AuroracastUKLearn
Comets
Leftovers from the building of the solar system, most of them frozen solid until they come close.
Seeing it from the UK
Most comets are a job for binoculars, and even then you are looking for a small grey smudge rather than the picture above. A genuinely naked eye comet turns up perhaps once a decade, and when one does it is worth the drive out of town. The thing to know is what it will not do: a comet does not shoot across the sky. It sits there like a faint fuzzy star and only moves against the background over nights, so if something darts past in a second you have seen a meteor, not a comet. Look after dusk or before dawn, since a comet bright enough to see is usually close to the Sun and therefore low.
A comet is a lump of ice, dust and rock left over from the solar system being built, and it has been in the freezer ever since. Most are only a few kilometres across, which is nothing, and for almost all of their lives they are dark and invisible somewhere out past the planets.
What makes one worth looking at is coming close to the Sun. The ice does not melt, it goes straight from solid to gas, and as it does it carries the trapped dust out with it. That is the bright head you see, a cloud of gas and dust around a tiny solid core, and it can end up larger than a planet even though the thing producing it is the size of a town.
There are usually two tails and they point in slightly different directions. The dust tail is the broad curved one, pale yellow, made of grains pushed gently by sunlight. The gas tail is straighter and bluer, blown back hard by the solar wind, the same stream of particles that causes the aurora. The thing that catches people out is that both tails point away from the Sun no matter which way the comet is travelling, so on the way out a comet flies tail first.
Some come back on a schedule. Halley returns every 76 years and is due in 2061. Others, like the one photographed here, are on orbits so long they are effectively one time visits: NEOWISE will not be back for about 6,800 years. How bright any of them gets is famously hard to predict, because it depends on how much ice is left near the surface, and comets have a long history of disappointing everybody.
The numbers
- What it is
- Ice, dust and rock left from the solar system forming
- Size of the solid core
- Typically a few kilometres
- Size of the head
- Can be larger than a planet
- Which way the tail points
- Away from the Sun, always
- Halley’s Comet
- Every 76 years. Next in 2061
- NEOWISE, pictured
- July 2020. Back in about 6,800 years
To find out whether it is up tonight, and when it gets properly dark where you are, see Sky Tonight.