Blue Collar Scientist

People who have heard my talks about asteroids, comets, and planetary impacts will probably remember me saying that I didn’t think there was much important about the distinction between a comet and an asteroid. My view is that these small bodies exist on a continuum. At one end are small bodies that are made of rock, which are fully asteroids in the prevailing model. At the other end are balls of ice, which are fully comets in the prevailing model. But astronomers have discovered a lot of small bodies that look like blends between the two. It has been known for some time that comets are dusty and gritty, and some more than others; meanwhile, some asteroids have orbital characteristics of comets, but they’d have to be comets whose volatiles have all evaporated. My thinking here is neither unique nor original - I’ve cribbed it from people like Don Yeomans, Alan Harris, and Tim Spahr, among others - genuine small-body experts.

Now, via the Knight Science Journalism Tracker, I learn of some results of the Startdust mission, which collected debris from comet 81P/Wild 2. It turns out that the grit looked like the stuff that makes up asteroids.

Huh. Imagine that.

To my mind, this is further confirmation that these objects exist on a continuum. They are all small bodies, some of them have more ice, some of them have more dust, grit, and rocks, and because of this the boundary line separating an asteroid from a comet is fuzzy and indistinct. Remember - it is only the human mind that wants to put things into such clear-cut bins; nature is under no obligation to conform to our systems of classification.

Update: I finally tracked down a press release informing this story, and note that the author of the study, Hope Ishii, says: “It’s a reminder that we can’t make black and white distinctions between asteroids and comets. There is a continuum between them.”

Always better to hear it from the experts, but nice to know that I can draw a valid conclusion from available data.

This entry was posted on Friday, January 25th, 2008 at 8:04 pm and is filed under astronomy. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.