The green comet Nishimura has been a source of excitement to astronomers since its discovery in August, but it is now heading away from us, slowly disappearing from view. It won’t be back for another 400 years, but luckily we know of a few other comets on their way towards Earth that will be visible over the next few months.
Comets are balls of dust and ice that come from the outer reaches of the solar system, in the cold climes far beyond the orbit of Neptune. They become visible from Earth when their orbits bring them near to the sun. The warmth of the sun’s rays turns their ice into a charged gas called a plasma, creating a plasma tail that stretches away from the comet. Dust also evaporates, creating a dust tail. This is what gives comets their recognisable shape: a nucleus in the centre with two long tails typically stretching for a few hundred thousand kilometres behind.
We usually know months or years ahead of time when a comet will appear, but sometimes, as happened with Nishimura, they surprise us. Nishimura was discovered just a month before its closest approach to the sun.
Short-period comets, which take just a few years to orbit the sun, are usually known to us because we have seen them many times before, while long-period comets like Nishimura, with orbits lasting hundreds of years, can turn up unexpectedly.
A handful of comets are known to be passing by the sun in the next few months and will become visible from Earth. It can be tricky to predict how bright a comet will appear in the night sky, but it is likely that you will need binoculars or a small telescope to see all of them – unless another bright surprise, like Nishimura, turns up.
Also known as 103P, comet Hartley was first spotted in 1986 at Siding Spring Observatory in Australia. It is a small, peanut-shaped comet about 1.6 kilometres across that takes about 6.5 years to orbit the sun. It last passed by in 2017, and the time before, in 2010, it was caught on camera by NASA’s Deep Impact spacecraft.
Hartley can already be seen with the aid of binoculars or a small telescope, but it will be most visible around 12 October. In the northern hemisphere on this date, it will be in the constellation Auriga, which contains the bright star Capella. It will be visible from the southern hemisphere too, but will be harder to spot due to being lower in the sky.
Comet Encke, also known as 2P, has one of the shortest periods of any known comet, with an orbit that takes it past the sun every 3.3 years. Its latest swing brings it closest to Earth on 24 September, but it will be easiest to see on 22 October, when it is closest to the sun. Like Hartley, it will be slightly too dim to see with the naked eye, but it will be visible through binoculars, and only in the morning just before sunrise. Encke was first spotted in 1786 and was named after German astronomer Johann Franz Encke, who calculated its orbit in 1819.
This year, comet Tsuchinshan, also known as 62P, will be a Christmas comet. Its orbit, which lasts around 6.5 years, will bring it closest to the sun on 25 December, when it will be visible from the northern hemisphere through binoculars, in the constellation Leo. It will then travel towards Earth, getting closest to us on 29 January.