Controllers of a European comet-hunting spacecraft say they will put it in deep-space hibernation for the next 31 months as it heads for its distant target.

During the loneliest leg of its 10-year mission, the European Space Agency's Rosetta craft will head ever closer toward comet 67-P, soaring to almost 620 million miles from Earth, an ESA release said Friday.

Controllers at ESA's European Space Operations Center plan to issue the final command next week to switch Rosetta into hibernation mode, they said.

"Rosetta is getting farther from the sun, and soon there simply isn't going to be enough sunlight to power its systems," said Paolo Ferri, Head of ESOC's Solar and Planetary Mission Operations Division.

"We already achieved a record in July 2010 when we reached 400 million km (250 million miles) from the sun and became the most distant spacecraft ever to operate on solar power alone. Rosetta will double the record distance during the hibernation period."

The only devices left running will be the onboard computer and several heaters, controllers said.

After the hibernation command is sent, there will be no signal to or from the spacecraft until 2014.

"We've planned for hibernation for some time, and it's a complex phase of the mission," says Andrea Accomazzo, Spacecraft Operations Manager at ESOC.

"Still, for the flight control team, it's an emotional moment," he said.

"We're essentially turning the spacecraft off. We're already looking forward to January 2014 when it wakes up and we get our spacecraft back."