Juno

Juno — properly 3 Juno — is one of the founding members of the asteroid belt: the third minor planet ever discovered and, for half a century, counted as a full-fledged planet of the Solar System. It was found on 1 September 1804 by the German astronomer Karl Ludwig Harding, one of the "Celestial Police" who were hunting for a missing world between Mars and Jupiter. What they kept turning up instead was a swarm of small bodies — and Juno was the third of them, after Ceres and Pallas.

Today Juno is recognised as a large stony asteroid roughly 250 km across, orbiting in the main belt about 2.7 times farther from the Sun than Earth. It is no longer a planet, but it holds a permanent place in history as one of the objects that revealed the asteroid belt for what it is.

A planet, then an asteroid

For decades after its discovery, Juno, Ceres, Pallas and Vesta were listed in textbooks as planets, each with its own planetary symbol. As telescopes improved and dozens more such bodies turned up in the same region through the 1840s and 1850s, astronomers realised these were members of a whole population rather than planets, and coined the term "asteroid". Juno was reclassified along with the rest in the 1850s — a demotion that foreshadowed Pluto's own by 150 years.

A stony, irregular world

Juno is an S-type (stony) asteroid, built largely of silicate rock and nickel-iron, with a relatively bright, reflective surface. It is distinctly irregular — very roughly 320 by 270 by 200 km — and spins once every 7.2 hours. Though one of the larger S-types, it holds only about one percent of the mass of the asteroid belt; Ceres alone dwarfs it. Ground-based studies have even hinted at a large impact crater on one side.

Tracking it from the ground

Because its orbit is fairly eccentric, Juno's brightness varies considerably; near a favourable opposition it can reach about magnitude 7.5, within reach of binoculars, making it one of the easier asteroids to hunt down. Astronomers also study it through stellar occultations — timing the instant Juno briefly blocks a background star — to measure its size and shape without ever leaving Earth.

How to observe it

Juno never becomes a naked-eye object, but during its best oppositions (roughly every 16 months) a good pair of binoculars or a small telescope will show it as a slowly moving "star" against the fixed background. You need a star chart or ephemeris to pick it out, since it looks like any other faint point of light until you watch it drift from night to night.