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The moon will return to earth to die in 7.6-billion years. Here’s how it may happen.

Debrief

After 51 years, the US returned to the Moon this week. Intuitive Machines Inc., the Houston-based maker of the Moon lander Odysseus, reported that its vehicle landed on its side after possibly breaking one of its legs upon landing.  A solar panel could have been damaged while some antennae are pointed downwards, limiting their ability to send data back to Earth.

The moral of this story: never wish a lunar lander to break a leg!

Given how challenging and expensive it is for us to ship cargo from Earth to the Moon, one may wonder whether the Moon will ever return back to Earth, its birth site? This question echoes the wish of parents to see their grown-up kid back home.

In these troubling times, it gives me pleasure to be the bearer of good news: “Yes, the Moon will be back home.” According to my theoretical calculations, the Moon will return to Earth in 7.6 billion years. Although it is unlikely that humans will survive that long to witness this thrilling return, plenty of Moon rocks will be raining back on the surface of Earth, making them common terrestrial commodities instead of the rare museum items they are today.

Let me elaborate.

In 7.6 billion years, the Sun is predicted to expand and engulf the Earth during its final evolution as a red giant. Within hundreds of millennia, after the Sun’s photosphere gets close to Earth, the drag of the Earth on the solar envelope will cause it to spiral into the core of the Sun. But long before it gets there, the same drag will cause the Moon to spiral quickly in its orbit towards the Earth.

By then, the Moon will appear twenty times bigger in our sky at a distance of about twenty thousand kilometers from Earth, and it would be tidally disrupted into a stream of rocks and dust around the planet resembling Saturn’s rings. The drag of these fragments on the envelope of the Sun will bring them quickly down to Earth, well before the Earth gets to the core of the dying Sun.

Eventually, the Earth will merge with the core of the Sun and join the metallic sphere that will condense there as a newly-born white dwarf. The white dwarf remnant will carry about three-fifths of the mass of the Sun and have a radius comparable to that of the Earth.

Terrestrial life as we know it would be eliminated long before the return of the Moon to Earth, within 1-2 billion years from now, when the brightening Sun will boil off all oceans on Earth.

What a dramatic sequence of events awaits in our future! But before mourning the future death of the Moon, we should recall how it was born.

The Moon was likely produced as a result of a giant impact by a Mars-size object, commonly called Theia, on Earth in the early solar system. The impact occurred during the first hundred million years after the Earth was born. It melted the Earth’s rock, turning it into a magma ocean.  The Moon and Mars were also magma oceans for a brief period…

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