March 06--Dawn has arrived. The signal was received at 5:36 a.m. PST Friday: NASA's Dawn spacecraft had successfully entered into orbit around Ceres, becoming the first NASA mission to visit a dwarf planet, and the first to go to two distinct bodies in the solar system.
Ceres is the largest object in the asteroid belt between Mars and Jupiter and the final stop on Dawn's journey in the solar system. The 4.5-foot-long spacecraft blasted off from Earth in 2007 and spent 14 months exploring the mega-asteroid Vesta in 2011 and 2012. It is scheduled to stay at Ceres through June 2016.
The science phase of Dawn's Ceres mission won't officially get underway for a few more weeks, but since the end of January the spacecraft has been sending back early-approach images of the icy world that have defied scientists' expectations and whetted their appetites for the more detailed images that should be coming soon.
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The early pictures reveal that the surface of Ceres is not smooth as researchers had anticipated, but rather dotted with craters. The images also show two mysterious and intriguing bright spots nestled close together inside a 57-mile-wide crater. Scientists are still unsure what they might be.
"What we expect at Ceres is to be surprised, so it's getting off to a good start," deputy principal investigator Carol Raymond told the Los Angeles Times in late January.
Ceres has an average diameter of 539 miles and was likely on its way to becoming a full-fledged planet until the gravity of Jupiter got in the way.
Researchers say Ceres has a rocky core, an ice mantle and a thin dusty crust, but they believe that in the past it had a liquid ocean like Jupiter's moon Europa or Saturn's moon Enceladus.
As Dawn flies closer to Ceres over the coming months, instruments on the spacecraft will get a better look at the topography of the world, find out if it still has an active geology, learn what it is made of and get a better understanding of its internal structure.
Raymond said Ceres and Vesta are fossils from the early solar system and could help scientists understand how the Earth formed and where its water might have come from.
"By studying Vesta and Ceres, we will gain a better understanding of the formation of our solar system, especially the terrestrial planets, and most importantly the Earth," she said in a statement.
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UPDATES
7:40 a.m. This story was updated with comment from Dawn mission investigator Chris Russell about the spacecraft's arrival at Ceres.
7:21 a.m. This story was updated with confirmation the spacecraft began orbiting the dwarf planet.
This story was originally published at 4:30 a.m.
