The Why Files The Why Files --

Comets: the universal recipe?

Comets: Poking through leftovers
As we've said, the reason NASA spends megabucks snapping snapshots of comets and punching them out with a hyper-expensive, hyper-velocity bullet is because comets are leftovers. Priceless leftovers. Poke your beak into yesterday's rice and beans, and you might figure out the recipe. Same deal with comets: They may tell us what raw material formed the planets and our sun about 4.5 billion years ago.

Gray comet with light beaming from behind.
Tempel 1 looks like an average comet, except this one posed for a nice close-up. Gotta lotta terrain, doesn't it? Courtesy of NASA. View a 76 Kb QuickTime movie showing the impact on Tempel 1. The movie shows impact of instrumented "bullet" on comet. Courtesy of NASA/JPL-Caltech/UMD

From eyeing half-baked planetary systems out in the universe, we think our solar system also began as a giant, whirling pizza of gas and dust. A dense blob formed at the center. Call it the sun. In the hinterlands, eight or nine planets came together. (Why eight or nine? Because Pluto may have been captured in the solar system after forming elsewhere...) Small, space traveling machine with lenses, scopes and cameras.

Here's Deep Impact, the spacecraft that sent a gut-punching impactor (bottom) into comet Tempel 1. Courtesy of NASA

Recipe: Makes one (1) solar system
But what were the exact ingredients, and were they beaten, folded, kneaded or whipped into planets? Were Mars Bars made from Milky Ways?

(We're trying to undercook this foodie riff, but this "leftover" bit wasn't even our idea. Back in 1996, we wrote about comets because comet Hale-Bopp was visiting. The comet's co-discoverer, Alan Hale, told us that comets, "are believed to be leftovers from the formation of the solar system." By studying them, he said, "you'd get insight into conditions when it first formed.")

About 4.5 billion years after the primordial pizza, the solar system has nine planets, one sun -- and a lot of orbiting offal (stuff that reminds us of those crumbs that we can never manage to fold into the bread dough).

Some of those leftovers are the rocky residents of the asteroid belt between Mars and Jupiter. A different bunch of rocky-icy leftovers lurks beyond Neptune, in the Kuiper Belt, and on the far side of Pluto, in the Oort Cloud.

Technically, those remote, icy objects don't become comets until they approach the sun, and solar heat causes the release of gas and dust. When the gas and dust form the tail and the gauzy "coma" that wraps around the nucleus, then we call the thing a comet. So here's the promise and the premise: comets are leftovers of the edges of the solar system that can tell us what was going on when the planets started forming.

Comet has two distinct tails. The blue ion tail is longer, thinner. The white dust tail is broad and shorter. Comet Hale-Bopp and its twin tails on March 8, 1997, seen from Earth. Elizabeth Warner, NASA-JPL

A tail to tell
Before those clumps of -- what? -- leave the Kuiper Belt or Oort Cloud, they orbit for billions of boring years in the deep freeze, where blistering cold and isolation shield them from much of the wear-and-tear that affects objects in the more "tropical" parts of the solar system. Then, when a heavy planet's gravity (Jupiter, are you listening?) yanks them just right, these comets-to-be get flung toward the inner solar system.

Jerked from the deep freeze, they start to roast in sunlight and begin to break apart. Comets put on quite a show during their brief lives. Nothing else in space has one tail, and comets sport two: The broader tail contains dust; the narrower tail contains ions, which are pushed away from the dust by the solar wind, causing the tails to separate.

Ten years ago, scientists put a lot of emphasis on the tails, which had the benefit, after all, of being easy to see. But in our annoying kitchen lingo, focusing on the tail of a comet is like studying the aroma of a food. You may learn a lot, but it's no substitute for studying the food itself.

Visited any comets lately? What's cookin'?