challenger did not surface until long after the explosion. A tractor carries one of the shuttle's elevons after it washed ashore on Cocoa Beach, Fla., on Dec. 17, 1996 ... almost 11 years after the loss of Challenger and its crew. (AFP-Getty Images) Back to slideshow navigation
  30. Day of remembrance

    Every January, NASA recalls the Challenger explosion as well as other space tragedies on a "Day of Remembrance." Here, NASA Administrator Sean O'Keefe lays a wreath at the Space Shuttle Challenger Memorial at Arlington National Cemetery in Virginia on Jan. 28, 2003. O'Keefe also paid tribute to the three astronauts of Apollo 1 who died in a launch pad fire on Jan. 27, 1967. Sadly, seven more astronauts died just days after this picture was taken, on Feb. 1, 2003, when the shuttle Columbia broke up during re-entry. (Bill Ingalls / NASA via Getty Images) Back to slideshow navigation
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  1. Christa McAuliffe
    Jim Cole / AP
    Above: Slideshow (30) The Challenger tragedy in pictures
  2. Image:
    Y. Beletsky / ESO
    Slideshow (12) Month in Space: January 2014

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