Vagabonds in space: asteroids, comets, and moons, oh my!

Natural History, July-August, 2004 by Neil deGrasse Tyson

For many centuries the inventory of our celestial neighborhood was quite stable. It included the Sun, the stars, the planets, a handful of planetary moons, and the comets. Even the addition of a planet or two to the roster didn't change the basic organization of the system.

But on New Year's Day 1801, a new category arose: the asteroids, so named a year later by the English astronomer Sir William Herschel, the discoverer of Uranus. During the next two centuries, the family album of the solar system became crammed with the data, photographs, and life histories of asteroids, as astronomers located vast numbers of these vagabonds, identified their home turf, assessed their ingredients, estimated their sizes, mapped their shapes, calculated their orbits, and crash-landed probes on them. Some investigators have suggested that the asteroids are kinfolk to comets, and possibly even to planetary moons. And at this very moment, some astrophysicists are plotting methods for deflecting any big ones that may be planning an uninvited visit.

To understand the small objects in our solar system, one should look first at the big ones, specifically the planets. A curios fact about the planets is captured in a relatively straight-forward mathematical rule proposed in 1766 by a German astronomer named Johann Daniel Titius. A few years later, Titiuss colleague Johann Elert Bode, giving no credit to Titius, began to spread the word about the rule, and to this day it's often called the Titius-Bode law or even, erasing Titius's contribution altogether, Bode's law. This handy-dandy formula yielded pretty good approximations of the distances between the planets and the Sun, at least for the ones known in Titius's time: Mercury, Venus, Earth, Mars, Jupiter, and Saturn. Herschel's discovery of Uranus, in 1781, in an orbit that matched the expectations of the Titius-Bode law, lent the formula credibility and spurred astronomers to look around carefully for more planets in the solar system. So either the law is just a coincidence, or it embodies some fundamental fact about how solar systems form.

It's not quite perfect, though. At least three shortcomings plague it.

Problem number 1: You have to cheat a little to get the right distance for Mercury, by inserting a zero where the formula calls for 1.5. Problem number 2: Neptune turns out to be much farther out than the formula predicts, orbiting more or less where a ninth planet would go. Problem number 3: Pluto, which some people persist in calling the ninth planet (for our exhibits at the Rose Center for Earth and Space, we think of icy Pluto as the "king of comets"), isn't even close to where the Titius-Bode law predicts.

In addition, the law would have a planet orbiting in the space between Mars and Jupiter--at about 2.8 astronomical units, or AU, from the Sun (one AU is the average distance between Earth and the Sun). Encouraged by the fact that the newly discovered Uranus orbited at more or less the distance Titius-Bode said it would, astronomers in the late eighteenth century thought it would be a good idea to check out the zone around 2.8 AU. And sure enough, on that first day of 1801, the Italian astronomer Giuseppe Piazzi, founder of the Observatory of Palermo, discovered something there. Subsequently that something disappeared into the glare of the Sun, but exactly one year later, with the help of brilliant computations by the German mathematician Carl Friedrich Gauss, the new object was rediscovered in a different part of the sky. Everybody was excited: a triumph of mathematics and a triumph of telescopes had led to the discovery of a new celestial object. Piazzi himself named it Ceres (as in "cereal"), for the Roman goddess of agriculture, in keeping with the tradition of naming planets after ancient Roman deities.

By now, many tens of thousands of asteroids have been catalogued, and they're still being discovered. Altogether there are probably many more than a million that measure more than half a mile across. As far as anyone can tell, even though Roman gods and goddesses did lead complicated social lives, they didn't have 10,000 friends; astronomers had to give up on that source of names long ago. So asteroids are now named after actors, painters, philosophers, and playwrights; cities and countries; dinosaurs, flowers, seasons, and all manner of miscellany. Even regular people have asteroids named after them. Harriet, Jo-Ann, and Ralph each have one: they are ca]ld 1744 Harriet, 2316 Jo-Ann, and 5051 Ralph, with the number indicating the sequence in which each asteroid's orbit became firmly established. David H. Levy, a Canadian-born amateur astronomer who is the patron saint of comet discoverers but has discovered plenty of asteroids as well, was kind enough to pull an asteroid from his stash and name it after me: 13123 Tyson.

Most asteroids are made entirely of rock, though some are entirely metal and some are both; most inhabit what's often called the main belt, a zone between Mars and Jupiter. Asteroids are usually described as being formed of material left over from the earliest days of the solar system--material that never got incorporated into a planet. But that explanation is incomplete at best: some asteroids are pure metal. To account for their metallic composition, it helps once again to begin with the planets--specifically, how they formed.