NASA Studies Asteroids for Clues of Early Solar System

By Tom Whaley, a solar system ambassador in the outreach program of NASA's Jet Propulsion Laboratory. He lives in Vero Beach.

Asteroids are rocky fragments left over from the formation of the solar system about 4.6 billion years ago. Most can be found in orbiting the Sun in a belt between Mars and Jupiter. This Asteroid Belt probably contains millions of asteroids ranging in size from Ceres, which at 585 miles in diameter is about one-quarter the diameter of our Moon, to bodies that are less than one-half mile across. Scientists believe that the strong influence of Jupiter's gravity overcame the gravitational attraction of the asteroids for each other and prevented them from coming together to form a planet.

As asteroids revolve around the Sun, Jupiter's gravity and occasional close encounters with the planet Mars or with another asteroid knock them out of the Main Asteroid Belt and send them into space across the orbits of the planets. Asteroids are composed of rock and metals and are normally dark in color making them difficult to see since they reflect little light. When their orbit brings them toward Earth from the direction of the Sun, they are basically unobservable with current tracking techniques. Asteroids from this "blind spot" have created quite a stir in the past year with close passages to the Earth.

NASA's Galileo spacecraft was the first to observe an asteroid close-up, flying by asteroids Gaspra in 1991 and Ida in 1993 on its way toward its main mission at Jupiter. During its visit with Ida, scientists were surprised to find that the asteroid has a small moon, named Dactyl, the first known moon of an asteroid.

In 1996, NASA launched the Near Earth Asteroid Rendezvous spacecraft, called NEAR for short and later renamed NEAR Shoemaker in honor of Dr. Eugene Shoemaker, a legendary geologist who influenced decades of research on the role of asteroids and comets in shaping the planets. In June 1997, enroute to its primary target, the asteroid Eros, NEAR flew by asteroid Mathilde, making high-resolution pictures, measurements of brightness, and studies of chemical composition.

NEAR began orbiting the asteroid Eros on February 14, 2000, returning the highest resolution images ever made of an asteroid, as well as measuring its physical properties. Eros is the largest of the near-Earth asteroids whose orbits cross that of the earth. These near-Earth asteroids are of particular interest because of their potential for collision with Earth, as well as for the clues they hold about the nature of the small bodies from which the inner planets, including Earth, were formed. On February 12, 2001, NEAR Shoemaker made the first controlled descent to the surface of an asteroid and continued to transmit radio signals.

NASA is planning to launch the DAWN mission in 2006 to visit two of the largest asteroids in the main belt, Ceres and Vesta. DAWN will reach Vesta in 2010, study it for about a year, and then move on to Ceres where it will arrive in 2014. The mission aims to achieve an understanding of the conditions and processes acting in the first few million years of the solar system. DAWN will investigate the structure, density and other properties of these two asteroids that have remained intact since their formation.

Italian astronomer Giuseppe Piazzi discovered Ceres, named after the Roman goddess of agriculture, in 1801. It revolves around the Sun in 4.6 years and has a diameter of about 585 miles. Ceres appears to be a very primitive asteroid, still containing water-bearing minerals, and possibly having a weak atmosphere and frost on the surface.

Heinrich Olbers discovered Vesta, named after the Roman goddess of the hearth, in 1807. It takes 3.6 years to revolve around the Sun and has a diameter of about 320 miles. Vesta is dry and has a surface formed by early lava flows. Meteorites found on the Earth, believed to be from Vesta, indicate that it formed within five to fifteen million years of the birth of the solar system.

DAWN will also make use of an ion propulsion engine to get the additional velocity required to reach Vesta after leaving its booster rocket, and to move on to Ceres after completing its mission at Vesta. Ion propulsion makes efficient use of the onboard fuel by accelerating the spacecraft to a velocity ten times that of ordinary chemical rockets. It also permits a much smaller engine, and therefore a smaller spacecraft, saving weight and liftoff costs.

NASA's continuing study of asteroids will provide scientific information about the early solar system as well as early warning about those on collision course with the Earth.