A Quick Look at Asteroid 433 Eros
433 Eros is one of the largest and best-observed near-Earth asteroids. Discovered independently by Gustav Witt (Germany) and August H.P. Charlois (France) in 1898, Eros accounts for nearly half the volume of all near-Earth asteroids. The potato-shaped space rock's dimensions of 21 miles (33 kilometers) long, 8 miles (13 kilometers) wide and 8 miles thick make it about twice the size of Manhattan.Eros orbits the Sun with a perihelion of 1.13 Astronomical Units (169,045,593 kilometers) and an aphelion of 1.78 AU (266,284,209 kilometers), and it rotates once every 5 hours and 16 minutes.
The gravity on Eros is very weak but enough to hold the NEAR Shoemaker spacecraft. Depending on where they stood, a 200-pound person on Earth would weigh about 2 ounces on Eros. A rock thrown from the asteroid's surface at 22 miles an hour (10 meters per second) would escape into space.
The estimated "daytime" temperature is about 212° F (100° C), while at "night" it plunges to -238° F (-150° C).
Eros is an S-type asteroid, the most common type found in the inner asteroid belt . Asteroids are classified by their albedos and colors as determined by spectrographic observation. The spectra of S-types imply a composition of iron- and magnesium-bearing silicates (pyroxene and olivine) mixed with metallic nickel and iron. Scientists try to match the asteroid's spectra with the mineralogy of meteorites found on Earth.
Ordinary chondrites, the most common meteorites, seem primitive and relatively unchanged since the solar system formed 4.6 billion years ago. Stony-iron meteorites, on the other hand, appear to be remnants of larger bodies that were once melted so that the heavier metals and lighter rocks separated into different layers.
Eros is spectrally similar to both ordinary chondrite and stony iron meteorites, but its composition more closely matches the ordinary chondrites.
Though it is an S-type asteroid, Eros may be somewhat varied in its chemical composition. Ground-based telescopic data suggest that its opposite sides have slightly different mineralogies. NEAR Shoemaker is searching for spectral and compositional variations at Eros.
The Debate:
There has been a long-standing scientific debate over whether S asteroids are undifferentiated bodies related to ordinary chondrite meteorites, or whether they are geochemically processed bodies akin to stony-irons. The issue is an important test of our understanding of the connection between asteroids and meteorites. If S asteroids are unrelated to ordinary chondrites, then where do these most common of all meteorites come from? If the two are related, then why don't they look spectrally identical?Three decades of increasingly detailed telescopic observation have not resolved the dilemma. Even Galileo's flyby observations of two S-type asteroids, Gaspra and Ida, have not provided a solution. The difficulty is that remotely sensed spectral data cannot accurately determine the relative abundance of the three key minerals present in such surfaces: olivine, pyroxene, and nickel–metal. A precise measurement of the abundances of key elements (iron, silicon, magnesium, and so on) associated with such minerals would resolve the dilemma once and for all. This is a major goal of the X-ray and gamma-ray investigations on NEAR.
