By the time you read this you may have seen the first close up pictures of the Asteroid Vesta – the space probe Dawn is scheduled to enter orbit on July 16th. For a brief time you, too, can glimpse Vesta – the only main belt asteroid visible to the unaided eye. (At least forty are visible with 9×50 binoculars at some point in their orbits.)
Vesta is an asteroid, an ill-defined term that basically means nowadays any large rock closer to the Sun than Jupiter. Vesta resides in the main asteroid belt, a broad region between the orbits of Mars and Jupiter. Giant Jupiter’s gravitational field prevented any planet from forming there during the creation of the Solar System and even Mars wound up being smaller than it should have been. What did form here were millions of leftover chunks of matter which in total are less than the mass of the Moon. Despite images in print media and movies such as Star Wars, space is so large that even with millions of asteroids it would be unusual to see another one if you were standing on the surface of Vesta.
Vesta is large for an asteroid, perhaps 300 miles across, and might be reclassified as a dwarf planet after Dawn arrives, joining Ceres and Pluto as a member of the group. It is composed of lighter color rock than most asteroids and, when taken together with its size, makes Vesta briefly visible to the naked eye when we are closest to it. Tonight it can be found in the constellation Capricornus at magnitude +5.47 which renders it just barely visible from a dark sky site, but probably not in suburban Marco. With binoculars though, it would be easy to spot.
A much more difficult target would be the nearby Neptune at magnitude +7.83, well below naked eye visibility. At nearly 2.7 billion miles distance, Neptune only appears as a star-like point to all but the most powerful telescopes. It will show up well in binoculars though – the problem is in identifying it from many similarly bright stars.
Neptune was first identified as a planet in 1846, but was no doubt seen many times before and mistaken for a star. Just a couple of weeks ago, on July 12, Neptune completed exactly one full orbit about the Sun since its discovery, but is some distance away from its original location – the Earth is in another portion of its orbit, so the viewing angle is different.
See you next time!
Mr. Usher is President of the Everglades Astronomical Society which meets every second Tuesday at 7:00PM at the Norris Center, Cambier Park, Naples.