Sungrazers shed light on comet break-up
An analysis of comets called sungrazers suggests conventional theories about how comets break up may be wrong, according to a US astronomer.
Dr Zdenek Sekanina, a senior research scientist at NASA's Jet Propulsion Laboratory has studied images of sungrazers - small comets that pass close to the sun - and concluded these may break up far away from the sun.
His research is published in this week's issue of The Astrophysical Journal.
Most sungrazing comets are tiny - the smallest could be less than 10 metres across (compared to other comets with a nucleus of between 1 and 10 kilometres) - and move in a highway-like formation of comets that pass near the sun and disintegrate.
In contrast, the sungrazer images reveal many sungrazer comets arrive at the sun in clusters and on parallel paths. Normally, these tiny fragments would have disintegrated if on an earlier trip they had come so close to the sun. Therefore the parents of these tiny sungrazers must have broken up after their previous encounter with the sun and continued to break up far from the sun on their journey through the solar system, according to Dr Sekanina.
The idea of comets breaking up in a fairly orderly pattern is a new one, Dr Sekanina writes, describing how one comet cascades into large families of smaller comets in a process that could be an important part of a comet's natural life cycle.
This replaces a previous theory that comets broke up occasionally and only very close to the sun, as the strong pull of gravity cracked their loosely piled chunks of dust and ice.
In the past seven years of observation by The Solar and Heliospheric Observatory (SOHO), about 400 sungrazers in the immediate vicinity of the sun have been observed, although Dr Sekanina estimates there may currently be as many as 200,000.
The sungrazers move in tight pairs and occasionally SOHO has been able to capture them simultaneously. By devising a standard model for split comets using the offsets between the sungrazers, Dr Sekanina is able to work out when they separated and when their 'parents' did.
It is not just sungrazers that can break up far from the sun, notes Dr Sekanina. New observations of comet 57P/du Toit-Neujmin-Delporte show fragmentation has also resulted in a 'highway' of tiny comets. The separation occurred beyond the orbit of Mars.