Many of us have been hunting up comet 46P/Wirtanen these December nights, but there’s another solar system critter near the comet you might want to check out — asteroid 3 Juno. They’re both in the same region of the sky, so after you’re finished observing 46P, you can nudge your binoculars or telescope about 10° (one fist) to the southeast to spot the asteroid.
While Wirtanen is fuzzy from all the dust and gas it’s releasing, Juno’s a point of light slowly moving among the stars of the constellation Eridanus (ear-RID-duh-nuss). At mid-month it shines at magnitude 7.8, equal to Neptune, which just had a close conjunction with Mars last week. I can see Neptune in 8×40 binoculars from a moderately light-polluted sky, so for many skywatchers, Juno will be a binocular object.
The asteroid fades slowly through December and January but stays as bright as magnitude 8.5 through mid-January, keeping within the range of 10×50 binoculars. Any telescope will show it throughout the current apparition. The best time to see Juno is from around 8 p.m. local time, when it’s well up in the southeastern sky, until 1 a.m. That’s a nice, big window. Conveniently, it makes a nifty equilateral triangle with the bright stars Rigel and Aldebaran.
3 Juno is a main belt asteroid located that orbits the sun between Mars and Jupiter. Today it’s 100 million miles (160 million km) from Earth. The number “3” in front of its name tells us it was the third asteroid discovered. The lucky person to first see it was German astronomer Karl Harding on Sept. 1, 1804 back when asteroids were still called planets. With a diameter of 145 miles (233 km), Juno’s the 11th largest asteroid in the main belt. A study of reflected sunlight from its surface indicates it’s a rocky object with a composition similar to common meteorites called chondrites picked up on Earth.
Because Juno is relatively close to the Earth, it’s brighter than usual, so now is a great time to take a look. Once you find it come back and look again a few nights later to see that it’s moved, unlike the stars around it which stay put. Let us know if you find it — good luck!