Comets and Asteroids
Follow 15 Eunomia as its skims along the horizon to meet Centaurus
Asteroid 15 Eunomia was discovered by Annibale de Gasparis in 1851 and is the largest known S-type, or silicaceous, asteroid – these are characteristically metallic, reflective and bright. In May, Eunomia will hover around mag. +10, making it an easy target for a small telescope or large binoculars. It reaches opposition on 8 May when it will appear as a mag. +9.8 dot in the north of Centaurus.
Centaurus isn’t normally associated with northern hemisphere viewing but it’s a glorious sight to behold from more southerly climes. However, the extreme northern part of the constellation just manages to rise above the UK’s southern horizon. In order to see it, you’ll need a good, flat horizon in this direction. Aim to view from 01:00 BST (00:00 UT) at the start of the month. A bright, almost full Moon will spoil the show during early May and a severe lack of night will make things harder at the end of the month. Ironically this is when the asteroid will be climbing higher in the sky. The best time to catch Eunomia will be mid-month from around 00:00 BST (23:00 UT).
At 330kmx245kmx205km Eunomia is between the eighth and 12th largest asteroid in the main-belt between Mars and Jupiter. It’s not possible to rank accurately as there’s uncertainty about some diameters. Its orbit swings from 2.149 AU out as far as 3.138 AU from the Sun. The orbit is chaotic and will change randomly over time because of the gravitational influence of the planets. It currently takes 4.3 years to complete one orbit and rotates once on its axis every 6.1 hours.
It can appear as bright as mag. +7.9 during favourable oppositions, with an angular diameter of 0.29 arcseconds. Its mass is estimated at slightly in excess of 1% of the entire asteroid belt at 3.12x1019kg.