Publications Topics:

Books

ASP Conference Series

Monograph Publications

IAU Publications

Books of Note

Purchase through the AstroShop

Journals

Publications of the ASP (PASP)

Magazines

Mercury Magazine

Archive

Guidelines for Authors

Order Mercury Issues

Mercury Advertising Rates

Newletters

The Universe in the Classroom

ASP E-mail Newsletters

Special Features

Astronomy Beat

Contact Us

Mercury, May/June 1996 Table of Contents

by James C. White II, Middle Tennessee State University

(c) 1996 Astronomical Society of the Pacific

Orbiting between Mars and Jupiter are thousands of rocky bodies. These "minor" planets give up their secrets reluctantly. Let's spy on one of the larger ones.

This Month's Project: (re)discovering the asteroid Vesta

Where was the missing planet? Late in the 18th century, astronomers were sure that planets should pop up at certain distances from the Sun -- including between the orbits of Mars and Jupiter. Yet no planet had ever been seen there.

The German baron Francis Xavier von Zach was determined to find it. In 1800, he divided the sky into segments and assigned an astronomer to each. Sicilian astronomer Giuseppe Piazzi got one of those segments, but unbeknownst to his boss, kept up his own, separate project. On Jan. 1, 1801, in a part of the sky outside his assigned segment, Piazzi saw a faint dot he initially believed to be a comet. By tracking the object, he found that it orbited the Sun between Mars and Jupiter. Piazzi named it Ceres (SEE-reez), after Sicily's protective deity. Van Zach was ecstatic.

But in March 1802, before the celebratory din had died down, another German astronomer, Heinrich Olbers, found a second planet-like object between Mars and Jupiter. This one was named Pallas. In 1804 came Juno; in 1807, Vesta. By the end of the century, astronomers had found another 300 of these roving objects, called asteroids; today, 7,000 are known.

Most minor planets, as asteroids are sometimes affectionately called, hum in elliptical orbits between Mars and Jupiter. Some cross our orbit, a fact the dinosaurs would rather forget. Most are less than a kilometer across; only Ceres (1,000 kilometers), Pallas (600 kilometers), and Vesta (500 kilometers) have diameters larger than 300 kilometers. If you lumped them all together, you'd wind up with a hunk of rock only 15 percent the diameter of Earth.

We have Jupiter to thank for the asteroids. The pernicious gravity of the largest planet stirs up the material between it and Mars. That material might have formed a planet, but every time it tried to coalesce, Jupiter ripped it apart. What was bad news for a planet is good for astropaleontologists trying to reconstruct the evolution of the solar system: As leftover scraps of planet formation, asteroids contain information about events long ago. And their rich veins of heavy elements make them targets for future mining operations (the Miner 2549ers?).

Not exactly planets, but not exactly rocks, these mysterious objects are often ignored because they don't have the romance of Saturn, the might of Jupiter, or the sweaty glare of Venus. It's just bad press, or rather, a lack of press. "Stop the presses! We're running a front-page scoop on that cute little minor planet they just discovered." Not.

Observing Guidelines

Although Vesta isn't the largest asteroid, it is the most reflective and geologically diverse. The Hubble Space Telescope has revealed that Vesta looks much like our Moon, with old lava flows and a large impact basin that dips right down into the minor planet's mantle.

This May is a wonderful opportunity to catch Vesta surfacing from the murk of the asteroid belt. On May 8, it reaches perihelic opposition, rising in the east just as the Sun sets in the west. It will be closest to Earth at this time, and almost as close to the Sun as it gets.

At its brightest, the asteroid will shine at magnitude 5.6, just bright enough for observers in dark locations to see it with the naked eye. The rest of us should bring along binoculars. Floating through Libra during May, Vesta will be 15 times dimmer than the star beta Librae. On 8 May, beta Librae and Vesta will be less than five degrees apart -- you'll be able to fit both in your binocular's field of view.

Begin your observations at the beginning of May. Use the finding chart to locate the small world, and night by night throughout the month, sketch its location relative to background stars or photograph it. If you choose photography, I suggest you use fast film, 20-second exposures, and a sturdy tripod. Your sketches or photographs will reveal the asteroid's motion.

In your written record, note your location (latitude and longitude or the nearest large city), the time of day of each photograph or sketch, the sky and weather conditions, and the approximate altitude of Vesta above the horizon -- recall that your fist at arm's length is about 10 degrees.

What to Do With the Observations

Incorporate your observations into a document with the following information: name of the project (such as,"Charting Vesta's Motion"), your name or the name of your group, details of the observing location, mailing address, telephone number, and email address, if available. We welcome reports from observers of all ages in all countries. In your report, please provide accurate time and date information and details of your observations. We want to know where you were, when you were there, what the weather and skies were like, and what you were thinking. This helps us as we select the Guest Observer, and it helps readers to understand what you studied.

Please submit your completed report by July 31, 1996 by email to 2032694@mcimail.com or by regular mail to John Isles, Attn: Guest Observers, 1016 Westfield Drive, Jackson, Mich. 49203-3630.

JAMES C. WHITE II is a professor in the Department of Physics and Astronomy at Middle Tennessee State University in Murfreesboro. His astronomical research focuses on cataclysmic variables. White writes a monthly astronomy column carried by newspapers in Tennessee. His email address is jwhite@physics.mtsu.edu.