Between 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 largest, Ceres, was discovered Jan. 1, 1801, by the Italian astronomer Father Piazzi (1746–1826). Its orbit was calculated by the German mathematician Karl Friedrich Gauss (1777–1855), who invented a new method of calculating orbits on that occasion. Ceres was initially declared to be a new planet, then reclassified as an asteroid. In August 2006, the International Astronomical nion declared Ceres to be a dwarf planet (see Astronomical Terms, p. 381–382).

Ceres is approximately 580 mi (930 km) across, and contains about a quarter of the asteroid belt's total mass. NASA's Dawn mission is planned to visit Ceres in 2015, after first flying by the second-largest asteroid, Vesta, in 2011.

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 mi (4.8 to 8.0 km) 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 mi (16,000 km) 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 mi (20 km) by 7.5 mi (12 km) by 7 mi (11.2 km) in size. Its surface is covered with a layer of loose rubble and its terrain is marked by 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 by 0.7 mi (1.44 by 1.12 km). 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 mi (1,200 km) 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 mi (52.8 km). 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 lit side are larger than 12 mi (19.2 km).

On Feb. 14, 2000, NEAR successfully entered into orbit around Eros and remained in orbit for one year, taking photographs of the asteroid and gathering information about its composition, structure, size, and shape. The spacecraft landed safely on the surface of Eros in a controlled crash on Feb. 12, 2001, relaying information for another two weeks before being shut down. NEAR measured Eros to be 21 mi (33.6 km) long by 8 mi (12.8 km) wide and 8 mi (12.8 km) deep. It rotates once every 5.27 hours and has no visible moons. NEAR data also showed that the asteroid's ancient surface is covered with craters, ridges, boulders, and other complex features. NEAR was the first spacecraft to orbit an asteroid and the first craft to operate on solar power so far from the Sun.

NEAR gathered about 160,000 images of Eros, about 10 times more than was planned. The spacecraft was renamed NEAR-Shoemaker in honor of geologist Dr. Eugene M. Shoemaker (1928–1997), who researched the influence of asteroids and comets in shaping planets.

In March 2004, a 90-foot (30-meter) diameter asteroid made the closest-ever asteroid flyby of Earth, coming within 26,500 mi (43,000 km) of our planet. Scientists believed that had the asteroid (named 2004 FH) entered Earth's atmosphere, it would have broken up with no damage to the planet. A half-mile-wide asteroid flew by Earth in July 2006, just slightly farther away than the Moon.

The First Ten Asteroids Discovered¹