C/2012 S1 (ISON): Updates from Spitzer

Posted July 24, 2013 by User 1

Comet C/2012 S1 (ISON), popularly known as just Comet ISON, had been, at the time of its discovery in 2012, been referred to as the "Comet of the Century." Since then, the comet has experienced a downturn in activity, falling behind predictions by 2 magnitudes in brightness (a factor of six) by Spring of 2013. In the past few weeks, the comet passed behind the Sun, as viewed from Earth, and thus, there have been no observations for the past month. Before disappearing into the daylight, the Spitzer Space Telescope made some last observations of the comet in infrared.

The comet shows a well defined dust tail on the left, similar to what was captured by Hubble in April. The image on the right, which has the dust subtracted out, shows an orb of gas -- likely CO₂ -- which is being released from the comet at the rate of about 1000 metric tons per day. As the comet nears the Sun, water will take over and drive the activity of the comet at its peak.

Unfortunately, due to the lack of earlier comparisons, these observations cannot be used to predict the activity of the comet beyond what we have already. At the time of the observation, C/2012 S1 (ISON) was approximately magnitude 15.5, about 4000 times fainter than the faintest stars visible to the unaided eye in a reasonably dark sky.

The comet will reappear in the morning sky in August and new observations will be available then. If the current rate of brightening holds, the comet should reach magnitude 14 by the end of August. Only time will tell how well that prediction holds.

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