SOHO Supersonic snowballs: Comets interacting with the Sun's atmosphere

This coronagraph image from the Solar and Heliospheric Observatory (SOHO) shows Comet Lovejoy receding from the sun after its close encounter.

The horizontal lines through the comet's nucleus are digital artifacts caused by saturation of the detector. Yes, Lovejoy is that bright! To view a movie of Comet Lovejoy's path please go here. 

Since the 1980s astronomers have seen thousands of comets falling towards the Sun, most of them too small to survive a close approach, let alone to re-emerge.

Until recently no such objects had been seen very close to the Sun as the glare of sunlight made them impossible to observe.

Now a team of scientists led by Professor Emeritus John Brown, Astronomer Royal for Scotland (Edinburgh's Royal Observatory) and former Regius Professor of Astronomy at Glasgow University, have worked out which comets make it through this fiery journey, which fizzle out high up and which explode just above the surface.

Prof. Brown presented this new work in a paper at the National Astronomy Meeting in Manchester on Friday 30 March.

Comets are giant dusty snowballs believed to date from the epoch of the formation of the Sun and planets, so carry important information about the early history and composition of the Solar system.

The comets we see spend most of their time very far from the Sun, orbiting in the so called Oort Cloud, before being disrupted into orbits that carry them towards our nearest star over tens of thousands of years.

When comets reach the inner Solar System, their dusty ices melt and vapourise to form huge tails blown back by the solar wind and by sunlight.

The largest, like the famous Comet Hale Bopp seen in the late 1990s, have nuclei tens of kilometres across and masses of 10 million million tonnes.

Objects this large only lose a tiny fraction of their material on each passage around the Sun, so are able to survive thousands of journeys through the Solar System.

In contrast, the smallest objects may only be 10 metres across with a mass of 1000 tonnes. If these small comets make a close approach to the Sun, they are vapourised by sunlight and by the friction of the atmospheric gas.

In the culmination of work carried out over the last few years, Professor Brown and his colleagues are now able to predict how comets lose their mass and are destroyed in the solar atmosphere, their behaviour depending on whether or not their orbital path reaches into the 'lower atmosphere' 7000 km (roughly 1% of the solar radius) from the top of the brightest visible solar layer, the photosphere.