Juno in the Spotlight

Few stargazers bother to track down asteroids in the night sky, even though many of them are quite easy to spot. For example, in 2007 asteroid 4 Vesta passed unusually close to Earth and could be glimpsed with the unaided eye from a dark location.

Right now you can look for another famous minor planet. Asteroid 3 Juno is having an especially favorable apparition, making it about a magnitude brighter than it usually gets. It's magnitude 8.2 on September 1st, 7.7 at opposition on September 21st, 7.8 on October 1st, 8.2 on November 1st, and 8.9 on December 1st.

According to Don Yeomans, manager of the Near-Earth Object Program at NASA's Jet Propulsion Laboratory, "This is going to be as bright as it gets until 2018."

Juno is positioned beneath the Great Square of Pegasus near the intersection of Pisces, Cetus, and Aquarius. This region of the sky is low in the east after evening twilight but higher up and better placed for viewing around midnight.

As the chart at right shows, Juno is currently passing not far from Uranus (itself easy to see at magnitude 5.7), as the chart at right shows. Click on the chart for a larger, black-on-white chart that you can use with binoculars or a small telescope.

German astronomer Karl Ludwig Harding first glimpsed Juno in September 1804, only three years after the discovery of the first asteroid, 1 Ceres. Yet it was a lucky find. With a diameter of 145 miles (234 km), Juno is only the 10th largest asteroid, and it's by far the smallest of the four found from 1801 through 1807.

After that no others turned up until 1845, long enough for the idea of "four minor planets" to become entrenched in astronomy literature. In fact, astronomers of that era considered Ceres, Pallas, Juno, and Vesta part of the Sun's planetary retinue and even assigned them planetary symbols.

The spectrum of Juno suggests that it's composed of silicate rock, and dynamicists believe it is the source of many of the meteorites that rain on Earth. Several years ago astronomers used adaptive optics to record Juno at several wavelengths with the 100-inch Hooker telescope atop Mount Wilson. It's an intriguingly lumpy body, and apparently parts of it have been chipped away over the eons by collisions with other asteroids.

A NASA spacecraft named Dawn is en route to Vesta and Ceres, but it might be a very, very long time before we get close-up views of Juno. So take advantage of this window of opportunity to add another famous celestial object to your "life list."