It Keeps On Going, and Going, and Going…

Last month I mentioned seeing Comet 17/p Holmes. It’s still quite visible, especially now that the moon isn’t flooding the sky with light.

That little p in it’s name indicates it’s a periodic comet, and yes, it has been seen many times since it’s discovery in 1892. The comet orbits the sun in the area between Mars and Jupiter. I went looking for it’s period – the time it takes to make an orbit – and came up empty. It turns out that it has no real regular period, because the orbit of Holmes is strongly affected by Jupiter. In fact, in 1908 the orbit was changed enough that astronomers “lost” the comet until 1964.

The big mystery is what caused the comet to brighten so dramatically in October. So far there is no good explanation.

Studies with ground-based observatories show that Comet Holmes has been emitting a rather typical combination of water and other compounds. Weaver, Lisse, and others managed to commandeer the Hubble Space Telescope for one orbit’s observations on each of three nights in late October and early November. The blandness of these images argues that the nucleus didn’t break apart wholesale, as was the fate of Comet 73P/Schwassmann-Wachmann 3 in April 2006.

So was it hit by something? – an asteroid perhaps?

One possible explanation that’s almost certainly not correct is that it was struck by an asteroidal fragment. Although this comet occupies the realm between Mars and Jupiter, its orbital inclination (19°) actually keeps 17P/Holmes well separated from most denizens of the asteroid belt. Moreover, counting the outbursts that occurred during and after its discovery in 1892, three collisions would be needed to explain all the activity.

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