Many of you may have tracked down an asteroid – a shattered fragment of a planetesimal that never grew large enough to become a planet between the orbits of Mars and Jupiter – with binoculars or a telescope, but have you ever seen one with the naked eye? If not, then June presents you with an opportunity to see the brightest, 4 Vesta. What’s more, ringed planet Saturn happens to lie close by to act as a convenient guide.
On Tuesday, 19 June, Vesta comes to opposition in the constellation of Sagittarius. At 07:50 UT on that day, the asteroid makes its closest approach to Earth for the year at a distance of 1.141568 astronomical units, or 170¾ million kilometres (106.1 million miles).
With a mean diameter of 326 miles (525 kilometres) and an orbital period of 3.63 years, potato-shaped Vesta is the second-largest main-belt asteroid after dwarf planet 1 Ceres between the orbits of Mars and Jupiter. Vesta was discovered by German astronomer Heinrich Wilhelm Olbers on 29 March 1807.
Currently shining at close to magnitude +5.6 and peaking at +5.3 at opposition, Vesta is potentially a naked-eye object for eagle-eyed observers under dark, moonless skies, particularly from the Southern Hemisphere where it is very high in the sky (the glow from a waxing Moon will interfere for roughly a fortnight from 20 June). For the rest of us with average vision, the asteroid is still an easy binocular object — if you know exactly where to look.
Where and when to see Saturn and Vesta
The ringed planet (which reaches opposition on 27 June; more on that nearer the time) and asteroid lie in a region of the constellation Sagittarius that is highest in the southern sky of Western Europe (including the UK) at 2am local time in mid-June, or by 1am local time at the end of the month. As seen from the heart of the British Isles, Saturn attains a peak altitude of just 15 degrees high in the south; Vesta transits about 1½ degrees higher.By mid-June, Vesta lies just under 8 degrees, or 1½ 10×50 binocular fields of view, to the upper right of Saturn, as seen from the Northern Hemisphere. The asteroid passes a lunar diameter (28 arcminutes, to be precise) south of magnitude +5.5 open cluster Messier 23 in the small hours of Friday, 15 June in the UK, a fine visual and astrophotographic opportunity.
Given that Vesta attains a peak altitude of just 17 degrees in the southern sky as seen from the centre of the British Isles mid-month, dimming due to atmospheric extinction amounts to around half a magnitude. Hence, Vesta will appear about magnitude +6 at best for UK observers. Southern Hemisphere observers will see both Saturn and Vesta very high in the sky, so the asteroid will appear at full brightness and a comfortable naked-eye target from moonlight-free dark sky sites.