(CNN) -- An upper layer of Earth's atmosphere recently shrank so much that researchers are at a loss to adequately explain it, NASA said on Thursday. The thermosphere, which blocks harmful ultraviolet rays, expands and contracts regularly due to the sun's activities. As carbon dioxide increases, it has a cooling effect at such high altitudes, which also contributes to the contraction. But even these two factors aren't fully explaining the extraordinary contraction which, though unlikely to affect the weather, can affect the movement of satellites, researchers said.