Minor Objects in the Solar System

The nine planets and at least 39 moons to the Sun than Mercury. It came are only a tiny part of the population that circles the Sun. There are huge numbers of smaller objects: comets, asteroids, large cosmic rocks and small pebbles, and innumerable particles of dust. Each object makes its own con- tribution to the solar scenery, and each has its own secrets to reveal. These small objects have been less studied, and none have yet been visited by a spacecraft. Our main source of information has been observations through the telescope and study of meteorites, which are part of the extraterrestrial materials that fall to Earth at a rate of several hundred tons per day.

Asteroids and meteorites

There is no planet between Mars and Jupiter. Instead, there are a great many small objects called asteroids where a planet might have formed. About 2000 have been observed sufficiently so that their orbits are known. Most of them are irregular rocks a few kilometers across, but a dozen are about 250 kilometers (160 miles) or more in diameter. The largest, Ceres, has a diameter of about 1000 kilometers (about 600 miles) and is roughly the size of Texas. It has been estimated that there may be a half-million asteroids larger than one kilometer (0.6 miles) in diameter, nearly all too small to be observed by current methods.

New member in the family.
A tiny trail of light across afield of fixed stars reveals the track of a recently discovered member of the small group of asteroids that cross the orbit of the Earth. The new arrival, Ra-Shalom, was 29 million kilometers (18 million miles) from Earth when this picture was taken. The grainy background is due to the great enlargement from the original negative. (Courtesy of Eleanor F. Helin, California Institute of Technology.)

Rare chunk of cosmic history.
White fragments in this dark, carbon-rich meteorite that fell on Mexico in 1969 contain minerals formed at high temperatures, perhaps among the first substances to condense as the solar system began to form from a huge cloud of hot gas and dust. Some of the white pieces also contain material that is chemically different from the rest of the meteorite, and which may have come from a nearby star that exploded shortly before the solar system was born. (Courtesy of Brian Mason, National Museum of Natural History, Smithsonian Institution.)

Most asteroids have orbits that remain between Mars and Jupiter, in a region called the asteroid belt, but a few have orbits that cross the orbit of the Earth. These "Earth-crossers" are called the Apollo-Amor asteroids (named after two of their number). The most famous member of the group is Icarus, discovered in 1949, and so named because it actually goes closer within 6.5 million kilometers (4 mil- lion miles) of the Earth in June, 1968.

Recent observations have detected other members of this Apollo-Amor group in the sky. The geological record on Earth also bears witness to these Earth-crossers, for about 100 ancient meteorite impact craters have been detected, each one possibly representing an asteroid that didn't quite make it past the Earth. Some of these collisions must have been unbelievable catastrophes; the two largest known craters, one in Canada and one in South Africa, are more than 100 kilometers (60 miles) in diameter.

Smaller bits of asteroids also collide with the Earth, although less violently and more frequently. These objects, called meteorites, have provided us with extraterrestrial samples for centuries, long before we were able to collect rocks from the Moon. We think that they come from the asteroid belt. If this is right, then there must be a great variety of objects out there, for meteorites are very different. Most meteorites are stones containing many glass droplets, but others are pieces of lava flows, chunks of solid nickel-iron, and even bits of dark, carbon-rich materials containing significant amounts of water.

Meteorites are among the most ancient solar system samples we have. They date from the formative stages of the solar system 4.5 billion years ago, and they are the only source of direct information about the physical and chemical processes that went on as the Sun and the planets grew from a collapsing cloud of dust and gas.

Studies of meteorites during the Space Age provide specific details about the solar system's birth and early years. Some meteorites seem to be pieces of actual lava flows that poured from ancient volcanoes on small asteroids heated by primordial radioactivity. The carbon-rich materials in some meteorites contains amino acids-so-called "building blocks" of life-which give us new insight into the possible occurrence of life in the universe. White rock fragments from a meteorite that fell in Mexico in 1969 are composed of high-temperature minerals that may have been the first materials to form as the solar system came into being.

illustration of the possible process by which our solar system was created

Sample from the stars.
An artist's impression shows how material from another star might have been trapped in meteorites as the solar system formed. The explosion of the star (supernova) sends a shock wave through interstellar dust clouds. Material from the star, carried along by the wave, is trapped in the first meteorites formed and thus preserved before it is strongly diluted with other solar system materials. It is possible that passage of the shock wave also induced the cosmic cloud to begin the condensation that gave birth to the solar system.

