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originally posted by: SeaWorthy
originally posted by: wildespace
a reply to: SeaWorthy
The blue colour was added in the picture, the original image is black-and-white. hubblesite.org...
The comet would most probably look greenish due to diatomic carbon (C2) sublimating off its surface. www.slate.com...
Thanks can you link where they say it has been tinted blue? I can't find that info on your link or elsewhere and when I look at the full-size original here it is still blue. Not that it matters that much but they always show it blue everywhere I look.
These images represent several exposures acquired by the WFC3/UVIS instrument through the F350LP filter on the Hubble Space Telescope. Color has been applied to the grayscale (black&white) images
Figure 4A shows the best ﬁt model for a nucleus radius of 14 km for sublimation from CO or CO2, forced to match the photometric data at the time of discovery at TA=-140.8◦.
We also show a ﬁt for an 80 km radius nucleus.
The data from 2017 May to September showinsuﬃcient range along the orbit (TA) to distinguish between sublimation from CO or CO2as a driver of the activity. At these distances, there is no contribution from H2O sublimation CO-driven Activity in Comet C/2017 K2... (PDF Download Available). Available from: www.researchgate.net... [accessed May 05 2018].
originally posted by: MarsIsRed
Here's the abstract from arXiv: A Comet Active Beyond the Crystallization Zone
We present observations showing in-bound long-period comet C/2017 K2 (PANSTARRS) to be active at record heliocentric distance. Nucleus temperatures are too low (60 K to 70 K) either for water ice to sublimate or for amorphous ice to crystallize, requiring another source for the observed activity. Using the Hubble Space Telescope we find a sharply-bounded, circularly symmetric dust coma 10^5 km in radius, with a total scattering cross section of ∼10^5 km2. The coma has a logarithmic surface brightness gradient -1 over much of its surface, indicating sustained, steady-state dust production. A lack of clear evidence for the action of solar radiation pressure suggests that the dust particles are large, with a mean size ≳ 0.1 mm. Using a coma convolution model, we find a limit to the apparent magnitude of the nucleus V> 25.2 (absolute magnitude H> 12.9). With assumed geometric albedo pV = 0.04, the limit to the nucleus circular equivalent radius is < 9 km. Pre-discovery observations from 2013 show that the comet was also active at 23.7 AU heliocentric distance. While neither water ice sublimation nor exothermic crystallization can account for the observed distant activity, the measured properties are consistent with activity driven by sublimating supervolatile ices such as CO2, CO, O2 and N2. Survival of supervolatiles at the nucleus surface is likely a result of the comet's recent arrival from the frigid Oort cloud.
Interestingly, it was first observed in 2013 at a distance of 23.7 AU, and even at that distance it was showing signs of activity! That's one of the reasons the astronomers are certain this is the closest this comet has ever been to the sun. This also means a reasonably accurate assessment of it's orbit is possible - an object from interplanetary space would be traveling faster and be on a hyperbolic trajectory, so the Oort cloud is the most likely origin.