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Why Voyage to an Asteroid?
Rocks on Earth are a record of our planet's history, telling of dinosaurs, ancient oceans, and shifting continents. But Earth is so dynamic that the record of our planet's earliest history, the first billion years, has been largely lost to erosion and plate tectonics. Rocks on the airless Moon record earlier events, but even the Moon was active enough to obliterate the record of the earliest formation of the planets. Most asteroids, however, are believed to be nearly unchanged since the Solar System's first hundred million years.
Earth rocks, rocks on the Moon, and rocks on asteroids contain
information about the various periods in our Solar System's history.
Asteroids and pieces of asteroids regularly fall to Earth. The impacts of the largest incoming asteroids devastate the environment and are believed to have caused mass extinctions of many living species, including the dinosaurs. Smaller pieces of asteroids which survive the fall, are called meteorites. Their chemistry records the happenings during formation of our planet from the solar nebula (the whirling disk of dust and gas that condensed to become the Solar System). Unfortunately meteorites are like pages torn from an ancient manuscript; we don't know which asteroids are the sources of which meteorites, so the "big picture" of the evolution of the early Solar System remains mysterious and controversial.
A giant asteroid impact into the shallow seas north of Mexico's Yucatan peninsula
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is thought to have created the environmental devastation that killed the dinosaurs.
The Peekskill meteorite left a blazing trail through
Earth's atmosphere before it reached the ground.
Asteroids, or "minor planets," are numbered in order of their discovery. The first asteroid (1 Ceres) was discovered on January 1, 1801, by Italian astronomer Guiseppe Piazzi. Most asteroids orbit the sun in a broad disk between the orbits of Mars and Jupiter, the so-called "asteroid belt." Others have orbits that cross inside of the orbit of Mars and approach Earth's orbit. The latter are "near-Earth asteroids." Asteroids are thought largely to be debris from solar system formation that never accreted into major planets. A small fraction of asteroids are also thought to be dead comets.
Asteroids come in two main flavors. "C-type" asteroids are dark and gray, and thought to be rich in carbon. They prevail in the outer part of the Solar System. "S-type" asteroids are brighter and rich in minerals that make up the interiors of Earth, the Moon, and Mars. Prior to NEAR, the only up-close information on asteroids came from the Galileo spacecraft as it flew by 951 Gaspra and 243 Ida on its way to Jupiter. Both Gaspra and Ida proved to be irregularly shaped, heavily cratered, and mottled with subtle color variations that hint at different rock types. Ida even has a small moon, named Dactyl.
Ida, and its small Moon Dactyl, shown here in false color,
were imaged by the Galileo spacecraft in 1994.
Gaspra, shown here in enhanced color, was the first asteroid
imaged by a spacecraft, the Galileo spacecraft in 1991.
This "ordinary chondrite" meteorite is made of the same minerals
that form the interiors
of Earth, the Moon, and Mars, but it is little changed since formation
of the Solar System.
The images of Gaspra and Ida showed us irregularly-shaped, cratered surfaces, but there is much more we don’t know. For example, are they solid chunks of rock or piles of rubble bound loosely by their own gravity? In what manner, chemically or physically, have their surfaces been altered by hundreds of millions of years of exposure to space? How are they related to meteorites? And what does it all mean in our understanding of Solar System evolution?
That's where NEAR comes in. NEAR will address these questions with detailed, systematic measurements that can only be made from orbit. 433 Eros was chosen as NEAR's target because it is both relatively large and close to Earth. Eros is elongated and shaped like a potato, about 35 kilometers long and 14 kilometers wide (22 miles long, 9 wide). It orbits the Sun once every 643 days -- one Eros year. One "day" on Eros, its period of rotation, is only 5 hours and 17 minutes long.
This montage of asteroids visited by spacecraft shows their sizes
relative to Eros.
Comments are welcome
Copyright © 1998-1999 JHU APL All Rights Reserved
Created: 20 Dec, 1998
Revised: 22 Jan, 1999
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