The main belt
Billions of asteroids are confined within a wasteland between the orbits of Mars and Jupiter, like so much rubble left over from the creation of the solar system. Most of them occupy a great ring, known as the asteroid belt, at mean distances of 2.2 to 3.3 AU from the Sun and with orbital periods of 3 to 6 years. For comparison, the mean distance between the Earth and the Sun is roughly 1 AU, or one astronomical unit, about 150 billion, or 1.5 x 1011, meters. Not all asteroids lie in this belt, but those that do are said to belong to the main belt.
Not all asteroids are found in the main belt. An especially interesting type is further away from the Sun than the asteroid belt. They move along Jupiter’s orbit, keeping pace with the giant planet. The first known one, 588 Achilles, was discovered photographically by the Heidelberg astronomer Max Wolf (1863-1932) in 1906. Hundreds of them are now known, travelling on both sides of Jupiter in two clouds, one preceding the giant planet and one following it. As with Achilles, they are all named after heroes of the Trojan War and they are therefore collectively known as the Trojan asteroids.
The Trojan asteroids are held captive by the gravity of both Jupiter and the Sun. They are found near two of the five Lagrangian points, named after the Italian-born French mathematician Joseph Louis Lagrange (1736-1813) who predicted their existence 134 years before the discovery of 588 Achilles. At these points, the gravitational force of Jupiter is equal to that of the Sun, which is much more massive than Jupiter but also a lot further away from the asteroids. These Lagrangian points lie in the corners of equilateral triangles that have Jupiter and the Sun at the other corners.
Although the vast majority of asteroids travel in the main belt lying between the orbits of Mars and Jupiter, there are some notable exceptions that reside within the inner solar system. Known as the near-Earth asteroids, they move inward toward our planet as they travel around the Sun.
There are three populations of near-Earth asteroids, called the Atens, Apollos and Amors. Both the Aten and Apollo asteroids move on eccentric orbits that can cross the Earth’s path in space. The Atens are always close to the Sun, never moving out as far as the orbit of Mars. The elongated orbits of the Apollo objects loop in from the main belt to within the Earth’s orbit. The Amors travel around the Sun between the orbits of Mars and the Earth, and often cross the orbit of Mars.
Why do some asteroids move near the Earth, while most of them stay in the asteroid belt? Gaps of missing asteroids in the main belt provide some clues to these wandering interlopers. These are the Kirkwood gaps, discovered long ago by the American astronomer Daniel Kirkwood (1814-1895).
The locations of these clearings correspond to orbital resonance with Jupiter, in which the orbital periods are exact fractions of the giant planet’s period. Any object that orbits the Sun at the 3:1 resonance, for example, would have exactly one third, or 1/3, the orbital period of Jupiter, and it would complete three circuits around the Sun for every one that Jupiter completes. Such an asteroid would revolve around our star at a distance of 2.5 AU with a period of 3.95 years, compared with Jupiter’s orbital distance of 5.2 AU and orbital period of 11.86 years. An asteroid that happened to stray into this resonance would come close to Jupiter at almost the same part of the asteroid’s orbit at regular 11.86-year intervals and the accumulated gravitational interaction with Jupiter could dislodge the asteroid form its orbit.
A satisfactory explanation of the Kirkwood gaps was not achieved until the 1980s when powerful computers were used to study Jupiter’s influence on the motion of asteroids. The computer simulations showed that Jupiter induces a chaotic zone in the vicinity of an orbital resonance, and that an asteroid that moves into the resonant orbit will eventually be tossed out of it. An asteroid in the chaotic zone can spend tens of thousands of years in a well-behaved, near-circular orbit. But that ordered, placid behavior can be unexpectedly interrupted after 100 thousand years or so, when the orbit is suddenly stretched and elongated in a chaotic way.
In the past, there have been two extreme theories for the origin of asteroids. According to the first, the asteroids represent the fragments of a former planet that has been torn apart. The second theory proposes that the asteroids are the pieces of a planet that never formed. Today, astronomers favor a theory that lies between the extremes. It is now known that the combined mass of all asteroids is far too small to make up a major planet. If all the known asteroids were brought together, they would create a body less than five percent the mass of the Moon. So the first extreme must be discarded. On the other hand, the second extreme can also be excluded because there is strong evidence that many of the asteroids were once collected into a relatively small number of slightly larger parent bodies.
