Asteroid in Jupiter’s orbit goes its own way

One of Jupiter’s companions is a bit of a nonconformist.

The gas giant shares its orbit around the sun with a slew of asteroids, but scientists have now discovered one that goes against the flow. It journeys around the solar system in reverse — in the opposite direction of Jupiter and all the other planets. Asteroid 2015 BZ509 is the first object found that orbits in the same region as a planet but travels backward, researchers from Canada and the United States report March 30 in Nature.
The asteroid was discovered with the Pan-STARRS observatory in Hawaii in 2015, and the researchers made additional observations with the Large Binocular Telescope Observatory in Arizona.

Backward-going asteroids are rare — only 0.01 percent of known asteroids are in retrograde orbits. Until now, none were known to share a planet’s orbit. It was thought that asteroids going in reverse couldn’t coexist with a planet because interactions with the celestial body, twice each orbit, would knock the asteroid off track. But because 2015 BZ509 passes on alternating sides of Jupiter, the interactions cancel each other out, the researchers say. The first flyby in orbit pulls the asteroid outward, and the next tugs it inward — keeping the maverick asteroid in line.

The asteroid’s relationship with Jupiter is no short-term fling: The researchers determined that the two have shared an orbit for a million years.