When astronomers decided in 2006 that Pluto never should have been classified as a planet in the first place, they made it the charter member of a new class of objects called dwarf planets — objects that just didn’t quite fit the concept of a planet. Five other solar system objects were quickly added to the ranks of dwarf planet, including Ceres, the largest member of the asteroid belt, and the only one large enough to pull itself into a spherical shape.
Although Ceres is the largest denizen of the asteroid belt, it is only the second brightest. The brightest asteroid — and third largest — Vesta, occasionally shines brightly enough to barely reach human-eye visibility, which it was in the middle of last month. Now, it is slightly fainter, so that only the sharpest-eyed individuals can pick it out in an extremely dark sky. For all of this month and into July as they orbit the sun, the paths of Ceres and Vesta keep them close enough together that you can see both within the field of view of a pair of binoculars.
Researchers anticipate arrival
NASA’s Dawn spacecraft achieved orbit around Vesta on July 16, 2011, and will arrive at Ceres in February.
At Vesta, Dawn discovered odd channels running down the sides of impact crater walls. At first glance, they resemble water erosion here on Earth, but Vesta is thought to be too small to have water in any significant amounts. Also, the surface of Vesta is much younger than other asteroids. What resurfaced Vesta is unknown.
Dawn flies now toward a rendezvous with Ceres. On Jan. 22, NASA scientists announced the discovery of water vapor surrounding Ceres. Scientists theorized that Ceres had surface ice, but couldn’t verify that. That discovery confirms the theory.
Tonight, Ceres and Vesta will be separated in our sky by about four times the diameter of the full moon: 2 degrees.
The asteroid pair form an equilateral triangle with yellow-orange Mars and Spica, the 10th brightest star we can see from Oklahoma. Only one-quarter of the moon’s diameter will separate them on June 30, and then their orbits begin to diverge.
Continue reading this story on the...