Atmosphere of Earth they penetrate more deeply. The various layers of Earth's ionosphere, important to HF radio propagation, begin below 100 km and extend beyond 500 km. By comparison, the International Space Station and Space Shuttle typically orbit at 350–400 km, within the F-layer of the ionosphere where they encounter enough atmospheric drag to require reboosts every few months. Depending on solar activity, satellites can experience noticeable atmospheric drag at altitudes as high as 700–800 km. The division of the atmosphere into layers mostly by reference to temperature is discussed above. Temperature decreases with altitude starting at sea level, but variations in this
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