Dawn craft
An artist's interpretation of the Dawn spacecraft above Ceres. NASA

A NASA spacecraft lost the use of a third wheel that was critical to its functionality and ability to reposition in space, the space agency said. The Dawn spacecraft, which set out for exploration in the asteroid belt in 2007, lost operation of the wheel Sunday afternoon.

Experts at NASA realized the craft had lost the wheel during routine communications Monday. Upon learning of the wheel malfunction NASA returned the craft to regular flight configuration and safe mode. This is the third wheel Dawn has lost since it launched.

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Dawn set off to observe the protoplanet Vesta in 2007. Vesta is the largest protoplanet still in existence and sits in the asteroid belt between Mars and Jupiter. Protoplanet Ceres also sits in this part of the solar system. The two are the largest objects in the asteroid belt.

After completing two years of observations of Vesta the craft made its way to Ceres. The third wheel malfunctioned Sunday while the craft was repositioning to complete an observation of the Occator Crater.

The first wheel malfunctioned in 2010 and the second in 2012 while the craft was finishing up its study of Vesta. Researchers were prepared for the third wheel to fail at some point. One more wheel remains operable.

The craft has been relying on hydrazine propellant for repositioning amid the wheel failures. The propellant comes from small jets, and as it is used up, Dawn's ability to navigate is diminished.

NASA says the malfunctioning wheel won't significantly impact the rest of Dawn's extended mission.

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