The white fragments document more than just the beginning of the solar system: They may contain records that actually date from "before the beginning."' Some of the fragments contain anomalous material that is chemically unlike the matter that makes up the solar system. (The chemical elements are the same, but the abundance patterns of certain elements are different.) This chemically unusual material did not come out of our own Sun. It may have come from another nearby star, perhaps flung out in a violent supernova explosion to enrich the cloud of gas and dust where the solar system later formed. The discovery also suggests that shock waves from the supernova passed through the cloud, triggering the condensation that eventually produced the Sun and the planets. Thus, through laboratory analyses of the meteorites, we can now see back in time beyond the formation of the solar system itself.

Meteorites also tell much about what the asteroids are like. Using ground-based telescopes and spectrometers, we are able to make crude chemical analyses of individual asteroids by analyzing the light that they reflect. Comparing these data with the results of meteorite studies, we have determined that the asteroids can be divided into families that are similar to the different groups of meteorites, although we have not yet been able to link a meteorite group uniquely to a specific asteroid.

The origin of asteroids has also been illuminated by meteorites. The asteroids were once thought to be the remnants of an exploded planet. Meteorite studies have disproved this idea, and the asteroids are now thought to have formed as small objects in a region where the gravitational tug-of-war between Jupiter and the Sun prevented a larger body from coming together.

We now know enough about asteroids to ask some exciting questions. Do they hold records of the actual formation of the solar system? How can such small bodies have developed with such a variety of compositions and histories? Among the unanswered questions are some that may be of more than scientific importance. How many asteroids are there? How many undiscovered ones cross the Earth's orbit? When might another catastrophic collision occur?

Comets

Comets are the shining wanderers of the solar system. With their glowing tails that may stretch 100 million kilometers (60 million miles) through space, they are conspicuous, remarkable, and exciting objects. Their appearances often have been superstitiously associated with disasters and (more recently) with brief periods of intense scientific study.

Most comets reside in the outer fringes of the solar system, far beyond Pluto. In fact, the solar system probably is surrounded by a huge cloud of more than a hundred billion comets that may stretch a third of the way to the nearest star. Almost all remain there in the frozen darkness, but on rare occasions they are perturbed (perhaps by the gravity of a nearby star) and a comet may be sent on a long trip down to the Sun and back out again.

For all their apparent size in the sky, comets are actually fairly small objects. When a comet begins its trip down past the Sun, it is probably a chunk of "dirty ice," a mixture of rock dust and ice a few kilometers across, much smaller than the typical observed asteroid. As it speeds toward the Sun, the heat from the Sun evaporates the ice, and the gases thus released blow dust particles outward from the solid body or nucleus. Radiation from the Sun ionizes the released atoms, producing a tail that glows in the sky like a neon sign; the dust particles reflect sunlight and form another, smoother tail. The tails seem white to the eye, but color photography reveals that the ionized gas tail is blue and the dust tail yellow. Although the tails may be bright, they are thin, and stars show through.

The small nucleus, the only even near-permanent part of a comet, is surrounded by the coma or head of the comet, a large, hazy structure formed by the liberated gas and dust.

From the coma, the tails sweep back in the direction opposite the Sun, driven by the pressure of sunlight and by the solar wind. In 1970, an instrument on the Earth-orbiting satellite OAO-2 (named the Orbiting Astronomical Observatory) revealed that comets are surrounded by huge clouds of hydrogen gas, produced when ultraviolet light from the Sun decomposes the cometary gas. Later, the hydrogen cloud of the bright Comet Kohoutek was scanned by a photometer on Mariner 10, and the comet was photographed in ultraviolet light from Skylab. Aboard Skylab, astronaut Ed Gibson was able to view the comet when it was very close to the Sun, and he discovered a unique "anti-tail" pointing toward, rather than away from, the Sun.

