ISON Image of the Week

STEREO Coming Into Focus  (Sep 23, 2013)

On October 10, 2013 the NASA STEREO-A satellite will observe ISON for the first time. After that, the comet will be observable to STEREO almost continuously through to as late as January 2014!
As we have reported over the past couple of weeks, ground-based observers are finding it increasingly possible to obtain some very nice images of Comet C/2012 S1 (ISON) with only modest sized telescopes. As long as the comet continues to behave itself -- as it seems to be doing right now -- then those viewing conditions should improve at least through much of October until it begins to get close to the Sun in the sky again. Depending on its brightness at the time, ground-observers may lose sight of Comet ISON for a while in November as it races towards the Sun., but no matter how close ISON gets to it, there will soon be no place for the comet to hide from our space-based eyes!

In just a couple of weeks, around October 10, 2013, Comet ISON will enter the field of view of the Heliospheric Imager 2 ("HI-2") instrument on the NASA STEREO-A spacecraft. From that moment on, we will have a clear view of ISON, 24-hours a day, 7-days a week, until early January 2014! Even for the couple of hours that ISON will be hidden "behind" the Sun at perihelion, it will still remain visible from the STEREO-B spacecraft, which is positioned on the far side of the Sun.

The STEREO mission comprises two near-identical satellites (known as "Ahead", or "A", and "Behind", or "B") in slightly different orbits around the Sun. Both contain the suite of SECCHI imaging instruments that between them cover the region of space from the solar surface out to way beyond Earth's orbit. The HI-2 instrument takes long exposures at visible wavelengths of light, and has the widest field of view of all the STEREO/SECCHI instruments - about 70-degrees - so the pixels are big and thus resolution is low. But it can detect objects down to around magnitude 13, so we should definitely see comet ISON in those images.

Comet ISON will slowly cross the HI-2A field through October and November as seen in this week's image, and enter the 20-degree HI-1A camera in late November. However, by late October it will have already appeared in the HI-1 field of view on the STEREO-B spacecraft, giving us near stereoscopic views of comet ISON! These HI-1 cameras have higher resolution than HI-2, and can return truly spectacular images of comets approaching the Sun! In the days surrounding perihelion, comet ISON will then pass through the COR-2 and COR-1 coronagraphs, and for the STEREO-B spacecraft the Extreme Ultra-Violet Imager (EUVI) solar disk imager, as it makes its passage past the Sun. Assuming it survives this passage, both STEREO spacecraft will continue to observe it recede from the Sun, and back through the HI-1 cameras, for at least a few days.

Beyond that it gets more complicated. In their nominal (i.e. "normal") orbits and positions, the STEREO spacecraft will lose sight of ISON in early December. However, in support of the CIOC, the STEREO mission teams are looking to perform regular spacecraft roll maneuvers that will extend observations of the comet through to as late as early January! These rolls can only be temporary each day as they affect the routine operations of the spacecraft, and the STEREO team are still nailing down the specifics of the plan (which do, in part, depend on how ISON performs post-perihelion), but details will be posted on the STEREO Comet ISON Observation website once they are known for sure.

The continuous observations that STEREO offers us from their two different vantage points in space, in combination with data from similar coronagraphs on ESA/NASA's SOHO satellite, should make for popular and spectacular viewing throughout November in particular, and return a wealth of information about ISON's brightness, it's tail, and it's interaction with the solar wind and solar corona.

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!