[meteorite-list] MRO Will Attempt to Image Comet C/2012 S1 (ISON)

[Ron Baalke]

http://mars.jpl.nasa.gov/mro/blogs/index.cfm?FuseAction=ShowBlogs&BlogsID=281

All Eyes on ISON
By Sarah Milkovich
August 13, 2013

There is a comet currently approaching the inner solar system that has 
been generating a lot of interest since its discovery last year: comet 
C/2012 S1 (ISON).

ISON is a "sun grazer" comet, which means it will get unusually close 
to the sun. This is also ISON's first journey around the sun, and so it 
may have a lot of water and carbon-dioxide ice to burn off, which should 
create a lovely tail. Astronomers are very interested in observing ISON 
to gather more information on these types of comets, and we're all hoping 
that it will survive the journey around the sun to put on a spectacular 
display for us as it passes by Earth in December. However, before ISON 
gets close to Earth on its outbound swing, it gets even closer to Mars 
on its inbound journey.

In August, ISON ought to be getting more active and developing its tail 
as it warms up, and astronomers want to get more information on this stage 
of the comet's development. Due to solar system geometry, ISON cannot 
be observed easily from Earth at this time - but it can be from Mars! 
In addition, we have a fabulous camera (HiRISE) and imaging spectrometer 
(CRISM) on the Mars Reconnaissance Orbiter in orbit at Mars. MRO is going 
to attempt to make measurements of ISON on August 20.

The catch is MRO was designed to look at the surface of Mars not a comet! 
During normal operations, we point our instruments straight down much 
of the time, or target interesting locales by rolling MRO up to 30 degrees 
to either side. In order to look at ISON, we have to turn the spacecraft 
over so that we can point those same instruments straight up, away from 
Mars! Furthermore, HiRISE is a high-resolution "pushbroom" camera, which 
has a very skinny field of view. HiRISE uses the motion of MRO as the 
spacecraft orbits Mars to take lines of little pictures that build up 
to a full picture. This means that in order to fully image the comet, 
we need to flip the spacecraft AND do a series of small motions to drag 
the HiRISE field of view across the area where we think the comet and 
its tail will be. Since we don't know enough about the comet to predict 
how bright it will be, we don't know exactly what camera settings to use, 
so we want to take several images with different exposures in the hopes 
that one of them will turn out well.

This is clearly a complicated challenge, involving the HiRISE team, the 
CRISM team, and the spacecraft team all working together to get the timing 
and commands correct and to make sure that this sequence doesn't cause 
any harm to the spacecraft itself. We've done a few unusual observation 
campaigns before, including HiRISE observations of Mars' moons, Jupiter, 
and the Earth-Moon system (images can be found here: 
http://hirise.lpl.arizona.edu/releases/), so we're very hopeful that we 
can get successful measurements of comet ISON.

Even if the comet is not bright enough to get good images this time since 
it is still 93 million miles (150 million kilometers) away!, this will 
be good practice for a second observing campaign, including observations 
with more of the MRO instruments, as ISON passes by Mars at just 6.8 million 
miles (11 million kilometers) in late September and early October. Stay 
tuned!