433 Eros
Published on Feb 14, 2008 at 9:10 pm.
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Filed under asteroids.
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Discovered in 1898 by Carl Witt, asteroid 433 Eros orbits the Sun once every 643 days. Unlike most asteroids, though, its orbit is closer to the Sun than the traditional asteroid belt between Mars and Jupiter. Eros comes as close to the Sun as 1.13 AU (just 12 million miles outside of Earth’s orbit) and gets as far as 1.46 Au from the Sun (about as far as Mars). Eros does not get as far from the Sun as Mars’ average distance from the Sun, but it does get farther than Mars’ perihelion distance, making Eros a Mars crossing asteroid. The asteroid’s orbit does not cross Earth’s orbit, but it does come close, making Eros a Near Earth Object. It will be passing within 17 million miles of Earth in about 4 years. Orbits like this, though, are not stable over long periods of time in the Solar System, so the orbit will be shifted eventually. Eros may eventually evolve into an Earth crossing asteroid, making it a threat to us.
If Eros became a threat, it would be a significant one. It is big. Eros has an elongated shape, about 13 kilometers across and 33 kilometers long. That makes Eros much larger than the object that struck Earth near the end of the age of the dinosaurs. But, unlike many asteroids that we can just take guesses as to the mass of, we know a bit more about Eros. 433 Eros was a target of study for NASA’s NEAR spacecraft. On February 14, 2000, the NEAR spacecraft entered orbit about Eros. After a year’s study, NEAR was directed to approach closer to the asteroid. NEAR approached close enough to the asteroid to make contact with it (effectively landing on its surface). From NEAR, we know that Eros has a mass of about 7.2 trillion metric tons, and it has a density of about 2.4 g/cm3. That density suggests that Eros may be a solid body, unlike other asteroids that are believed to be piles of loose debris. This mass and density give Eros a surface gravitational acceleration about 1/1600 that of Earth. That means that if you were to drop a rock from a height of about 1 meter above the surface of Eros, it would take at least 18 seconds to fall to the ground. It also means that if I were standing on the surface of Eros, I’d weigh less than 2 ounces!
As NEAR studied Eros, it became apparent that the asteroid seemed to have striations or layers along its length. One possible explanation for this is that Eros may have once been part of a larger planetesimal that was shattered by a collision long ago. Again, that may be consistent with it being a mostly solid body. Part of Eros’ odd shape comes from a very large impact crater on one side. The interior of this crater is much less cratered than other parts of Eros’ surface, indicating that the impact may have been of fairly recent origin. When NEAR got close to the surface of Eros, images showed a lot of large rocks laying around. These rocks appear to be laying on the surface of Eros, rather than the asteroid being simply a pile of rocks, like the asteroid 25143 Itokawa that was visited by Japan’s Hayabusa spacecraft. These rocks laying all over Eros may be material thrown out by the large impact that created the large saddle shaped impact crater. Furthermore, craters near to and on the opposite side of Eros from this crater are fewer and rounder than other craters on Eros. Other asteroids, such as 253 Mathilda, have many such smooth craters. Those asteroids are believed to have smooth craters because those bodies are more porous and not solid like Eros. But, Eros does have some sharper defined craters, showing that such craters can form there. It has been speculated that the smooth craters near and opposite the larger impact feature may have been shocked and shaken by seismic waves from the large impact. That may have broken up the craters and caused them to soften in appearance.
 Thanks to NEAR, 433 Eros is one of the best studied asteroids to date. But, there is always a lot more to learn.
 -Astroprof
Images courtesy NASA, JPL, JHUAPL
jj fullam on May 4, 2009 at 8:31 pm: 1
Eros is very impressive and frightening , how mwny more exist without our best astronomers knowing about them ?