Comets and asteroids

Catch asteroid 4 Vesta as it reaches opposition in the constellation of Leo

4 Vesta holds the accolade of being the brightest of the asteroids. During a favourable opposition this 525km object can reach mag. +5.1, bringing it well into naked-eye territory. This month, Vesta reaches opposition on 4 March, slightly off its most favourable brightness at mag. +5.9, but still theoretically within reach of the unaided eye from a dark-sky site. Our ‘Sky Guide Challenge’ on page 55 is to try and achieve this rare sighting.

Fortunately, Vesta is located in a region of sky that reaches a decent altitude from the UK as it crosses southern skies around midnight UT. The asteroid is located in the eastern portion of Leo, the Lion, starting the month slightly less than 2˚ east of mag. +3.3 Chertan (Theta (θ) Leonis). On the 1st, it shines at mag. +6.0 and slowly brightens to mag. +5.9 by the 4th.

As the month slips by, Vesta tracks northwest, further into the body of the Lion. It ends its monthly track 2.5˚ southwest of mag. +4.4, 60 Leonis and 5˚ west-southwest of Zosma (Delta (d) Leonis). This part of the sky is away from the Milky Way and therefore devoid of lots of stars that are a similar brightness to Vesta, which might cause confusion: along its track in March, the two brightest stars it passes are of magnitudes +6.8 and +6.4! Vesta is a large object and it orbits the Sun once every 3.63 years, with an orbit that takes it out as far as 2.57 AU from the Sun and as close as 2.15 AU. At its dimmest, Vesta can be mag. +8.5. As its distance from us changes over the course of our respective orbits, its angular diameter changes from 0.2 to 0.7 arcseconds.