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 will get 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.
Once the Moon will appear twenty times bigger on our sky — at a distance of about twenty thousand kilometers from Earth, it would be tidally disrupted into a stream of rocks and dust around the Earth, 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 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 of time.
In our recent expedition to the Pacific Ocean, we retrieved meteoritic spherules of differentiated composition, which likely originated from a planet with a magma ocean. When the rock is molten some elements join the iron core of the planet, leaving behind a differentiated composition on what later becomes the planet’s crust. Our recent paper showed that the elemental abundance pattern in the meteor site does not match the crust on Earth, the Moon or Mars, and is likely associated with a magma-ocean planet outside the Solar system. Interstellar meteors with this composition can be naturally produced by the tidal disruption and melting of a rocky planet like the Earth when it comes close to a common dwarf star, as I showed in a new publication with my postdoc, Morgan MacLeod.
Recent computer simulations of the Moon forming impact, suggest that the ejected mass ended up as an orbiting ring of debris around Earth, with some of this matter condensing into the Moon. As a result of the impact, the Earth was spun up to about a four-hour day. In one of my recent classes at Harvard, I asked the students to imagine their daily routines in a four-hour day. A long two-session class would have consumed most of their day on the early Earth.
The Moon’s birth and death on Earth is a reminder of the humbling phrase: “Ashes to ashes, dust to dust.” Apparently, this circular journey applies to giant rocky bodies in space, not only to humans.
For now, we must follow Francis Bacon’s wise advice from 1625: “If the Mountain won’t go to Mohammed, then Mohammed will go to the Mountain”. NASA’s Artemis program has no choice but to ship cargo from Earth to the Moon for the next 7.6 billion years until the Moon will return to Earth. At that time, all cargos will return to Earth by nature, free of charge.
In the context of the 1942 philosophical essay by Albert Camus, titled “The Myth of Sisyphus,” the absurdity of life can be summarized by the act of lifting a cargo up to the Moon only for the cargo to roll back down to Earth 7.6 billion years later. Camus summarized the human condition by affirming: “The struggle itself towards the heights is enough to fill a man’s heart. One must imagine Sisyphus happy.” Accepting this state of affairs, we can enjoy our life under the Sun while it lasts.
ABOUT THE AUTHOR
Avi Loeb is the head of the Galileo Project, founding director of Harvard University’s — Black Hole Initiative, director of the Institute for Theory and Computation at the Harvard-Smithsonian Center for Astrophysics, and the former chair of the astronomy department at Harvard University (2011–2020). He chairs the advisory board for the Breakthrough Starshot project, and is a former member of the President’s Council of Advisors on Science and Technology and a former chair of the Board on Physics and Astronomy of the National Academies. He is the bestselling author of “Extraterrestrial: The First Sign of Intelligent Life Beyond Earth” and a co-author of the textbook “Life in the Cosmos”, both published in 2021. His new book, titled “Interstellar”, was published in August 2023.
