That’s no moon: Earth’s tiny tagalong | Science News for Students

That’s no moon: Earth’s tiny tagalong

Asteroid orbits sun but sticks close to Earth
Jul 20, 2016 — 7:00 am EST
asteroid orbit

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.


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 orbitThis 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.

Power Words

(for more about Power Words, click here)

asteroid   A rocky object in orbit around the sun. Most orbit in a region that falls between the orbits of Mars and Jupiter. Astronomers refer to this region as the asteroid belt.

astronomy    The area of science that deals with celestial objects, space and the physical universe. People who work in this field are called astronomers.

gravity    The force that attracts anything with mass, or bulk, toward any other thing with mass. The more mass that something has, the greater its gravity.

moon    The natural satellite of any planet.

orbit    The curved path of a celestial object or spacecraft around a star, planet or moon. One complete circuit around a celestial body.

planet   A celestial object that orbits a star, is big enough for gravity to have squashed it into a roundish ball and it must have cleared other objects out of the way in its orbital neighborhood. To accomplish the third feat, it must be big enough to pull neighboring objects into the planet itself or to sling-shot them around the planet and off into outer space. Astronomers of the International Astronomical Union (IAU) created this three-part scientific definition of a planet in August 2006 to determine Pluto’s status. Based on that definition, IAU ruled that Pluto did not qualify. The solar system now includes eight planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.

quasisatellite     A body that orbits the sun but appears to orbit a planet.

sun   The star at the center of Earth’s solar system. It’s an average size star about 26,000 light-years from the center of the Milky Way galaxy. Or a sunlike star.


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Further Reading

C. Crockett. “Mini moons may zip around Earth.” Science News. Vol. 188, September 5, 2015, p. 9.