Any astrophotographers among you who may have just imaged the glorious globular cluster Messier 15 in the constellation of Pegasus may care to check your images carefully, for there may be an unexpected interloper in the foreground — and I’m not referring to a Perseid meteor. On the UK night of 12-13 August, second-largest asteroid 2 Pallas passed just 1.3 degrees to the southeast of M15’s ball of stars.
On Saturday, 20 August 2 Pallas also reaches opposition in the constellation of Pegasus bordering on Equuleus (the foal). This fascinating asteroid, which is likely a remnant protoplanet, is therefore at its brightest and lies closest to Earth for 2016. As seen from the centre of the British Isles, Pallas currently attains a maximum altitude of almost 46 degrees above the southern horizon at 1am BST, ensuring that it is very well placed for observation from northern temperate latitudes.
That Pallas should currently be found in excess of 24 degrees from the ecliptic immediately reveals an important fact about this asteroid: a high orbital inclination. In fact, the orbit of Pallas is tilted by almost 35 degrees to the zodiacal plane.
Unlike 1 Ceres and 4 Vesta that have been photographed in great detail from close orbit by NASA’s Dawn spacecraft, 2 Pallas has not thus far received a robotic emissary from Earth. Pallas has, however, passed in front of stars on a number of occasions and an occultation event accurately timed by 140 observers on 29 May 1983 led to a precise determination of its size and cross-sectional shape. Further refined by other observations, the accepted values for Pallas’ dimensions are 362 x 345 x 311 miles (582 x 556 x 500 kilometres). To put this in perspective, Pallas has a diameter one-sixth that of our Moon.
Pallas possesses some other odd characteristics. It has a very high axial tilt of 84 degrees, meaning that areas near the asteroid’s poles experience continuous sunlight for up to two years, while other parts of its surface can remain in constant darkness or constant sunlight for about an Earth year. Pallas rotates on its axis in a period of 7 hours 48¾ minutes with an orbital period of 4 years 223 days at a mean distance of 2.77 astronomical units from the Sun.
A brief history
Ceres, the largest object in the asteroid belt (now classified as a dwarf planet), was discovered by Italian Catholic priest and astronomer Giuseppe Piazzi on the first day of the 19th century. However, owing to a poorly determined orbit before it slipped into conjunction behind the Sun, 1 Ceres was lost for a time.
German physician and astronomer Heinrich Wilhelm Matthäus Olbers (1758-1840) was observing Ceres three months after he helped recover it based on a refined orbital calculation by the young Carl Friedrich Gauss when, on 28 March 1802, Olbers found another seventh-magnitude moving object just 7½ degrees away; he had found asteroid number two, which he called Pallas. Five years and a day after the discovery of 2 Pallas, Olbers added 4 Vesta to his asteroid tally.
Heinrich W. M. Olbers was evidently a fascinating character. He was a medical practitioner in Bremen by day, but by night studied the sky from an observatory in the upper storey of his home — apparently, he never slept more than four hours. His diligence paid off with the discovery of periodic comet 13P/Olbers on 6 March 1815. He also dabbled in cosmology, as anyone who has encountered Olbers’ paradox will know.
Observing Pallas with large binocular or a small telescope
Owing to its rather eccentric orbit, the opposition distance of 2 Pallas varies considerably. At rare perihelic oppositions, Pallas can attain magnitude +6.5 making it an easy binocular object and a potential naked-eye target for particularly keen-sighted individuals. At its faintest, 2 Pallas can sink to almost magnitude +10.7.Pallas reached aphelion at the end of March 2016, so at its closest approach to us on the UK afternoon of Wednesday, 24 August — which is two days after opposition — the asteroid is 2.398 AU (223 million miles) from the Earth and 3.349 AU (311 million miles) from the Sun.During this opposition, 2 Pallas peaks at magnitude +9.2, crossing the constellation border from Pegasus into Equuleus on 22 August. Observers in the UK and Western Europe will benefit from the darker skies in the last few days of the month when the old Moon is a waning crescent and rises after midnight local time.
Inside the magazine
For a comprehensive guide to observing all that is currently happening in the late summer sky tailored to the UK and Western Europe (but including the Southern Hemisphere sky), obtain a copy of the August 2016 edition of Astronomy Now.
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