Encounters with Jupiter send asteroid on a bizarre backward spin
Jupiter is widely credited with providing Earth with a bit of protection. The immense gravity of the gas giant typically either sucks in asteroids and comets or flings them out into orbits where they pose our planet little danger. But astronomers have now identified an asteroid that's in a stable orbital interaction with Jupiter. That interaction sends the asteroid around our Solar System backward and causes it to shift between two radically different orbits without ever settling into either.
The planets and other bodies in the Solar System mostly orbit in a single direction, inherited from the spinning disk of material from which they formed. A few bodies orbit in the opposite direction-called retrograde-but these tend to have odd, highly elliptical orbits. They're also very rare; only 0.01 percent of the known asteroids have retrograde orbits. Orbiting in the wrong direction around the Solar System tends to bring an object into relative proximity to a planet twice an orbit, and the resulting gravitational interactions will eventually destabilize the orbit.
Or so we thought. Some mathematical work showed that it's possible to have a stable retrograde orbit that overlaps with the orbit of a planet. In this case, the gravitational interactions are the key to stability. Each of the two passes of a single orbit would provide a nudge that counteracted the results of the previous one. While the orbit would shift with each nudge, it would remain stable due to this cancellation.
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