BBC Sky at Night Magazine

Comets and asteroids

Minor planet 42 Isis reaches opposition in Scorpius towards the month’s end


Minor planet 42 Isis reaches opposition on 23 May when it appears a little brighter than 10th magnitude in the constellat­ion of Scorpius, in the region of sky to the north of the Scorpion’s claws. Its monthly journey begins in Ophiuchus less than half a degree to the south of mag. +5.7 TYC 5639-1020-1, which itself lies a little over 2˚ north of mag. +4.3 Phi (φ) Ophiuchi and 1.5˚ south-southwest of the mag. +7.8 globular cluster M107. On 1 May, 42 Isis shines at mag. +10.7, brightenin­g toward opposition as it tracks west. It crosses the border into Scorpius on 10 May, crossing a thin vertical strip of Scorpius during the rest of the month, creeping into Libra on the 31st. At opposition, on the 23rd, it reaches mag. +9.8 and is suited for small-scope viewing. By the 31st it will have faded to mag. +10.0.

42 Isis is a substantia­l body, measuring 103km in diameter. It was discovered by Norman Pogson, a comet hunter who introduced the mathematic­al scale known as Pogson’s ratio which described the 2.512 (square root of five) brightness difference between two successive magnitudes. This was his first asteroid discovery and is named in honour of his daughter, Elizabeth Isis Pogson. Isis was the Egyption god of magic, fertility and motherhood.

It is a main belt asteroid with an orbit that extends out as far as 2.986AU from the Sun and as close as 1.898AU. Isis is a siliceous or S-type asteroid which means it is of mineralogi­cal compositio­n

– ie, stony. It takes 42 Isis 3.82 years to complete one orbit.

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