In the early hours of September 27, 2007, a Delta II rocket lifted off its launch pad at Cape Canaveral in Florida. It was the first day of the Dawn mission, an ambitious undertaking by NASA to send a spacecraft to the two most massive members of the asteroid belt — Ceres and Vesta. Dawn arrived at Vesta on July 16, 2011, orbiting and studying the Arizona-sized asteroid for more than a year. On September 5, 2012, the ion-propelled craft bid adieu to Vesta and began its journey toward a Ceres encounter in 2015.
What would prompt NASA to incur the expense of a mission to two asteroids, instead of one?Astronomy magazine subscribers can read the full column for free. Just make sure you're registered with the website.