One of NASA’s six-year-old Mars rovers missed its winter wake-up call, prompting concerns that it may never recover from the frigid cold.

During previous winters, Spirit was able to get to a sun-facing slope in Gusev Crater before the deep cold set in, a position that left it able to draw enough power from its solar cells to keep heaters turned on and to phone home on occasion.

GALLERY: Saving Spirit

Mired by two broken wheels and stuck in the sand for more than a year, Spirit

went into hibernation in March, suspending all communications and other activities so

any available energy can be used to recharge and heat batteries and keep its

mission clock ticking.

Ground control teams began sending out a beacon for Spirit on July 26, but the rover has not yet responded. In a press release on Friday, NASA put out the word that it may never wake.

“It will be the miracle from Mars if our beloved rover phones home,” Doug McCuistion, director of NASA’s Mars Exploration Program, said in a statement. “It’s never faced this type of severe condition before — this is unknown territory.”

SLIDE SHOW: Phoenix Mars Lander’s First Images

Engineers estimate Spirit may be experiencing temperatures as low as minus 67 degrees Fahrenheit, compared to minus 40 degrees during previous winters when its heaters were working.

Writes NASA:

“Spirit is designed to wake up from its hibernation and communicate with Earth when its battery charge is adequate. But if the batteries have lost too much power, Spirit’s clock may stop and lose track of time. The rover could still reawaken, but it would not know the time of day … Spirit would start a new timer to wake up every four hours and listen for a signal from Earth for 20 minutes of every hour while the sun is up. “The earliest date the rover could generate enough power to send a beep to Earth was calculated to be around July 23. However, mission managers don’t anticipate the batteries will charge adequately until late September to mid-October. “Based on models of Mars’ weather and its effect on available power, mission managers believe that if Spirit responds, it most likely will be in the next few months. However, there is a very distinct possibility Spirit may never respond.”

Spirit and a twin rover named Opportunity arrived on Mars in January 2004 for what was expected to be a three-month mission. Opportunity, which is en-route to a large crater named Endeavour, remains in good health. This week, it sent home its first picture of a Martian dust devil:

Top image: Spirit’s last stand. Bottom image: Opportunity sightseeing on Mars. Credit: NASA/Jet Propulsion Laboratory