A NASA science satellite on Friday will wrap up a seven-and-a-half-year journey to Ceres, an unexplored dwarf planet in the main asteroid belt between Mars and Jupiter, scientists said on Monday.
The Dawn spacecraft visited the asteroid Vesta before firing its electric ion engine to continue on to Ceres, a round, 600-mile-wide (970 km-wide) mini-planet that is the largest body in the asteroid belt.
Earth's moon, by comparison, is about 2,160 miles (3,480 km) in diameter.
The solar-powered probe is expected to put itself into orbit around Ceres at 7:20 a.m. EST (1220 GMT) on Friday. However, radio telescopes on Earth will not be in position to pick up Dawn's signal until later in the day, National Aeronautics and Space Administration officials said at a news conference.
"The approach has gone flawlessly so far," said Dawn Project Manager Robert Mase of NASA's Jet Propulsion Laboratory in Pasadena, California.
Scientists are eager for their first close-up look at a dwarf planet, believed to be a building block left over from the formation of the planets 4.6 billion years ago.
"They're literally fossils that we can investigate to understand the processes that were going on at that time," said Dawn scientist Carol Raymond, also with Jet Propulsion Laboratory.
Another NASA spacecraft, New Horizons, will fly by the distant dwarf planet Pluto in July. Pluto, once considered one of the planets of the solar system, was later downgraded to a dwarf planet.
Ceres, namesake of the Roman goddess of agriculture, is already providing intrigue. Pictures relayed from Dawn last month show bright streaks on its surface, including two very bright spots inside a crater.
"These spots were extremely surprising," Raymond said.
Scientists suspect Ceres may have had an underground ocean early in its history that later froze. Impacting asteroids or comets could then have exposed patches of highly reflective ice.
Europe's Herschel space-based telescope previously detected water vapor on Ceres, a clue that impacting bodies may periodically send plumes of watery material shooting into space.
"In the initial views of Ceres, we see many strange features: smooth areas, areas that chaotically fractured and craters of all shapes and sizes," Raymond said. "Of particular interest are the bright spots ... which stand out against Ceres' dark surface."
It will take Dawn about a month to position itself for 14 months of observations of Ceres. In all, the mission is costing NASA $473 million.