What would prompt NASA to incur the expense of a mission to two asteroids, instead of one? They’re all the same, right? Not really. The 2006 International Astronomical Union’s redefinition of solar system bodies elevated the much larger and more spherical Ceres to dwarf planet status. Vesta remained an asteroid — more specifically, a minor planet.
The differences, however, go beyond mere definition. Vesta is a rocky body similar in makeup to the inner planets. Ceres, on the other hand, appears to have an icy composition comparable to the moons that occupy the outer solar system. Both likely have changed little since the early stages of solar system formation. Studying two dissimilar bodies that originated in the gap between Mars and Jupiter may provide important clues about the formation of the planets.
During its visit, Dawn mapped Vesta’s surface, studied its composition, and analyzed its internal structure. Among the discoveries was a 310-mile-wide (500 kilometers) impact basin — subsequently named Rheasilvia — occupying much of Vesta’s south pole. Near the crater’s center, Dawn imaged a mountain nearly three times as tall as Mount Everest. The spacecraft’s cameras also captured an intriguing series of grooves girding Vesta’s equator, possibly a result of the impact that formed Rheasilvia.
The asteroid’s varied surface composition and a differentiated (layered) interior that includes a basaltic crust and iron core indicate that Vesta is more than your run-of-the-mill asteroid. Indeed, researchers now consider Vesta a “protoplanet” not unlike Earth during the early stages of its formation. What will Dawn reveal about Ceres? Stay tuned!
The finder chart on page 43 of this issue’s “The Sky this Month” will help you navigate your way to Vesta and Ceres. Your prime launch window will occur during the first half of January. At this time, the Moon is either absent from the evening sky or not yet bright enough to interfere with your observing. Pair your telescope with a low-power eyepiece; its wide field of view will embrace a larger chunk of sky than would a high-power ocular, making a star-hop search easier.
The journey ends when you encounter what appears to be two 7th-magnitude “stars” in the locations indicated on the chart. Don’t bother switching to higher powers. Vesta and Ceres are so small and remote that they appear starlike when viewed with backyard scopes, no matter how high the magnification. To be absolutely sure you’ve spotted the two, make follow-up observations a night or two later. If the “stars” have moved, you’ve officially captured Vesta and Ceres and completed your Dawn mission.
Your voyage to Vesta and Ceres may seem underwhelming when compared to what the Dawn mission has revealed — all you get to see are ordinary stellar specks, while the spacecraft sent back spectacular close-up images of Vesta’s cratered surface. Consider that the price tag for the Dawn mission runs nearly a half-billion dollars, and, upon completion of its mission, Dawn will have traveled 3 billion miles (5 billion km). Your telescope and accessories probably only set you back several hundred or thousand dollars, and you didn’t even have to leave your backyard. Not a bad deal!
Questions, comments, or suggestions? Email me at firstname.lastname@example.org. Next month: the dance of the spirits. Clear skies!