A Trojan in retreat

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

The asteroid is the only one in the solar system known to have an opposite, or retrograde, orbit around the sun while at the same time sharing a planet's orbital space, says researcher and co-author Paul Wiegert of University of Western Ontario’s Department of Physics and Astronomy.

Jupiter shares its orbit with more than 6,000 Trojan asteroids (white), which travel in the same direction as the planet. But one of the planet’s companions is an outlier, traveling in the opposite direction. Planets and asteroids  have been enlarged to make them visible. 

All but fewer than 100 of the million or so known asteroids in our solar system travel around the sun in the same direction as Earth and the other planets (prograde motion). But asteroid 2015 BZ509 (“BZ” for short) circles the other way around – moving against the flow of all other asteroids in the giant planet’s orbital entourage (retrograde motion).

Put another way, it’s as if Jupiter were a monster truck on a track circling the sun, and the asteroids in Jupiter’s orbit are sub-compact cars all whizzing along in the same direction. BZ is the rogue — driving around the track in the wrong direction — and it does so every single lap, and has done so for thousands of laps for a million years or more.

So how does it avoid colliding with Jupiter? Jupiter’s gravity actually deflects the asteroid’s path at each pass so as to allow both to continue safely on their way, Wiegert says. Co-author Martin Connors of Athabasca University, adds: “Passes relatively near Jupiter take place twice on each body's orbit around the sun, but one is inside Jupiter's orbit, the other outside, so the disturbing effects of Jupiter, remarkably, cancel out.“

Little is known about the asteroid, which was discovered in January, 2015. It has a diameter of about three kilometers and may have originated from the same place as Halley's comet, which also has a retrograde orbit. The team hasn’t been able to determine yet if BZ is an icy comet or a rocky asteroid.

Images of 2015 BZ509 obtained at the Large Binocular Telescope Observatory (LBTO) that established its retrograde co-orbital nature. The LBTO has two 8.4 meter-wide main mirrors side-by-side, hence the two images taken in different color filters. The bright stars and the asteroid (circled in yellow) appear black and the sky white in this negative image. What are those weird white dots, spots and stripes? They are imaging artifacts in these raw images.

But their analysis – based on complex calculations and on observations through the Large Binocular Camera on the Large Binocular Telescope (LBT) on Mt. Graham, Arizona, during a span of 300 days — show BZ is somehow able to maintain a stable orbit even as an outlier. For co-author Christian Veillet of the LBT Observatory, this is a new step in a 15-year-long collaboration among the three co-authors, which until now, has been devoted to prograde asteroids sharing Earth’s orbit.

The calculations conducted by the team show the orbit has been stable for at least a million years and will be stable for at least a million more. Learning more about the asteroid provides another intriguing glimpse into previously unknown and unmapped features of our solar system, says Wiegert, adding that “the detective work has just begun.”

More information and video clips are available here. The Letter in Nature can be found here.