In terms of composition, the majority of asteroids appear to be carbonaceous in nature, having dark, low reflectance surfaces. The significant minority have silicate surfaces, are relatively reflective, with a material composition similar to the surface of the Earth. The rarest asteroids appear metallic in nature. Even visual observations show that the brightness varies over a time period of about 5 hours. The light curve is superficially very much like that of an eclipsing binary star.
Photometric observations show that while the brightness varies, the colour is effectively constant.
Subsequent analysis of all the data shows that an irregular shape is the only possible mechanism to explain the light and colour curves. There have been fly-bys of asteroids before, usually with spacecraft on the way to somewhere else. Although it moved out well beyond the orbit of Mars, NEAR used only solar panels for its power supply, to avoid the weight and expense of a nuclear reactor traditionally used at such distances. With this complement of instrumentation it was hoped to obtain an accurate picture of the structure, geography and composition of Eros. This embarrassing failure was turned to triumph, when by vary careful calculations followed by much judicious manipulation of the thrusters to minimise this unanticipated fuel burn, the orbit was able to be repositioned so that on the second fly-by a year later, NEAR did manage to get into orbit about Eros.