Asteroids are small celestial bodies ranging in size from a mile or two in diameter to almost 500 miles. They orbit the Sun in the same direction as the principal planets, although their orbits, as a rule, are far more eccentric; most revolve about the Sun in a cosmic zone between the orbits of Mars and Jupiter.
This February 1996 launch from Kennedy Space Center sent the Near Earth Asteroid Rendezvous (NEAR) spacecraft off on a three-year flight to a rendezvous with a 25-mile-long asteroid known as 433 Eros.
Asteroids have on occasions in the distant past collided with Earth and left evidence of great devastation; one theory holds that an asteroid impact with Earth caused the demise of the dinosaurs. These mysterious cosmic objects clearly have potential for influencing the evolution of the atmosphere and life on Earth. They also offer clues to the nature of the early solar system processes and conditions, which have long been erased by evolution on Earth and other large bodies, but which are preserved in various forms on these small bodies that do not have sufficient gravitational power to retain an atmosphere.
All of which makes asteroids prime targets for comprehensive scientific investigation. Such an investigation is under way; before the century ends, scientists will know a great deal more about asteroids than they do today.
The investigation formally began on February 17, 1996 with the launch of the Near Earth Asteroid Rendezvous (NEAR) spacecraft on a three-year odyssey toward a rendezvous with a 25-mile-long asteroid known as 433 Eros. In February 1999, a firing of NEAR's main thruster will put the spacecraft into an orbit around the asteroid-the first spacecraft ever to accomplish such a feat-to circle Eros for a year, coming at times as close as 10 miles from its surface.
An international project conducted in cooperation with Germany and France, NEAR is designed to image the asteroid and measure its size, shape, volume, mass, gravity field and spin; its elemental and mineral composition; its mass distribution and magnetic field. Built by Johns Hopkins University's Applied Physics Laboratory (APL), the spacecraft measures approximately nine feet in length, five feet in diameter and weighs some 1,800 pounds. APL is NASA's manager for the project.
NEAR is an exciting and important project in its own right, but it takes on added significance by virtue of its status as the prototype, or pathfinder, of a relatively new NASA program known as Discovery, an ongoing effort to foster development of low cost spacecraft that will enable frequent solar system exploration missions. As the first flight project of that program, NEAR bears the responsibility for demonstrating that the concept of low-cost planetary exploration is valid.
NEAR was built under a ceiling of $150 million, a fraction of the cost of some interplanetary missions, and it was completed in only two years under a new streamlined management approach. Although cost was a primary consideration throughout the development period, NASA and APL did not compromise capability in the search for economies. NEAR has a full complement of five sophisticated instruments and multiple redundancies in its operating systems.
NEAR exemplifies one aspect-solar system exploration-of NASA's broad space science program, whose primary missions and goals embrace these general areas of investigation:
Examining the content, structure, origin and evolution of the galaxy and the universe;
Defining the relationships among the Sun, Earth and the heliosphere;
Seeking greater understanding of the origin and evolution of planetary systems;
Pursuing greater understanding of the origin and distribution of life in the universe;
Observing Earth's air, land, water and life resources toward greater understanding of the complex mechanisms that control Earth's behavior.
Selected examples of ongoing and upcoming programs in these areas are contained in the following pages.