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.
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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.
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| 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.”
