Anticipated STEREO observations of Comet ISON
Comet D/2012 S1 was discovered in September 2012 by Russian astronomers Vitali Nevski and Artyom Novichonok using data from the International Scientific Optical Network (ISON). For that reason, it is also known as Comet ISON. This comet is on a close encounter with the Sun on November 28, 2013 (Thanksgiving day in the U.S.), when it will pass at a distance of only 2.7 solar radii from the center of the Sun. Since Comet ISON was discovered so far out, beyond the orbit of Jupiter, and will pass so close to the Sun, many think that this could turn out to be a major comet.
If Comet ISON works out as expected, the STEREO spacecraft should have a spectacular view. The movie below shows the geometry of the STEREO Ahead (red) and Behind (blue) spacecraft during the passage of Comet ISON (orange). The top panel shows the view from above looking down on the orbital plane of the planets (the ecliptic plane), while the bottom panel shows the view from the side. Note that the comet’s orbital path is highly inclined to the ecliptic plane. As the comet approaches the Sun, it comes in at a fairly shallow angle, but leaves at a much steeper one, carrying it well above the ecliptic plane. If one only looked at the top-down view, it looks like the comet passes close to the Earth around the end of December, but the view from the side shows that this is not really the case.
The first STEREO telescope to see Comet ISON will be the large angle Heliospheric Imager #2 on the Ahead spacecraft (HI2-A). The first image below shows the projected day-by-day location of the comet in the HI2-A field-of-view from October 10, 2013, when it is expected to enter on the left side of the field, through November 23, 2013, when it is expected to leave on the right side of the field. Next to this is the comet’s passage through the smaller HI1-A field-of-view November 21-28, 2013. Click on each image for a larger version.
Both STEREO spacecraft will have a view of the comet in the COR1 and COR2 coronagraphs in the hours around closest approach on November 28. This is shown for the Ahead and Behind spacecraft respectively in the images below. Click on each image for an enlarged view. In each case, the comet enters from the lower left and exits near the top of the image. The coronagraphs on STEREO Ahead will be able to see the comet for about a day and half between about 04:00 UT on November 28, and 14:00 UT on November 29. The Behind coronagraphs will have a longer look at the comet, from about 06:00 UT on November 26 until the end of the day on November 29. During the period when Comet ISON is closest to the Sun, it will actually pass in front of the Sun as seen from Behind. This opens up the exciting possibility that we might see extreme-ultraviolet emission from the comet, as was seen recently with the bright sungrazing Comet Lovejoy.
|Predicted position of Comet ISON in the EUVI-B field-of-view in ten-minute intervals between 18:10 UT and 20:10 UT on November 28, 2013.|
The LASCO C2 and C3 coronagraphs on the Solar and Heliospheric Observatory (SOHO) satellite will also have a view of the comet as it passes through their fields-of-view, as shown below. From SOHO’s viewpoint the comet enters from the lower right early on November 27 and exits towards the top near the end of November 30. There’s also a possibility that the Solar Dynamics Observatory (SDO) satellite might be able to observe Comet ISON in extreme-ultraviolet when it’s closest to the Sun. However, at the currently estimated apparent distance of 1.8 solar radii, SDO would have to point away from the Sun to see the comet.
|Predicted hour-by-hour position of Comet ISON in the LASCO C3 (blue) and C2 (red) fields-of-view on SOHO, November 27-30, 2013. Click on the image for a larger version.|
After the comet swings around the Sun, it passes once again through the HI1-A field-of view during the first week in December, as shown below. At this point, the comet is leaving at such a steep angle that the nucleus never passes through the HI2-A field-of-view without rolling the spacecraft, though a tail might be visible.
|Predicted day-by-day position of Comet ISON on the outbound journey in the HI1-A field-of-view from December 1-7, 2013, moving from right to left. Click on the image for a larger view.|
All the plots on this page are based on preliminary estimates of Comet ISON’s orbit. As more observations of the comet are collected, these orbital estimates will be improved over time.