Take The Lazy River To The Asteroid Juno

Images of asteroid 3 Juno taken in two different wavelengths of light with the 100-inch Hooker telescope at Mt. Wilson Observatory show what appears to be a 60-mile-wide crater at lower left in each photo. Material excavated by the collision that produced the crater “bite” shows very clearly at 934 nm. Sallie Baliunas et al.

When the word Juno shows up on this blog it’s usually in reference to the NASA spacecraft that swings around Jupiter every 53 days to take mind-blowing photos of the planet’s polar regions. Not this time. This time we’re going to find the asteroid 3 Juno which just happens to be making a close approach of Earth in the evening sky this month.

The “3” in front of Juno indicates the order of its discovery. Juno was first spotted by a German astronomer in 1804, the third small body found between Mars and Jupiter after 1 Ceres and 2 Pallas. It’s the 11th largest asteroid at 146 miles (233 km) across and has an elongated elliptical orbit around the sun, so Juno’s distance varies during its orbital period of 4.4 years from 186 million miles (300 million km) to 312 million miles (502 million km).

In comparison, the Earth’s distance varies over the year by only about 3 million miles. Our planet’s orbit is closer to a circle than Juno’s so the variation is less.

This map will get you oriented. Start at the Pleiades and Hyades clusters (or Orion’s Belt) and star hop to 32 and 35 Eri in Eridanus. From there the detailed chart (below) will guide you to Juno. Stellarium

Every 13 years or so, Juno passes relatively near the Earth at opposition, the time when the asteroid and Earth are lined up on the same side of the sun and closest. That happens on Nov. 16 when the two bodies will be just 96.3 million miles (155 million km) apart. Coincidentally, this happened with another asteroid this past summer — 4 Vesta. Maybe you got out to see it back in June and July when it brightened to magnitude 5.3. Vesta was super easy to pick up in binoculars and even became faintly visible with the naked eye. Juno won’t get that bright because it’s less than half the size of Vesta, but it will still show in a pair of 7×35, 10×40 or similar binoculars.

Using the Hyades star cluster, I’ve sketched out a route to Juno. Its position is marked for 10 p.m. Central Time every 5 days. For a particular day just interpolate between the tick marks. I’ve also labeled several dim naked eye stars near the asteroid that will make good guides: 35, 32, 30, 21, 22 and 17 Eridani. West is up on the map and stars are shown to magnitude 8. Chris Marriott’s SkyMap with additions by the author

Juno’s a rocky body with a bright surface, so it will hang around magnitude 7.5 now through the end of November, fading to 8.2 by the end of the year. Look for it when the moon’s either out of the sky or no more than half. The asteroid will be traveling through a unremarkable section of the constellation Eridanus the River located below Taurus the Bull and to the right of Orion. Now through Nov. 15 will be an ideal time to take a look. Another dark, moonless window opens again in late November through mid-December.

This “big picture” map shows Juno’s travels across the sky from July 2018 to April 2019. Tom Ruen

I’ve included a couple maps to help you find Juno. In early November, from 10 o’clock on is the best time. Late in the month, you can start an hour earlier at 9 p.m. The general map will get you looking at the right area of the sky, then use the more detailed map and follow the arrows to 5.3 magnitude 35 Eridani and 4.5 magnitude 32 Eridani. These stars will be faintly visible with the naked eye and easy in binoculars. They along with 21 and 22 Eri will serve as key signposts to help you “star hop” with binoculars or a telescope to the asteroid.

It’s fun getting to know the asteroids. While they look no different than stars even in telescopes, they’re our neighbors and circle the sun as surely as the planets.