That’s no moon: Earth’s tiny tagalong

Asteroid orbits sun but sticks close to Earth

The orbit of 2016 HO3 (yellow) keeps it close to Earth. From our vantage point, it appears as though this asteroid orbits Earth. It actually orbits the sun.

JPL-CALTECH/NASA

Earth has a newly discovered companion. It’s not another moon, though. Asteroid 2016 HO3 is a quasisatellite (KWAH-zee-SAT-uh-lyte). This space rock appears to orbit Earth. But that’s an illusion. It’s just looping around the sun and playing leapfrog with our planet.

asteroid orbit
Asteroid 2016 HO3 orbits the sun, but its path makes it appear to orbit Earth. JPL-Caltech/NASA
This temporary tagalong was discovered on April 27 in images from the Pan-STARRS observatory in Hawaii. The asteroid’s orbit around the sun is similar to Earth’s. One year on 2016 HO3 is just about 16 hours longer than an Earth year. Earth’s gravity keeps the asteroid from wandering. It never strays farther than about 40 million kilometers (25 million miles) from Earth. And it never comes closer than about 14 million kilometers (7 million miles). That’s 38 times the distance between the Earth and the moon.

The asteroid’s path sometimes takes it speeding ahead of Earth. At other times, it falls behind. Because the orbit of the space rock is tied to the sun and not Earth, the asteroid doesn’t qualify as a full-fledged moon. But its constant proximity to us is enough to make it the only known quasisatellite of our world.

The tiny rock is no more than about 100 meters (330 feet) across. Astronomers suspect it has probably tagged along with Earth for only about a century. And orbital calculations suggest that it will continue to do so for several centuries to come.

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