The Moon is unusually large for a rocky planet's companion, and for centuries nobody could say where it came from. Today most planetary scientists favor one answer: early in the history of the solar system, a Mars-sized body struck the young Earth, and the debris gathered into the Moon. The idea is called the giant-impact hypothesis. It is not a settled fact in every detail, but it explains more of the evidence than any rival.
Before the Apollo era, three explanations competed. In the fission idea, a fast-spinning young Earth flung off a piece of itself. In the capture idea, the Moon formed elsewhere and was caught by Earth's gravity. In the co-formation idea, Earth and Moon grew side by side from the same cloud of material.
Each failed a key test. Capture is very hard to do without a lot of energy loss, and a captured body would probably have a different composition. Fission and co-formation struggle to explain how much angular momentum the Earth-Moon system has. Co-formation also cannot easily explain why the Moon has so little iron compared with Earth.
The modern version took shape in the mid-1970s, helped by data from the Apollo missions. It says that about 4.5 billion years ago, a planet-sized body hit the proto-Earth. That impactor is nicknamed Theia, after the Greek Titan said to be the mother of the Moon goddess Selene. Early models described a body roughly the size of Mars, about half of Earth's diameter.
In the simulations, the collision melts and vaporizes large parts of both bodies and throws a hot disk of rock and vapor into orbit. Within a period of years to perhaps a few centuries, much of that disk clumps together. The Moon that results starts out molten and very close to Earth, far closer than today.
Between 1969 and 1972, six Apollo landings returned about 382 kilograms (842 pounds) of lunar rock and soil, roughly 2,200 separate samples according to NASA-linked summaries. Several observations from them fit a violent, hot beginning:
Here is where the story gets less tidy. Different parts of the solar system have slightly different mixes of isotopes, the heavier and lighter versions of the same element. If the Moon were made mostly from Theia, it should look different from Earth. Yet for oxygen, titanium and several other elements, Moon rocks look almost the same as Earth rocks.
Researchers have proposed several ways out. One is that Theia happened to form from similar material to Earth. Another is that the impact mixed everything so thoroughly that the differences were erased. Newer models use more energetic collisions or a fast-spinning Earth, and some describe a doughnut-shaped cloud of vaporized rock called a synestia. A 2014 study in Science reported a tiny but measurable difference in oxygen isotopes between Apollo samples and Earth rocks, which it read as a sign of Theia's contribution. Later work has debated how large that difference is, so the question is still open.
The Moon is also drifting away from Earth by about 3.8 centimeters a year, measured by bouncing laser pulses off reflectors left by Apollo crews and later missions. Running that backward fits a Moon that began much closer. Tides raised by the Moon slow Earth's rotation, which is why the length of a day has been growing over geological time.
The Moon's surface itself is a record of the impacts that followed. Its craters are well preserved because it has no weather and little erosion. Counting craters is how scientists estimate the age of surfaces there, a method also applied to other worlds. Our own planet has lost most of its scars, but cases such as the Chicxulub impact show how violent the later history was.
The giant impact is the largest collision in Earth's history, but it belongs to the same family as every smaller event. Leftover rocks from planet building still cross Earth's path today as near-Earth objects. Most are far smaller than Theia, and the scales involved are very different: a body like Apophis is a few hundred meters wide, while Theia was thousands of kilometers across. Yet the physics of energy, speed and size is the same, and it is what lets scientists reason about modern events such as Tunguska and Chelyabinsk, or test deflection methods covered in planetary defense.