Comet ISON Meets STEREO
For the next couple of months, Comet ISON will be the subject of countless photographs. Amateur astronomers across the globe are recording this sun-grazer's progress as it passes each milestone, including its closest approach to Mars on October 1st, and are hoping for a spectacular performance in mid-autumn. There are also dozens of professional observatories that are dedicating time to observations of this comet, and over time, a plethora of information will be available regarding the life, and possible death, of Comet ISON.
Still, our information would be restricted were we dependent on ground-based telescopes alone. Not only does the atmosphere get in the way, but earth-based information of celestial objects is "one-sided". With few exceptions, our experience of objects as seen from Earth can only include features that face in our direction.
In 1959, the Soviet space probe, Luna 3, recorded images of the far side of the Moon for the first time., and we experienced what it was like to see one of our celestial neighbors in its entirety. The far side of the Sun was first imaged just a few years ago, on June 6, 2011, by NASA's two STEREO spacecraft, STEREO-Ahead and STEREO-Behind, and it is this mission that will give us a more complete view of Comet ISON.
The STEREO (Solar TErrestrial RElations Observatory) mission was launched in October, 2006 to help us better understand coronal mass ejections and help alert the arrival time of CME's directed towards Earth.
STEREO consists of two observatory satellites placed in slightly different orbits. STEREO-Ahead (A) was placed ahead of Earth's orbit, and STEREO-Behind (B) trails behind. Both observatories have cameras and other instruments aboard, and will be used to investigate Comet ISON from early October through the end of the year. This means that there will be "eyes" on the comet even in the days before and after perihelion.
On or about October 10th, Comet ISON will enter the field of view of STEREO-Ahead's Heliospheric Imager #2 (HI2-A). Since the HI2-A's field of view is so large (70º), the comet will stay in range until at least November 23rd.
On November 21st, STEREO-Behind gets into the act. It's smaller, higher resolution 20º field of view camera will track the comet's passage until the 28th of November. Note the overlap of a few days (the 21st - 23rd of November); where both cameras will have ISON in their sight, allowing for possible 3-D images!
Both STEREO spacecraft will also view ISON in their coronagraphs around closest approach (November 28th), and SOHO coronagraphs will also get a look during that timeframe. There is also a chance that the Solar Dynamics Observatory (SDO) will be able to observe ISON in extreme-ultraviolet.
Now, here's a little tidbit that could extend the role that STEREO plays in capturing data on Comet ISON: The normal orbits of STEREO A & B would result in losing sight of the comet in late November or early December. However, mission scientists are planning to roll the spacecrafts so that their orientation allows them to see ISON for a longer period of time! Since rolling the satellites would disrupt their normal mission schedules, they cannot be left in that position for long, but plans are in the works to re-orient both Ahead and Behind for four hours a day during ISON's observing schedule so that they can give their full attention to the comet. If the current schedule goes ahead as planned, STEREO will still be gathering data on ISON after the New Year, just as its winging its way away from the Sun, towards a final encounter with Mars.
Whether Comet ISON ends up being the "comet of the century" or not, there is no doubt we will study it as though it were. Now, who's going to wade through all that data? Not me, I've got to wash my hair.
--by Penny Distasio