ISON Image of the Week

The SWAN Diver  (Oct 07, 2013)

It's faint (for now...) but it's there: comet ISON in the "SWAN" instrument on the NASA/ESA SOHO satellite. What we see here is the SWAN instrument detecting hydrogen in the coma of comet ISON - information that we may be able to use to estimate the rate at which ISON is producing water.
On October 1st, 2013 comet ISON flew just 0.07AU (10.8 million km, 6.7 million miles) from Mars, during which time a fleet of spacecraft at the Red Planet turned their gaze towards the comet. We haven't heard results from all of the missions, but we do know at least one instrument -- HiRISE, on the Mars Reconnaissance Orbiter -- successfully imaged comet ISON! Much fuss was publicly made of this observation, and rightfully so, but what is less well known is that the day before HiRISE took its first set of images of ISON, another spacecraft also obtained its first images of comet ISON...

To our delight this week we heard that comet ISON has now become visible in the Solar Wind ANisotropies, or SWAN, instrument on the ESA/NASA Solar and Heliospheric Observatory (SOHO) satellite! Right now it is barely visible, requiring a keen eye to pick it out from the noise. But over the coming weeks, as the comet inevitably brightens, it should become increasingly visible in those images until it gets washed out by the Sun's brightness in late November.

Granted, the SWAN images aren't the prettiest you'll ever see but they can yield some valuable results. First, SWAN is an "all-sky" imager, which means it has a 360-degree view around itself. There are only two areas it can't look at - the Sun and the Earth - and so those are the blacked out parts of the image opposite. SWAN does not take images like an ordinary camera either. The full explanation gets a little technical, but basically SWAN is looking at the imprint of the solar wind (the Sun's constant outward flow) upon hydrogen in our solar system. It sees what are called solar "Lyman-alpha" photons, which is essentially light that is reflected at a certain wavelength in the ultraviolet part of the spectrum. This means that a comet needs to have a pretty healthy amount of hydrogen in order for us to see it in SWAN, as what we're looking for is the solar wind "reacting" to this hydrogen. Bigger comets tend to have enough of a gassy coma that we can see them in SWAN images, and these observations can be used as a proxy for water production rates of comets. We hope that in time, comet ISON will enable exactly that!

With the images taken by MRO/HiRISE and SOHO/SWAN, we have now had six different spacecraft observe comet ISON... and the fun is only just beginning! In one week from today, the NASA STEREO-A spacecraft begins observing, and two weeks after that the STEREO-B spacecraft joins the fun. Those two, along with SOHO, will observe ISON continuously through to early December, with extended operations plans hopefully pushing that into mid-January 2014! There is much to look forward to, and the best really is yet to come for observing this comet!

Every week this year we will put up a new image related to Comet C/2012 S1 (ISON). If you have a cool image you'd like us to consider, please send it to, along with a description and any credits you would want applied. We'll contact you if we choose to use your image on the CIOC Website.

See our ISON Image of the Week Archives for earlier picks!