Asteroids are rocky and metallic objects that orbit the Sun, but are too small to be considered planets.  They are also known as minor planets and have diameters up to about 1000 km.  Each asteroid is designated by a number showing the order of discovery and a name: 1 Ceres, was the first asteroid discovered (in 1801) and is the largest asteroid.  The majority of asteroids are contained within the main belt that exists between the orbits of Mars and Jupiter, roughly 2-4 AU from the Sun.  An AU or astronomical unit is the average distance from the Earth to the Sun, and equals 149,597,870 km (or 97,750,679 miles).  However, a substantial number of asteroids have orbits inside that of Mars that bring them closer to Earth.  These are the Near-Earth Asteroids (NEAs).  While more than 6000 asteroids have been identified in the main belt, only 250 NEAs have been discovered with only 3 having diameters greater than 3 km.  NEAs are thought to originate as either main-belt asteroids that have been perturbed by Jupiter into their present orbits or possibly extinct comets in short-period orbits.

Three main-belt asteroids have been imaged in some detail by spacecraft as they passed by on their way to their primary objective.  The Galileo spacecraft on the way to Jupiter imaged asteroids 951 Gaspra on October 29, 1991 and 243 Ida on August 28, 1993.  The NEAR (Near Earth Asteroid Rendezvous) Mission imaged asteroid 253 Mathilde on June 27, 1997 during its cruise to the near-Earth asteroid 433 Eros.
 
 

                                                Asteroid Gaspra imaged by the Galileo spacecraft.

 
 
                                                    Asteroid Ida imaged by the Galileo spacecraft.
 
 

                                                        Asteroid Mathilde imaged by the NEAR
                                                                        spacecraft.

Asteroids appear to have differences in composition based on their albedo and the spectrum of sunlight reflected from their surfaces.   The reflected spectrum contains characteristic features of minerals on the asteroid.   Based on these reflectance spectra, most asteroids are classified into one of three main groups.

• C-type: This type includes more than 75% of known asteroids.   They are extremely dark and are thought to consist mostly of rock and carbon-hydrogen compounds.  They may be associated with carbonaceous-chondrite meteorites.
• S-type:   This type includes about 17% of known asteroids.   They are relatively bright and are thought to be made of various silicate mixtures with iron and magnesium and may be associated with common meteorite types such as ordinary chondrites.
• M-type:   This type includes most of the rest of the known asteroids.   They are also relatively bright and are thought to contain nickel-iron along with silicates.   These are thought to be associated with iron meteorites.

Primary scientific goals of the mission are to measure:

• Bulk properties: size, shape, volume, mass, gravity field, and spin state.
• Surface properties: elemental and mineral composition, geology, morphology, and texture.
• Internal properties: mass distribution and magnetic field.

                                        Artist concept of the NEAR spacecraft orbiting Eros.

To meet these science objectives, the NEAR spacecraft has the following scientific instruments onboard:   a multi-spectral imager, X-ray and gamma-ray spectrometers, a near-infrared spectrometer, a magnetometer, a laser range finder and the use of the telemetry system for radio science.