Saturday 10 March 2012

Comet C/2011 W3 (Lovejoy) survives a close encounter with the sun.

Comet C/2011 W3 (Lovejoy) was discovered in November 2011 by Australian Terry Lovejoy, an amateur astronomer and astrophotographer. It was quickly discovered to belong to a group of comets known as Kreutz Sungrazers, which have extremely eccentric, long period-orbits that take them very close to the Sun. A typical Kreutz comet takes over a thousand years to orbit the Sun, is more that 100 AU at its aphelion (more than 100 times as far from the Sun as the Earth at the furthest point in its orbit, and passes within a few solar radii of the Sun at its perihelion (the closest point in its orbit to the Sun. Since orbiting objects speed up when they are close to the Sun and slow down when they are far away, this means that Kreutz comets only make rare and brief visits to the Inner Solar System. It is thought that the Kreutz Group comets are the result of the breakup of a single large object; since passing very close to the Sun the sun is unhealthy for a comet (an object made largely of ice), this breakup event is thought to have happened in the fairly recent past.

The orbit of C/2011 W3 (Lovejoy). Wikimedia Commons.

On 16 December 2011 C/2011 W3 (Lovejoy) reached its perihelion, coming within 1.2 solar radii, and disappearing behind the Sun as seen from the Earth. It was not thought that the comet would survive this encounter, but it re-emerged from the other side of the Sun, and continued on its journey back into the Outer Solar System.

This came as somewhat of a surprise to astronomers. Not only had C/2011 W3 (Lovejoy) been exposed to extreme heat from the Sun, but extreme tidal forces. In 1992 comet Shoemaker Levy 9 was torn apart by a close encounter with the planet Jupiter, but C/2011 W3 (Lovejoy) had survived a much closer encounter with the much larger Sun, and survived largely unscathed.

Comet C/2011 W3 (Lovejoy), bottom left, close to the Sun. Image taken by NASA's Hinode Satellite.

In a paper published in the online arXiv database at Cornell University Library on 8 March 2012, and submitted to the planetary science journal Icarus, a team of scientists from the Institut für Geophysik und extraterrestrische Physik at the Technische Universität Braunschweig, led by Bastian Gundlacha, propose an explanation for the survival of C/2011 W3 (Lovejoy).

Gundlacha et al. propose that C/2011 W3 was able to survive the tidal influence of the Sun because of the solar heat it was simultaneously subjected too. This caused material to erupt from all over the surface of the comet, effectively the equivalent of a rocket on every point on the surface of the comet, all pushing inwards towards the center. This pressure held C/2011 W3 together despite the tidal forces exerted upon it by the Sun, whereas Shoemaker Levy 9 was cold and inert at the time of its encounter with Jupiter, and was therefore torn apart by the much weaker tidal forces from the planet.

The forces exerted on comet C/2011 W3 at the time of its encounter with the sun. Although tidal forces were pulling the comet apart, this was more than compensated for by the pressure generated by material evaporating from the surface. From Gundlacha et al. (2012).