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Asteroid Juno at opposition
BY MARK ARMSTRONG
ASTRONOMY NOW

Posted: 2 August 2013


Asteroid Juno comes to opposition on 4 August and will be observable all night among the stars of western Aquarius, right on the border with Aquila and well above the plane of the ecliptic. Shining at magnitude +9 it's probably best to track it down with a small telescope or through large binoculars.

Asteroid Juno
Juno reaches opposition on 4 August. AN graphic Greg Smye-Rumsby.

Juno was discovered on 1 September 1804 by the German astronomer Karl L. Harding, the third asteroid to be discovered after Ceres and Pallas. Juno is named for the mythological highest Roman goddess. Juno is the smallest of the 'big four' asteroids, with dimensions of 320x267x200 kilometres; Ceres is much larger at 950 kilometres across and Pallas and Vesta are both around 525 kilometres in size. Juno has a large orbital eccentricity of 0.2559, as does Pallas, larger in fact than that of dwarf planet Pluto. This brings it closer to the Sun than either Ceres or Vesta, some 297 million kilometres (1.988 AU) at its closest (perihelion) and further out than both at aphelion, a distant 502 million kilometres (3.356 AU). This month it will be 2.702 AU from the Sun. Juno takes 4.36 years to complete its journey around the Sun.

Juno can get as bright as mag. +7.4, not far short of Ceres' mag. +6.6 despite Ceres being much larger. This is due to Juno's highly reflective surface, a measure of which is termed albedo. Along with Vesta, which is the brightest asteroid at mag. +5.1 at best, Juno has a high albedo of 0.238, with Ceres' much lower at 0.090.

It's well worth trying to track down Juno this month as it is quite well placed, culminating at over 30 degrees above the southern horizon. Juno is moving retrograde and its motion will take it out of Aquarius into neighbouring Aquila in the second week of August, then into Capricornus at the start of September, where it reaches its second stationary point on the 20th. Juno then resumes its prograde or easterly motion, re-entering Aquarius at the end of October. By this time it will have faded to mag. +10.

Observers won't be able to resolve Juno's disc as its too small and too far away but once you have the correct field then its motion over the course of several nights will reveal its presence; try sketching the field stars. Imagers will easily record it in brief exposures and blinking images taken over the course of a night will show its movement. On opposition night it lies at the precise position RA 20h 42.2m, DEC -04o 49.5', about 1.8 degrees west of 3 Aquarii (mag. +4.4). An ephemeris for your location can be generated at the Minor Planet Centre website. The Astronomical Almanac and the Handbook of the British Astronomical Association have ephemerides too.

The Planets
From tiny Mercury to distant Neptune and Pluto, The Planets profiles each of the Solar System's members in depth, featuring the latest imagery from space missions. The tallest mountains, the deepest canyons, the strongest winds, raging atmospheric storms, terrain studded with craters and vast worlds of ice are just some of the sights you'll see on this 100-page tour of the planets.
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Hubble Reborn
Hubble Reborn takes the reader on a journey through the Universe with spectacular full-colour pictures of galaxies, nebulae, planets and stars as seen through Hubble's eyes, along the way telling the dramatic story of the space telescope, including interviews with key scientists and astronauts.
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3D Universe
Witness the most awesome sights of the Universe as they were meant to be seen in this 100-page extravaganza of planets, galaxies and star-scapes, all in 3D!
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