Defending Earth from a Colliding Object

Even with the best of search programs, whether anything can be done about an object found to be on a collision course with Earth depends on many factors. The most important are the amount of lead time and the physical properties of the object—its size, shape, spin rate, density, strength, and other characteristics. Scientists believe that kinetic energy interception is adequate for the majority of objects, including those of intermediate size and most likely to cause destructive tsunamis. Such a strategy would involve the use of a nonexplosive projectile sent to strike the object in a particular location at high speed to change its orbit and possibly to fragment it.

For the remainder, more aggressive measures, likely involving the use of powerful thermonuclear devices, are thought to be necessary to achieve the same results. Because the physical properties of NEOs are so poorly known, however, it is possible that such measures could do more harm than good—e.g., by breaking a large object into numerous smaller, but still potentially destructive, pieces without deflecting them enough to miss Earth. Validating these options requires additional theory, laboratory experiments, and safe experiments involving actual NEOs in space. In the early years of the 21st century, few, if any, such efforts were being made.

The Moon

The Moon is Earth's sole natural satellite and nearest large celestial body. Known since prehistoric times, it is the brightest object in the sky after the Sun. It is designated by the symbol C. Its name in English, like that of Earth, is of Germanic and Old English derivation.

The Moon's desolate beauty has been a source of fascination and curiosity throughout history and has inspired a rich cultural and symbolic tradition. In past civilizations the Moon was regarded as a deity, its dominion dramatically manifested in its rhythmic control over the tides and the cycle of female fertility. Ancient lore and legend tell of the power of the Moon to instill spells with magic, to transform humans into beasts, and to send people's behaviour swaying perilously between sanity and lunacy (from the Latin luna, "Moon"). Poets and composers were invoking the Moon's romantic charms and its darker side, and writers of fiction were conducting their readers on speculative lunar journeys long before Apollo astronauts, in orbit above the Moon, sent back photographs of the reality that human eyes were witnessing for the first time.

Centuries of observation and scientific investigation have been centred on the nature and origin of the Moon. Early studies of the Moon's motion and position allowed the prediction of tides and led to the development of calendars. The Moon was the first new world on which humans set foot;

The familiar near side of Earth's Moon, photographed on December 7,1992, by the Galileo spacecraft on its way to Jupiter. Two primary kinds of terrain are visible—the lighter areas, which constitute the heavily cratered and very old highlands, and the darker, roughly circular plains, traditionally called maria, which are relatively young lava-filled impact basins. NASA/JPL/Caltech the information brought back from those expeditions, together with that collected by automated spacecraft and remote-sensing observations, has led to a knowledge of the Moon that surpasses that of any other cosmic body except Earth itself. Although many questions remain about its composition, structure, and history, it has become clear that the Moon holds keys to understanding the origin of Earth and the solar system. Moreover, given its nearness to Earth, its rich potential as a source of materials and energy, and its qualifications as a laboratory for planetary science and a place to learn how to live and work in space for extended times, the Moon remains a prime location for humankind's first settlements beyond Earth orbit.

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