Would you like to pretend you’re riding piggyback on NASA’s Dawn mission hopping from the asteroid Vesta to its next target Ceres? All you need is a pair of 7×35 binoculars, a reasonably dark sky and a bit of imagination. November may not be the most pleasant month for skywatching in the northern hemisphere, but Ceres and Vesta, two of the brightest, biggest asteroids, are making their closest approach to the Earth in 2012 at nearly the same time. Surely this fortunate coincidence will inspire you to bundle up and brave the chill.
Vesta comes to opposition (closest to Earth) on December 9 when it will shine just below the naked eye limit at magnitude 6.4; Ceres reaches opposition only nine days later at magnitude 6.7. While the two almost-planets will be brightest then, they’re already nearly as bright and an easy catch now that the moon has retired from the evening sky.
Ceres is especially easy to spot over the next few nights since it keeps close company with the naked eye star Eta Geminorum in the toes of Gemini the Twins. Watch for it to buzz just south of M35 – one of the prettiest star clusters in the sky for both telescopes and binoculars – around Thanksgiving time. Consider their juxtaposition another reason to be thankful.
Ceres begins November at magnitude 8.0 but brightens to 7.3 by month’s end. Vesta meanwhile sidles up the south side of Taurus the Bull, home to the V-shaped Hyades star cluster and brilliant Jupiter. At magnitude 7.2 this week, you’ll find it a tad brighter and easier to see than Ceres. On the map, notice that Vesta glides through the northern edge of the bright, loose star cluster Collinder 65. Another bonus.
Both asteroids are presently traveling west – opposite their normal orbital motion – in what astronomers call a retrograde loop. The outer planets perform the same crazy backwards move around the time of their oppositions, too. Ceres and Vesta aren’t defying orbital mechanics. They only appear to move backwards because the faster Earth is passing them by, much like a car in the right lane appears to move backward as you pass it on the left. Soon enough, they’ll resume their routine eastern motion through the constellations as the viewing geometries of Earth and asteroids change.
Dawn spent about a year in orbit around Vesta and finally departed late this summer with its sights set on a February 2015 encounter with Ceres. We learned lots at Vesta including that it differentiated into core and crust much like planets do. Rocks in its crust studied from orbit are a match for a suite of meteorites found on Earth called eucrites, diogenites and Howardites. No need for a sample return mission – pieces of Vesta have been falling from the sky for centuries.
Ceres is the largest asteroid – a dwarf planet actually – and another place altogether. By examining sunlight reflected from its surface, Ceres appears more carbon and water-rich than Vesta and a closer match to carbonaceous chondrite meteorites.
You can share in these discoveries and the road ahead by stepping outside the next clear night with binoculars and chart in hand. There’s nothing like seeing the real thing.
Let me know how you like the maps. I reversed the stars, going with black-on-white for greater clarity. Hopefully, they’re easier to read. To use a map, right-click and save to your desktop and then print out a copy. I’ll update them later this month, so you can continue to “mission along”.