Astronomers map unique ‘wrong-way’ asteroid


For at least a million years, an asteroid orbiting the ‘wrong’ way around the sun has been playing a cosmic game of chicken with Jupiter and about 6,000 other asteroids sharing the giant planet’s space, according to a report published in the latest issue of Nature.

Asteroid Bee-Zed (BZ509) zips along like a wrong-way driver on the interstellar speedway that marks the orbital flow of our solar system’s planets and asteroids. Somehow, this odd little chunk of ice or rock manages to dodge traffic with every pass – even as logic suggests it should have been annihilated long ago by the thousands of other vehicles headed in the other direction.

A team of researchers, led by astronomer Paul Wiegert of Western’s Department of Physics and Astronomy and the Centre for Planetary Science and Exploration, has spent hundreds of hours observing and computer-simulating the orbit of Bee-Zed since its discovery in 2015.

In the newly published paper, Wiegert’s team notes a number of curious discoveries about the cosmic outlier that is Bee-Zed. For one, it is the only asteroid among a million or so in our solar system known to travel clockwise around the sun at the same time as it is also travelling in a planet’s orbital space.

“Everybody knows the Earth goes around the Sun and all the other planets go around the Sun as well,” Wiegert explained. “They all go the same direction – counter-clockwise – when seen from above. And 99.9 per cent of the asteroids go around the Sun in exactly the same direction.”

The remaining 0.1 per cent tend to head off in their opposite (retrograde) paths far from where they might intersect with another planet’s orbit.

But Bee-Zed is different – it is the only one known to jump into the fray against thousands of Jupiter’s Trojan asteroids.

To understand how rare that is, Wiegert offers the following analogy.



“If you imagine the solar system as a racetrack, the planets are these monster trucks and Jupiter is the biggest monster truck of them all. Asteroids are like these ridiculously small clown cars going around the sun, all going in one direction … except for Bee-Zed,” he said.

An animation his team created shows Bee-Zed swinging wide around Jupiter in its 12-year orbit, then jumping full speed into Trojan traffic. The asteroid has safely navigated its course many thousands of times, for at least a million years, Wiegert’s team has found.

So how does it avoid colliding with Jupiter? The team’s hypothesis is Jupiter’s gravity actually helps prevent impact.

“Bee-Zed manages to avoid Jupiter by swerving around it at just the right moment and Jupiter’s gravity actually helps it do that. It traps it in this orbital state and protects it,” so that both can continue safely on their way,” Wiegert explained.

As for how it manages to evade head-on hits with other asteroids, the research team is less certain. “Bee-Zed just has to be lucky enough not to hit them,” he added.

Their analysis is based on complex calculations and 300 days of observations through the Large Binocular Camera on the Large Binocular Telescope in Mt. Graham, Arizona.

By the team’s calculations, Bee-Zed’s orbit has been stable for a million years and will be stable for at least a million more.

Little is known about the asteroid, which was discovered in January 2015. It has a diameter of about three kilometres and it may have been a spin-off of Halley’s comet, which also has a retrograde orbit. The team hasn’t been able to determine if Bee-Zed is an icy comet or a rocky asteroid.

Learning more about the asteroid provides another intriguing glimpse into previously unknown and unmapped features of our solar system.

“The detective work has just begun,” Wiegert said.

His team brings together researchers from around the globe and includes Martin Connors of Athabaska University Observatories in Alberta, University of Calgary’s Department of Physics and Astronomy and the Institute for Space-Earth Environmental Research at the University of Nagoya, Japan; and Christian Veillet of the Large Binocular Telescope Observatory in Tucson, Arizona.

Their work was supported in part by the Natural Sciences and Engineering Council of Canada.