My first Magazine Sky & Telescope - 02.2019

Spring Equinox Voyager 1 Encounter March 1980 August 2009 January 2039 November 12, 1980 u LONG VISIT Cassini-Huygens reached Saturn just after the planet’s perihe- lion. After sending Huygens to Titan’s surface in 2005, Cassini surveyed the moon 127 times at various distances. N Summer Solstice December 1987 May 2017 October 2046 n si yge u i-H is s i o ns M January 14, 2005 n Perihelion S S Aphelion January 1974 July 2003 November 2032 Cassini-Huygens Saturn Orbit Insertion July 1, 2004 Cassini End of Mission N 9 AU September 15, 2017 10 AU September 1988 April 2018 July 2047 Winter Solstice S May 1973 October 2002 April 2032 Saturn’s axial tilt 26.7° N S Autumn Equinox November 1995 April 2025 October 2054 Huygens Descent N 1 Titan year = 29.5 Earth years 1 Titan day = 15.9 Earth days heaviest molecule discovered before Cassini arrived in the system and characterize the plasma environment. CAPS Saturn system (C 6 H 6 , mass of 78). Scientists thought that was capable of measuring a much larger range of ions than would be good enough. INMS, but it only had sufﬁ cient mass resolution It wasn’t. INMS found molecules all the way to identify the composition of the light ions it was up to its upper mass limit. That was frustrating: designed to study. It meant we wouldn’t be able to use INMS to tell However, when CAPS starting making measure- #4 what the heaviest species are in Titan’s atmosphere. ments of Titan’s atmosphere during close ﬂ ybys, Acetylene Luckily, the orbiter had a variety of instru- it found something startling: negatively charged ments. Another one, called the Cassini Plasma Spectrometer ions with a mass-to-charge ratio of up to 10,000 atomic mass (CAPS), was designed to analyze light ions in the Saturn units. That means that, instead of recording ionized mol- sk yandtele scope.com • FE B RUA RY 2 019 25

My first Magazine Sky & Telescope - 02.2019 | Page 27