A pair of unmanned NASA spacecraft that are orbiting the Moon were renamed Ebb and Flow on Tuesday by a middle school class in Montana, the US space agency announced.

The original names for the twin probes Gravity Recovery and Interior Laboratory (GRAIL) -- A and B -- were not very inspired, admitted principal investigator Maria Zuber of the Massachusetts Institute of Technology.

"We were so busy in the design and getting these two spacecraft launched on time that when we gave them names, we gave them names of A and B, and that isn't too creative. So we asked the youth of America to assist us," she said.

More than 11,000 students took part in the contest to rename the twin craft which aim to map the Moon's surface, determine its gravity field and reveal the contents of its inner core.

The winners were a fourth grade classroom of nine- and 10-year-olds at Emily Dickinson School in Bozeman, Montana.

"They noted the fact that GRAIL is going to be studying gravity on the Moon, and that the effect of gravity on the Earth is seen every day in terms of tides," said Zuber.

"So they chose Ebb and Flow because it was the daily example of how the Moon's gravity is working on the Earth," she added, describing the idea as "very simple" but also "sophisticated."

Other such contests have been held in the past to name NASA spacecraft, such as the Mars rover Curiosity, coined in 2009 by a 12-year-old essay winner in Kansas named Clara Ma.

The two GRAIL spacecraft, a $500-million pair of washing machine-sized satellites, launched in September and reached lunar orbit at the turn of the New Year. Their work mapping the Moon is set to begin in March.