Could Earth Have Two Moons?

It’s our closest neighbor in the solar system and the only one we’ve set human feet on so far. But there’s still plenty of mystery surrounding our orbital partner, the moon.


—Image courtesy NASA

Perhaps one of the biggest questions is why we have a lone natural satellite, and a pretty big one at that.

Today’s prevailing theory is that the moon is made from bits of Earth that broke off during a collision between our planet and a Mars-size body about 60 million years after Earth was born. This material formed a ring around Earth that condensed into the moon.

The theory explains the moon’s size as well as why Earth’s mantle and the moon both lack iron—because the heavy element would have sunk down to form a core.

The trouble is, if the moon has a core, why does it have no magnetic field?

Earth’s metallic core is the source of our magnetic field, which protects us from damaging solar radiation. Today’s moon has no such protective shield, which is one of the main challenges in building a lunar base that people could inhabit for extended periods.

This week a team led by an MIT researcher reported that rocks collected during the Apollo missions support theories that the moon once had a molten core.

Reporting on the paper in the journal Science, NatGeo News editor Christine Dell’Amore notes that 4.2-billion-year-old moon rocks exhibit magnetism that’s consistent with a magnetic field and with the time when the moon could have had an ancient, active core.

But just because our moon formed this way, doesn’t mean that’s how all moons came to be. Many moons in our solar system were most likely passing asteroids that got trapped by a larger planet’s gravity.

This notion was in fact one of the earlier theories for how Earth got its moon—and it’s possible this is how our moon could one day get a mate.

Plenty of programs are up and running around the world to monitor the skies for the next killer asteroid, a space rock that comes too close to Earth and causes an impact on the scale of the one we think wiped out the dinosaurs.

But could a so-called near-Earth object (NEO) decide not to beat us, but join us?

The good people at Earth & Sky explain on their Kids page that an asteroid of just the right size moving at just the right speed and just the right angle could in fact get captured by Earth’s gravity into a stable orbit and become a second moon.

I’d be curious if any of the NEO watchers out there have calculated not only which objects might hit Earth, but which could one day add to our immediate family.

It actually wasn’t too long ago that astronomers were seriously looking for a second moon hypothesized to be too small, too fast, or in too odd of an orbit for humans to find.

The idea of an “invisible” second moon got widespread attention when writer Jules Verne used a French astronomer’s theory for how a second moon could go unnoticed in one of his most famous novels.

And an amusing TIME article from as recent as 1954 speculated that the astronomer who found Pluto, Clyde Tombaugh, was engaged in a secret search for a second moon on behalf of the American government.

With all the satellites, shuttles, and telescopes we’ve sent into space, it’s an easy guess as to whether a mystery moon could be circling Earth. But it’s fun to think that in the future some special asteroid might come along and make the moon a little less lonely.

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