This Backwards-Orbiting Asteroid Has Been Flirting With Death For A Million Years

Most asteroids orbit the Sun in a counterclockwise fashion, but a newly-discovered object nicknamed Bee-Zed goes against the grain, spinning around the Solar System the opposite way. Not only that, it frequently ventures within Jupiter's orbital space — putting it on a potential collision course with the gas giant and its 6000 co-orbiting asteroids.

The retrograde asteroid is shown in green. (Credit: Paul Weigert/Western University)

Of the millions of documented asteroids in the Solar System, a scant 82 of them, or 0.01 per cent, orbit the Sun in a retrograde motion. But as a new study in Nature points out, asteroid Bee-Zed, or 2015 BZ509, is exceptional even among these backwards-orbiting misfits. It has the distinction of being the only known retrograde object in the Solar System that shares its orbital plane with another planet, in this case mighty Jupiter.

What makes this celestial anomaly stranger still is that Jupiter is accompanied by 6000 "Trojan" asteroids, the vast majority of which follow the gas giant in a prograde orbit. Similar to a racecar driver going the wrong way around a track, Bee-Zed is careening towards these objects with each trip around the Sun. According to calculations made by Western University astronomer Paul Weigert, Bee-Zed has been doing this for at least a million years, amounting to tens of thousands of successful "laps" around the Sun. So far, it has emerged unscathed from these close encounters.

Bee-Zed's success may not be an accident. As noted in the study, Jupiter's gravity is causing the rogue asteroid to weave in and out of the planet's path each time the two objects pass. It's the only asteroid known to have this relationship with a planet, and this state of "synchronicity" should allow Bee-Zed to avoid a catastrophic collision with Jupiter for the next million years at least. This analysis is based on calculations and observations made with the Large Binocular Camera on the Large Binocular Telescope in Mt Graham, Arizona.

With each orbit Bee-Zed and Jupiter make around the Sun, the retrograde object passes once inside and once outside the gas giant. This results in two opposing gravitational nudges that keeps the object on a safe path. Even though Bee-Zed crosses Jupiter's orbital plane, it never actually gets too close; the nearest the two objects get to each other is about 175 million km, roughly the distance between Earth and the Sun. So for Bee-Zed, it's like playing "chicken" with a massive truck — but the space rock only ventures onto its path when the truck is still far, far away.

The asteroid may not smash into Jupiter any time soon, but it's less clear if Bee-Zed will smash into one of the Trojans.

"We don't know where all Jupiter's Trojans are yet so we can't definitely determine the odds," Weigert told Gizmodo. "Given their relative sizes and the volume of space they orbit within, the odds are only about one in one billion of a collision each time BZ goes around its orbit. So the chances of BZ colliding with another Trojan asteroid are small. But the Trojan asteroids don't have the same 'dynamical protection mechanism' that protects BZ from collisions with Jupiter: They just have to trust in luck."

Not much is known about Bee-Zed, which was discovered by the Panoramic Survey Telescope And Rapid Response System (Pan-STARRS) in 2015. And although astronomers presume it to be a rocky asteroid, they aren't even entirely sure — it could be an ice-covered comet. In fact, it may have originated from the same place as Halley's Comet, perhaps the most famous retrograde object in the Solar System.

[Nature]

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