On July 6, 2011, solar physicist C.J. Schrijver of the Lockheed Martin Advanced Technology Center and colleagues became the first to directly witness a comet falling within the solar corona, a sort of blazing-hot atmosphere that surrounds the sun. Labeled C/2011 N3 (SOHO), the comet is from the Kreutz family, the source of about 80 percent of the comets that pass so close to our star. The comet, moving at roughly 1.3 million miles per hour, was only visible to scientists for 20 minutes before vaporizing.
Schrijver and his colleagues describe their observations of the destructing comet in a study released in Science this week. A series of extreme ultraviolet (EUV) images captured by the Atmospheric Imaging Assembly (AIA) on NASA?s Solar Dynamics Observatory (SDO) satellite enabled researchers to document the comet?s death plunge into the sun.
Before astronomers had the AIA, those studying the sun had just one megapixel camera at their disposal and could take a picture of a sun-grazing comet only about once every 5 to 15 minutes. But the 16-megapixel cameras aboard SDO can capture eight pictures every 10 seconds, and a dedicated ground station allows for much more information to be sent down from the satellite at one time. The sequence of frames enabled Schrijver and his team to track the comet?s glowing tail as it moved within about 62,000 miles of the solar corona before disintegrating.
Schrijver?s study provides a fascinating window into something that had always been hidden from view by the sun?s intense radiation. But researching the death throes of these sungrazing travelers could reveal some surprises about the sun and comets.
The researchers think the comet glows as it tears apart because charge-exchange collisions excite electrons into emitting EUV light as they transition to lower energy states. This glow lets researchers track the tail and thereby estimate the comet?s deceleration, as well as the total mass of the comet that was lost.
From the images, the researchers could also tell that the comet wasn?t one solid body. Instead, it was made of a number of clustered fragments that began to break off the closer the comet moved to the sun. Some of those fragments were larger than 10 meters in diameter.
Sun-grazing comets could also increase our understanding of the sun itself. Schrijver says that a comet can act almost like a probe, moving into otherwise obscured regions of the solar atmosphere and showing what?s going on in there. For example, the researchers suspect there are small variations in the density and speed of the solar wind?the stream of charged particles steadily emanating from the sun?and hope they can determine these variations by seeing how much of the comet?s tail is blown away from the sun?s surface.
These probe comets could also reveal new information about the sun?s mysterious magnetic fields, according to Karl Battams, a researcher at the Naval Research Lab and co-author on the paper. Unlike the earth?s magnetic field, the sun?s is constantly changing, creating sunspots, solar flares, coronal mass ejections, and other general outflows of matter that have a massive effect on earth, Battams says.
On December 15, 2011, an even bigger and brighter comet than C/2011 N3?this one called Lovejoy?cruised into the solar corona. But unlike its predecessor, this comet traveled through the corona and back out again. "[Lovejoy] gave us opportunity to see what the sun?s magnetic field is like," Schrijver says. "We can?t see it, we can only sense it because it does something to material that can glow."
The AIA images open an entirely new field of science that we never knew about before, Schrijver says. "It?s nice that nature gives us a tool to go there?by throwing a big piece of rock into that area and seeing what happens."
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