The twilight comet: Comet PanSTARRS

Over the last few nights, bright Venus, Jupiter and the lovely crescent moon have been capturing people’s attention in the western sky. But hidden from view has been an interloper making a rare trip through the inner solar system.

Comet PanSTARRS (C/2014 Q1) follows a 38,000-year orbit around the sun. Most of the time it is adrift at the outer edge of the solar system, but on July 6 it reached perihelion, which is to say that its orbit brought it past the sun.

At that time it travelled 48 million km from the sun (that’s within the orbit of Mercury) and its brightness peaked at a magnitude of 4. It passed closest to Earth on July 20, and is now heading away from the sun and transitioning into the night-time sky.

The comet has been too faint to be seen with the naked eye against the twilight sky. But it can be viewed with binoculars or a small telescope over the next few weeks (see here for a finder chart).

Although it is fading in brightness, the comet is also slowly drifting into darker skies as it moves a little higher in the west each night and stays in the sky for longer after sunset.

A comet with three tails?Rob Kaufman

Two-pronged comet

Comet hunters across Victoria have been taking advantage of some lovely clear skies to photograph the comet. It’s very photogenic, with three stunning and distinct comet tails – an ion tail and a dust tail that both point away from the sun, and then a second unusual dust tail that pokes out perpendicularly to the main tail.

Amateur astronomer Michael Mattiazzo suggests that this unusual tail may be the result of a massive dust shedding around the time of perihelion.

A comet produces its magnificent tail when it ventures close to the sun. The comet heats up and sheds material into space, which is then blown away from the comet’s nucleus through solar interactions.

Comet PanSTARRS from Swan Hill, Victoria, July 19.Michael Mattiazzo

The straight and narrow ion tail has a characteristic blue glow. It’s formed from gas that has been ionised by the sun’s ultraviolet light. The tail is primarily composed of ionised carbon monoxide and this molecule is responsible for the blue colour.

The ions, being charged particles, are pushed back by the solar wind that streams swiftly past the comet. The solar wind carries the sun’s magnetic field across the solar system and in the images, it is possible to see structure within the comet’s ion tail which may trace variations in the embedded magnetic field.

The dust tails shine yellow with reflected sunlight.Rob Kaufman

Aligned with the ion tail, but much broader in shape, is the comet’s main dust tail. Made of small smoke-like particles, the dust reflects sunlight to provide a yellow shine. The dust is carried away from the comet’s nucleus by solar radiation, which is much weaker than the driving force of the solar wind. The smaller the dust particle, the more easily it is pushed back by sunlight, so the tail tends to be broad and gently curving.

Keeping up with comets

The Southern Comets website, provides great information to stay up-to-date with Comet PanSTARRS and all other comets that are due to appear in our southern skies. Through August as Comet PanSTARRS fades, Comet Catalina (C/2013 US10) will be another binocular comet to spot, appearing towards the south. It’s heading towards perihelion on November 15, when it will be a northern hemisphere target.