Asteroid 4 Vesta looks starlike from Earth, but up close it is a fascinating world. This colour image from NASA’s Dawn spacecraft captured at a distance of about 5,200 kilometres (3,200 miles) shows Vesta on 24 July 2011. With a mean diameter of 326 miles (525 kilometres), it is the second-largest main-belt asteroid after dwarf planet 1 Ceres. Click the picture for a larger-scale version. Image credit: NASA / JPL / MPS / DLR / IDA / Björn Jónsson.On Wednesday 18 January, brightest asteroid 4 Vesta comes to opposition in the constellation of Cancer bordering on Gemini, ideally placed for observation by Northern Hemisphere skywatchers. Two days later, Vesta makes its closest approach to Earth for this year at a distance of 1.5217 astronomical units, or 141 million miles (227 million kilometres).
With a mean diameter of 326 miles (525 kilometres) and an orbital period of 3.63 years, Vesta is the second-largest main-belt asteroid after dwarf planet 1 Ceres that lie between the orbits of Mars and Jupiter. This year marks the 210th anniversary of Vesta’s discovery by German astronomer Heinrich Wilhelm Olbers.
Presently shining at close to magnitude +6.3, Vesta is therefore on the verge of naked-eye visibility for eagle-eyed observers under dark, moonless skies. For the rest of us with average vision, the asteroid is still an easy binocular object — if you know exactly where to look.This binocular finder chart shows the position of asteroid Vesta at 0h UT (12am GMT) every three days over the course of one month, starting 15 January 2017. For scale, the starchart spans the width of a fist held at arm’s length and depicts an area of the UK sky two-thirds of the way from the southern horizon to overhead at midnight. Prominent stars Castor and Pollux in the constellation of Gemini lie 4½ degrees apart — easily encompassed by a typical 10×50 binocular. Vesta is about 1½ magnitudes brighter than the faintest stars shown that are of magnitude +8. Click the graphic for a grayscale version suitable for printing and use outside. AN graphic by Ade Ashford.Prominent stars Castor and Pollux in eastern Gemini may be found due south at midnight in mid-January (or 11pm GMT by month’s end), two-thirds of the way from the southern horizon to overhead as seen from the UK. The celestial twins lie just 4½ degrees apart, so the pair can be seen in the same field of view of a typical 10×50 binocular.
If an imaginary line drawn from Castor through Pollux is extended a similar distance, it marks the position of Vesta around 24 January — an additional aid to identifying the asteroid around that time. Given the brightness of Vesta, it will be easy to spot the interloper amid the stars of Gemini shown in the finder chart above that shows binocular stars down to magnitude +8.
As a further guide to identifying Vesta, the asteroid passes within half a degree (less than a lunar diameter) north of magnitude +3.6 naked-eye star kappa (κ) Geminorum early on 3 February. Pollux and κ Geminorum are separated by just 3.6 degrees, another binocular pairing that will help you find Vesta in early February.
Inside the magazine
For a comprehensive guide to observing all that is happening in this month’s sky, tailored to Western Europe, North America and Australasia, obtain a copy of the January 2017 edition of Astronomy Now.
For ten nights starting 5 April 2019, asteroid 2 Pallas and unmistakable Arcturus – the northern celestial hemisphere’s brightest star – lie within the same field of view of typical 8× binoculars. Also, don’t miss magnitude +7.9 Pallas’s very close conjunction with magnitude +2.7 star eta (η) Boötis at 22h UT (11pm BST) on the night of 10 April.
While you may not relish the prospect of waking up in the small hours most Monday mornings, observers in the British Isles and Western Europe will want to set their alarms no later than 5am GMT on 21 January to see this month’s showstopper celestial event — a total lunar eclipse of an unusually close ‘supermoon’. The total lunar eclipse is also visible in its entirety (weather permitting) from the Americas.
Possibly a kilometre or more in size, Apollo asteroid 2002 AJ129 passes just 10.9 lunar distances from Earth at 21:30 UT (9:30pm GMT) on 4 February — its closest approach for 114 years. For a few nights around this date the magnitude +12.6 body is well placed for observers as it gallops through the constellations of Virgo and Leo into Cancer at a rate of up to 40 degrees/day. We show you where and when to look for it.
