The Mars Climate Orbiter will arrive at the red planet September 23 to begin a 2-year mapping and weather-observation mission. Initially, it will enter a highly elliptical orbit that will send the spacecraft at a grazing angle through the thin Martian atmosphere to gradually slow it down. This maneuver, called aerobraking, is designed to conserve propellant that would otherwise be needed to reduce the orbiter's speed before entering martian orbit.