The next two weeks are an excellent opportunity to spot the brightest of the asteroids, Vesta.
In the first six years of the 19th century, astronomers discovered four new members of the solar system. All four were small objects moving in orbits between the orbits of Mars and Jupiter. Initially they were called planets, but by mid century, enough new objects had been found in this area that they were given a category of their own, much as Pluto was reclassified from a planet to a dwarf planet. They were called “asteroids” because all were so small that they looked just like stars in the telescopes of the day. Now there are tens of thousands of known asteroids.
Vesta is the brightest of all the asteroids, ranging between magnitudes 5 and 8, and one of the largest, measuring 318 miles (512 km) across. It reached 6th magnitude at opposition on September 29, meaning that it could just barely be seen by someone with perfect eyesight at a perfectly dark site.
The rest of us have to make do with binoculars. Here’s how to find it.
The first chart shows its overall position among the constellations of autumn. The two left-hand stars of the Square of Pegasus, Alpheratz and Algenib, point southward across the circlet of Pisces to the constellation Cetus, the Whale. Look for a large triangle formed by Eta and Iota Ceti and Deneb Kaitos. The last is easy to spot because, although only second magnitude, it is by far the brightest star in this rather dim part of the sky. Eta and Iota are both magnitude 3.5, so quite a lot dimmer than Deneb Kaitos.
The second chart shows these three stars in detail, and the path of Vesta over the next two weeks. The end of Vesta’s path with the label is its position on Wednesday, September 30, and the points on the trail to the right show its position each night after that.
Vesta should be quite easy to spot, since it is about two magnitudes brighter than any of the stars along its track. Just to be sure, make a simple plot of the stars in its vicinity, and then check again a night or two later. The “star” that has moved is certain to be Vesta.
Vesta is now one of the best known objects in the solar system because it had the NASA spacecraft Dawn orbiting it for over a year (July 2011–September 2012). This is a great chance for you to see it with your own eyes.