Comet dies on film, leaving trail of mystery tagged: , , , , , ,

Comet dies on film, leaving trail of mystery

Posted by in Space

Courtesy of NASA and World Science staff

A comet has been caught doing something never seen before: die a scorching death as it flies too close to the sun, scientists say.

That a comet met this fate was no surprise, but the chance to watch it first-hand – in a video taken July 6 – amazed even seasoned comet watchers, and left many scratching their heads.

‘Comets are usually too dim to be seen in the glare of the sun’s light,’ said Dean Pesnell at NASA’s Goddard Space Flight Centre in Greenbelt, Md. ‘We’ve been telling people we’d never see one’ from the agency’s Solar Dynamic Observatory, which filmed the video and where he is project scientist.

The gleaming comet overturned these assumptions, but exactly how it managed this is unclear, the investigators said.

The object – from a group of comets known as the Kreutz comets – was seen moving in over the right side of the sun, disappearing 20 minutes later as it evaporated. The movie is more than just a novelty, astronomers said. As detailed in a paper in the journal Science published 20 January, watching the disintegration provides a new way to estimate the comet’s size and mass. It turned out to be between about 50-100m long, and about the weight of an aircraft carrier.

Evaporating as it got closer to the Sun

‘It was moving along… and was literally being evaporated away,’ said Karel Schrijver, a solar scientist at Lockheed Martin in Palo Alto, California, co-author of the paper and principal investigator of the Atmospheric Imaging Assembly instrument at the observatory.

Typically, comet-watchers see the Kreutz-group comets only through images snapped by coronagraphs – special telescopes that view the Sun’s atmosphere, or corona, by using a round obstruction to block the direct blinding sunlight. On average, a new member of the Kreutz family turns up every three days, with some of the larger members being seen for some 48 hours or more before disappearing behind the obstructing disk. They are never seen again. They obviously disintegrate when they get close to the Sun, according to researchers, but this had never been witnessed directly.

On 6 July, Schrijver spotted the bright comet in a coronagraph produced by the Solar Heliospheric Observatory, or SOHO. He looked for it in images from his Solar Dynamic Observatory and much to his surprise found it. Soon a movie of the comet circulated to comet and solar scientists.

Sceptical at first, but then…

Karl Battams, a scientist with the Naval Research Laboratory in Washington, DC, who has studied many comets with SOHO and is also an author of the paper, was sceptical when he first got 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.’

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, researchers reason.

The paper describes the comet and its last moments as follows: it was travelling some 640km/second and made it to within 99 200km 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 the camera, the comet weighed an estimated 50 million kilograms. It had also broken up into a dozen or so large chunks with sizes estimated between 10-50m. These were embedded in a ‘coma’ – the fuzzy cloud surrounding the comet – about 1300km, and followed by a glowing tail some 16 000km long.

A pulsing tail

It is actually the coma and tail seen in the video, not the comet’s core. Close examination shows that the light in the tail pulses, getting dimmer and brighter alternately. The team suspects these fluctuations result from successive break-ups 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,’ said Schrijver. ‘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.’ Figuring out exactly why this happens is one of the mysteries remaining.

High on the list is to answer the not-so-simple question of why we can see the comet at all, scientists said. Certainly, a few basic characteristics of this situation help. For one, this comet was big enough to survive long enough to be seen, and its orbit took it right across the face of the Sun. It was also, said Battams, probably one of the top 15 brightest comets seen by SOHO, which has observed over 2 100 sun-grazing comets to date. The filming camera also contributed by taking a picture every 12 seconds, whereas most other similar instruments capture images only every few minutes.

Ultimately, however, the fact that one can see this comet against the background of the sun means there is some physical process not yet understood, Pesnell said. ‘Normally,’ he explained, ‘a comet passing in front of the sun absorbs the light from the sun. We would have expected a black spot against the sun, not a bright one.’ Figuring out this question should offer information not only about material in the comet, but also about the sun’s atmosphere – and so this opens up the door to a new niche of study, according to astronomers. Assuming, of course, that one can spot some more comets.

This image is from a computer artist’s animation of the break-up of a single comet near the Sun. The fragmenting comet is the white object on the right. Many of the SOHO comets are believed to be fragments of larger comets. (Image: NASA/Walt Feimer)

Source: World Science, http://www.world-science.net