Of the 100 billion comets that may exist, less than 1000 have been observed thus far as they make the longjourney down to the heat of the Sun. Some comets now are trapped in smaller orbits, taking from a few to a few thousand years to complete one lap. Even so, most of them spend the bulk of their time in the cold, dark, outer solar system and just a tiny portion of their lives speeding in glowing splendor past the Sun.

Photo of a shooting star descending behind the horizon

Shining wanderer.
Shining wanderer. A visitor from far beyond Pluto, Comet West seems to hover above Table Mountain in California shortly before sunrise in March 19 76. The bright head of the comet is seen just above the mountains, while its long broad dust tail sweeps up and back from the nucleus, pushed outward by the pressure of sunlight. Comet West passed within 118 million kilometers (73 million miles) of Earth and will not return for another 560,000 years. (Courtesy of the Jet Propulsion Laboratory.)

Comets probably formed in the freezing darkness beyond Jupiter and perhaps beyond Pluto where they spend most of their lives. As a result, comets are probably the most primitive and the most unchanged original solar system materials that we can ever hope to sample and to study.

We have never yet sent a spacecraft to a comet. What we have learned during the Space Age has come from ground-based telescopes, from a few unmanned satellites and sounding rockets, and from studies made in the Skylab space station. But excitement is building for a major event: the reappearance of Halley's Comet in 1985. Swinging on a long orbit that brings it by the Sun every 76 years,

Halley's is one of the best-known and brightest comets, and scientists are already planning observing programs and possible spacecraft missions that may remove much of the mystery from this famous visitor.

A speck from a comet?.
Displayed in a close-up under an electron microscope, this tiny bit ofcosmic dust may be ourfirst sample of a passing comet. Less than one-tenth of a millimeter across, the particle is composed ofmillions ofeven tinier crystals. Although chemically similar to some meteorites, itsfluffy, crystalline structure is unlike that of any known meteorite. Interplanetary dust particles like this are trapped in special collectors flown aboard high-altitude aircraft. Their interplanetary origin is established by analyzing the gases that they trapped from the Sun while still in space. The interplanetary dust is believed to come from comets, which shed material as they are warmed by the Sun. It may be possible to collect material from a particular comet when one passes close enough to the Earth some day.

Dust

There is no lower limit to the sizes of the solid particles that move around the Sun. Small asteroids grade downward into large meteoroids and then into smaller pebbles and so on down to the tiniest particles of dust. The most numerous particles are the smallest ones. A particle larger than a millimeter (about one twenty-fifth of an inch) in diameter is a relative rarity in space, but even smaller particles exist by the uncountable billions. There are enough of them to reflect sunlight in a faint glow, called the zodiacal light.

Unlike planets and other large objects, dust particles are not permanent residents of the solar system. They spiral slowly inward toward the Sun. Over a million years or so, a typical particle will fall into the Sun, so that the current dust population must consist of fairly new arrivals, presumably shed from comets.

Because interplanetary dust particles may be actual samples of comets, strenuous efforts have been made to collect them. Many efforts failed because of the rarity of the particles and the contamination of collecting devices by terrestrial dust. Recently, however, extraterrestrial dust particles have been successfully trapped with collectors mounted on high-flying aircraft.

The yield has been small so far: only about a hundred particles a few thousandths of a millimeter across. But recently developed instruments are so sensitive that even these tiny objects can be usefully studied. They are definitely extraterrestrial, for their chemical composition is like that of common meteorites (and not like that of the Earth), but they are fluffy, fragile objects, each particle a mosaic of millions of tiny crystals.

As we look ahead to the reappearance of Halley's Comet, we are continuing to collect and study the dust that may have been shed by comets in the past. Perhaps when Halley does appear, we may be able to do more than just look at it. We may be able to collect and analyze the very dust that it sheds as some of those tiny fragments drift down to our planet.