If you’ve never seen an asteroid, now’s the time. Asteroid 4 Vesta — the “4” stands for the 4th asteroid discovered — will shine brighter this season than it has for years and for years to come. The 329-mile-wide ball of rock orbit the in the main asteroid belt between Mars and Jupiter at an average distance of 219 million miles (353 million km) or about 1.5 times farther than Mars.
On June 19, it will be at opposition to the Earth and even closer to us than the close approach of 2007. Back then, it was faintly visible with the naked eye from dark skies if you knew exactly where to look. At opposition, the Earth and Vesta line up on the same side of the sun just 106 million miles (170.6 million km) apart. Close means bright in the world of astronomy. Vesta will shine at magnitude 5.3 on and around that date, about as bright and close as it ever gets.
The average person can see a magnitude 6.0 star from a very dark, moonless sky. Vesta’s more than half a magnitude brighter. Not only will any pair of binoculars show it, but lots of us have a shot at spotting without any optical aid at all — as long as you’re watching from outside the city. The best viewing period for the minor planet, when it’s brightest with no interference from the moon, will be from June 8-22.
Vesta spends most of the month moving westward in Sagittarius not far from the planet Saturn. The planet offers an easy way to navigate the constellation’s star fields to the asteroid. Use the map to locate Saturn and then star-hop with the naked eye or binoculars to 3.8-magnitude Mu (μ) Sagittarii. The asteroid is located 2.5°–4° northwest of that star through mid-June. Despite its location in this star-rich region of the sky, Vesta has little competition from similarly bright stars, making it fairly easy to spot.
On the nights of the June 14–15, it will pass less than ½° southeast of the bright open cluster M23 before slipping into the constellation Ophiuchus. Come early August, the asteroid does an about-face and turns back east, looping back into Sagittarius in early September. On the final night of summer, it slides 1° south of the bright Lagoon Nebula (a.k.a. M8). You can use the map above or this one, created by Sky & Telescope. Vesta’s location on the Sky & Telescope map is shown for 0 hours Universal Time, equal to 8 p.m. Eastern the previous night, 7 p.m. Central and so on. So June 12 on that map is the asteroid’s location at 8 p.m. on June 11.
Vesta was first seen by German astronomer Heinrich Olbers on March 29, 1807. In those days, asteroids were brand new, with the first, 1 Ceres found only 7 years earlier. As their number multiplied with every new discovery their number astronomers switched from calling them planets to minor planets or asteroids. The word means “star-like.”
These days, we have information on some 760,000 asteroids, an incredible number, right? Unfortunately, it’s pitifully small when compared with the total estimated in the billions. Even if we limit asteroid size to 328 feet (100 meters), there are still more than 150 million just in the main belt! And yet space in the region between Mars and Jupiter is so vast, the average distance between asteroids is 2.5 times the distance of the Earth to the moon. That’s why not a single space probe has ever gotten whacked while crossing the belt.
If you own an 8-inch or larger telescope, you can see a little more. Color for instance. Many observers report that Vesta looks yellow. I’m excited to try and see Vesta as a disk, er, oval. It’s not a sphere in part because an earlier asteroid impact tore off its bottom, so it looks more like a spinning potato now. If you’re ready for the challenge, pick a night when Saturn appears sharp and crisp at high power through your scope, then point at Vesta and up the magnification to 500x or higher. Does it look oval?
I was up late last weekend and watched Saturn rise above the trees in a dark sky. With the help of binoculars I found Vesta. Then, knowing exactly where to look, I lowered the glasses and saw the faint pinprick of light beaming from the asteroid belt.