12:25 p.m. EST March 6, 2015 After a 7-1/2 year journey through the solar system, NASA's Dawn spacecraft settled into orbit Friday around the mini-planet Ceres that lies in the asteroid belt between Mars and Jupiter.
Mission controllers at NASA's Jet Propulsion Laboratory in Pasadena, Calif., 310 million miles away, received a signal from the spacecraft at 8:36 a.m. EST that Dawn was healthy and thrusting with its ion engine, the indicator Dawn had entered orbit as planned.
"Confirmed: I am in orbit around #Ceres," Dawn said in a Twitter message.
"Since its discovery in 1801, Ceres was known as a planet, then an asteroid and later a dwarf planet," said Marc Rayman, Dawn chief engineer and mission director at JPL. "Now, after a journey of 3.1 billion miles (4.9 billion kilometers) and 7.5 years, Dawn calls Ceres, home."
The spacecraft, trailing a beam of blue-green vapor behind it, was captured by the dwarf planet's gravity at about 4:39 a.m. EST, the mission team said.
"We feel exhilarated," said Chris Russell, principal investigator of the Dawn mission at UCLA, according to a NASA statement."We have much to do over the next year and a half, but we are now on station with ample reserves, and a robust plan to obtain our science objectives."
Dawn's job is to tell all about Ceres, which was discovered more than 200 years ago but has let slip very little about itself. Does it have polar ice caps? Does it have a liquid ocean? And could it have once harbored – or could it still harbor -- life?
"We've never een sent a spacecraft there," says Marc Rayman, Dawn's chief engineer and mission director. "This is a first reconnaissance of an entirely new world."
The planet has been like "your secretive neighbor. Ceres would tell us nothing," says Dawn principal investigator Christopher Russell of the University of California, Los Angeles. "Everything we're seeing was unexpected."
Among the most unexpected sights are the two astonishingly bright spots nestled in a crater on Ceres's surface. Photographed by Dawn's camera Feb. 19, the dots have the experts buzzing with speculation. Maybe a direct hit from an asteroid -- a space rock – excavated some buried ice. Maybe the bright spots are a sheen of minerals deposited by gushing geysers.
The spots "are unique in the solar system," Carol Raymond, deputy principal investigator of the Dawn mission said at a NASA briefing Monday. "The mystery will be solved, but it's really got us at the edge of our seats."
Like the big boys of the solar system, Ceres first took shape some 4-1/2 billion years ago from gas and dust. Unlike the true planets, Ceres froze in a state of arrested development, never growing beyond roughly a third the size of our moon.
Ceres "made it to the last rank before planet," says planetary scientist Andy Rivkin of the Johns Hopkins University Applied Physics Laboratory, who has studied the body. "The planets formed out of things that were Ceres-sized." Ceres, he says, can teach us what it takes to build an object such as the icy moons adorning some of the solar system's bigger planets.
Though it's not a full-fledged planet, Ceres does merit the consolation title of "dwarf planet," a title awarded to round bodies that, though big, are too small to make everything else get out of their way. Ceres is one of only known five dwarf planets – Pluto is another – and the first to host an orbiting spacecraft.
