- G. Smith, Richmond, VA
Greetings! I will take each question in turn. First of all, yes, it is true. Lunar meteorites, pieces from the Moon, are on Earth and have been throughout natural history. Since 1979, when Japanese scientists positively identified the first lunar meteorite (Yamato 791197), about fifty such meteorites have been discovered. Scientists are confident that many more remain undiscovered.
Asteroid impacts displace swarms of fragments from the lunar surface. The Moon's surface gravity is comparatively weak. The Moon's escape velocity - the speed a body must attain to escape its gravitational field - is 2.4 kilometers per second, as opposed to Earth's 11.2 km/sec escape velocity. Since the Moon's grip on these scattered fragments is tenuous, many of them escape from it into outer space. As Earth is so close to the Moon - generally about a quarter of a million miles away - and much more gravitationally powerful, it will capture many of these lunar projectiles, which settle onto the surface as meteorites.*
An example of a lunar meteorite
Geologists can ascertain that the such meteorites originated from the Moon by analyzing their chemical compositions. Through Moon rock examinations, these geologists know the chemical composition of lunar material. For instance, they recognize that almost 99% of the lunar crust's crystalline material is composed of four minerals: ilemnite (iron-titanium oxide), pyroxene (magnesium-iron calcium), olivine (magnesium iron silicate), and plagioclase feldspar (calcium-aluminu-sodium-silicon-oxygen). If the meteorites' composition is consistent with that of the Moon, we can infer its lunar origin.)
Long before the Apollo astronauts traveled to the Moon to pilfer rocks, lunar fragments were collecting on Earth's surface. Only after we began our lunar explorations, however, were humans in the position to identify lunar meteorites.
*Let's quickly review some terms. "Meteoroids" are fragments of asteroids, comets, the Moon or planets that are suspended within outer space. "Meteors." are those light streaks produced when meteoroids descend through the upper atmosphere. "Meteorites," are pieces of meteoroids that remain intact after their descent and are collected on Earth.