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Ida probably has a composition similar to a metal-rich chondrite or perhaps even a stony-iron meteorite. Its overall surface and potato-like shape are similar to Eros. Although both Ida and Eros are essentially big rocks, many asteroids are actually masses of rubble held together by their weak gravity. Some have such low densities that they may be more space than rock! One way that scientists get a basic idea of what asteroids and comets are made of is looking at the light reflected from the surface. Certain wavelengths are absorbed by certain materials. The overall reflectivity of an object is called its albedo. The Earth has an average albedo of 30%, meaning that 30% of sunlight is reflected back to space, mostly by our clouds. Venus, with its bright clouds, has an albedo of 65%. The rocky surface of Mercury has an albedo of only 11%, and coal dust has an albedo of a mere 3%. Ida has an albedo of 21% and Eros has an albedo of 16%. These are typical for rocky surfaces. The asteroid Mathide has an albedo of only 4%, which suggests that it may be have the composition of the darker carbonaceous chondrites. I'm telling you about albedo now so you can appreciate the surprisingly low albedos of cometary nuclei. Next: Halley's CometImage credit: National Space Science Data Center |
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