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The AAS Nova site is featuring a recent publication in the AAS journal by Paul Bryans of HAO. In the past sungrazing comets have produced extreme ultraviolet emissions as they passed through the sun's corona. So when the sungrazing comet ISON made its perihelion passage within two solar radii of the Sun's surface, it was expected also to be a bright emitter.
However, despite solar EUV telescopes repointing to track the orbit of the comet, no emission was detected. This "null result" is interesting in its own right, offering the possibility of placing limits on the size and composition of the nucleus. We explain the lack of detection by considering the properties of the comet and the solar atmosphere that determine the intensity of EUV emission from sungrazing comets. By comparing these properties with those of sungrazing comet Lovejoy, which did emit in the EUV, we conclude that the primary factor resulting in non-detectable EUV emission from ISON was an insufficiently large nucleus. We conclude that the radius of ISON was at least a factor of four less than that of Lovejoy. This is consistent with white-light observations in the days before perihelion that suggested the comet was dramatically reducing in size on approach.
Paul Bryans and W. Dean Pesnell 2016 ApJ 822 77. doi:10.3847/0004-637X/822/2/77