posted on Dec, 7 2013 @ 02:58 AM
Comet ISON was a really small one, around 1 km in diameter, perhaps even less. No wonder it broke up! Here's the assesment of some scientists using
the Mars Reconnaissance Orbiter's images of ISON as it passed by Mars (emphasis mine):
COMET C/2012 S1 (ISON)
W. A. Delamere, Delamere Space Sciences; A. S. McEwen, Rod Heyd, and Sarah Mattson, University of Arizona; J.-Y. Li, Planetary Science Institute; and
C. M. Lisse, Applied Physics Laboratory, Johns Hopkins University, report detection of comet C/2012 S1 using the Mars Reconnaissance Orbiter's High
Resolution Imaging Science Experiment (HiRISE) on Sept. 29-Oct. 2 UT. The comet was detected in all five orbits (ten pointings) of MRO, with the
signal-to-noise ratio increasing as the comet approached Mars and the phase angle of the observations rose from 47 to 89 deg. The authors find total
V-band magnitudes of 16.2, 16.5, 16.7, and 16.5 in the 0".2 aperture at phase angles of 47, 47, 51, and 51 degrees for the first four observations. At
closest approach, the HiRISE pixel scale at the comet was 13 km. Assuming all the observed flux to be due to scattered sunlight from the surface
(noting that dust from the inner coma dominates the higher-phase-angle observations) and assuming a nucleus surface with an albedo of 0.03 with an
adopted nucleus phase law of 10**(-0.016*phase angle), the authors find upper limits for the nucleus diameter of 1.25, 1.12, 1.05 and 1.12 km for
the first four observations, with phase angles of 47 and 51 degrees for the broadband red filter. Similarly for the narrower blue-green filter, the
upper limits are 1.25, 1.15, 0.95 and 1.12 km.
Note that those are upper limits to ISON's size, so in fact it could have been something like 900 meters in diameter.
This assesment corresponds to one amateur astronomer's calculations, presented below (column "Rcomet (m)" represents radius of ISON's nucleus in
So next time some crank Youtube video tries to tell you that a comet is gigantic, keep in mind that there are real experts out there doing real
science, measurements and calculations.
To round my post up, here's the latest hi-res STEREO-A HI1 image of ISON, from Dec 04:
ISON's remnant is hardly brighter than the nebulosity around the Pleiades, according to amateur astronomer Ian Musgrave. By the time earth telescopes
can see it it will be fainter still.
(The comet visible at the bottom of the image is Encke. It might be just image processing artifact, but to me it looks like it's got fainter and is
perhaps losing its life too)
edit on 7-12-2013 by wildespace because: (no reason given)