The fate of comets nose-diving towards the Sun and how easily they give up their mass can be predicted according to how deep into the Sun’s atmosphere they plummet, say scientists presenting their research at the National Astronomy Meeting today.

Comets spend most of their lifetime in the cold outer reaches of the Solar System before being dislodged from orbit and sent headlong towards the Sun. While some comets will loop around the Sun relatively unscathed, putting on beautiful displays as their gases and ices heat up and stream out into a tail, others may not be quite so lucky. Professor Emeritus John Brown, Astronomer Royal for Scotland presented research at the National Astronomy Meeting in Manchester today that predicts which comets will fizzle up in the Sun’s outer atmosphere and which ones will plunge much deeper.

“Three things can happen,” he tells Astronomy Now. “Small comets tend to vaporize in sunlight quite high up and before they get near the Sun. The second type are large enough to go further in and they vaporise in sunlight but they do it in the Sun’s lower atmosphere. The third type no one has ever seen before and they go deeper into the atmosphere, but they burn up not by sunlight but by friction. They melt and explode in the lower Sun’s atmosphere very quickly.”

Two examples of the second type Brown describes were seen by NASA’s Solar Dynamics Observatory last year. C/2011 N3 burned up after passing 100,000 kilometres above the solar photosphere, while a larger comet, C/2011 W3 (Lovejoy), surprised onlookers by surviving passage at a similar altitude of 140,000 kilometres.

The observations were in line with Brown’s predictions in the way comets give up their mass, momentum and energy according to how deep into the Sun’s atmosphere they plummet. “Some just fizzle out in the corona over about an hour, while others that go into the chromosphere explode like a solar flare in seconds,” he explains, adding that the heavier they are the longer they survive, so by tracking how long they survive you can estimate how heavy they are.

Since comets are believed to date from the earliest epoch of Solar System formation, probing these icy snowballs as they meet a fiery demise could give up vital clues to how planetary bodies first formed. “Potentially the most exciting thing is that as the whole comet burns up, vaporizes and explodes you’ll see a spectra of the whole structure, so you'll potentially see the chemical abundances right through the whole mass of the comet,” says Brown. “Everything we’ve done so far has just been scratching the surface.”

Watching comets skim through the atmosphere also reveals details of the upper solar atmosphere. As Comet Lovejoy shot through the atmosphere it passed through dense strips of solar magnetic field, causing the comet’s initially straight tail to briefly lash like a whip. “We’ve been given new probes into the solar atmosphere as well,” adds Brown.

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