2013 holds the promise of not 1 but 2 bright comets! Comet ISON will be best visible in November and December 2013 (see: Comet ISON Discoverd). The second comet is named Panstarrs (C/2011 L4) and it will grace the evening sky in March.
Comet Panstarrs was discovered in June 2011 by the Panoramic Survey Telescope and Rapid Response System (PannSTARRS) in Hawaii. Since then, the comet has slowly grown brighter as it approaches the inner Solar System. Between June 2011 and May 2012, its apparent magnitude increased from 19 (extremely faint) to 13.5 (visible in large amateur telescopes). By August, it was magnitude 11.5.
Comet Panstarrs will be closest to Earth on March 05 (distance of 1.09 astronomical units) and it reaches perihelion (closest point to the Sun) 5 days later on March 10. It is during this period that Panstarrs is at its brightest, but just how bright will it be?
From one analysis of 35 measurements made between between February and October 2012, the comet’s brightening trend indicates that it should reach a maximum magnitude of -1 (as bright as Sirius, the brightest star in the sky) and should be visible in a dark sky a week later near magnitude +1 with a 10° to 20° long tail. This would place it in the exclusive category of brightest comets seen in the past 100 years. But predicting the brightness of comets is fraught with uncertainty. To understand why, we need to known something about the physical nature of comets.
The nucleus of a comet is a solid body typically several kilometers in diameter. It is composed of a mixture of frozen gasses (water, carbon dioxide, ammonia, carbon monoxide, methane, etc.) and dust – this is known as the dirty snowball model. The exact proportions of the gasses and dust, and the structure of these components, varies from comet to comet.
As a comet approaches the Sun, solar radiation slowly vaporizes the outer layers of the nucleus, spewing gas and dust particles into space. This expanding cloud of material forms an enormous bubble around the nucleus called the coma, which can be over a million kilometers in diameter. Solar radiation pushes against the coma and forces dust particles to stream away from the Sun to produce the dust tail. The solar wind – a continuous stream of electrically charged particles from the Sun – interacts with cometary gas to produce ions (electrically charged atoms). The ions also stream away from the comet in the direction opposite the Sun to form the ion tail. Both of these tails are visible in telescopes, and – if the comet is especially bright – to the naked eye. The tail of a bright comet may be over 100 million kilometers long!
Predicting the future brightness of a comet is a notoriously difficult business because it depends on the exact size, structure and composition of the nucleus as well as how rapidly it rotates. This information is essentially unknown, so brightness estimates are largely based on a comet’s past behavior during the previous months. But just like the stock market, “past performance is no guarantee of future results”. This is especially true since the comet heats up more rapidly as it gets increasingly closer to the Sun.
Comet Panstarrs has a slightly hyperbolic orbit suggesting it is a new comet from the outer Oort Cloud and making its first encounter with the Sun. In the past, comets with similar orbits have shown rapid brightening as they approach the Sun, thereby promising spectacularly bright apparitions as they pass perihelion. Unfortunately, the growing brightness of these comets quickly tapers off as a thin veneer of fresh volatiles surrounding the nucleus evaporates into space. This is exactly what happened to the over-hyped Comet Kohoutek in 1973.
Comet Panstarrs’ observed rate of brightening has slowed in December and January, leading to a new projected maximum brightness of magnitude +3 (as bright as the stars in the “Big Dipper”) instead of magnitude -1. This would still make Panstarrs visible to the naked eye, but not nearly as impressive.
So which brightness prediction is correct? It’s still anyone’s guess given the capricious nature of comets. Just make sure you watch the evening sky shortly after sunset throughout mid-March.