The Asteroid Belt

On New Year's night in 1801, the Italian astronomer Giuseppe Piazzi found a small object wandering across the sky. Piazzi at first thought that he had discovered a new comet. After study however it was found that the object was moving in a nearly circular orbit around the Sun. Piazzi named the little body "Ceres" for the Sicilian goddess of grain25. Ceres turned out to be way too small to be a planet, measuring 933 kilometers in size. Even at that size, Ceres contains about 25% of all the mass in the asteroid belt and is by far the largest object in the belt. It orbits the Sun at a mean distance of 414 million kilometers and orbits the Sun each 4.6 years. That places it about 162 million kilometers outside the orbit of Mars and about 305 million kilometers inside the orbit of Jupiter. Ceres' orbit lies not far out of the plane of the solar system inclined only about 10 degrees from the ecliptic. It was not long before Ceres had company. The next year a second body was discovered by Heinrich Olbers and he named it Pallas. Olbers was attempting to recover Ceres and refine our knowledge of its orbit when he happened on Pallas. Pallas' has a mean distance from the Sun about equal to that of Ceres but travels in an orbit that is somewhat more eccentric than that of Ceres. While Ceres orbit has an eccentricity of about .07, Pallas is about .23. This is far more than any other planet. Pallas orbit is inclined substantially as well, more than 34 degrees. Pallas is considerably smaller than is Ceres measuring about 525 kilometers in diameter. Olbers named Pallas for the ancient Roman god of wisdom. Juno was the third asteroid to be discovered by Karl Harding in 1804. Juno is barely half the size of Pallas. In 1839, Juno's orbit was determined to change appreciably. Today, Juno's orbit is actually slightly more eccentric than is that of Pallas and is inclined 13 degrees to the ecliptic plane. Juno orbits the Sun each 4.35 years. In 1807, Olbers found his second

25 By tradition asteroids are named by their discoverers. Early asteroids were named for ancient Greek or Roman deities as the planets were. Many asteroids are now named for contemporary heroes (3,350 Scobee) or family members or even pets. Today, an asteroid is always referenced by a catalog number issued in order of its discovery and a proper name.

asteroid and named it "Vesta" for the Roman goddess of virtue. Vesta measures 285 miles in diameter (460 kilometers) and orbits the Sun each 3.63 years, the shortest period of the four largest asteroids. Vesta is very different from the first three asteroids. Though it is approximately the same size as Pallas, its surface is much brighter than any other asteroid and belongs to a special class of asteroids different from the others. Vesta's surface is so reflective that it is the only asteroid that ever reaches naked-eye visibility. Today we know of several asteroids that share similar spectral properties with Vesta and occupy similar orbits. Some call them "Vestoids." Scientifically they are given a special classification based on composition and albedo. We'll discuss different asteroid classes in just a bit. These first four asteroids came to be generally known as the "Big Four" among the asteroid belt. By the time the century had ended, several hundred asteroids had been found. Twenty-six of those were found to be larger than 200 kilometers. As we learned more about the asteroids, we discovered that the idea that they were possible leftovers from a shattered planet began to lose appeal. All the known asteroids added together have less total mass than the Moon. Scientists actually do not have a great deal of interest in the large ones because we've found them all and know their orbits very precisely. It's the small ones that worry astronomers because we have probably found only a fraction of 1% of the estimated one million asteroids in the one-kilometer size range.

Asteroids are organized into groups by means of their orbital characteristics. Most asteroids are found in the area between Mars and Jupiter between 1.7 and 3.0 astronomical units from the Sun. These asteroids are called main belt asteroids. The main belt is divided into several different sub-groupings each named for a principal asteroid within that group. Such groups include Hungarias, Floras, Phocaea, Koronis, Eos, Themis, Cybeles and Hildas. Small relatively empty zones called Kirkwood Gaps separate each of these groups from each other. The Kirk-wood Gaps are kept relatively free of asteroids as a result of gravitational interaction with Jupiter. An object within a Kirkwood Gap would likely have an orbital period that would equal a simple fraction of Jupiter's (one-quarter, one-third, one-half). An object in such an orbit would resonate with Jupiter causing energy to be added to the asteroid and causing it to move into a higher orbit.

