Atmospheric duct by the ionosphere. The reduced refractive index due to lower densities at the higher altitudes in the Earth's atmosphere bends the signals back toward the Earth. Signals in a higher refractive index layer, "i.e.," duct, tend to remain in that layer because of the reflection and refraction encountered at the boundary with a lower refractive index material. In some weather conditions, such as inversion layers, density changes so rapidly that waves are guided around the curvature of the earth at constant altitude. Phenomena of atmospheric optics related to atmospheric ducting include the green flash, Fata Morgana, superior mirage, mock mirage
2002 AA29 reflects radio waves only weakly. In the former case it would have to have an unusually high albedo. This would be evidence in support of the speculation that it, or at least the material of which it is composed, is different from most other asteroids so far discovered on near-Earth orbits, or represents a fragment thrown off by the collision of a medium-sized asteroid with the Earth or the Moon. Using radar echo measurements at the Arecibo radio telescope the rotational period of could be determined. In this radar astronomy procedure radio waves of known wavelength are emitted from a