# Why do comets appear to have a continuous supply of dust?

Comets orbiting the sun have a tail that points away from the Sun, due to the solar wind.

(Strictly speaking, a comet can have both a type I tail - ion tail, and a type II tail - a dust tail. This question is about the type II tail - the dust tail).

Now I understand that some of the source of the dust is volatile materials in the comet oxidising due to the comet's proximity to the sun. But surely some of these comets have been orbiting for hundreds of thousands of years (if not millions). Why do they still have unspent fuel? Why are they still dusty? Hasn't all the dust blown off by now?

My question is: Why do comets appear to have a continuous supply of dust?

• The dust ejected is a small fraction of the overall comets mass, and the proportion of a time a comet spends close to the sun is low compared to the duration of their orbits, so they don't always appear so "dusty". – Dean Mar 7 '16 at 12:27
• My outside take on it is that once they run out of exposed surface volatiles to supply a tail or coma in strong sunlight, it is simply no longer classified as a comet. This as a matter of our conventional historical accounting rather than as a matter of relevantly describing nature as it is understood today. And a comet turns off when most of the volatiles on its surface has sublimated. Like snow plowed up at a street corner. In early spring the surface snow melts, which increases the concentration of sand and dirt on its surface, blocking sunlight and slowing down further melting. – LocalFluff Mar 7 '16 at 15:03
• Don't know enough to call this an answer, but when the comet is inactive, wont it "gather" a little "dust" as it's essentially the only mass around. Not a lot of dust, but If it "burns" up 100kg of dust per orbit and gains 2kg of dust per orbit this would stretch out it's lifespan. – coteyr Mar 7 '16 at 16:42
• It's a vacuum, no oxygen- does it actually burn? – user11095 Mar 7 '16 at 20:40

The rate of loss of mass from a comet is perhaps surprisingly low. A paper The calculation of $Af\rho$ and mass loss rate for comets gives a rate for a "typical" comet at 1.29AU as 153kg/s. A typical comet has a mass of about $10^{13}kg$. Even if the comet is constantly active (and they are not) it would have enough material for over 2000 years.