In Chapter 7 we talked a bit about near-Earth asteroids (NEA). NEAs are divided into three classes. Atens orbit inside of Earth's orbit with an aphelion that is greater than Earth's perihelion. Apollos orbit outside of Earth's orbit with a perihelion distance that is less than Earth's aphelion distance. Amours circle the Sun just outside Earth's orbit and approach Earth's aphelion distance but never actually cross Earth's orbit. This makes them less threatening than Apollos and Atens, which actually pass through the same space that Earth does.

Astronomers have also charted two groups of asteroids,which are called Trojans. The two Trojan groups are in gravitational balance between the Sun and Jupiter at the Lagrangian points 60 degrees ahead and 60 degrees behind Jupiter and travel together along with Jupiter in this odd gravitational dance. Astronomers also speculate that there may be some Trojan-type asteroids bound to the Lagrangian points of Venus and Earth. The asteroid 5621 Eureka is bound to one of Mars' Lagrangian points.

Asteroids are classified (different from specifying orbital categories) by what they are made of and by how much light they reflect back into space (albedo). Most asteroids are what are called C-type. These asteroids are rocky in nature, made mostly of carbonaceous materials and are similar to the Sun except for a lack of hydrogen,helium and other volatiles. These make up about 75% of all known asteroids, but because they tend to be so dark and hard to see, they may actually be underrepresented in the total asteroid population. About 1 in 7 asteroids are classified as S-type. These asteroids are made of metallic nickel-iron and iron and magnesium-silicates. These asteroids are much brighter than the C-types are. The last major classification is the M-type asteroids. These asteroids are the brightest being nearly pure nickel-iron. They make up only about 7% of all known asteroids. Over the years several different systems for differentiating asteroids have emerged and new categories have emerged in old systems. But for simplicity, it is probably easiest to simply discuss asteroids as "rocky" and "metallic." Ceres, Pallas and Juno are rocky. Vesta is heavily metallic.

We did not know much more about asteroids until the opportunity came along to explore them with spacecraft. Asteroids were not considered to be objects of sufficient interest by NASA or other space agencies to send dedicated missions to them, but NASA planners saw an opportunity when redesigning the trajectory of the Galileo probe to Jupiter. Galileo was to be launched directly to Jupiter by the Space Shuttle Challenger in May 1986 with a two engine Centaur upper stage. But Challenger was destroyed on the mission immediately preceding its scheduled Galileo flight and in the aftermath of the accident, NASA decided against flying the hydrogen-fueled Centaur in the shuttle cargo bay. A less powerful two-stage solid fuel rocket called an Inertial Upper Stage would instead launch Galileo. The IUS did not have the power to propel the massive Galileo probe directly to Jupiter, so a series of slingshots would have to be devised. Galileo would reach Jupiter by gaining speed after launch from the Space Shuttle Atlantis by making a flyby of Venus, then Earth. Along the way, planners realized that Galileo would fly close by two asteroids. First it would fly by the asteroid 243 Ida. When it did so, astronomers turned Galileo's cameras on it and got a completely unexpected shock. Ida had a satellite! A tiny companion that astronomers named "Dactyl" was circling Ida. Later, Galileo also flew past the asteroid 951 Gaspra. In later years the advanced technology demonstrator spacecraft, Deep Space 1 flew past the asteroid 9969 Braille before traveling on to its planned destination, Comet Borrelly. The Stardust spacecraft made a flyby of the asteroid 5535 Annefrank before going on to its dust-collecting mission at Comet Wild 2. The only dedicated mission to an asteroid to date was the Near Earth Asteroid Rendezvous mission in the late 1990s. NEAR nearly ended in disaster. It conducted a successful flyby of the asteroid 253 Mathilde before flying to 433 Eros and entering orbit. But when it was time to enter orbit around Eros, a software error caused the engine to fail to fire. The trajectory gnomes jumped in and saved the day and after flying an orbit around the Sun, NEAR caught Eros again one year late and slipped into orbit. After circling the rock for a year, NEAR gently settled on the surface of Eros and transmitted data from the surface for a short time. What these missions have taught us is that C-type asteroids are not always the solid rocky bodies we thought they were but rather amount to little more than lumps of detritus and rock weakly held together by gravity.

An exciting future mission is NASA's DAWN mission scheduled to launch in 2007. This will be the first mission to travel to any of the Big Four. DAWN carries a re-startable ion engine that will allow it to first enter orbit 4 Vesta in June 2010. After spending a year studying 4 Vesta, DAWN will then blast out of orbit and fly to 1 Ceres, entering orbit there in 2014.

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