Last year, the Martian rover Spirit became stuck in loose sand. This prevented the rover from driving to a favorable sun-facing slope for its fourth Martian winter, which runs from May to November. The low angle of sunlight during the long winter limits the power generated from the rover's solar panels.

Mission managers at NASA's Jet Propulsion Laboratory in Pasadena, Calif., have not heard from Spirit since March 22. They believe Spirit is probably in a low-power "hibernation" mode. During hibernation, the rover suspends communications and other activities so available energy can be used to recharge and heat batteries and keep the mission clock running.

Because most of Spirit's heaters are not being powered this winter, the rover is probably experiencing its coldest internal temperatures yet, minus 67 degrees Fahrenheit. During the rover's three previous winters, Spirit communicated once or twice a week with Earth and used its heaters to stay warm while parked on a sun-facing slope. As a result, the heaters were able to keep internal temperatures above minus 40 degrees Fahrenheit.

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 while the sun is up.

At the end of July, mission managers began using a paging technique called "sweep and beep" in an effort to communicate with Spirit. The rover should send back a signal to Earth if it hears the commands.

Based on models of Mars' weather, managers believe that if Spirit responds, it most likely will be in the next few months.

Doug Oster writes a blog, "Growing With Doug," exclusively at PG+, a members-only web site of the Pittsburgh Post-Gazette. Our introduction to PG+ gives you all the details.