Mission managers have been steering Dawn ever closer to the 310-mile-wide (500-kilometer-wide) asteroid, and they expect the craft will be captured into orbit around Vesta at about 1 a.m. ET.

Dawn had to travel more than 1.6 billion miles (2.5 billion kilometers) to reach Vesta, which is part of the main asteroid belt, between the orbits of Mars and Jupiter. (Explore an interactive solar system.)

"This is truly an exciting mission, not only because this is the first time ever we will enter orbit around a main-belt asteroid, but because we get a chance to unlock the earliest chapters in our solar system's history," said Chris Russell, principal investigator for the Dawn mission.

Main-belt asteroids are considered to be cosmic fossils