Question of the Week Podcast

Question of the Week episode

Thu, 4th Apr 2013

How long does it take for comets to melt?

Comet Hale-Bopp as it flies over the sky of Pazin in Istria, Croatia (c) Philipp Salzgeber -

We find out how long it takes for a comet to melt and ask could we ever get a cometless sky? Plus we wonder the power of plants.

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  • How long does it take for comets to "melt"?

    How long does it take for a comet to completely 'melt' away from repeated exposures to the sun? (Composition and size would certainly have something to do with it, I'm sure) Will there come a time when all the comets are gone from the Solar System because they have sublimated?



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At one limit, just one approach, if the comet has an extremely elliptical orbit that takes it directly into the Sun.
At the other extreme, a comet on a circular orbit well beyond Pluto will hardly sublimate at all. evan_au, Fri, 22nd Mar 2013

There are some estimates that periodic comets such as Halley's Comet will eventually melt away.  Estimates vary from 10,000 years to 10 million years, and that it may have already lost as much as 90% of its mass during the last few million years.

Measurements of the age of Comet Wild 2 indicate that it likely was formed at the same time the solar system was formed, or or about 4.5 billion years ago.  So, either the comets orbits vary significantly over their lifetime, or they may have some extraordinary longevity.

One possibility with a comet such as Halley's comet is that it might spend a few months loosing material as it passes the sun, then it may spend the next 75 years vacuuming up micrometeorites, and small molecules from its path.  And, thus the overall loss may not be as much as might be expected.  However, with a return every 75 years, it could take centuries of precise observations to get precise data about its loss.
CliffordK, Fri, 22nd Mar 2013

Comets have been called "dirty snowballs", and it is evaporation of the volatiles as they approach the Sun which give comets their characteristic tails.

If a comet on a very elliptical orbit meets with another orbiting object (eg micrometeorites), they are likely to have a closing velocity of 1-30 km/sec. This is more likely to turn all volatiles in the micrometeorite into a plasma, as well as a similar volume of material from the comet. This plasma will most likely be lost to the comet since it has almost no gravity. So I suspect that any vacuuming will be mutually destructive.

On the other hand, dirty snowballs in the Oort cloud which have nearly circular orbits could collide at much lower speeds, and succeed in producing one large snowball (plus some splatter which goes into independent orbits). evan_au, Sat, 23rd Mar 2013

I think ill eat pizza on friday Bob Zimbobway, Tue, 10th Dec 2013

makes sense Andy, Sat, 24th Oct 2015

Any rocks in the comet will be left over as asteroids. Steve. Meyers, Thu, 22nd Sep 2016

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