BBC Sky at Night Magazine
Comets and asteroids
Minor planet 19 Fortuna reaches opposition on 11 September in the constellation of Pisces, south of the faint asterism of the Circlet. Shining at mag. +9.2 it should be a relatively easy target for a small scope. It tracks southwest throughout September, pushing Fortuna into the neighbouring constellation of Aquarius by the month’s end. Fortuna’s track location also keeps it relatively close to the Solar System’s outermost planet Neptune throughout the month. Medium wide images of the area taken several days apart should reveal the motion of both bodies. Neptune will appear brighter than Fortuna at mag. +7.9.
19 Fortuna is a large main belt asteroid, with a diameter of 225km. It’s quite dark in appearance with an albedo – how much light it reflects – of just 3.7%. Its peak brightness at favourable oppositions can reach mag. +8.9, but at its dimmest it sinks to mag. +13.0. As a carbonaceous asteroid, it falls into an uncommon class known as a G-type. Approximately 5% of asteroids are G-type including the largest, dwarf planet Ceres.
Fortuna’s orbit takes it out as far as 2.8 AU from the Sun and in as close as 2.1 AU. It takes 3.81 years to complete each orbit, spinning on its axis once every 7.4 hours. It’s had close encounters with other bodies, its orbit being perturbed by the 76km asteroid 135 Hertha. This allowed the mass of Fortuna to be derived to be 1.08 x 1019kg, a value which has more recently been refined to 1.27 x 1019kg. In 2012 Fortuna passed within 6.5 million km of the small 25km asteroid 687 Tinette, with no significant consequence.
In 1993 the Hubble Space Telescope was able to determine that Fortuna was virtually spherical. Incredibly, at the time the asteroid presented an apparent size of just 0.2 arcseconds.