Moonlanding - Scientific Background

Scientific Background

In order to go to the moon, a spacecraft must first leave the gravity well of the Earth. The only practical way of accomplishing this currently is with a rocket. Unlike other airborne vehicles such as balloons or jets, a rocket is the only known form of propulsion which can continue to increase its speed at high altitudes in the vacuum outside the Earth's atmosphere.

Upon approach of the target moon, a spacecraft will be drawn ever closer to its surface at increasing speeds due to gravity. In order to land intact, a spacecraft must either be ruggedized to withstand a "hard landing" impact of less than about 100 miles per hour (160 km/h) (not possible with human occupants), or it must decelerate enough for a "soft landing" with negligible speed at contact. The first three attempts by the Americans to perform a successful hard moon landing with a ruggedized seismometer package in 1962 all failed.

The Soviets first achieved the milestone of a hard lunar landing with a ruggedized camera in 1966, followed only months later by the first unmanned soft lunar landing by the Americans. The escape velocity of the target moon is roughly equivalent to the speed of a crash landing on its surface, and thus is the total velocity which must be shed from the target moon's gravitational attraction for a soft landing to occur. For Earth's Moon, this figure is 2.38 kilometres per second (1.48 mi/s).

Such a change in velocity (referred to as a delta-v) is usually provided by a landing rocket, which must be carried into space by the original launch vehicle as part of the overall spacecraft. An exception is the soft moon landing on Titan carried out by the Huygens probe in 2005. As the only moon with an atmosphere, landings on Titan may be accomplished by using atmospheric entry techniques that are generally lighter in weight than a rocket with equivalent capability.

The Soviets succeeded in making the first crash landing on the Moon in 1959. Crash landings may occur because of malfunctions in a spacecraft, or they can be deliberately arranged for vehicles which do not have an on board landing rocket. There have been many such moon crashes, often with their flight path controlled to impact at precise locations on the lunar surface. For example, during the Apollo program the S-IVB third stage of the Saturn V moon rocket as well as the spent ascent stage of the lunar module were deliberately crashed on the Moon several times to provide impacts registering as a moonquake on seismometers that had been left on the lunar surface. Such crashes were instrumental in mapping the internal structure of the Moon.

To return to Earth, the escape velocity of the moon must be overcome for the spacecraft to escape the gravity well of the moon. Rockets must be used to leave the Moon and return to space. Upon reaching Earth, atmospheric entry techniques are used to absorb the kinetic energy of a returning spacecraft and reduce its speed for safe landing. These functions greatly complicate a moon landing mission and lead to many additional operational considerations. Any moon departure rocket must first be carried to the Moon's surface by a moon landing rocket, increasing the latter's required size. The moon departure rocket, larger moon landing rocket and any Earth atmosphere entry equipment such as heat shields and parachutes must in turn be lifted by the original launch vehicle, greatly increasing its size by a significant and almost prohibitive degree. This necessitates optimizing the sizing of stages in the launch vehicle as well as consideration of using space rendezvous between multiple spacecraft.