Earth observations taken from the shuttle Challenger during STS-41C

41C-31-990 (6-13 April 1984) --- Southwestern Algeria's Erg Chech shows long lines of parallel sand dunes called siefs. The Erg (sand desert) is in a remote (26.5 degrees north by 1.5 degrees west) of harsh desert, uninhabited and rarely visited. These parallel sand dunes are about 100 miles in length and 5 to 10 miles apart and are found in very few areas of the Earth. Most sand dunes are traverse dunes, or perpendicular to the general direction of the wind.