Asteroid Eros Is Very, Very Ancient Say NEAR Scientists
Dr. Andrew F. Cheng of the Johns Hopkins University Applied Physics Laboratory in Laurel, Md., which manages the NEAR mission said “we can now say that Eros is an undifferentiated asteroid with homogeneous structure, that never separated into a distinct crust, mantle and core. 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.” Eros is classified as being an S type asteroid, one which has a composition of iron- and magnesium-bearing silicates such as pyroxene and olivine mixed together with metals such as nickel and iron. Asteroids with this mixed composition were thought to be remnants of larger asteroids that were originally large enough to have differentiated into different layers as they formed. Over time, these larger asteroids were broken into pieces by repeated impacts. With regard to Eros’ putative parent body, these new findings suggest otherwise. According to a Johns Hopkins University press release, “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.” One of the other mysteries of Eros has to do with its surface. Given its small size the innumerable impacts it has experienced should have knocked much of the debris off of the asteroid all together, Instead, the surface has been found to be littered with debris of all sizes. According to a Cornell press release, Veverka says “What is striking about Eros is that if I look at the moon in great detail, I see lots of tiny craters and fewer blocks of rock. But on this object, when I get down to sizes the size of a car, there are very few craters and lots of boulders.” And yet, he says, the surface of Eros shows clear evidence of violent impacts. Veverka suggest several explanations: ” one is that we simply don’t understand cratering events on small objects, and somehow the debris gets thrown out at very low speeds. Or the ejected material ends up in the same orbit as Eros, and over time the asteroid runs back into its own debris and gathers it up, which is equally bizarre. We simply don’t understand this.” Eros was discovered in 1898 and is one of the largest asteroids known. Eros has an elongated peanut-like shape measuring 40 x 14 x 14 kilometers and rotates once every 5.27 hours. Eros is classified as an Earth Approaching Asteroid. Specifically it is placed in the Amor category of asteroids. Amors have orbits that bring them to within 1.3 AU of the sun (1.3 times Earth’s average distance from the sun). Eros orbits around the Sun with a perihelion (closest approach) of 1.13 AU (169,045,593 km) and an aphelion (greatest distance) of 1.78 AU (266,284,209 km). The primary mission of NEAR is to orbit asteroid Eros for approximately 1 year. During that time it will conduct a variety of measurements designed to help understand the composition, mineralogy, and overall characteristics of Eros. If all works out after its baseline mission is completed, NEAR will circle in closer and closer until mission controllers may actually attempt a quasi-soft landing (i.e. a slow motion collision) on Eros itself. Related Sites ° Near Earth Asteroid Rendezvous Mission ° Focus on NEAR, SpaceRef ° NEAR mission discoveries highlighted in latest issue of Science, Johns Hopkins University press release, 21 September 2000 ° Mystery of tiny asteroid Eros — so much rock but so little gravity — detailed in Science, Cornell University press release, 21 September 2000 ° Asteroid Eros Yields Secrets from the Time Before Earth Was Born, NASA press release, 21 September 2000 Current Articles From Science Magazine °22 September 2000: Radio Science Results During the NEAR-Shoemaker Spacecraft Rendezvous with Eros, Science, [summary – can be viewed for free once registered. A subscription fee is required for full access.] “We determined the mass of asteroid 433 Eros, its lower order gravitational harmonics, and rotation state, using ground-based Doppler and range tracking of the Near Earth Asteroid Rendezvous (NEAR)-Shoemaker spacecraft and images of the asteroid’s surface landmarks. The mass of Eros is (6.687 ± 0.003) ? 1018 grams,which, coupled with our volume estimate, implies a bulk density of 2.67 ± 0.03 grams per cubic centimeter. The asteroid appears to have a uniform density distribution. The dynamical environment of Eros suggests that it is covered with regolith and that one might expect material transport toward the deepest potential wells in the saddle and 5.5-kilometer crater regions.” °22 September 2000: NEAR at Eros: Imaging and Spectral Results, Science, [summary – can be viewed for free once registered. A subscription fee is required for full access.] “Eros is a very elongated (34 kilometers by 11 kilometers by 11 kilometers) asteroid, most of the surface of which is saturated with craters smaller than 1 kilometer in diameter. The largest crater is 5.5 kilometers across, but there is a 10-kilometer saddle-like depression with attributes of a large degraded crater. Spectra (800 to 2500 nanometers) are consistent with an ordinary chondrite composition for which the measured mean density of 2.67 ± 0.1 grams per cubic centimeter implies internal porosities ranging from about 10 to 30 percent.” °22 September 2000: The Shape of 433 Eros from the NEAR-Shoemaker Laser Rangefinder, Science, [summary – can be viewed for free once registered. A subscription fee is required for full access.] “Measurements from the Near Earth Asteroid Rendezvous (NEAR)-Shoemaker Laser Rangefinder (NLR) indicate that asteroid 433 Eros is a consolidated body with a complex shape dominated by collisions. Impact crater morphology is influenced by both gravity and structural control. Small-scale topography reveals ridges and grooves that may be generated by impact-related fracturing. “ °22 September 2000: The Elemental Composition of Asteroid 433 Eros: Results of the NEAR-Shoemaker X-ray Spectrometer, Science, [summary – can be viewed for free once registered. A subscription fee is required for full access.] “Low aluminum abundances for all regions argue against global differentiation of Background Information ° Asteroids and Comets, SpaceRef Directory ° NEAR Shoemaker Moves Closer to Asteroid 433 Eros, SpaceRef ° Putting NEAR’s Images in Perspective, SpaceRef ° NEAR gets Nearer to Eros, SpaceRef ° NEAR’s Near-Infrared Spectrometer No Longer Works, SpaceRef ° More Color Photos of Eros from NEAR, SpaceRef ° NEAR Rendezvous Burn a Success, SpaceRef
Results from the NEAR Shoemaker mission to asteroid 433 Eros show that the asteroid is very ancient. So ancient, in fact, that it may have witnessed the earliest days of our solar system’s history.
Joseph Veverka, a professor of astronomy at Cornell University and principal investigator on the NEAR spacecraft’s multi-spectral imager (MSI), or camera, and the NEAR infrared spectrometer (NIS) said in a press release that “we basically know that Eros is an example of a very primitive body in which nothing much has happened other than formation and cratering. If you want the most pristine material in the solar system that has had the least happen to it, then Eros is a good example.”
Eros. Magnesium/silicon, aluminum/silicon, calcium/silicon, and iron/silicon ratios are best interpreted as a relatively primitive,
chondritic composition. Marked depletions in sulfur and possible aluminum and calcium depletions, relative to ordinary chondrites, may represent signatures of limited partial
melting or impact volatilization.”
