If the Moon crashed into Earth, the collision would melt the planet’s entire surface, blast away most of the atmosphere, and sterilize every square meter of land and ocean. The Moon’s diameter of roughly 3,474 kilometers puts it well above the size threshold researchers estimate is needed to fully liquefy Earth’s crust, and the energy released would dwarf anything in human experience by many orders of magnitude. This is not a survivable event for any complex life, and the planet that emerged afterward would bear little resemblance to the one we know.
What Happens as the Moon Gets Closer
Long before impact, a Moon spiraling inward would wreak havoc through tidal forces alone. The Moon already raises ocean tides of a meter or two from its current distance of about 384,000 kilometers. Tidal force scales with the inverse cube of distance, so halving the distance increases that pull roughly eightfold. As the Moon drew nearer, tides would grow from meters to tens of meters, then hundreds. Coastlines would be submerged and exposed in cycles so violent that no coastal infrastructure could survive. The same tidal stress would flex Earth’s crust, triggering earthquakes along every major fault system and potentially reactivating volcanic systems worldwide.
At a certain distance, tidal forces acting on the Moon itself become important. Every large body has what physicists call a Roche limit, the distance at which tidal stretching overcomes the object’s own gravity. For an object held together mostly by gravity rather than internal strength, this limit sits at roughly two to three planetary radii from the host world’s center. Simulations of Mars’s moon Phobos, a loosely bound rubble pile, show it would be torn apart beyond about two Mars radii, shedding mass in stages before undergoing complete tidal disruption and forming a broad ring of particles.
Our Moon, though, is a different beast. It is a solid, differentiated body with a crust, mantle, and small iron core. Solid rock has far more internal strength than a rubble pile. The Moon would resist tidal breakup much longer than Phobos would, likely surviving intact or nearly intact until it was extremely close to Earth’s surface. A rubble-pile version of the Moon would shatter into a ring; the real Moon would more likely slam into Earth as a single massive body, making the impact far more concentrated and violent.
The Collision and Surface Melting
An object falling from the Moon’s current orbit would accelerate under Earth’s gravity and arrive at something close to Earth’s escape velocity of 11.2 kilometers per second, possibly faster depending on how the scenario plays out. At that speed, the kinetic energy of a body with the Moon’s mass is staggering. For context, the asteroid that ended the dinosaurs was perhaps 10 to 12 kilometers across and hit at around 20 kilometers per second. The Moon is nearly 300 times wider and incomparably more massive.
Modeling of giant impacts tells us what happens to the target planet under conditions like these. Even the original Moon-forming collision, which involved a Mars-sized body striking a proto-Earth at about 15 kilometers per second with a projectile-to-planet mass ratio of around 0.14, melted somewhere between 30 and 65 percent of the planet depending on its initial temperature.1Journal of Geophysical Research: Planets. Magma ocean formation due to giant impacts That ancient impactor, called Theia, was actually larger than the Moon. But the Moon itself, at a diameter of roughly 3,474 kilometers, still exceeds the threshold that more recent modeling sets for total surface melting. Researchers studying large early-Earth impacts found that completely melting Earth’s surface requires an impactor in the range of 2,000 to 2,700 kilometers in diameter, while vaporizing the oceans takes a body larger than about 700 kilometers.2The Planetary Science Journal. Large Impacts onto the Early Earth: Planetary Sterilization and Iron Delivery The Moon clears both bars easily. Every ocean would boil away, and the entire crust would become a global sea of molten rock.
Why Nothing Survives
The sterilization question is not really a question at all. That same research on early-Earth bombardment found that sterilizing impacts require larger impactors than earlier estimates suggested, but even so, the Moon far exceeds the necessary size.2The Planetary Science Journal. Large Impacts onto the Early Earth: Planetary Sterilization and Iron Delivery When the surface is fully molten and the oceans have been vaporized, there is no habitat left for any form of life, not even the hardiest deep-subsurface microbes. The energy delivered to the crust would heat rock to thousands of degrees, and the thermal pulse would penetrate deep underground. Life on Earth, from bacteria in ocean-floor vents to every tree, fish, insect, and person, would be gone in a geologically instantaneous event.
This stands in contrast to “mere” mass-extinction impacts. The asteroid that killed the non-avian dinosaurs 66 million years ago left the oceans liquid, much of the deep biosphere intact, and allowed recovery within a few million years. A Moon-scale impact operates in a fundamentally different regime. It does not just disrupt ecosystems; it eliminates the physical conditions that make ecosystems possible.
