NASA's Hubble Space Telescope has found a binary asteroid pair that exhibits characteristics of a comet:
Hubble was used to image the asteroid, designated 300163 (2006 VW139), in September 2016 just before the asteroid made its closest approach to the Sun. Hubble's crisp images revealed that it was actually not one, but two asteroids of almost the same mass and size, orbiting each other at a distance of 60 miles.
Asteroid 300163 (2006 VW139) was discovered by Spacewatch in November 2006 and then the possible cometary activity was seen in November 2011 by Pan-STARRS. Both Spacewatch and Pan-STARRS are asteroid survey projects of NASA's Near Earth Object Observations Program. After the Pan-STARRS observations it was also given a comet designation of 288P. This makes the object the first known binary asteroid that is also classified as a main-belt comet.
The more recent Hubble observations revealed ongoing activity in the binary system. "We detected strong indications for the sublimation of water ice due to the increased solar heating — similar to how the tail of a comet is created," explained team leader Jessica Agarwal of the Max Planck Institute for Solar System Research, Germany.
The combined features of the binary asteroid — wide separation, near-equal component size, high eccentricity orbit, and comet-like activity — also make it unique among the few known binary asteroids that have a wide separation. Understanding its origin and evolution may provide new insights into the early days of the solar system. Main-belt comets may help to answer how water came to a bone-dry Earth billions of years ago.
The team estimates that 2006 VW139/288P has existed as a binary system only for about 5,000 years. The most probable formation scenario is a breakup due to fast rotation. After that, the two fragments may have been moved further apart by the effects of ice sublimation, which would give a tiny push to an asteroid in one direction as water molecules are ejected in the other direction.
Also at HubbleSite, New Atlas, ScienceAlert, and Space.com.
A binary main belt comet (preprint)
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(Score: 1) by Booga1 on Monday September 25 2017, @04:47PM
So the twin-tailed comet from Warhammer might not be such a rare thing after all...
(Score: 0) by Anonymous Coward on Monday September 25 2017, @05:16PM (2 children)
The boundary between "comet", "asteroid", and "planet" (including dwarf planet) is just plain fuzzy. Our vocabulary is an oversimplification that often hides the complexity and variation of the "things" buzzing around the Solar System.
I don't know if there's a fix other than drawing arbitrary lines in the sand, such as something has to have a mass of at least X to be called a Y, be a certain % of water/ice to be called a "comet", etc. But even hard lines can be problematic. For example, if a given body has most of its ice on the outside, it can look & act like a fantastic comet as it approaches the sun even though its inner body and total composition is mostly rock.
(Score: 4, Interesting) by takyon on Monday September 25 2017, @05:46PM (1 child)
We can just get more generic. It's a binary object. Binary meaning two parts with similar diameters orbiting each other, and object meaning rock(s).
As for comets:
Ceres, the asteroid dwarf planet, has a lot of ice water. But it will never approach too close the Sun (aside from being heated or destroyed when the Sun goes red giant).
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(Score: 1, Interesting) by Anonymous Coward on Monday September 25 2017, @09:02PM
I believe "atmosphere" in your description is just water vapor/ions from melting ice. Any icy body would probably generate one if close enough to the sun. In that case, "comet" perhaps can be seen as a condition rather than a type of object, although tradition is to still call it a comet. If we talk about Halley's Comet in its current (outer) state, we still call it a comet even though it's not spewing gases right now and may not be notably different than an icy asteroid. The further away from the sun you go, the more icy asteroids you find, which you kind of expect because solar radiation evaporates ice.
If by chance an encounter with say Planet X hurled it toward the sun, perhaps it could then be called a "comet". That much evaporating ice would make a masterful show in the sky: a mega-comet from our perspective.