The Hubble Space Telescope has presented us with
telling images of a peculiar body that seems to share the attributes of an
asteroid and a comet. It has been
christened with the double name 300163 2006 VW139/288P, alias P/2006 VW139, acknowledging
its asteroid-like and comet-like traits and declining to make a decision
between the two categories.
The first part of the name attests that the body was
discovered as an asteroid in October 2006, but the recognition of its strange
appearance and its ambiguous identity had to await the Hubble observations a
year ago. The second part of its name
identifies it as a body with cometary appearance (a tail) and a “periodic”
orbit, meaning that it has a known orbital period. And what should we call it? The exceptional clunkiness of the name
suggests nicknames such as “2006 VW”, which unfortunately conjures up a
misleading (non-duplicitous) image.
A “video” of the body, constructed out of a number
of snapshots taken as it rotates, can be found at http://hubblesite.org/videos/news/release/2017-32.
Form your own opinions (and suggest your own
nicknames).
So what do we see when we look at that video? Two bright, roughly equal-sized apparently
solid bodies, just a few body diameters apart, trailing a bright “tail” of
generally cometary appearance. The tail
appears to be a dust stream rather than a gaseous tail, tempting us to
speculate about the chain of events that produced this strange display.
The “just-so” story has to take into account the
duplicity, proximity, and activity of the body.
There are several possible scenarios to consider:
1a and b. Collisional Fission. An asteroid in the Belt was recently
clobbered by a glancing impact sufficiently violent to get it rotating fast
enough to fission into two bodies in very close orbit about each other, perhaps
even a contact binary. This happened so
recently that the dust sprayed out by the impact has not had time to dissipate. Sounds improbable, but there are examples
galore to whet our imagination. The
Trojan asteroid 624 Hektor was studied in 1980 by a former student of mine, Stu
Weidenschilling, and interpreted as a contact binary; a Wikipedia article on
“Contact Binary: Asteroid” lists 13 strong candidates for the same
distinction. At least two comets
(67P/Churyumov-Gerasimenko and 8P/Tuttle) also appear to have the same
structure. Such structures can arise
either by (1a) the glancing collision of two large independently-orbiting
bodies or by (1b) rotational fission of a single asteroid that was spun up to
the point of splitting apart by a relatively small but sufficiently violent impact
event. Since the encounter velocity of
two asteroids in independent orbits around the Sun is typically several
kilometers per second, the overwhelming majority of asteroid/asteroid
collisions must result in explosive disruption rather than capture into closed
orbits. Low-velocity encounters must be
rare; but contact (or near-contact) binaries ARE rare. The dust-generating activity can then be
attributed to the collision event, which must have been fairly recent;
otherwise, the dust would have been swept up by the bodies or blown away by solar
radiation pressure. Oh, we also know
from independent evidence that 2006 VWs may survive low-velocity
collisions.
2. It is just
a comet whose gas emission is undetectably small, but sufficient to maintain a
dust cloud, according to H. H. Hsieh and 41 other coauthors, Discovery of
main-belt comet P/2006 VW139 by Pan-STARRS1, Astrophysical Journal
Letters, 748 (1). (L15) 1-7 (2012). They
interpret observations by the Pan-STARRS1 survey telescope as showing a
striking similarity to the main-belt cometary object 133P/Elst-Pizarro, which
has a rather similar orbit. Both may be
outliers of the Themis family of main-belt asteroids. They argue that the stable maintenance of the
dust cloud is indicative of cometary outgassing, most likely of water vapor, but
they are also unable to detect any gaseous molecular emissions
spectroscopically.
If I had to bet on
the strength of present evidence, I’d have to prefer the latter
explanation. The implication of a very
high water content in some Belt asteroids is worth remembering—especially if
you are a water-based spacefarer.
See also:
https://www.space.com/38214-spitting-asteroids-hubble-telescope-discovery.html?utm_source=sdc-newsletter&utm_medium=email&utm_campaign=20170922-sdc
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