What Happens to the Atmosphere
A collision of this scale does not just heat the atmosphere; it removes a large fraction of it entirely. Giant impacts drive atmospheric loss through two main channels. Near the impact site, massive ejecta plumes launch material upward at velocities exceeding escape speed, physically sweeping away the overlying air. In the far field, on the opposite side of the planet, the impact shock propagates through the ground and “breaks out” at the surface, kicking the ground upward violently enough to accelerate the atmosphere above it to escape velocity. Three-dimensional simulations show that both mechanisms contribute, with the near-field plume and the far-field ground kick each stripping away significant portions of the gas envelope.3The Planetary Science Journal. Atmospheric Loss during Giant Impacts: Mechanisms and Scaling of Near- and Far-field Loss
But the immediate blowoff is only part of the story. A giant impact also converts a huge amount of kinetic energy into heat, warming the rocky surface and whatever atmosphere remains. That thermal energy causes the surviving gas envelope to expand dramatically. Research on hydrogen-helium envelopes around rocky planets shows that this thermal expansion can drive a sustained, rapid outflow of gas, partly or completely eroding the atmosphere over time even after the initial shock has passed.4Monthly Notices of the Royal Astronomical Society. Atmospheric mass-loss due to giant impacts: the importance of the thermal component for hydrogen–helium envelopes Earth’s nitrogen-oxygen atmosphere behaves differently from a hydrogen-helium envelope, but the underlying physics of thermal expansion driving mass loss still applies. A Moon-scale impact would leave the remnant atmosphere superheated and bloated, bleeding gas into space for an extended period.
There is also a third, slower mechanism that researchers have only recently quantified. Not all the debris from a giant impact escapes into space. Much of it goes into orbit and eventually falls back. Those secondary impacts, raining down over millions of years, drive their own atmospheric erosion. Modeling of the canonical Moon-forming impact found that debris re-accretion alone could gradually erode an atmosphere similar to present-day Earth’s in roughly 30 million years. Any planet growing through giant impacts within about two astronomical units of its star is likely to experience significant atmospheric loss unless the initial atmosphere was at least five times more massive than Earth’s.5Monthly Notices of the Royal Astronomical Society. Re-accretion of giant impact ejecta can drive significant atmospheric erosion on terrestrial planets So even if a fraction of the atmosphere survived the initial collision and the thermal blowoff that followed, the rain of returning debris would continue stripping it away for tens of millions of years.
Acid Rain and Toxic Fallout
Whatever atmosphere did cling to the planet in the immediate aftermath would be profoundly hostile. Research on the environmental chemistry of large impacts, particularly work on the Cretaceous-Tertiary boundary event, shows that high-energy collisions generate enormous quantities of nitrogen oxides by shocking atmospheric nitrogen and oxygen. A cometary impact can produce globally distributed nitric and nitrous acid rain with a pH between 0 and 1.5, acidic enough to dissolve calcium carbonate and strip vegetation. An asteroidal impact produces similarly extreme acid rain near the impact site and moderately acidic rain globally.6Earth and Planetary Science Letters. Bolide impacts, acid rain, and biospheric traumas at the Cretaceous-Tertiary boundary The Moon dwarfs any asteroid, so the chemistry of the post-impact atmosphere would be orders of magnitude worse. Nitrogen dioxide alone would block sunlight, and the acid deposition would mobilize toxic metals from whatever rock had re-solidified. None of this matters for life in the short term, since there would be no life left, but it illustrates how thoroughly uninhabitable the post-collision Earth would be even for hypothetical organisms that somehow survived the heat.
A Magma Ocean and a Debris Ring
After the collision, Earth would be covered in a global magma ocean, a layer of molten silicate rock potentially hundreds of kilometers deep. The planet would glow visibly, radiating heat into space at a furious rate. Simulations of giant-impact debris show that material launched into orbit follows a predictable evolution: it starts as a clump near the impact site, spreads into an asymmetric ring over the first 10,000 years, and gradually settles into a smooth, symmetric ring around the planet by roughly a million years after the event.7Monthly Notices of the Royal Astronomical Society. Debris from terrestrial planet formation: the Moon-forming collision For a time, Earth would look something like Saturn, with a glowing molten surface encircled by a ring of rocky debris. Some of that debris might eventually coalesce into a new moon, just as the original Moon is thought to have formed from the debris of the Theia impact billions of years ago.
