What shape are comets?

The classic shape that most people think of when imagining a comet is that of a big fuzzy blob with a long tail. In reality, the fuzzy blob is actually a thinly spread but large cloud of gas and dust blown out from the comet as it is heated as it nears the sun. This cloud is called the 'coma'. The tail is caused by the solar wind blowing the comet's thin atmosphere away from it.

A comet's coma only starts to form when it gets close enough to the Sun, and is many times larger than the nucleus (the 'middle', solid bit of the comet), which is usually only a few kilometres in diameter.

What shape is a comet's nucleus?
As of last week, five comets have had their nucleus photographed by spacecraft. These are:

1986: Halley's comet, photographed by Giotto

Halley's Comet Nucleus (ESA probe Giotto, 1986)

2001: Borrelly, photographed by Deep Space 1

By NASA/JPL, via Wikimedia Commons

2004: Wild 2, photographed by Stardust

By Davodd at en.wikipedia, from Wikimedia Commons

2005: Tempel 1, photographed by the Deep Impact impactor

By NASA/JPL-Caltech/UMD, via Wikimedia Commons

2010: Hartley 2, photographed by EPOXI (a re-named Deep Impact)

No photo for this one, just a short but sweet (and very cool) video compiled by the EPOXI team from photographs taken by the spacecraft:

So, as you can see from the photos and video above, comets are actually sort of lumpy, bumpy, potatoey, peanutty shapes. And that's the scientific term for it*.

* It's not really.