Spaceflight Now: Breaking News

Tiny robotic probe unlocking secrets of ancient asteroid
Posted: September 22, 2000

NEAR Shoemaker obtained the two images in this mosaic on September 16 from a vantage point 62 miles) above Eros. This view shows one of the asteroid's ends, and is interesting because the area appears very flat and smooth, with few impact craters. The largest craters (one just left of center and another at bottom right) have degraded to where they no longer have rims, which implies that they are relatively old. Photo: JHU/APL
Findings from NASA's Near Earth Asteroid Rendezvous (NEAR) mission confirm that asteroid 433 Eros is a consolidated, primitive sample from the solar system's beginnings.

"We can now say that Eros is an undifferentiated asteroid with homogeneous structure, that never separated into a distinct crust, mantle and core," says NEAR Project Scientist Dr. Andrew F. Cheng of the Johns Hopkins University Applied Physics Laboratory in Laurel, Md., which manages the NEAR mission. "We have definitive mass and density measurements plus spectacular images and movies showing ridges, pits, troughs and grooves that provide fascinating clues about its history."

NEAR is the first in-depth study of an asteroid. Now more than halfway through a yearlong orbit mission that began Feb. 14, 2000, the NEAR Shoemaker spacecraft has taken more than 103,300 images and extensive measurements of Eros' composition, structure and landforms, at distances ranging from 22 to 220 miles (35 to 350 kilometers).

NEAR Shoemaker's multispectral imager and now-silent infrared spectrometer have returned a flood of observations revealing heavily cratered expanses abutting relatively smooth areas. The asteroid's largest crater measures 3.4 miles (5.5 kilometers) wide and sits opposite from an even larger 6.2-mile (10-kilometer), saddle-shaped depression. Ejecta blocks - rocks and boulders created by impacts - are abundant and measure up to 330 feet (100 meters) across, although they are not distributed evenly on the asteroid.

Some areas are heavily saturated with craters wider than 660 feet (200 meters). Images taken from lower orbits also reveal "younger" sections where craters have been filled or covered by loose material.

More than 8 million measurements taken by the laser rangefinder to definitively establish Eros' shape have determined the asteroid is a consolidated body rather than an agglomerate of loosely bound, much smaller components. Such a "rubble pile" structure has been inferred for many asteroids, but does not apply to Eros. Its irregular, peanut shape - which a body as large as Earth could not maintain - houses a homogeneous internal structure. Although Eros is consolidated, the ubiquitous fabric of ridges and grooves suggests an extensively fractured interior.

This montage shows structural features on Eros obtained by NEAR Shoemaker from several different orbits. Any underlying, deep-seated structures probably formed as a result of large impacts to the asteroid's surface, in the same way that a car windshield fractures when it is hit by a stone. Later, movement of regolith may have modified the structure's surface expressions. Photo: JHU/APL
NEAR Shoemaker's X-ray spectrometer has detected low levels of aluminum relative to magnesium and silicon, indicating an undifferentiated composition. Eros, or the parent body it could have broken from, has not experienced the extensive melting process that planets like Earth undergo in their development. This finding leads researchers to believe that Eros may be related to the primitive ordinary chondrites, the most common type of meteorite. NEAR Shoemaker's imager and infrared spectrometer have also found spectral properties consistent with a primitive, chondritic composition.

Using ground-based Doppler and range measurements - and by tracking surface landmarks - scientists have determined the asteroid's mass is 6.687 x 1015 kilograms and density is 2,700 kg/m3, which is about the average density of Earth's crust. The density is relatively uniform throughout the asteroid.

Eros has a stable rotation and an escape velocity that ranges from 3.1 to 17.2 meters per second, which would allow a baseball thrown from its surface to leave forever. The acceleration of gravity on the surface of Eros varies from 2.3 to 5.5 millimeters per second squared, thousands of times smaller than on Earth. A person who weighs 150 pounds on Earth would weigh from 0.56 to 1.3 ounces on Eros - about as much as one or two bags of airline peanuts.

The research detailed in the four Science articles covers the first six months of orbit around Eros. NEAR Shoemaker moves in for a low- altitude flyover of Eros on Oct. 25, 2000, coming within 3.7 miles (6 kilometers), and will end the mission in February 2001 with a slow, controlled descent to the asteroid's surface. The spacecraft is currently 109 million miles (176 million kilometers) from Earth, circling Eros at just under 5 miles per hour.