Asteroid | Research & Encyclopedia Articles

Asteroid

Johannes Kepler was the first to postulate the existence of a hidden planet between Mars and Jupiter, a theory long pondered by astronomers. Then, in 1766, Johannes Titius (1729-1796), a professor of mathematics and physics in Germany, developed a formula for calculating planetary distances that seemed to suggest a planet belonged between Mars and Jupiter. When the planet Uranus was discovered in 1781, it fit into the formula, causing many scientists to be even more certain a hidden planet existed. One astronomer, Franz Xaver, proposed the formation of a society of astronomers that would be responsible for looking in assigned areas of the sky for the mystery planet.

One man, Father Giuseppe Piazzi (1746-1826), was involved in such a search at this time. During the night of New Year 's Eve, 1800, he saw a small star in Taurus. Since he could not find it listed in star catalogues, he observed it over several nights; Piazzi discovered that the body moved relative to the fixed stars, so it had to be an object that belonged to the solar system. Piazzi gave this object the name of Ceres, the patron goddess of Sicily. Piazzi was unable, however, to calculate Ceres's orbit from so few observations. A German mathematician, Carl Friedrich Gauss, became intrigued with the problem and invented a new method for orbit calculations. Using his technique, the small object was rediscovered in the winter of 1801-02. That same winter, another German, Heinrich Olbers (1758-1840), found a second planetoid: Pallas.

This second discovery sparked a debate: were these two objects remnants of some planet's catastrophe, or did they always exist in their present form? We now know that all the asteroids together would produce an object much smaller than our moon, so it is unlikely they were ever in one piece. The asteroids appear to be leftovers from the formation of the solar system out of the solar nebula.

In 1804 and 1807 two more asteroids were found. The third was called Juno, and the fourth was dubbed Vesta. These were the only planetoids found until the mid-1800s, when telescopic equipment and techniques improved. From 1854 until 1870 five new asteroids were discovered every year. The all-time champion asteroid hunter in the days before photography was Johann Palisa (1848-1932) who found 53 by 1900 and added many more before his death.

In 1891 the German astronomer Maximilian Wolf (1863-1932) began using photographic techniques to search for asteroids. He had his telescope set up to follow the apparent motion of the stars, so that any other object like an asteroid would produce a short line in a photographic image rather than a dot like the stars. There had been about 300 asteroids found up until his time, but the use of photography opened the floodgates. Wolf alone discovered 228 asteroids. Astronomers now estimate that roughly 100,000 asteroids exist that are bright enough to appear on photographs taken from Earth. Several research programs are underway as of 1998 to scan the sky for this multitude of undiscovered asteroids, particularly those that may pass close to Earth (see below).

Unmanned spacecraft have returned close-up photos of several asteroids. The Galileo spacecraft, bound for Jupiter, passed by and photographed asteroid Gaspra in October 1991 and asteroid Ida in August 1993. (The spacecraft itself was in little danger, since even in the main asteroid belt, between the orbits of Mars and Jupiter, the asteroids are no more crowded than would be a pair of flies with the state of Massachusetts all to themselves.) To astronomers' surprise, Ida was found to have a tiny moon, subsequently named Dactyl, that was only about 1.5 kilometers (1 mile) across. The Near-Earth Asteroid Rendesvous (NEAR) spacecraft returned photos of asteroid Mathilde in June 1997.

Not all asteroid reside peacefully in the main belt. The huge planet Jupiter has captured some planetoids, called Trojan asteroids, which are found in two clusters ahead and behind the giant planet. They gather at these two points because of the gravitational forces of the Sun and Jupiter.

In addition to the Trojans, there are other asteroids that have orbits which bring them into the inner regions of the solar system. Some of these asteroids have orbits that pass close to Earth's orbit; these are appropriately termed near-Earth objects (NEOs). In 1937, the NEO Hermes swept within 600,000 miles of the Earth (only twice the distance from the Earth to the Moon); in 1989 another asteroid came within 500,000 miles of our planet. A considerable stir was raised when astronomers announced, in early 1998, that the newly-discovered asteroid 1997 XF11 would pass perilously close to Earth in the year 2028, swinging a mere 30,000 miles from our planet. Unfortunately the initial data were released prematurely, and subsequent analysis and observations showed that 1997 XF11 would pass no closer than 600,000 miles -- close enough for Earth's gravity to alter the asteroid's orbit, but leaving the risk of an impact effectively zero.

Geologic evidence makes it clear, however, that NEOs do not always miss. One of the best-preserved of these non-misses is Meteor Crater in northern Arizona. It is 600 feet deep and more than half a mile wide, yet it was blasted into the Earth by an object no more than 30 meters (100 feet) across. It is now generally accepted that the impact of a much larger asteroid, in what is now the vicinity of the Yucatan peninsula, was responsible for the extinction of the dinosaurs.

Because they are remnants of the beginnings of our solar system, asteroids can provide astronomers with valuable information about the conditions under which the solar system was formed. Perhaps more profoundly, the extinction of the dinosaurs by an asteroid impact may have given the struggling mammals the foothold they needed to develop and thrive as a viable group of animals -- a group from which humans eventually evolved. Quite possibly the impact 65 million years ago that devastated the Cretaceous Earth set the stage for the life that would ultimately develop, thrive, and ponder its origins.