We went out on Christmas night and found Comet Machholz just about where the star chart says it’s supposed to be. With a pair of binoculars we could make out a nice fuzzy glob of light. If you could take a picture of it over several minutes’ time, it would look about like this image from two of Europe’s better-known comet photographers. Notice in their view that Machholz seems to have a couple of tails. The one sweeping out from the comet to the upper left is the dust tail and the one sweeping out to the lower left is the plasma tail.
For several years while I was in graduate school I studied comets, including one named Machholz! Our team’s interest was in tracking the evolution of a comet’s plasma tail, which is formed in an interaction between cometary gases and the solar wind. The dust tail, by contrast, is made up of microscopic dust particles streaming off the comet. When I joined the team I began working on analyzing images of Comet Halley and its nicely active plasma tail. We wanted to track it throughout its tour around the Sun, and later on we did the same with others. The biggest result was a book published as part of the International Halley Watch project called the International Halley Watch Atlas of Large-Scale Phenomena. It was published through the University of Colorado in a limited edition run, and I was astonished to find it the other day on Amazon.com (although I did know it was selling at various times on Ebay). In fact, I have eight copies of it here at my office.
For the project I and other team members (Jack Brandt, Yu Yi, Marty Snow, Marlon Caputo) went through about 2,500 images of Comet Halley submitted by astronomers from around the world. We measured the size and angular orientation of the plasma tail and correlated it with the comet’s location in the solar wind. Then we arranged the images in a time sequence starting from late 1985 to about June 1986. If you flip through the book, you can see the tail change drastically as Halley neared the Sun and visited various latitudes of the solar wind.
My job was mostly to analyze the images, and then lay them out in the book for publication. So, while I’m not listed as one of the book’s authors, I am acknowledged for my contributions, and it was a most satisfying and interesting project with which to be associated. And, I learned a lot about comets and the solar wind in the process.
We went on to study other comets: Machholz, Schwassman-Wachman, DeVico, Mueller, Hyakutake, and Hale-Bopp. We gathered up images for each comet, and correlated their positions in the solar wind with their appearances, and also with data from the Ulysses spacecraft, which was in the same “regime” of the solar wind as each of the comets for various periods during their respective orbits. For Hale-Bopp, we took observations at the University of Hawai’i 88-inch telescope on Mauna Kea during November 1996 to see if we could catch the “turn on” of the plasma tail. That was a personal thrill for me, even though our data showed that at the time the plasma tail hadn’t yet started to grow.
Nowadays I’m out of the plasma-tail research business, although some of my colleagues, most notably Jack Brandt (team leader, close friend and co-author, currently at the University of New Mexico), continue to pursue the study of correlations between cometary plasma tails and the solar wind. He continues to publish papers based on data we took during my tenure at CU, as well as on data he’s gathered since that time on other comets. We made some good contributions to the comet research literature. Plasma tails are a fascinating way to see the Sun’s influence on things even as small and evanescent as a comet!

A couple of weeks ago we went out to find Comet NEAT. After some starhopping around in the general direction of where the starmaps said it should be, we found it — looking like a little smudge of light. I know there are folks who would say, “so what?” and then shrug their shoulders as if we were crazy. But, to me that little smudge was fascinating. It was the outward manifestation of a block of dirty ice in orbit around the Sun. That ice is left over from the creation of the solar system, some five billion years ago. And, as it goes around the Sun, it leaves little bits of itself behind in the form of a tail and a sprinkle of particles. Eventually those particles will find their way into our atmosphere as the Earth plows through the cometary wake in its own orbit. We’ll see them as meteors.

Nothing goes to waste in the solar system, or in the universe, for that matter. The comet we see today leaves behind stuff that we see later as meteorites. The Sun puts out a huge stream of particles that flows past the Earth and out into interplanetary space. Eventually it thins out and the “edge” of that stream is, essentially, the “edge” of our solar system. The particles in that stream interact with planetary magnetic fields, and on some worlds (Earth, Jupiter, Saturn, some of the larger moons) we see the interaction as auroral glows.

The solar wind is a form of mass loss that enriches the interstellar medium with elements that eventually get used in a new generation of stars. Some five billion years from now the Sun will swell up to become a red giant, and unleash more of itself to the space between the stars. All that stuff will also become part of the seedbeds for the next stellar families to spring up, complete with stars, planets, asteroids, moons, and comets. Larger stars die in supernova explosions which also recycle stuff into the interstellar medium. That, too, goes into the stellar formation factories of the future. The result? More stars. More worlds, moons, asteroids, and comets. It’s lovely cycle of birth, life, death, and rebirth.

So, next time there’s a comet for us to see, think about this cycle as you spot the lovely shrouded coma and tail that stream out from the comet. It’s part of the life-dance of the universe.