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Last updated on April 19, 2014 at 12:41 EDT

Some Comets Like It Hot

January 13, 2012

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Comets are icy and fragile. They spend most of their time orbiting through the dark outskirts of the solar system safe from destructive rays of intense sunlight.  The deepest cold is their natural habitat.

Last November amateur astronomer Terry Lovejoy discovered a different kind of comet.  The icy fuzzball he spotted in the sky over his backyard observatory in Australia was heading almost directly for the sun.  On Dec. 16th, less than three weeks after he found it, Comet Lovejoy would swoop through the sun´s atmosphere only 120,000 km above the stellar surface.

Astronomers soon realized a startling fact: Comet Lovejoy likes it hot.

“Terry found a sungrazer,” says Karl Battams of the Naval Research Lab in Washington DC.  “We figured its nucleus was about as wide as two football fields–the biggest such comet in nearly 40 years.”

Sungrazing comets aren’t a new thing. In fact, the orbiting Solar and Heliospheric Observatory (SOHO) watches one fall toward the sun and evaporate every few days. These frequent kamikaze comets, known as “Kreutz sungrazers,” are thought to be splinters of a giant comet that broke apart hundreds of years ago.  Typically they measure about 10 meters across, small, fragile, and easily vaporized by solar heat.

Based on its orbit, Comet Lovejoy was surely a member of the same family–except it was 200 meters wide instead of the usual 10.  Astronomers were eager to see such a whopper disintegrate.  Even with its extra girth, there was little doubt that it would be destroyed.

When Dec. 16th came, however, “Comet Lovejoy shocked us all,” says Battams.  “It survived, and even flourished.”

Images from NASA’s Solar Dynamics Observatory showed the comet vaporizing furiously as it entered the sun’s atmosphere–apparently on the verge of obliteration–yet Comet Lovejoy was still intact when it emerged on the other side.  The comet had lost its tail during the fiery transit–a temporary setback. Within hours, the tail grew back, bigger and brighter than before.

“It’s fair to say we were dumbfounded,” says Matthew Knight of the Lowell Observatory and the Johns Hopkins Applied Physics Lab. “Comet Lovejoy must have been bigger than we thought, perhaps as much as 500 meters wide.”

That would make it the biggest sungrazer since Comet Ikeya-Seka almost 40 years ago.  With a tail that stretched halfway across the sky, Ikeya-Seki was actually visible in broad daylight after it passed through the sun’s atmosphere in October 1965. In Japan, where observers spotted the over-heated comet only 1/2 degree from the sun, it was described as 10 times brighter than the Full Moon.

Comet Lovejoy wasn’t that bright, but it was still amazing. Only a few days after it left the sun, the comet showed up in the morning skies of the southern hemisphere.  Observers in Australia, South America, South Africa, and New Zealand likened it to a search light beaming up from the east before dawn.  The tail lined up parallel to the Milky Way and, for a few days, made it seem that we lived in a double-decker galaxy.

Astronauts on the International Space Station also witnessed the comet. ISS Commander Dan Burbank, who has seen his share of wonders, even once flying directly through the Northern Lights onboard the space shuttle, declared Comet Lovejoy “the most amazing thing I have ever seen in space.”

An armada of spacecraft including SOHO, the Solar Dynamics Observatory, NASA’s twin STEREO probes, Japan´s Hinode spacecraft, and Europe’s Proba2 microsatellite recorded the historic event.

“We’ve collected a mountain of data,” says Knight. “But there are some things we’re still having trouble explaining.”

For instance, what made Lovejoy’s tail wiggle so wildly when it entered the solar corona?  Perhaps it was in the grip of the sun’s powerful magnetic field.

What caused Lovejoy to lose its tail inside the sun’s atmosphere–and then regain it later? “This is one of the biggest mysteries to me,” says Battams.

And then there is the ultimate existential puzzle: How did Comet Lovejoy survive at all?

As January unfolds, the “Comet that liked it Hot” is returning to the outer solar system, still intact, leaving many mysteries behind.  “It´ll be back in about 600 years,” says Knight.  “Maybe we will have figured them out by then.”

Image Caption: Comet Lovejoy as seen by the Solar Optical Telescope (SOT) on Hinode as it approached the sun early on 16 December 2011. Hinode purposely adjusted its instruments during this time period to observer the side of the sun and track the comet. Scientists were rewarded with two images, taken 30 seconds apart before Comet Lovejoy vanished into the glare of scattered light from the sun. Using these images, scientists believe the coma — the visible cloud of ice and dust surrounding the comet — is about 450 miles across. Since the comet is so much fainter than the sun, the SOT’s original image showed a saturated, over bright sun next to the comet; this image has been processed with a replacement, less bright image of the sun that was captured simultaneously. Credit: JAXA/Hinode/LMSAL

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Source: Tony Phillips, Science@NASA