[meteorite-list] First MRO HiRISE Images of Comet ISON

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Wed, 2 Oct 2013 11:58:19 -0700 (PDT)
Message-ID: <201310021858.r92IwJNQ005533_at_zagami.jpl.nasa.gov>

http://www.uahirise.org/releases/ison.php

First HiRISE Images of Comet ISON
Written by: Alan Delamere and Alfred McEwen
University of Arizona

[Images]
Comet ISON Captured by HiRISE
Image credit: NASA/JPL/University of Arizona

On 29 September 2013, the Mars Reconnaissance Orbiter (MRO) maneuvered
to point its High Resolution Imaging Science Experiment (HiRISE) camera
at ISON, a new comet passing by Mars on its way into the inner Solar
System.

HiRISE saw a small spot at the position of ISON that is relatively
bright, like a star, but moving relative to actual stars. The comet's
coma is apparently very faint, so these data provide useful constraints
on the size of the comet nucleus and its overall brightness, key
measurements to understand its behavior and useful knowledge to
subsequent observers.

These images show a 256 x 256 pixel patch of sky at the range to the comet
of 8 million miles and when the solar phase angle is 47 degrees. Three more
observations of ISON are planned for 1 and 2 October as the comet moves
through closest approach to Mars at 7 million miles, but with less
illumination as seen from Mars.

Based on preliminary analysis of the data, the comet appears to be at
the low end of the range of brightness predictions for the observation.
As a result, the image isn't visually pleasing but low coma activity is
best for constraining the size of the nucleus. This image has a scale of
approximately 8 miles (13.3 km) per pixel, larger than the comet, but
the size of the nucleus can be estimated based on the typical brightness
of other comet nuclei. The comet, like Mars, is currently 241 million
kilometers from the Sun. As the comet gets closer to the sun, its
brightness will increase to Earth-based observers and the comet may also
become intrinsically brighter as the stronger sunlight volatilizes the
comet's ices.

Comet ISON (officially known as C/2012 S1) is believed to be in its
first pass through the inner solar system from the distant Oort Cloud, a
roughly spherical collection of comets and comet-like structures that
exists in a space between one-tenth light-year and 1 light-year from the
sun. The comet will pass within 724,000 miles (1.16 million kilometers)
of the Sun on 28 November 2013. It was discovered on 21 September 2012,
roughly between Jupiter and Saturn, by Vitali Nevski and Artyom
Novichonok at the International Scientific Optical Network (ISON) near
Kislovodsk, Russia.
Received on Wed 02 Oct 2013 02:58:19 PM PDT