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Astronomers capture
comet's dive into Sun

Astronomy Now
Posted: 20 January 2012

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The first observations and analysis of a comet disintegrating within the Sun's atmosphere on 6 July 2011 are presented this week in the journal Science.

Comet C/2011 N3 was first discovered by the Solar and Heliospheric Observatory (SOHO) on 4 July 2011, and an alert was sent to Lockheed Martin Solar and Astrophysics Laboratory (LMSAL) scientists to watch with NASA’s Solar Dynamics Observatory (SDO) as the comet neared the Sun. NASA's twin Solar-Terrestrial Relations Observatories (STEREO) also tuned in to watch events unfold.

Comet C/2011 N3 plunges towards the Sun in front of the SOHO's LASCO C2 camera in July 2011. The Sun's bright light is blocked out by a coronagraph. Image: SOHO (ESA & NASA).

Usually comets are too dim to be seen against the glare of the Sun but, hailing from a group of ultrabright comets known as the Kreutz comets, C/2011 N3 broke all the rules. Traveling at nearly 650 kilometres per second, it was tracked for 20 minutes until it disintegrated just 100,000 kilometres above the solar surface into chunks of ice and dust.

"Most comets evaporate well before coming near the Sun," says solar physicist Karel Schrijver, lead author of the Science paper and SDO's AIA principal investigator. "Some have been seen to reach about three solar radii from the Sun when they disappeared behind a disk placed in front of the Sun in coronagraphs so that the sky can be viewed without blinding the instrument, but none could be observed while evaporating close to the Sun until we observed C/2011 N3 with SDO's AIA (Atmospheric Imaging Assembly) instrument."

The observations gave the scientists a unique opportunity to study properties of the comet otherwise difficult to obtain. “As we witnessed this comet evaporate as it traversed a known amount of space over a specific period of time, we were able to work backward to estimate its mass just before it reached the Sun," says Schrijver.

The team bracket its size as a maximum 90 metres across, weighing as much as an aircraft carrier before it broke apart into smaller fragments varying in size from 10 to 45 metres. The pieces remained embedded within the comet's nebulous coma, a diffuse cloud of ice, dust and gas that surrounds the nucleus of a comet. Streaming out from the comet's core was a tail extending 16,000 kilometres, seen to vary in brightness as the comet broke apart.

“I think the light pulses in the tail were one of the most interesting things we witnessed,” says 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. Only because of these pulses can we measure how fast the tail falls behind the comet as its gases collide with those in the Sun’s atmosphere. And that, in turn, helps us measure the comet’s weight.”

During 15 years of operations, SOHO has witnessed more than 2,000 comets approaching the Sun, and as well as opening up new ways to study these outer Solar System objects closer to home, these so-called Sun-grazing comets also provide insight to solar activity.

"Observing Sun-grazing comets opened up a way to learn something about how the solar wind is driven," explains Schrijver. "Since C/2011 N3, a second and even brighter comet (Lovejoy) was sighted to enter and successfully leave the Sun's corona on December 15-16, 2011. As it was almost half a year later, and the Earth had moved to almost the opposite side of the Sun in its orbit, the perihelion of Lovejoy lay behind the Sun. But SDO observed the approach and then the departure from the low corona in exquisite detail: we saw the comet's tail form striations, guided by the Sun's magnetic field, and we saw it bend rapidly as the comet moved through the high solar corona."

Schrijver adds that they are working with an international team of experts to use these observations to learn how the Sun manages to sustain a hot, million-degree, fast solar wind. "We are hoping to use these Sun-grazing comets as probes of the high solar atmosphere."

Watch the unique video of the comet as it passes in front of the Sun here.

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