Earth has a tiny tagalong, and no, it’s not a moon

Quasisatellite orbits sun, but stays close to Earth

illustration of orbit of asteroid 2016 HO3

THAT’S NO MOON The orbit of asteroid 2016 HO3 (yellow), discovered April 27, keeps it close to Earth; from our vantage point, it appears as though 2016 HO3 orbits Earth when it actually orbits the sun.

JPL-Caltech/NASA

Quasisatellite
KWAH-zee-SAT-ah-lite n.

A body that orbits the sun and appears to orbit Earth.

Asteroid 2016 HO3 appears to orbit Earth, but that’s just an illusion. As the space rock loops around the sun, it plays leapfrog with our planet, sometimes speeding ahead sometimes falling behind. The asteroid’s suncentric orbit keeps it from qualifying as a full-fledged moon of Earth, but its constant proximity to us is enough to make it the only known “quasisatellite” of our world.

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 400 million kilometers from Earth and never comes closer than about 14 million kilometers (38 times Earth’s distance to the moon).

The tiny rock — no more than about 100 meters across — has probably tagged along with Earth for about a century, and orbital calculations suggest that it will continue to do so for several centuries to come. 

headshot of Associate News Editor Christopher Crockett

Christopher Crockett is an Associate News Editor. He was formerly the astronomy writer from 2014 to 2017, and he has a Ph.D. in astronomy from the University of California, Los Angeles.

More Stories from Science News on Planetary Science

From the Nature Index

Paid Content