The magma ocean itself would cool surprisingly quickly by geological standards, though “quickly” here still means thousands of years. Modeling of deep terrestrial magma oceans shows that solidification proceeds from the bottom up, because the melting temperature of rock increases with pressure. The mantle becomes significantly more viscous within about 20,000 years, while a crust forms at the surface due to radiative heat loss. The last pockets of fully molten silicate end up restricted to the upper mantle.8Earth and Planetary Science Letters. On the cooling of a deep terrestrial magma ocean More recent work suggests the full timeline can stretch further depending on atmospheric conditions and the planet’s chemical makeup, with solidification of an Earth-like magma ocean taking anywhere from a fast track of thousands of years to as long as a million years under certain conditions.9PubMed Central. Magma Ocean Evolution at Arbitrary Redox State
Could the Moon Actually Fall Into Earth
The short answer is no, not under any natural process operating today. The Moon is currently moving away from Earth at about 3.8 centimeters per year, driven by tidal interactions that transfer angular momentum from Earth’s rotation to the Moon’s orbit. This has been happening since the Moon formed, and it will continue for billions of years. There is no known mechanism that would reverse this process and send the Moon spiraling inward.
In the very distant future, billions of years from now, the Sun will expand into a red giant. Its outer envelope will extend past Earth’s current orbit, and the drag of that tenuous gas could theoretically alter the orbits of both Earth and the Moon. But by that point, the Sun’s expansion will have already rendered Earth uninhabitable, and the planet may be consumed by the Sun entirely. The Moon crashing into Earth is not something that happens in any realistic astrophysical timeline.
For the Moon to fall inward, you would need to somehow drain angular momentum from the Earth-Moon system, a process that does not occur naturally in the current configuration. Tidal friction works the other way: Earth’s rotation is faster than the Moon’s orbital period (even though Earth’s day is lengthening over time), so the tidal bulge leads the Moon and pushes it outward. If Earth’s rotation ever slowed to the point where the Moon’s orbital period was shorter than a day, the tidal interaction would reverse, and the Moon would begin spiraling inward. But the timescale for that to happen naturally is far longer than the remaining lifespan of the Sun.
When Moons Actually Do Fall
While our Moon is safe, not every moon in the solar system has such a comfortable future. Mars’s moon Phobos is one of the best-studied examples of a moon on a collision course with its planet. Phobos orbits so close to Mars that tidal friction is pulling it inward rather than pushing it outward. At its current rate, Phobos will reach Mars’s Roche limit in roughly 50 million years.
Phobos, however, is tiny compared to our Moon, only about 22 kilometers across, and it appears to be a loosely consolidated rubble pile rather than a solid body. Simulations of its tidal disruption show that it would begin shedding mass at about 2.25 Mars radii, losing chunks from structurally weak areas like the rim of its large Stickney crater. Several larger shedding events follow at progressively closer distances, until at about 2.09 Mars radii a major breakup event destabilizes the remaining mass entirely, ending its synchronous rotation and leading to complete disruption. The result would be a broad ring of particles orbiting Mars, with orbital eccentricities in the range of one-hundredth to one-thousandth.10Astronomy & Astrophysics. Tidal disruptions of rubble piles: The case of Phobos Mars would get a ring, not a crater. The contrast with a hypothetical Moon-Earth collision is instructive: Phobos is small and weak enough to be shredded by tides before it reaches the surface, while our Moon is large and strong enough to survive intact all the way down.
What the Rebuilt Planet Would Look Like
Assuming no further catastrophes, the post-impact Earth would eventually cool, re-solidify, and develop a new atmosphere outgassed from its mantle. The composition of that atmosphere would depend heavily on the chemistry of the magma ocean and whatever volatiles remained trapped in the interior. Early Earth went through a similar process after the Theia impact roughly 4.5 billion years ago, and over hundreds of millions of years it developed oceans and, eventually, conditions hospitable to life.
But the rebuilt planet would differ from the one we know in several ways. Its mass would be slightly larger, having absorbed the Moon’s mass. Its rotation rate would be dramatically altered; depending on the angle and speed of impact, it might spin much faster or much slower than it does now. Without a large moon to stabilize its axial tilt, the planet’s obliquity could wander chaotically over millions of years, producing extreme swings in climate. The debris ring would eventually dissipate, either falling back to the surface or coalescing into one or more small moons, but those moons would be far less massive than the one that was lost.
Whether life could arise again on such a world is an open question. Life appeared on Earth within the first billion years after the Moon-forming impact, possibly within a few hundred million years. If the same geochemical ingredients were present, there is no obvious reason a second genesis could not occur. But it would start from scratch, with no inherited biology, no DNA, and no continuity with anything that existed before. The planet might eventually become habitable again. It would not be Earth in any recognizable sense.