Remnants of a planet that never formed
Why did the asteroids fail to coalesce into a single planet? It is likely that gravitational forces from the rapidly forming and massive Jupiter took charge of its neighborhood, stirring it up and keeping the original asteroids from growing too large. Numerous asteroids in resonant orbits with youthful Jupiter probably permeated the region of the asteroid belt, between 2 and 4 AU. Chaotic zones in the vicinity of these resonances would have pumped up the eccentricities of initially circular orbits, flinging the resident bodies into elongated and inclined orbits, accelerating them to high velocities, and causing them to crash into each other. The colliding objects would be moving too fast to stick to each other. Instead, they would break apart into fragments.
A lifetime of catastrophic collisions
Encounters among the earliest asteroids became increasingly violent as Jupiter stretched and twisted their orbits into eccentric and inclined orientations. These orbits criss-crossed each other, resulting in violent collisions at the place where they meet. Instead of continuing to grow, the largest asteroids were chiseled and blasted apart by mutual collisions. So the original asteroids never grew larger than about 1 million meters across, and they never accumulated into a major planet. The pulverized remnants of these former worlds became the present asteroids, often orbiting the Sun in families with common orbital characteristics and spectral properties.
The Japanese astronomer Kiyotsugu Hirayama (1874-1943) discovered asteroid families. He noticed that groups of main-belt asteroids share very similar orbits, suggesting that they are the broken fragments of larger objects. Hirayama called each group a family because he believed the members shared a common origin as the children of a bigger parent body. He also named a number of families after their largest member asteroids, such as the Eos, Koronis and Themis families.
The size of asteroids
Due to its small size, an asteroid remains an unresolved point of light in even the best telescopes on Earth, just like a faint star. This explains the name asteroid, which comes from a Greek word that means “starlike”. Although the name describes the visual appearance of these objects in a telescope, it is totally inappropriate to their physical nature. Using our instruments on Earth, we can determine the sizes of these objects, and they are much smaller than either a star or a major planet.
Even the biggest asteroids are smaller and less massive than the Moon. Ceres is by far the largest asteroid, having a radius of about 475 thousand meters and a mass of 1.17 x 1021 kilograms. That is about a third the radius of the Moon and only 0.016 the mass of the Moon. The brightest asteroid, Vesta, is even smaller, a little less than half the size of Ceres. There are many more small asteroids than big ones. About 1,000 asteroids are larger than 15 thousand meters in radius. Surveys of the faintest asteroids suggest that there are about half a million asteroids in the main belt larger than 1.6 thousand meters. Yet, despite their vast numbers, the total mass obtained by adding up the contributions of all asteroids, of all sizes, is far less than the mass of any major planet. The entire asteroid belt is only 0.05 the mass of the Moon and just 0.0006 the mass of the Earth.
An asteroid’s color
Because asteroids display no visible disk, Earth-based observers must infer their physical characteristics from the intensities and spectral properties of their reflected sunlight. By comparing an asteroid’s reflected light, wavelength by wavelength, with that of the incident sunlight, it is possible to deduce its surface composition. Astronomers divide the amount of incident sunlight at each wavelength by the amount of reflected sunlight at that wavelength, and the ratio tells them how much light of each color is reflected compared to any other color. Such spectral measurements have revealed the physical diversity of the asteroids, and shown that their compositional differences tend to depend on distance from the Sun.
The bulk of main-belt asteroids can be divided into two broad spectral categories, known as the S, for silicate, and C, for carbonaceous, types. The bright S-types have a reddish color and exhibit spectral dips identified with absorption by silicate minerals. They prevail in the inner part of the asteroid belt, orbiting closer to the Sun than the belt mid-point.
In contrast, the C-type asteroids are darker, bluer and richer in carbon, with relatively flat and featureless spectra at visual wavelengths. The C-type asteroids far outnumber all types, possibly composing three-fourths of the main belt. The C-type asteroid 1 Ceres is a representative example; it has a smooth visible spectrum with an infrared absorption feature attributed to water embedded in its mineral structure. The S-type asteroids probably account for up to 15 percent of all asteroids. Some of the less common M-type asteroids reflect sunlight in a way that suggests that their surfaces are composed of nickel and iron, hence the designation M for metallic for at least some of them. These objects could be the metal cores of larger parent bodies, stripped bare by collisions. The M-types are most common in the middle of the asteroid belt. Future space entrepreneurs may want to mine them for valuable metals.
