During this period, the comet was passing through the plane of the Milky Way, and background OH contaminated the signal. Solar radiation in the ultraviolet region of the spectrum interacts with OH molecules in the cometary coma. As the comet's orbital velocity with respect to the sun changes, the solar spectrum as seen at the comet undergoes a Doppler shift. Spectra taken by Amy Lovell, Chris DeVries, Pete Schloerb, Al Wootten, Matthew Senay, William Irvine and the 1996 REU students. As a comet warms on its journey around the sun, molecules boil off of the icy nucleus.
Water boils off in the inner solar system, as the nucleus is warmed by the sun.
Solar ultraviolet radiation destroys the water molecule, sundering it into OH and H. Ultraviolet radiation also excites the OH molecule, ultimately leading to observable emission in the radio lines as seen here. For this reason, OH is a good monitor of the rate at which the comet produces water, the most abundant molecule in the nucleus of a comet. 43m Radiotelescope Figure 1. The positions of hyperfine structure components are plotted at the expected frequencies as vertical lines at their relative intensities. The spectra are superposed upon a photograph of the comet setting over the telescope taken on 1997 April 16.