Spacecraft have been studying the Moon for nearly half a century. The history of direct lunar exploration formally began in 1959, when probes from the Soviet's Luna spacecraft first flew by and then impacted the Moon's surface. Around the same time, NASA's Pioneer 4 passed within 37,000 miles of the Moon. Subsequent U.S. missions, such as Ranger, Surveyor and Lunar Orbiter photographed the lunar surface in preparation for landing astronauts. On July 20, 1969, the dream of putting a human on the Moon became reality when Neil Armstrong stepped off the Apollo 11 spacecraft and onto the rocky, dusty lunar terrain. Ensuing Apollo missions returned the first scientific samples from an extraterrestrial body to Earth - nearly 850 pounds (400 kilograms) of Moon rock. While both Soviet and U.S. lunar missions have returned to Earth a wealth of data, including breathtaking pictures and surface rock samples, many scientific questions remain unanswered. Much of the composition and structure of the Moon is still a mystery to scientists - less than a quarter of its surface has been mapped in detail, and little is known about how it formed, what it is made of, and how it has evolved over time.
Analysis of the Apollo rock samples revealed that the Moon consists predominantly of volcanic materials, and that its composition is very similar to that of the Earth. Apollo's seismic studies and density measurements also suggested that the Moon has only a tiny metallic core, roughly 15 times smaller than that of the Earth.
Lunar surface samples, however, contain important clues not only to the origin of the Moon itself, but to the beginnings of the entire Solar System. Data obtained from these Moon rocks, for instance, has led scientists to conclude that the Moon may have formed when, 4.5 billion years ago, the Earth collided with a very large object (the size of Mars or larger), ejecting raw materials that eventually became the Moon. This is known as the impact theory. Currently, the impact theory is perhaps the one most widely accepted by planetary scientists, but other theories are still plausible. Coaccretion theory holds that the Moon formed in Earth's orbit along with all the other planets in the Solar System. Fission theory states that, when the Solar System was very young, the Earth was spinning so fast (nearly 10,000 miles per hour (16,000 km/hr), or ten times its current speed), that it threw off a large chunk of material in order to stabilize itself; that chunk, the theory holds, became the Moon. Capture theory holds that the Moon was formed elsewhere in the Solar System and was seized by the Earth. Of all these theories, the impact theory fits best with all the data that has thus far been accumulated about the Moon. Future lunar exploration, especially that which will determine global bulk composition of the Moon, should help scientists settle this issue.
The surface of the 38 mile (60 km)-thick lunar crust is covered with a layer of powdery dust called regolith. The Moon's terrain is a combination of heavily cratered highlands and smooth maria. The older and more rugged highlands were sculpted early in lunar history (more than four billion years ago), when crystals floated to the surface of the Moon - at that time a liquid "magma ocean." The younger, smoother maria - large, dark plains - were formed three to four billion years ago after huge asteroid impact basins were later filled with molten lava.
Unlike the Earth, the Moon does not have a significant magnetic field, and it has no significant atmosphere. Its unprotected regolith is thus constantly being bombarded by the solar wind. The Sun continuously embeds chemical elements, such as hydrogen ions, into the lunar surface. Thus, by studying the surface of the Moon, scientists can learn a lot about the Sun.
Many other questions remain. For instance, scientists still don't understand why the crust is thicker on one side, or what the Moon's volcanic history is. Lunar Prospector will attempt to answer some of the lingering questions scientists have about the Moon and its history.