There is an intriguing connection between the composition of asteroids and their distance from the Sun. The innermost asteroids, with orbits closest to the Sun, are rocky, siliceous and dry, while the outer ones are carbonaceous with water-rich, clay-like minerals. The igneous asteroids, found closer to the Sun, have fewer volatile compounds and less water, and they have been subject to greater heating. The primitive asteroids, which are located farthest from the Sun, are primarily rich in carbon and water. There is a related, progressive decrease in an asteroid’s reflecting power with increasing distance from the Sun. The brightest asteroids that reflect the most sunlight tend to lie near the inner edge of the main belt, closest to the Sun, while the most distant asteroids are, on the average, the darker ones with the lower reflecting power. The very darkest are found in the remote regions near Jupiter’s orbit. These differences in the composition and reflecting power of asteroids are probably related to conditions in the primeval solar nebula – the interstellar cloud of gas and dust from which the solar system originated. They may be a consequence of a decrease in temperature with increasing distance from the young Sun when the asteroids were formed.
The spin of an asteroid
Asteroids do not shine like a steady beacon with constant brightness. They instead reflect a varying amount of sunlight toward the Earth. The observed brightness variation, also known as a light curve, is periodic, often with two maxima and two minima. The overall repetition is due to rotation, while the double pattern of variability results from alternating side views of an asteroid’s elongated shape. When we see the biggest side of an asteroid, with the greatest area, the asteroid is brightest, while the smaller area reflects less sunlight and the asteroid is dimmer.
Almost all asteroids spin about a single axis. The period of rotation is inferred from the amount of time that it takes for the complete pattern of brightness variation to repeat itself. The rotation periods are usually between 2.4 and 24 hours, although a few of them rotate with longer periods, such as 253 Mathilde with a rotation period of 17.4 days and some have periods of only a few minutes. Frequent oblique collisions can increase the rate of rotation or decrease it, depending on whether the collision is in the direction or rotation or opposite to it.
Some asteroids are probably rotating as fast as they can. If an asteroid is not solid, and is thus bound only by its own gravity, it can only spin at a certain maximum rate before material is whirled off it. Asteroids larger than 200 meters seem to have reached this limit, for most of them do not rotate faster than once every 2.2 hours, suggesting that there is nothing stronger than gravity holding them together. If it lacks the material strength of a solid, an asteroid with a faster rotation rate, and a shorter period, will throw material off its surface and fly apart. Such an asteroid might resemble a rubble pile or gravel heap, formed after collisions have blasted a larger asteroid to bits, with the fragments reassembling into a loosely bound object with little internal cohesion. Close-up scrutiny by spacecraft has shown that large asteroids can be both rubble piles and solid rocks.
Asteroids smaller than 200 meters in diameter can rotate at faster rates, some turning once every few minutes. Their rotation is too rapid for these asteroids to consist of multiple components bound together by mutual gravitation. They must instead be rock solid. Some small asteroids rotate so swiftly that their day ends almost as soon as it begins. An example is the 30-meter asteroid 1998 KY26. Its day and night are only 5 minutes long; sunrises and sunsets on this asteroid take less than one second. By way of comparison, daylight at some places on Earth can last 12 hours or longer, and terrestrial sunrise or sunset usually takes about two minutes.
For more than two centuries, no one knew what an asteroid looks like. Since they are so small and far away, the surfaces of asteroids cannot be distinguished with telescopes on Earth, although modern technology has been used to image one of the largest ones, Vesta, with some success. The shape of an asteroid can nevertheless be inferred from the form and amplitude of its periodic light variations. They have told us that most asteroids are at least slightly elongated, chipped and pummeled into irregular shapes by eons of collisions. The stretched out, irregular shapes of some asteroids have also been determined from radar observations of near-Earth asteroids that travel close enough for scientists to detect the echoes of radio waves bounced off them. During their close approach, these asteroids speed by the Earth at distances of several hundred million meters, permitting brief, high-resolution radar images before they move on and fade from view.
The radar data indicate that the overall shape of some asteroids is dominated by two irregular, lumpy components that touch each other, something like a dumbbell. Each asteroid is a double object, that is, two bodies in contact. Examples are 4179 Toutatis, pronounced too-TAT-is and 4769 Castalia. The two pieces probably merged after a past catastrophic collision of a larger body; they may have been thrown apart and subsequently came together under their mutual gravity. Or they might be two former asteroids that joined in a gentle encounter.
