Comets and Asteroids
Take advantage of the 11th largest object in the asteroid belt, 3 Juno, at its brightest
Minor planet 3 Juno is one of the ‘Big Four’ asteroids. This is the name given to the first four objects discovered in the asteroid belt and includes 1 Ceres, 2 Pallas and 4 Vesta. Ceres was reclassified as a dwarf planet back in 2006 but nevertheless the Big Four retain a place in history having been identified in a relatively narrow window between 1 January 1801 and 29 March 1807. It took over 38 years to discover the next one, 5 Astraea.
Juno reaches opposition on 17 November in northern Eridanus. Throughout the month, Juno’s brightness hardly varies. It starts at mag. +7.6, increases to +7.5 on 4 November then stays at that value for the rest of the month. This is close to the brightest Juno can achieve, so November offers a great opportunity to observe it. At its dimmest, Juno can dip to mag. +11.6.
Although this month’s magnitude means it’s a
binocular-friendly object, the challenge will be to find it in what is a rather barren area of sky between northern Eridanus and western Taurus. It starts the month in a location slightly west of the mid-point between mag. +3.9 Nu Tauri and +4.0 Omicron1 Eridani, tracking west in an arc that takes it just south of mag. +4.7 32 Eridani
Taking 4.37 years to orbit the Sun, Juno is one of the larger asteroids and is estimated to contain 1% of the mass of the asteroid belt. It’s a siliceous or S-type asteroid with an unusually high reflectance of 23.8%. It measures 320x267x200km giving it a mean diameter of 233km. It rotates on its axis once every 7.2 hours.
An asteroid family known as the Juno clump consists of a number of small bodies each less than 6km diameter that orbit within the vicinity of Juno. As they have a similar reflectivity to Juno, it’s possible they are debris from impacts on Juno.