In video arcades across the country, teenage astronauts pay 25 cents to steer a laser-equipped starship through a gauntlet of floating space debris in a computer game called "Asteroids." Any asteroid that travels too close to the starship gets blasted into gravel.
Next spring, fact could mimic fiction when a Soviet space probe is scheduled to zap the surface of the Martian moon Phobos with laser and charged-particle beams. Phobos is thought to be an asteroid captured in the gravity of Mars. Although the laser on the Soviet probe will not significantly disturb the surface of Phobos, it should kick up enough stone and soil for capture and analysis as the spacecraft passes some 165 feet overhead.The Soviet mission comes at a time when interest in asteroids has reached an all-time high. Among other research objectives, "observation of asteroids and comets is essential to understanding how the solar system evolved," says Dr. Brian Marsden, an astronomer at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.
Asteroids - aka "minor planets," "planetoids" and, if they reach the Earth's surface "meteorites" - have long puzzled scientists trying to unlock the history of the solar system. Virtually billions of asteroids occupy the vast region between the orbits of Mars and Jupiter, ranging from irregular Texas-sized formations and Manhattan-sized boulders, to smaller rocks, gravel and sand. Each asteroid moves in its own orbit around the sun.
Astronomers first stumbled upon the asteroid belt in the early 1800s while searching for a missing planet thought to occupy that region of the solar system, roughly 170 million miles beyond Earth's orbit. Baffled by the tiny, moving pinpoints of light, astronomers labeled them "asteroids,' derived from the Greek word meaning starlike.
Today, space scientists study asteroid orbits, rotations and physical characteristics in an attempt to retrace the chain of events that gave rise to the solar system. Biologists look to asteroids as possible couriers of the first organic material on Earth some 3.5 billion years ago, while other scientists theorize that meteorite impacts caused major extinction events.
"Twenty years ago, asteroid science was in a rather inactive state," Dr. George Wetherill, as astronomer at the Carnegie Institution in Washington, D.C., says. "Since then, there has been an explosive increase in observations of the physical properties of asteroids and theoretical studies relating to the asteroid belt. Many new asteroids have also been discovered."
According to Wetherill, asteroids are primitive chunks of rocky and metallic matter that never coalesced to form planets. Nearly five billion years ago, he explains, a swirling cloud of interstellar dust and gas filled the solar system. In time, the dust coagulated into small bodies which, through additional accumulation, became the primary planets orbiting the sun.
"This accumulation was aborted in the asteroid belt, most probably by the gravitational effects of Jupiter, Saturn and the large quantities of dense gas in the outer solar system," Wetherill says.
Some process, possibly the gravitational pull of Jupiter and Saturn, "pumped up" the velocity of the asteroids into a "fragmentation state." Asteroids stopped clumping together and, instead, began fracturing upon impact with each other. "At present, the asteroid belt is slowly grinding itself down into smaller and smaller pieces," Wetherill says. "It has too much energy for the formation of a planet."
In certain regions of the asteroid belt, he continues, Jupiter's massive gravity pulled some of the swiftly moving asteroids out of their original orbits. Some crashed into Jupiter, others were catapulted into outer space or began wandering journeys outside the asteroid belt, crossing the paths of planets such as Mars and Earth. Gaps inside the asteroid belt indicate areas once populated by missing asteroids. "The cratering record on the moon, Mars or Earth is evidence of the final resting place of many wandering asteroids," Wetherill says.
Astronomers discover new asteroids, Marsden says, by taking time-exposure photographs of the sky through special telescopes. Because asteroids move, they appear as streaks on the photographic plates. An asteroid may be bright enough to be detected for only a few days or, at most, a month or two. Only after repeated sightings, perhaps over years or decades, is a new asteroid given an official number by the Center.
"A good deal of detective work is required to improve what we know about the orbit of a single asteroid," Marsden says. "So far, 3,904 minor planets have been assigned numbers, which means we know exactly where to find each one at any given time."
Much of what is known about asteroids, in fact, comes from the study of meteorites. "Most meteorites found on Earth are fragments from a larger asteroid," Wetherill says. "In turn, that `parent' asteroid was once probably part of an even larger body."
Minerals found in meteorites can sometimes indicate the size and details of how large asteroids formed, says Dr. Robert Fudali, a geologist and meteorite expert at the Smithsonian's National Museum of Natural History in Washington, D.C. Moreover, because the Earth and asteroids formed at the same time, meteorites offer a pristine record for studying, indirectly, geological changes on Earth over the millennia. The nature of the minerals found in meteorites can also give astronomers information about conditions, such as temperature, in the early solar system.
A closer look at asteroids might also lend insight to the origin of life on Earth. Many meteorites, for instance, have been found to be rich in amino acids, the building blocks of all living things. Asteroids, scientists say, may well be the source of the first organic material delivered to Earth.
Comets, too, may have delivered organic matter to Earth, "but generally, very little cometary material ever survives the plunge through the Earth's atmosphere," Wetherill says.
According to Marsden, comets and asteroids formed at roughly the same time. Asteroids formed closer to the sun and were baked dry. Comets formed in the cold, outer regions of the solar system and developed as so-called "dirty snowballs," hard-packed conglomerates of ice, stones and dust. Heat from the sun causes comet ices to vaporize, creating fiery-looking halos and long, glowing tails.
While some scientists credit asteroids as catalysts for life, others credit them with its disappearance. "Evidence seems to point to asteroid or comet collisions with Earth as the main drivers of mass extinctions," Dr. Eugene Shoemaker, a geologist at the United States Geological Survey in Flagstaff, Ariz., says. When large impacts occurred, he explains, the enormous quantities of dust thrown into the upper atmosphere may have blocked sunlight for months or years, causing temperatures to cool below tolerable limits for many life forms.