The inquisitive eyes of spacecraft were nevertheless required for the full resolution of the surface details of asteroids, and to turn these moving points of light into real places. The first glimpse was provided when the Galileo spacecraft flew close by two asteroids, 951 Gaspra and 243 Ida on its way to Jupiter, revealing details of these ravaged, misshapen worlds.
Another sideways glance was obtained when the NEAR Shoemaker spacecraft flew past 253 Mathilde on its way to a rendezvous with the asteroid, 433 Eros. Like Gaspra and Ida, asteroid Mathilde has survived blow after blow of cosmic impacts. Its surface is covered with the crater scars of past collisions that have disfigured the asteroid’s shape, like the battered and scarred face of a professional boxer who has just lost a fight. Huge pieces have been removed from Mathilde, leaving four enormous craters tens of thousands of meters across.
On Valentine’s day 14 February 2000, the Near Earth Asteroid Rendezvous, abbreviated NEAR, spacecraft became the first to orbit an asteroid, arriving at 433 Eros after a four-year journey from Earth. The NEAR craft was the first in NASA’s Discovery Program of no-frills, scientifically focused, low-cost missions, designed to do quality science in a “faster, better, cheaper” mode. The mission took just 26 months from start to launch, at a bargain total cost of $224 million. The car-sized vehicle circled Eros for a year, landing on the asteroid on 12 February 2001, another historic first. In the meantime, NASA renamed the spacecraft NEAR Shoemaker in honor of the astronomer-geologist Eugene M. Shoemaker (1928-1997), a pioneering expert on asteroid and comet impacts.
Radio tracking of the orbiting spacecraft was used to determine the mass of Eros, which weighed in at 6.687 million billion, or 6.7 x 1015, kilograms, about one-billionth the mass of Earth. That means that most adults would weigh less than a few ounces if standing on Eros, about as much as a bag of airline peanuts. And on Eros you could jump thousands of meters high, never to return. The gravity is so slight that NEAR Shoemaker had to keep its speed down to about 5 thousand meters per hour to stay in orbit, moving about as fast as a casual bicyclist. If it moved at a faster speed, the spacecraft would escape the asteroid’s feeble gravity and move into interplanetary space.
Eros is a warped and misshapen world, with heavily cratered expanses abutting relatively smooth areas. The asteroid’s biggest crater measures 5.5 thousand meters across, and most of the surface is peppered with smaller craters. The NEAR scientists spotted 100 thousand of them.
Eros has also been smoothed and rounded by glancing blows during its catastrophic past. This cosmic sculpture rivals the smaller bronze and marble forms of Constantin Brancusi (1876-1957) and Henry Moore (1898-1986). Equally beautiful is the broad, curved, saddle-like depression that connects two mountains on Eros, each thousands of meters high.
Far from being a barren lump of rock, Eros has a dusty, boulder-strewn landscape. Despite its weak gravity, the diminutive asteroid has managed to hold on to about 7 thousand boulders larger than 15 meters, forced out of craters and pulled back to the surface during the relentless bombardment of its past. Some of the isolated stones are as large as a house, and up to 100 meters across. The positions of the boulders on Eros indicate that at least 3 thousand of them were scoured out of a single crater by a colliding projectile a billion years ago. Some of these boulders went straight up and straight down. Most of the reminder traveled as far as two-thirds of the way around the asteroid, in all directions, before finally coming to rest on the surface.
Smaller rocks and a loose layer of dirty debris came into view when the NEAR Shoemaker spacecraft moved in to land on the boulder-strewn surface of 433 Eros. It took pictures as close as 125 meters above the surface, showing features as small as a golf ball. They indicate that, Eros is something between a very big rock and a planet, large enough to hold onto its pieces yet too small to lose its odd, distorted shape.
