Ten Things You Don't Know About Comets

By Phil Plait | April 20, 2010 6:00 am




Every time a comet comes near the Sun, it dies a little bit.


Like many things in life and the Universe, what makes a comet beautiful will eventually kill it. Well, kill it as a comet.

Some comets are held together by the frozen gases. As one of these passes the Sun, that gas blows away, and the comet loses some of its structural integrity. Eventually, if it’s mostly gas, it’ll disintegrate. We’ve seen comets do this! They fall apart as they near the Sun (FYI it’s called calving when a big piece breaks off, but disintegrating completely would be a catastrophic calving, I suppose) — like the one shown above, a Hubble image of comet 73P/Schwassmann-Wachmann 3 when it fell apart in 2006. Even comets that are mostly rock with only some gas can be weakened by losing that gas, because they would be left with large empty pockets inside the rock. If something like that got too close to Jupiter, say, the gravity of the planet (really, the tides) might tear the comet to shreds. Shoemaker-Levy 9 was a comet that broke into dozens of pieces and slammed into Jupiter in 1994. It may have been such a weakened comet that couldn’t survive Jupiter’s ferocious gravity.

Comets lose a lot of mass when they go by the Sun. A lot: some shed hundreds of tons of material per second. That’s actually a small fraction of the mass of a comet, but given time, and lots of solar passes, it adds up. Every comet we see is slowly dissolving in space. Eventually even the mighty Comet Halley will be gone, broken down into a swarm of rocks, gravel, and dust once its gas is gone.

But there will always be more to replace it.



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