This finder chart for magnitude +5.4 asteroid Vesta for the period June-July 2018 is centred on 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. For scale, the view is 25 degrees wide, or roughly the span of an outstretched hand at arm’s length (a 10×50 binocular field is also shown on the chart). At magnitude +0.1, Saturn is the brightest object in the area. Vesta’s position is for 0h UT on the dates shown with stars down to magnitude +7. Click on the graphic for a scaleable PDF chart suitable for printing and use outside. AN illustrations by Ade Ashford.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.
A colour image of asteroid 4 Vesta from NASA’s Dawn spacecraft captured at a distance of about 5,200 kilometres (3,200 miles) on 24 July 2011. Some 326 miles (525 kilometres) in diameter with an orbital period of 3.63 years, Vesta is the second-largest main-belt asteroid after dwarf planet 1 Ceres. As seen from Earth, however, Vesta appears starlike in typical backyard telescopes. Image credit: NASA / JPL / MPS / DLR / IDA / Björn Jónsson.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.This extract of the printable PDF finder chart from the top of the page shows northwestern Sagittarius bordering on Ophiuchus with a 10×50 binocular field for scale. Stars to magnitude +7 are shown, approximately 1½ magnitudes fainter than Vesta. Celestial north is up and east is left. AN graphic by Ade Ashford.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.
Although Saturn’s moons Dione (near) and Enceladus (far) are composed of nearly the same materials, Enceladus has a considerably higher reflectivity than Dione. As a result, it appears brighter against the dark night sky. This image was taken by NASA’s Cassini spacecraft in visible light with the narrow-angle camera on 8 September 2015.
You might think that, in the rings of Saturn, more opaque areas contain a greater concentration of material than places where the rings seem more transparent. But this intuition does not always apply, according to a recent study of the rings using data from NASA’s Cassini mission. The research also suggests that the planet’s brightest B ring could be a few hundred million years old instead of a few billion.
This false-color view from NASA’s Cassini spacecraft gazes toward the rings beyond Saturn’s sunlit horizon. Along the limb at left can be seen a thin, detached haze. This haze vanishes toward the left side of the scene.