There are two hypotheses for the internal structure of asteroids. According to the rubble pile hypothesis, an asteroid is the reassembled debris of previous impacts, with a porous interior that is literally filled with holes. In this interpretation, an asteroid consists of smaller pieces loosely held together by their mutual gravitational attraction. As mentioned earlier in this chapter, investigations of asteroid rotation rates suggest that many of them are not solid, and that there is nothing stronger than gravity holding them together. An alternative scenario proposes that asteroids are solid inside, and held together by their own material strength. It turns out that both ideas may be correct, depending on the asteroid. Over billions of years, asteroids on intersecting orbits have collided with enough force to shatter and break them into pieces. Instead of dispersing, the fragments might have re-accumulated into a rubble pile. Asteroid 253 Mathilde, a dark, carbon-rich, C-type asteroid is an example of such an asteroid. On the other hand, the collisional energy may have been enough to throw the fragments far into space, leaving a solid, chipped rock behind. The bright, silicate, S-type asteroid 433 Eros is this kind of object.
Most meteors, or shooting stars, are produced by tiny fragments of comets, which burn up in the air and never reach the ground, but occasionally a stone will fall from the sky, producing a brilliant trail of light flashing across the night sky. A rumbling sound and what appears to be a great burst of sparks may accompany it. These are fireballs and they are produced by tougher chunks of matter from space, resembling rocks.
Extraterrestrial chunks of rock and metal that survive the fiery descent through the atmosphere and reach the ground have been given the name meteorite. And strictly speaking a meteoroid is the solid object in space that appears as a meteor when it lights up in the atmosphere and becomes a meteorite if it reaches the ground.
The Antarctica lode
In 1969, a group of Japanese scientists discovered a bountiful source of meteorites on the blue ice fields at the bottom of the world, leading to a dramatic increase in the number of recovered meteorites. During the next three decades, about 20 thousand cosmic rocks were harvested from the Antarctic ice. The most productive areas were near the Allan Hills in Victoria Land and the Yamato Mountains in Queen Maud Land.
Most meteorites that have been seen to fall and then recovered are stones, rather than chunks of metal. About 94 percent of the fallen meteorites are stones, 5 percent irons and 1 percent stony-irons. About 90 percent of the stony meteorites are, in turn, classified as chondrites. So most of the meteorites that fall to Earth are chondrites. The name “chondrite” is derived from the ancient Greek word, chondros, meaning “grain” or “seed”. The other 10 percent of the stony meteorites are achondrites, which show signs of past igneous activity.
Rare and exotic finds
The frozen cargo of the Antarctica ice includes at least a dozen, greenish-brown meteorites that are strikingly similar to the welded highland rocks from the Moon. The abundance of various elements and gases in these meteorites are virtually identical to those found in lunar rocks; at the same time, they are unlike those found in any other known meteorite or terrestrial rock. These small stones were blasted off the Moon by impacting objects.
Out of the thousands of stony meteorites now found in terrestrial collections, roughly a dozen are believed to be pieces of Mars. They were blasted into space by impacting objects, with such force that they escaped the red planet’s gravitational pull and eventually reached Earth. One of them, dubbed ALH (for Allan Hills in Antarctica) 84001 contains controversial evidence for ancient microscopic life on Mars.
There is little doubt that most of the meteorites have come from the asteroid belt. They are probably chips off wayward asteroids, and there are two pieces of direct evidence for this conclusion.
Photography of meteorites as they descend through the Earth’s atmosphere can be used to determine their precise speed and direction of motion when they encountered the Earth. From these data, their orbits may be inferred, and many of the objects came from space beyond Mars, in the main belt of asteroids.
The surface composition of asteroids has been inferred by breaking down their reflected sunlight into its component colors. Such spectral displays are similar to those of meteorites, suggesting that the meteorites are the debris of colliding asteroids. The relative abundance of asteroid types are not like those of the fallen meteorites, but this may simply reflect the ease or difficulty in sending asteroid fragments to Earth.
In addition, there is some compelling indirect evidence
3. Crystalline structure
When the majority of iron meteorites are cut and polished and then are etched with acid, a delicate and complex pattern emerges. It is produced by regions of crystalline structure, depending on the local orientation of the crystals in the iron. The sizes and shapes of these crystals indicate that they grew very slowly, and that the meteorite must have been hot, almost to the melting point, for tens of million of years. It probably cooled at the rate of a few degrees in a million years.
So the crystalline patterns of meteorites also suggest an asteroid-meteorite connection. Additional suggestive evidence is the relationship among the sizes of asteroids, meteorites, and non-cometary meteoroids. The classes are not mutually exclusive, and there is considerable overlap. The simplest explanation of all the evidence is that meteorites are the debris of collisions among the asteroids.