Earth's Moon
Earth's Moon
The moon is our closest celestial body and has therefore been our first target. We've gone early and often. To date there have been 110 mission to our satellite. We are now used to expecting our missions into space to succeed, but that was certainly not the case in the early days of space exploration. The first 10 missions to the moon failed, though some returned partial information prior to failure. That changed on 14 September 1959, at 23:02:23 UT when Luna 2 (shown on the right) crashed into the Moon. While that doesn't sound like a successful mission, impacting on the lunar surface was actually one of the goals. Hence we can state that it successfully reached the surface and became "the first object of human origin to make contact with another celestial body."
And then, of course, we have this:
The first human landing on the moon was on 20 July 1969. During the Apollo missions of 1969-1972, 12 American astronauts walked on the moon and used a Lunar Roving Vehicle to travel on the surface and extend their studies of soil mechanics, meteoroids, lunar ranging, magnetic fields, and solar wind. The Apollo astronauts brought back 382 kilograms (842 pounds) of rock and soil to Earth for study.
Those lunar rocks were very important because they are pristine and we knew exactly where they were collected. We do have other samples of the moon, some coming from 3 Soviet Luna missions (16, 20, 24) as well as from lunar meteorites. Analysis of the Apollo mission provided many important insights into the early history of the solar system and enabled us to date some of the early solar system events precisely. We will return to some of those discoveries later. But for now, back to some more general lunar highlights.
The regular daily and monthly rhythms of Earth's only natural satellite, the moon, have guided timekeepers for thousands of years. Its influence on Earth's cycles, notably tides, has been charted by many cultures in many ages. The moon moderates Earth's wobble on its axis, leading to a relatively stable climate over billions of years. From Earth, we always see the same face of the moon because the moon is spinning on its axis at the same speed that it is going around Earth (that is, it is in synchronous rotation with Earth).
The light areas of the moon are known as the highlands. The dark features, called maria (Latin for seas), are impact basins that were filled with lava between 4.2 and 1.2 billion years ago. These light and dark areas represent rocks of different composition and ages, which provide evidence for how the early crust may have crystallized from a lunar magma ocean. The craters themselves, which have been preserved for billions of years, provide an impact history for the moon and other bodies in the inner solar system.
June 2019 Update: There has been exciting developments with lunar exploration, including some very exciting missions such as China's Chang'e 4 probe to the lunar far side and Israeli SpaceIL's Beresheet crash landing.The leading theory of the moon's origin is that a Mars-sized body collided with Earth approximately 4.5 billion years ago, and the resulting debris from both Earth and the impactor accumulated to form our natural satellite. The newly formed moon was in a molten state. Within about 100 million years, most of the global "magma ocean" had crystallized, with less-dense rocks floating upward and eventually forming the lunar crust. The early moon may have developed an internal dynamo, the mechanism for global magnetic fields for terrestrial planets.
Since the ancient time of volcanism, the arid, lifeless moon has remained nearly unchanged. With too sparse an atmosphere to impede impacts, a steady rain of asteroids, meteoroids, and comets strikes the surface. Over billions of years, the surface has been ground up into fragments ranging from huge boulders to powder. Nearly the entire moon is covered by a rubble pile of charcoal-gray, powdery dust and rocky debris called the lunar regolith. Beneath is a region of fractured bedrock referred to as the megaregolith.
