Comet 17P/Holmes has certainly given sky-watchers - backyard and professional astronomers alike - a thrilling chance to see a cometary outburst on a grand scale. After we posted my story about on-going speculation about what could have caused this outburst (and the one 115 years ago), many readers posted comments related to two questions: Could this have been triggered by a collision with an object in the main asteroid belt? And why can't we see more of a tail on this comet?

©Pic du Midi Observatory/Francois Colas/Jean Lecacheux/Boris Baillard

Here's what I found out: Michael Mumma at the Goddard Center for Astrobiology says such a collision in the asteroid belt is theoretically conceivable. He noted that comet guru Fred Whipple suggested that a collision with a small asteroid could have provided the right amount of energy to produce the ejecta and brightening observed in the comet's 1892 flare-up.

But Mumma himself thinks it would be "very surprising" if a collision were the cause of the outburst. He says part of the difficulty in weighing this possibility is that it's very hard to estimate how many small boulders are in the asteroid belt. These tiny objects - on the order of one-metre across - are beyond the detection limits of telescopes.

Brian Marsden, former director of the International Astronomical Union's Minor Planet Center, says plainly that he doesn't believe this to be a viable explanation for the outburst. He says it's hard to believe that this comet, among all those that pass through the asteroid belt, has been struck twice by objects in the belt - once in 1892 and again this year.

Many people have been discussing whether or not this comet has a tail. Comets typically have two types of tail - a dust tail and an ion tail. The dust tail is made of fine dust from the comet's main body, or nucleus, that has been swept out by the Sun's radiation. It usually points in the direction from which the comet came. The ion tail is caused by the Sun's magnetic field sweeping ions (which start out as neutral gas particles on the body of the comet) into a line that always points directly away from the Sun.

Marsden says the there isn't much of either type of tail. He says it's possible that there just isn't enough very fine dust in the material coming off the nucleus to be pushed by sunlight into a nice dust tail. (He says there may only be "fairly hefty dust" in the comet's coma.)

Some people have argued that we can't see the comet's ion tail because the orientation of the Sun, Earth and comet means the tail is mostly pointing away from Earth.

But though the tail does look fore-shortened, Mumma says the accumulated surface brightness would be greater seen from one end than if it were seen spread out, from the side. He likened it to looking at the contrail of a plane. If we saw the stream from the side, we would basically see right through it. But if we were in front of or behind the contrail, looking into it, it would appear many times brighter.

Marsden says the comet is so far from the Sun that the solar wind is not interacting strongly enough with the ionised gas to produce a fantastic ion tail. But he says it doesn't bother him that it doesn't have much of a tail. After all, the "fuzzy head" is putting on such a great show of its own.

Kimm Groshong, contributor