Monday, December 19, 2011

Lovejoy Survives Solar Passage

I am not so sure that the major constituent of a comet is water.  It is more likely to be a cocktail of frozen gases including water and plenty of elemental carbon.  This provides a plausibly much more survivable package while making the transit and explains the dust halo generated.

Even if it primarily ice, it is certain that repeated transits would have concentrated carbon dust on the surface.  On the other hand, the way the tail moved around suggests a lot of shearing forces are stripping gases and dust away from the comet.  It is dramatic.  Go to the quicktime video on the site to see the action.

At least this certainly provides a new lower limit to orbital survival for comets.  Maybe someday we will be dumb enough to send a vessel along capable of surviving the process.  One wonders at anything that could justify the effort.

Comet Lovejoy Plunges into the Sun and Survives

Dec. 16, 2011: This morning, an armada of spacecraft witnessed something that many experts thought impossible.  Comet Lovejoy flew through the hot atmosphere of the sun and emerged intact.

"It's absolutely astounding," says Karl Battams of the Naval Research Lab in Washington DC.  "I did not think the comet's icy core was big enough to survive plunging through the several million degree solar corona for close to an hour, but Comet Lovejoy is still with us."

The comet's close encounter was recorded by at least five spacecraft: NASA's Solar Dynamics Observatory and twin STEREO probes, Europe's Proba2 microsatellite, and the ESA/NASA Solar and Heliospheric Observatory.  The most dramatic footage so far comes from SDO, which saw the comet go in (movie) and then come back out again (movie).


NASA's Solar Dynamics Observatory caught Comet Lovejoy emerging from its scorching close encounter with the sun. [Entrance movie:Quicktime (22 MB)m4v (0.8 MB)] [Exit movie:Quicktime (26 MB)m4v (0.8 MB)]

In the SDO movies, the comet's tail wriggles wildly as the comet plunges through the sun's hot atmosphere only 120,000 km above the stellar surface. This could be a sign that the comet was buffeted by plasma waves coursing through the corona.  Or perhaps the tail was bouncing back and forth off great magnetic loops known to permeate the sun's atmosphere.  No one knows.

"This is all new," says Battams.  "SDO is giving us our first look1 at comets travelling through the sun's atmosphere. How the two interact is cutting-edge research." 

“The motions of the comet material in the sun’s magnetic  field are just fascinating,” adds SDO project scientist Dean Pesnell of the Goddard Space Flight Center.   “The abrupt changes in direction reminded me of how the solar wind affected the tail of Comet Encke in 2007 (movie).”

Comet Lovejoy was discovered on Dec. 2, 2011, by amateur astronomer Terry Lovejoy of Australia.  Researchers quickly realized that the new find was a member of the Kreutz family of sungrazing comets.  Named after the German astronomer Heinrich Kreutz, who first studied them, Kreutz sungrazers are fragments of a single giant comet that broke apart back in the 12th century (probably the Great Comet of 1106).  Kreutz sungrazers are typically small (~10 meters wide) and numerous. The Solar and Heliospheric Observatory sees one falling into the sun every few days.

At the time of discovery, Comet Lovejoy appeared to be at least ten times larger than the usual Kreutz sungrazer, somewhere in the in the 100 to 200 meter range.  In light of today's events, researchers are re-thinking those numbers.


This coronagraph image from the Solar and Heliospheric Observatory shows Comet Lovejoy receding from the sun after its close encounter. The horizontal lines through the comet's nucleus are digital artifacts caused by saturation of the detector; Lovejoy that that bright! [movie]

"I'd guess the comet's core must have been at least 500 meters in diameter; otherwise it couldn't have survived so much solar heating," says Matthew Knight. "A significant fraction of that mass would have been lost during the encounter. The remains are probably much smaller."

SOHO and NASA's twin STEREO probes are monitoring the comet as it recedes from the sun. It is still very bright and should remain in range of the spacecrafts' cameras for several days to come.

What happens next is anyone's guess.

"There is still a possibility that Comet Lovejoy will start to fragment," continues Battams. "It’s been through a tremendously traumatic event; structurally, it could be extremely weak. On the other hand, it could hold itself together and disappear back into the recesses of the solar system."

"It's hard to say," agrees Knight.  "There has been so little work on what happens to sungrazing comets after perihelion (closest approach).  This continues to be fascinating.”

Author:Dr. Tony Phillips| Production editor: Dr. Tony Phillips | Credit: Science@NASA

Footnote:1"When SDO was launched we thought we would see nothing besides the Sun and the dark disks of the Moon, Earth, Venus, and Mercury in our images," says SDO project scientist Dean Pesnell of GSFC. "No other bright object would be visible because our instruments are designed to look at the Sun. Now we are measuring the mass and composition of comets by turning the comet inside out."

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