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47

January/February 2018^ DISCOVER^53

❯

IN MARCH, ASTRONOMERS IN

CANADA identified an asteroid unlike

any other: It travels the same orbit as

Jupiter, but circles the sun in the opposite

direction. Only a few other objects in the solar

system are known to have such a “wrong

way” orbit, and asteroid 2015 BZ 509 alone

shares its path with a planet.

Astronomer Paul Wiegert, from the

University of Western Ontario, and colleagues

tracked the object during late 2015. They

found that its gravitational relationship with

the sun and Jupiter keeps it in a safe, stable

orbit — and it’s likely been there for around a

million years, they reported in Nature.

The questions now include figuring out if

2015 BZ 509 is more like an icy comet or a rocky

asteroid and how it ended up going the wrong

way in the first place. Those will help answer

what Wiegert calls the big question. “The first

asteroid discovered turned out to be one of

millions,” he says. “Is BZ 509 unique or just the

tip of an iceberg?”  STEPHEN ORNES

Earth orbit

Sun

Jupiter orbit

February

2018

September

2029

NASA/JPL-CALTECH; INSET: DAN BISHOP/DISCOVER

Asteroid 2015 BZ 509 orbit

Asteroid

Hits Reverse

This top-down view shows the shifting

orbit of asteroid 2015 BZ 509 around the sun,

and how it overlaps with Jupiter's.