Asteroid in Jupiter’s orbit goes its own way

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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.