When is a Comet not a Comet? Asteroid 2016 WF9 Blurs the Line between Comet and Asteroid

 A new asteroid was discovered by the Wide-Field Infrared Survey Explorer (WISE) mission in November of 2016, designated 2016 WF9. While the mission finally decided to designate the object as an asteroid a few days later, 2016 WF9 presented an interesting puzzle: while it appears to be a comet in structure, it also lacks the dust and gas cloud that is characteristic of what we expect from comets.

The researchers believe that, over time, 2016 WF9 has simply been depleted of the elements that would be ejected from the object to create its characteristic gaseous nucleus and tail. These features are formed as the icy objects near the Sun and are heated, causing the body to release gas and dust. The resulting cloud, and the tail strewn away from the comet by the solar wind, can be readily visible to the naked eye, and are the source of numerous legends over the course of human history.

Because of this unusual state of depletion, 2016 WF9 has presented an interesting example of what might become of comets that run out of gas, so to speak," explains James Bauer, NEOWISE’s Deputy Principal Investigator at JPL. “2016 WF9 could have cometary origins. This object illustrates that the boundary between asteroids and comets is a blurry one; perhaps over time this object has lost the majority of the volatiles that linger on or just under its surface.”

2016 WF9 is a fairly large object, somewhere between 0.5-1 kilometers (0.3 to 0.6 miles) across, but it poses no threat to us: the comet-oid’s orbit will only bring it within 51 million kilometers (32 million miles) of Earth’s orbit, nearly 133 times further away than the Moon. This is the closest 2016 WF9 is expected to come to the Earth before 2095.