Pretty much everything in the Solar System orbits in the same direction around the Sun (counterclockwise as observed from above Earth’s north pole, if you’re curious). But some objects just didn’t get the memo… Take the Trojan asteroid 2015 BZ509, for example. It orbits the Sun in roughly the same orbit as Jupiter, but in the opposite direction.

And according to a paper published today in Nature, it has been going the wrong way for about a million years!

That’s a little odd.

It’s a bit like this wee asteroid has been plowing the wrong way down a superhighway of other asteroids for a million years—without hitting anything! This kind of retrograde motion isn’t unique (other asteroids get away with it, too), but 2015 BZ509 (or “BZ,” pronounced “bee-zed” because it wasn’t discovered by Americans) also avoids colliding with the giant planet Jupiter itself. Today’s press release continues the traffic metaphor and compares BZ’s behavior to playing chicken with a monster truck.

Now, all of this is a bit of an exaggeration, since asteroid fields aren’t as densely packed as the one you might have seen in The Empire Strikes Back, for example. So it’s less of a traffic jam, more of a long-odds game of how many orbits a misfit like BZ can complete before eventually hitting something else. The likelihood of an individual collision are low, but over time, the odds shift against you…

We know of more than 6,000 Trojan asteroids sharing the same approximate orbit as Jupiter. Most of them have settled into gravitational sweet spots ahead of and behind the planet: they orbit with the same period as Jupiter, circling the Sun roughly every 12 years, but about half are clustered 60° ahead of the planet, the other half 60° behind. (Why the name “Trojans”? Because of the Trojan War, of course, with the Greeks versus the Trojans leading and following the planet Jupiter in its orbit. Who says astronomers aren’t cultured?) This stable geometry balances the gravitational influences from the Sun and Jupiter to allow the Trojan asteroids to live long lives in their respective orbits. (Unlike the mythological characters for which many of them are named.)

BZ passes headlong through both clumps of Trojans, which increases the chance of collision—and not once but twice for each of its orbits around the Sun.

And what about the risk of colliding with Jupiter itself? With each of its orbits, BZ receives a slight gravitational nudge from Jupiter—but it alternates between passing closer to and farther from the Sun than Jupiter. Thus, Jupiter nudges it one direction on one orbit and the opposite direction with the next orbit. The two effects cancel, leaving BZ in a fairly stable orbit.

The research team estimates that BZ has completed tens of thousands of trips around the Sun, and they calculate that it should be able to maintain its unusual orbit for another million years or so, before it loses sync with Jupiter.

Oh, and in case you were wondering, BZ’s errant path doesn’t take it anywhere near Earth. So we’re in no danger of this wrong-way Trojan hitting us. You can sleep easy.

But if you want to learn more about asteroids, comets, and the hard-hitting stories of our cosmic origins, check out the Academy’s planetarium show Incoming! You can see it every day at Morrison Planetarium, or look for it at other planetariums near you.

Share This