Comet ISON will reach perihelion on November 28, coming to within 1.7 solar radii of the photosphere. If ISON survives its closet approach, it is likely to reach negative magnitudes and become the brightest comet since Ikeya-Seki in 1965. However, whether it will survive perihelion is very difficult to predict. ISONfs nucleus is estimated to be 5 km in diameter, 5-10 times larger than Comet Lovejoyfs, which survived a much closer perihelion (0.2 R_s) in 2011. ISON also appears to be a dynamically young comet, fresh from the Oort cloud. This may suggest a lower bulk density than that of Lovejoy, which was a fragment of a much larger sungrazing comet, and mean that ISON will be more susceptible to fragmentation by tidal forces within the Sunfs Roche limit. As long as it survives to within 30 minutes of perihelion, it can be observed by XRT.
ISONfs trajectory will be accessible to XRT for about 55 minutes surrounding perihelion, from 11/28 18:12 to 19:07 UT. It will pass too far from the limb to be observed by SOT and may be observable to EIS for around 20 minutes. XRT will detect emission from the comet if the density of material lost from the nucleus combined with the coronal density is sufficient to generate O VII, which is what XRT is thought to have observed for Comet Lovejoy. Such observations would be useful to both the study of the comet and of the coronal environment. They can be used in combination with AIA observations to determine the density of the cometary material and to probe ionization timescales in the corona, since a detection would represent neutral material being fully ionized to coronal charge states. |
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