2. Asteroids 'n comets

3. Very variable

You can buy a telescope, but why do-it-yourself? This 12-inch mirror telescope was made by Jan van Gastel

The Hale Bopp comet of 1997. Photo: Amateur Astronomy

An infrared image of Jupiter showing the bright polar caps and the familiar red storm (at right). The bright spots in the southern hemisphere are the aftermath of fragments of comet Shoemaker Levy, which struck in 1994. The comet was first seen by semi-pro astronomers Gene and Carolyn Shoemaker, and David Levy. Photo: James Graham, Imke de Pater, Mike Brown, Mike Liu, Garrett Jernigan, Marina Fomenkova, Andy Ingersoll, Phil Marcus, W. M. Keck Observatory. NASA

The late Gene Shoemaker, David Levy, Carolyn Shoemaker the discoverers of comet Shoemaker Levy), and Charles S. Morris. This was taken during one of the last Shoemaker-Levy observing runs at Palomar. Courtesy ©1997 Jean Mueller.

Amateurs attack asteroid!
One of the most active areas for amateur astronomers is comets and asteroids. These small-but-potent visitors to the inner solar system are one of the few objects in astronomy that could actually affect Earth (an asteroid collision apparently dropped the curtain on the dinosaurs, to name one dramatic example).

Amateurs are commonly involved in nailing down the orbits of near-earth asteroids, says Robert Burnham of Astronomy magazine. "Typically, they follow up discoveries of asteroids that are made by professional telescopes, which do a snatch-and-grab and keep going, and don't do much follow-up."

After locating an asteroid, the amateur observer would create an image with a sensitive light detector called a charge-coupled device, and then return after an hour to see if it was in its predicted position, and then post refinements to the orbit on the Internet. "It helps refine the position of asteroids," says Burnham, and "helps professional astronomers work out whether there's a possible threat of collision." No Earth-whacking asteroids have yet been identified.

Alan Hale with Bill Bradfield (Australian), the most prolific visual comet discoverer of the 20th century.Courtesy © 1997 Kym Thalassoudis of the Astronomical Society of South Australia.

Finding asteroids
Amateurs can also use a similar technique to find asteroids in the first place, and it's a lot easier than it used to be. In the bad old days, astronomers found moving objects by scrutinizing films. In the good new days, software handles that menial chore, says Michael Bakich, also of Astronomy magazine. Software "overlays the images, and if any difference is noted, flags it." Then the observer must check: Was the difference caused by dust or an airplane "or have we discovered something here?"

Beyond finding and tracking asteroids, amateurs also make physical observation of the rocky little objects, which tend to have irregular shapes. "You only see a point of light," says Burnham, "but because they are irregular, there is a change in brightness," and that suggests the period of rotation. Asteroids have faint gravity, so if they spin quickly, they must be solid objects -- a pile of rubble would be torn apart by centrifugal force.

Meeting meteors
Another field for amateur stargazers is meteor showers, Bakich says. The most basic observation involves "just plopping down in a lawn chair, looking in a specific direction, and counting streaks." If that's too crude, you can use star maps to pinpoint the source of meteors; the technique has been used to discover many new showers, he says.

Largely due to amateur contributions, predictions of meteor showers have gone from generic 10 years ago ("The Persieds will be really strong"), to numeric in 2004: ("By August 12th when the shower peaks, sky watchers can expect to see dozens, possibly even hundreds, of meteors per hour").

The refinement of meteor predictions is not just grist for gawkers. Meteoroids -- the bits of junk that burn up in the upper atmosphere -- are the remains of busted-up asteroids. Observing meteors, Bakich says, is "how astronomers figure out how the meteoroid streams move through space; it gives us a better idea of our local neighborhood."

Perhaps the biggest score in amateur astronomy is when you are the first to find a comet, says Bakich. "If you've done a lot of work, and you find a comet... your name will be on it."

Just ask the folks who discovered comet Shoemaker-Levy, the fast-flying comet that whanged Jupiter 10 years ago.

What's new with variable stars?

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