One of the largest objects in the asteroid belt, 4 Vesta reaches opposition
Asteroid 4 Vesta has the potential to appear the brightest of the minor planets, reaching a respectable mag. +5.1 when it’s at a favourable opposition, easily visible to the naked eye from a dark-sky location. Vesta reaches opposition on 23 August and although not quite as bright as it can get, it will be on the threshold of naked-eye visibility from a dark sky site at mag. +6.0.
Vesta is currently located in southwest Aquarius, east of Saturn and triangular-shaped Capricornus, tracking in a southwest arc above NGC 7293, the Helix Nebula. One of the closest planetary nebulae to Earth, the Helix appears large with a very low surface brightness. Its inner ring is 8×19 arcminutes in size, extending out to appear almost 25 arcminutes across. Vesta begins the month 5º north-northeast of the Helix, ending the month 5º to the west of it. Use mag. +3.3 Delta (δ) Aquarii as the navigational starting point at the beginning of the month.
Vesta appears as a mag. +6.4 object on 1 August, brightening to its opposition magnitude of +6.0 on 18 August, a value it maintains through to 25 August. By the end of the month it will have dimmed to mag. +6.2, but is still an easy binocular target.
Vesta was discovered on 29 March 1807 by Wilhelm Olbers. As its prefix number suggests, it was the fourth minor planet discovered. It’s a large example, only beaten in size by dwarf planet Ceres. It completes an orbit of the Sun once every 3.63 years, its orbital path taking it out as far as 2.57 AU and in as close as 2.15 AU. Interestingly, its size combined with its varying distance from Earth means it presents an angular diameter that varies between 0.2 and 0.7 arcseconds.