How the Moon Formed: Theories, Collisions, and Scientific Discoveries

The question of how the Moon appeared remains one of the most intriguing mysteries of modern astronomy. Despite decades of research and numerous space missions, scientists continue to debate the exact mechanism behind the formation of our only natural satellite.

The Giant Impact Hypothesis: the generally accepted theory

The most widespread version today was proposed in 1975 by American astronomers William Hartmann and Donald Davis. According to their theory, Earth’s satellite formed as a result of a catastrophic collision between the young Earth and a large cosmic body roughly the size of Mars. This hypothetical protoplanet was named Theia, after the mother of the lunar goddess Selene in ancient Greek mythology. About 4.5 billion years ago, when the Solar System was still forming, Theia struck Earth at a specific angle. The powerful impact ejected a vast amount of material from Earth’s mantle, along with fragments of Theia itself, into near-Earth orbit.

How debris turned into the Moon

The material thrown into space did not disperse thanks to Earth’s gravity. Gradually, these fragments began to come together, forming a protomoon. The process of accretion—the sticking together of particles under the influence of gravity—continued until a single large satellite was formed. The impact theory explains several important features of the Moon. First, it consists mainly of lighter elements, and its density is lower than Earth’s, which is logical, since the satellite formed from mantle material rather than from the heavier terrestrial core. Second, the Moon orbits close to the plane of Earth’s orbit around the Sun—exactly where debris should have remained after the collision.

Problems with the traditional impact theory

Despite the popularity of the giant impact hypothesis, it has serious shortcomings. The main problem is related to the isotopic composition of lunar rocks. Samples of soil delivered by the Apollo missions showed a striking similarity between the material of the Moon and that of Earth—they are virtually identical in isotopic composition. According to computer models of the classical impact theory, more than 60% of the Moon’s material should have originated from Theia. However, if that were the case, the isotopic composition of the satellite would differ significantly from Earth’s, since Theia formed in another part of the Solar System from different source material. This inconsistency forced scientists to search for alternative explanations.

The multi-impact hypothesis: multiple collisions

In 2004, Russian astrophysicist Nikolai Gorkavyi proposed a bold solution to the problem of isotopic similarity. His multi-impact theory suggests that the Moon formed not as a result of a single catastrophic collision, but after an entire series of impacts by large celestial bodies on the young Earth.

Formation mechanism under the multi-impact scenario

According to Gorkavyi’s calculations, each collision created a debris disk around the planet, within which a satellite embryo gradually formed. These protomoons then merged with each other, forming increasingly larger bodies until a single Moon emerged. The key advantage of this version is that it explains the identical isotopic composition of Earth and the Moon. Multiple impacts ensured effective mixing of Earth’s material with the matter of various impactors, averaging the chemical composition of the planet and the forming satellite.

New research: four decisive collisions

In 2025, British researchers from the University of Bristol and Imperial College London conducted a series of computer simulations refining the multi-impact scenario. The scientists modeled Earth’s collision with four celestial bodies with masses ranging from a quarter to one and a half times the mass of Mars. The simulations produced impressive results. After the first impact, the isotopic composition of the forming Moon differed greatly from Earth’s, but with each new collision this difference decreased. In nine out of twelve simulations, the satellite acquired a mass equal to or exceeding that of the present-day Moon. One scenario demonstrated an especially precise match: after four impacts, the Moon attained exactly its current mass, a small iron core, and an isotopic composition virtually identical to Earth’s. The researchers suggested that the young Earth was rotating extremely rapidly—a full rotation took only three hours—which facilitated the ejection of debris into orbit.

Alternative theories of the Moon’s origin

Although impact hypotheses dominate modern science, there are other versions of how the Moon appeared.

The centrifugal separation hypothesis

This theory was proposed in 1878 by George Howard Darwin, the son of the famous naturalist. He suggested that a huge fragment broke away from the rapidly rotating young Earth under the influence of centrifugal forces and became the Moon. However, calculations showed that the planet’s rotation speed was insufficient for such an event.

The co-formation theory

According to the co-accretion hypothesis, proposed in the 18th century by philosopher Immanuel Kant and later developed by Soviet scientist Otto Schmidt, Earth and the Moon formed simultaneously from the same gas-dust nebula. A disk of cosmic dust and debris formed around the young planet, from which the satellite emerged. The main problem with this version is that it does not explain the differences in density and the presence of a large iron core in Earth, with its near absence in the Moon.

The gravitational capture hypothesis

This theory, proposed in 1909 by astronomer Thomas See, claims that the Moon was originally an independent celestial body that passed close to Earth and was captured by its gravity. This version is supported by the example of Mars’s moons, Phobos and Deimos, which are likely captured asteroids. However, this scenario is contradicted by the Moon’s nearly circular orbit and its regular spherical shape.

The significance of the Moon for Earth

Regardless of how exactly our satellite appeared, its influence on life on Earth is difficult to overestimate. The Moon acts as a cosmic shield, deflecting or intercepting some of the meteorites and asteroids heading toward Earth. Its gravitational influence causes ocean tides, which in the early period of the planet’s history contributed to the emergence of life from water onto land. The satellite stabilizes the tilt of Earth’s axis, ensuring stable climatic conditions over millions of years. Without the Moon, the climate on our planet would be far more chaotic, which could have hindered the development of complex forms of life.

Future research

A definitive answer to the question of how the Moon appeared may be provided by future space missions. Planned expeditions delivering samples of lunar soil from various regions of the satellite, including its far side, will help clarify the chemical and isotopic composition of the Moon. Comparative analysis of these data with terrestrial rocks and meteorites will make it possible to determine which of the existing theories most accurately describes the real events of the distant past. Modern computer modeling technologies are becoming increasingly sophisticated, allowing scientists to recreate the processes of celestial body formation with unprecedented detail. The combination of new observational data and powerful computational capabilities brings us closer to solving one of the greatest mysteries of the Solar System—the origin of our faithful cosmic companion.