Lately, I’ve talked about how to find planets, but there are lots of other planet-like bodies in the night sky — the asteroids. As of March 2016, we know of 1.3 million of them, 750,000 of which have calculated orbits. The name asteroid comes from “aster,” Latin for star, and it perfectly describes their star-like appearance in the telescope.
Unlike planets, which show nice disks in most scopes, asteroids are too tiny to look like anything more than stars, at least in amateur instruments. 4 Vesta is the second largest main-belt asteroid after 1 Ceres at 326 miles (525 km) and the brightest asteroid visible from Earth.
Tonight, Vesta reaches opposition along the border of the constellations Cancer and Gemini when it will shine at magnitude +6.3. That’s right at the naked eye limit for observers under rural skies, making finding the asteroid without optical aid a wonderful challenge for sharp-eyed skywatchers. But anyone with a pair of binoculars can find the asteroid even from suburban areas. Vesta will rise right around sunset and be well placed at a comfortable altitude in the eastern sky by 8 p.m. local time.
Two days after opposition, Vesta will make its closest approach to Earth for 2017 at a distance of 141 million miles (227 million km) or 1.5 times the average distance between the Earth and sun. You’ll recall that Vesta was the first target of NASA’s Dawn mission to orbit and explore two very different asteroids. Photos show a cratered, somewhat potato-shaped body with a huge, 250-mile-wide (400 km) crater centered on its south pole.
After a thorough study, astronomers concluded that Vesta was different from most asteroids. Like the early planets, it melted and “differentiated” into a layered body with a denser core and lighter, rocky mantle and crust. You’ll often hear Vesta described now as a protoplanet, a planetary embryo that never quite made it to adulthood.
Lucky for us, Vesta happens to be close to a pair of bright stars this month and next — first magnitude Pollux and his second magnitude brother, Castor, in Gemini the twins. To find the traveling protoplanet, face east around 8 o’clock and locate the stellar duo about four fists to the left and a little above Orion’s Belt. Next, point your binoculars at Pollux (the brighter) and use the chart to star-hop your way to Vesta.
Tonight, the asteroid passes very close to a similarly bright star, making a new, temporary “double star.” If you watch tonight and note how close the two are, you’ll easily see Vesta scooch to the west (right) the following night. On Feb. 2-3, watch again as Vesta skims just one full moon diameter (30 arc minutes) north of 3rd magnitude Kappa Geminorum.
It’s really fun seeing an asteroid, and Vesta’s bright enough for almost anyone to pick out. Happy protoplanet hunting!