Comet Lovejoy makes its closest approach to the Sun at the end of January. It is unusual for us to be able to observe a comet so close to its 'perihelion', as they are usually too close to the Sun in the sky to observe. We are lucky with this comet as it isn't actually getting that close to the Sun. Unlike comet ISON last year, which grazed the Sun at 1/100th of the distance the Earth is from the Sun, Lovejoy remains outside the Earth's orbit at closest approach. The orbits of Lovejoy and the Earth line up very well, putting the comet the opposite side of the Earth from the Sun, as this image shows.
This comet is a long period comet, visiting the Earth every 10 000 years or so. At the other end of its orbit, it is over 1 100 times further from the Sun than the Earth. It takes light from the Sun a week to travel out to that distance and from that far away, the Sun is fainter than a full moon on Earth.
Imaged through a colour filter, the head of the comet has a clear green hue. This colour comes from ionised Carbon as it is excited by ultraviolet light from the Sun. The blue tint of the tail is as a result of Carbon Monoxide ions.
Comets usually have two tails: a dust tail and an ion tail. The dust tail is left in a trail behind the comet as it travels around the Sun. It is made of inert particles of rock and dust thrown of by the comet as it is heated by the Sunlight. As well as inert particles, the comet also throws off ions: charged particles. These charged particles are influenced by the charged solar wind and always point away from the Sun.
This comet is unusually dustless, and its dust tail is not really visible. What we can see is the ion trail. At this point in its orbit, the ion tail is actually perpendicular to its direction of travel. Once it passes its closest approach and starts to move away from the Sun, its ion tail will actually be pointing ahead rather than behind.
The ion tail is rapidly evolving and changes in appearance from night to night.
Observing tips:
- Observe using Cluster Cam to see its full tail
- Using RGB Colour filter will show the distinctive colour of the comet
- Using a Clear filter gathers the maximum light so may show faint details better
- As the tail is quite faint, use an exposure time of 120 000 ms (or more) to see it clearly.
- Catch it quick as it will no longer be observable by March
