Washington: A “sun grazing” comet, which was caught by SOHO’s LASCO C2 camera as it dived toward the sun on July 5 and July 6, 2011, has amazed even the most seasoned comet watchers.
On July 6, 2011, a comet was caught doing something never seen before – die a scorching death as it flew too close to the sun. That the comet met its fate this way was no surprise, but the chance to watch it first-hand amazed all those who witnessed it.
“Comets are usually too dim to be seen in the glare of the sun’s light,” Dean Pesnell at NASA’s Goddard Space Flight Center in Greenbelt, said.
“We’ve been telling people we’d never see one in SDO data,” he said.
But an ultra bright comet, from a group known as the Kreutz comets, overturned all preconceived notions. The comet can clearly be viewed moving in over the right side of the sun, disappearing 20 minutes later as it evaporates in the searing heat.
“Of course, it’s doing something very different than what aircraft carriers do,” Karel Schrijver, first author of the study, said.
“It was moving along at almost 400 miles per second through the intense heat of the sun – and was literally being evaporated away,” he said.
Typically, comet-watchers see the Kreutz-group comets only through images taken by coronagraphs, a specialized telescope that views the Sun``s fainter out atmosphere, or corona, by blocking the direct blinding sunlight with a solid occulting disk.
On an average a new member of the Kreutz family is discovered every three days, with some of the larger members being observed for some 48 hours or more before disappearing behind the occulting disk, never to be seen again.
Such “sun-grazer” comets obviously destruct when they get close to the sun, but the event had never been witnessed.
The journey to categorizing this comet began on July 6, 2011 after Schrijver spotted a bright comet in a coronagraph produced by the SOlar Heliospheric Observatory (SOHO). He looked for it in the SDO images and much to his surprise he found it. Soon a movie of the comet circulated to comet and solar scientists, eventually making a huge splash on the Internet as well.
Karl Battams, a scientist with the Naval Research Laboratory in Washington, DC, who has extensively observed comets with SOHO and is also an author on the paper, was skeptical when he first received the movie.
“But as soon as I watched it, there was zero doubt,” he said.
“I am so used to seeing comets simply disappearing in the SOHO images. It was breathtaking to see one truly evaporating in the corona like that,” Battams said.
After the excitement, the scientists got down to work. Humans have been watching and recording comets for thousands of years, but finding their dimensions has typically required a direct visit from a probe flying nearby. This movie offered the first chance to measure such things from afar.
The very fact that the comet evaporated in a certain amount of time over a certain amount of space means one can work backward to determine how big it must have been before hitting the sun’s atmosphere.
The paper describes the comet and its last moments as follows. It was travelling some 400 miles per second and made it to within 62,000 miles of the sun’s surface before evaporating.
Before its final death throes, in the last 20 minutes of its existence when it was visible to SDO, the comet was some 100 million pounds, had broken up into a dozen or so large chunks with sizes between 30 to 150 feet, embedded in a “coma”, that is the fuzzy cloud surrounding the comet, of approximately 800 miles across, and followed by a glowing tail of about 10,000 miles in length.
It is actually the coma and tail of the comet being seen in the video, not the comet’s core. And close examination shows that the light in the tail pulses, getting dimmer and brighter over time.
The team speculates that the pulsing variations are caused by successive breakups of each of the individual chunks that made up the comet material as it fell apart in the Sun’s intense heat.
“I think this is one of the most interesting things we can see here,” Lockheed’s Schrijver said.
“The comet’s tail gets brighter by as much as four times every minute or two. The comet seems first to put a lot of material into that tail, then less, and then the pattern repeats,” he added.
The paper has been recently published in Science magazine.