A NASA spacecraft is about to reach the end of a nearly eight-year journey and make the first rendezvous with a dwarf planet.
The Dawn craft will slip into orbit on Friday around Ceres, a dwarf planet the size of Texas.
Unlike robotic landings or other orbit captures, the arrival won't be a nail-biter. Still, Dawn had to travel some five billion kilometres to reach the largest object in the asteroid belt between Mars and Jupiter.
"It's been a roller coaster ride. It's been extremely thrilling," project manager Robert Mase of the NASA Jet Propulsion Laboratory said on Monday.
Ceres is the first of two dwarf planets to receive visitors this year. NASA's New Horizons spacecraft is barrelling toward one-time planet Pluto where it will arrive in July.
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Launched in 2007, Dawn made the first stop of its journey at the asteroid Vesta. It beamed back more than 30,000 images of the rocky world inside the asteroid belt before heading to its final destination.
Dawn began its approach to Ceres in December, and last month it snapped pictures of the dwarf planet that revealed two mysterious bright spots inside a crater.
Scientists will have to wait until the craft spirals closer to the surface in the coming months to get sharper images.
It will get as close as 400km above Ceres' surface, or roughly the distance of the International Space Station above Earth.
Last year, European researchers not connected with the mission detected water plumes spewing from two regions on Ceres.
The source of the plumes remains unclear.
Deputy project scientist Carol Raymond said the shiny patches - possibly exposed ice or salt - were a surprise and could be related to the plumes.
Dawn carries an instrument that should be able to detect the plumes if the surface is still active.
"The team is really, really excited about this feature because it is unique in the solar system," Raymond said of the spots.
"We will be revealing its true nature as we get closer and closer to the surface. So the mystery will be solved, but it is one that's really got us on the edge of our seats."
The $US473 million ($A605 million) Dawn mission is the first to target two different celestial objects to better understand how the solar system evolved.
It's powered by ion propulsion engines, which provide gentle yet constant acceleration, making it more efficient than conventional rocket fuel.
With its massive solar wings unfurled, it measures about 20 metres, the size of a tractor-trailer.
