The moon is a slowly rotating spacecraft, 3,476 km in diameter, that always presents the same face to the earth. The moon lacks a significant atmosphere but has a rocky surface covered with dust. Its surface gravity is about one-sixth that of the earth. Table 6.1 describes a typical infrastructure and compares it to other remote observing sites. Table 6.2 lists some of the advantages and disadvantages of the moon as an observatory.
The moon has most of the advantages of any observatory in space. The absence of a lunar atmosphere and ionosphere permits observations over the entire electromagnetic spectrum with a resolution that is limited only by the characteristic size of the telescope. The lunar environment also lends itself to the construction of large, precise structures. The low lunar gravity and the absence of wind make possible telescope mirrors and support structures lighter than those constructed on the earth. The lunar night provides thermal stability, important for maintaining the precise alignment of a large telescope or the separations and orientations of an array of smaller ones. During the lunar night, telescopes can attain the low temperatures, less than 70 K, needed to improve infrared sensitivity. A major advantage of the moon compared with orbital observatory sites is the large, rigid lunar surface on which could be built arrays of telescopes extending over many kilometers to form interferometers.
The disadvantages of the moon compared with a remote site on the earth or in earth orbit include the limited mass that can be sent to the moon, the stringent requirements imposed on the design of instruments that must survive the rigors of space travel, and the need for assembly and operation of complex equipment with only a few workers. A rocket that can send 1,000 kg to low earth orbit, or 400 kg to high earth orbit, can send only 290 kg to the moon. Although lunar gravity is weaker than the earth's, supporting a few tons of telescope is a difficult task not faced by the designer of an orbiting telescope. Cosmic rays and the solar wind impinge directly on the lunar surface, unmoderated by a magnetosphere. Contamination of optical and mechanical components by lunar dust is a potential problem.
Detailed study will be required to determine whether, for any particular instrument, operation from high earth orbit offers advantages relative to operation on the moon. In the very distant future, mining or manufacturing operations on the moon might create an infrastructure that could make the moon an attractive site for many astronomical facilities.
The presence of astronauts offers both advantages and disadvantages for astronomy. Astronauts are able to install and repair astronomical facilities, albeit on a restricted work schedule and with dexterity limited by spacesuits. The direct