The giant impact that made the Moon
A Mars-size world strikes the young Earth
Quick facts
- When
- About 4.5 billion years ago
- Impactor
- An object roughly the size of Mars
- How the Moon formed
- From debris flung into orbit by the collision
- What the Moon does
- Steadies Earth's tilt and drives the tides
What happened
Not long after Earth formed, it suffered one of the most violent events in its history. The leading explanation, the giant impact theory, holds that an object roughly the size of Mars struck the young Earth and flung enough molten and vaporized rock into space that the debris settled into orbit and came together to form the Moon. NASA places this collision near the time of the Solar System's formation, about 4.5 billion years ago, and notes that any complete theory of the Moon still has to explain everything we observe about it today.
Why it matters
The Moon is not just scenery. Its gravity steadies the tilt of Earth's axis, which keeps the climate relatively stable over long spans, and it raises the ocean tides. Its ancient, cratered surface also preserves a record of the early Solar System that Earth's restless, weathered surface long ago erased, so the Moon is a kind of archive of the violence this event is part of.
How we know
NASA's explanation of how the Moon formed describes the Mars-size impactor, the molten and vaporized debris thrown into space, and the giant impact theory, while being clear that the theory is still tested against new observations.
Sources
- NASA. How the Moon formed (NASA Science) (2024) · Reputable sourcescience.nasa.gov · The domain "science.nasa.gov" is on our Reputable source registry. · Link is live and its text matches the event's key terms (Jul 2026)
- Identification of the giant impactor Theia in lunar rocks (Science, 2014, via PubMed) (2014) · Peer-reviewed (author-declared)pubmed.ncbi.nlm.nih.gov · Cited as a "journal" source (no stronger domain match). · Link is live and its text matches the event's key terms (Jul 2026)
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