Neptune has another moon

The smallest satellite of the planet Neptune is sufficiently difficult to image by earth-based telescopes so as to have precluded many efforts at detection. Now there is a new challenge for telescope astronomers in the recent report by H. Reitsema, W. Hubbard, L. Lebofsky, and D. Tholen of a third satellite moon in orbit around Neptune (Science, 215(, 289-291, 1982). The technique used to observe Neptune and investigate the upper atmosphere involves spectroscopic measurement by earth-based telescope during an occultation event of an uncatalogued star. The combined signals of Neptune and the stars were observed with a two-channel photometer gated at 10 ms. The two channels collected filtered light in red and blue bands. Neptune's atmosphere is blue as a result of strong methane absorption, so that the blue-filtered channel would experience only a slight drop in signal (½%) at full occultation. The red channel would experience a stronger drop in signal (4%). Shortly before 8 h 37 min coordinated universal time, as Neptune occultation of the star approached, an abrupt event occurred on the record that is interpreted as being due to passage of the previously unknown third moon in front of the star (see figure).