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Death of a comet brings new life to science

Dr Amanda Doyle, Feature writer
21 January 2012, 0:00 UTC

Sen—Comets crash into the Sun all the time, but NASA’s Solar Dynamics Observatory has just captured a debut image of a comet passing in front of the Sun before succumbing to a fiery death.

On 4 July 2011, NASA and ESA’s Solar Heliospheric Observatory, known as SOHO, discovered a comet. There was nothing out of the ordinary about this comet, which was given the name C/2011 N3 (SOHO). Like all comets it had a vivid tail stretched out against a dark sky. As SOHO is equipped with a coronograph to block out the intense light of the Sun’s disc, it could not pursue the comet’s journey into the Sun. However the Solar Dynamics Observatory (SDO) was up for the job.

Any attempts by the comet to sneak quietly into the Sun failed. For a twenty minute interval between 5 and 6 July, the Atmospheric Imaging Assembly (AIA) instrument on SDO was able to watch the passage of the comet across the face of the Sun, before its ultimate demise. “The Atmospheric Imaging Assembly on the SDO spacecraft is a technological leap forward compared to the older generation of instruments that we had available: the new telescopes are more sensitive, have high resolution, and take hundreds of images in the same time that the older generation would take a single image,” said Karel Schrijver from the Lockheed Martin Solar and Astrophysics Laboratory, who is lead author of the Science paper documenting the event.

“The comet that we detected was unusually heavy and bright: we looked back at other comets seen to fall towards the Sun in the same orbit over the 1.5 years that SDO had been in orbit when we saw C/2011 N3, but none of the others showed up. So it really was a combination of a good instrument and luck,” Schrijver told Sen. A large comet that approaches the Sun will start to evaporate and this is displayed by a magnificent tail streaming majestically behind the comet’s nucleus. The large comet can then swing around the Sun and continue along its journey unscathed. One unique example is Comet Lovejoy, which surprised scientists by actually passing through the Sun’s atmosphere in December 2011 and coming out the other side intact. Smaller comets will evaporate before they get too close to the Sun. However, C/2011 N3 was not too big, not too small, but just the right size so that it evaporated at its closest approach to the Sun, allowing the event to be witnessed.

“This became the first time we had seen a comet go across the face of the Sun,” said Dean Pesnell, project scientist for SDO at the Goddard Space Flight Center, for the Science podcast. “It was the first time that we had seen a comet in these kinds of telescopes that we use to study the Sun, and it was the first time that we had seen a comet literally just evaporate away in front of us.”

The comet travelled at a speed of around 600 kilometres per second through the scorching hot corona of the Sun, flying only 100,000 kilometres above the solar surface. Knowing both the time it took for the comet to disintegrate, along with amount of space crossed, meant that the astronomers could work backwards to estimate the mass of the comet before it plunged into the Sun. This is the first time such a method has been used to estimate a comet’s mass.

Both Comet Lovejoy and C/2011 N3 are sun grazing comets, otherwise known as Kreutz family comets. It is believed that they are all fragments of a larger comet, which could have been up to 100 kilometres across, that broke apart thousands of years ago as it neared the Sun. Of the 2000 or so comets that SOHO has detected, around 1400 of those are Kreutz comets. Most of the Kreutz comets are around 100 metres across and SOHO spots one every few days.

The tail of C/2011 N3 stretched for thousands of kilometres behind a fragmented nucleus. Several fragments of the nucleus can be seen in the images, and it is thought that the rapid variation of light coming from the tail was due to the rate that the icy nucleus vaporised. “These fragments appear to change rapidly in the image sequence, while the total intensity of them all together goes up and down by as much as a factor of four in just one to two minutes,” said Schrijver. “Taken together, this is evidence that the comet not only evaporates because of the intense heat of the Sun, but is in fact breaking up in the process.”

Observing the comet cross the Sun was a surprising event, but even more surprising was the fact that the comet was bright. Comets are expected to be bright against the night sky, and dark when in front of the Sun’s luminous face. However, the comet’s tail clearly showed up when it was in front of the Sun, glowing brightly as it was heated by the corona. The exact reason for the glowing tail has yet to be deciphered, and astronomers will eagerly await another opportunity to observe such a comet.