The AsteroidsBetween the orbits of Mars and Jupiter are an estimated 30,000 pieces of rocky debris, known collectively as the asteroids, or planetoids. The first and, incidentally, the largest (Ceres), was discovered during the New Year's night of 1801 by the Italian astronomer Father Piazzi (1746–1826), and its orbit was calculated by the German mathematician Karl Friedrich Gauss (1777–1855). Gauss invented a new method of calculating orbits on that occasion. A few asteroids do not move in orbits beyond the orbit of Mars, but in orbits that cross the orbit of Mars. The first of them was named Eros because of this peculiar orbit. It had become the rule to bestow female names on the asteroids, but when it was found that Eros crossed the orbit of a major planet, it received a male name. These orbit-crossing asteroids are often referred to as the “male asteroids.” A few of them—Albert, Adonis, Apollo, Amor, and Icarus—cross the orbit of Earth, and two of them may come closer than our Moon; but the crossing is like a bridge crossing a highway, not like two highways intersecting. Hence there is very little danger of collision from these bodies. They are all small, 3 to 5 miles (4.8 to 8.0 kilometers) in diameter, and therefore very difficult objects to identify, even when quite close. Some scientists believe the asteroids represent the remains of an exploded planet.
On Oct. 29, 1991, the Galileo spacecraft took a historic photograph of asteroid 951 Gaspra from a distance of 10,000 miles (16,000 kilometers) away. It was the first close-up photo ever taken of an asteroid in space.
Gaspra is an irregular, potato-shaped object about 12.5 miles (20 kilometers) by 7.5 miles (12 kilometers) by 7 miles (11.2 kilometers) in size. Its surface is covered with a layer of loose rubble and its terrain is covered with several dozen small craters.
Close-up photos of Asteroid 243 Ida taken by the Galileo spacecraft on Aug. 28, 1993, revealed that Ida had a tiny egg-shaped moon measuring 0.9 miles by 0.7 miles (1.44 by 1.12 kilometers). The moon has been named Dactyl.
NASA's Near-Earth Asteroid Rendezvous spacecraft was launched on Feb. 17, 1996. (Near-Earth asteroids come within 121 million miles [195 million kilometers] of the Sun. Their orbits come close enough that one could eventually hit Earth.) It flew within 750 miles (1,200 kilometers) of minor planet 253 Mathilde on June 27, 1997, and took spectacular images of the dark, crater-battered world. The asteroid's mean diameter was found to be 33 miles (52.8 kilometers). The NEAR spacecraft discovered that the carbon-rich Mathilde is one of the darkest objects in the solar system, only reflecting about 3% of the Sun's light, making it twice as dark as a chunk of charcoal. The asteroid is almost completely cratered, and at least five of its craters just on the lighted side are larger than 12 miles (19.2 kilometers).
The spacecraft reached asteroid 433 Eros in Dec. 1998, but aborted its mission due to engine problems. NEAR measured Eros to be 21 miles (33.6 kilometers) long by 8 miles (12.8 kilometers) wide and 8 miles (12.8 kilometers) deep. It rotates once every 5.27 hours and has no visible moons.
The spacecraft was renamed NEAR Shoemaker in honor of geologist Dr. Eugene M. Shoemaker (1928–1997), who influenced research on asteroids and comets in shaping the planets. It made a successful rendezvous with Eros on Feb. 14, 2000, and began a year-long orbit of the asteroid. NEAR Shoemaker data showed that the asteroid's ancient surface is covered with craters, ridges, boulders, and other complex features.
The First Ten Minor Planets (Asteroids)
(millions of miles)