mass (at the time of Mars atmospheric entry) of 2400 kg, of which only 390 kg was fuel. In comparison, the much lighter (292 kg) Surveyor 3 landed on the Moon in 1967 using nearly 700 kg of fuel. The lack of an atmosphere, however, removes the need for a Moon lander to have a heat shield and also allows aerodynamics to be disregarded when designing the craft. High gravity Although it has much less gravity than Earth, the Moon has sufficiently high gravity that descent must be slowed considerably. This is in contrast to an asteroid, in which "landing" is more often called "docking" and is a matter of rendezvous and matching velocity more than slowing a rapid descent. Since rocketry is used for descent and landing, the Moon's gravity necessitates the use of more fuel than is needed for asteroid landing. Indeed, one of the central design constraints for the Apollo program's Moon landing was mass (as more mass requires more fuel to land) required to land and take off from the Moon. Thermal environment The lunar thermal environment is influenced by the length of the lunar day. Temperatures can swing between approximately (lunar night to lunar day). These extremes occur for fourteen Earth days each, so thermal control systems must be designed to handle long periods of extreme cold or heat. Most spacecraft instruments must be kept within a much stricter range of between , and human comfort requires a range of . This means that the lander must cool and heat its instruments or crew compartment. The length of the lunar night makes it difficult to use solar electric power to heat the instruments, and nuclear heaters are often used. Landing stages Achieving a soft landing is the overarching goal of any lunar lander, and distinguishes landers from impactors, which were the first type of spacecraft to reach the surface of the Moon. All lunar landers require rocket engines for descent. Orbital speed around the Moon can, depending on altitude, exceed 1500 m/s. Spacecraft on impact trajectories can have speeds well in excess of that. In the vacuum the only way to slow down from that speed is to use a rocket engine. The stages of landing can include: Descent orbit insertion – the spacecraft enters an orbit favorable for final descent. This stage was not present in the early landing efforts, which did not begin