LOS ANGELES - Spirit, the scrappy robot geologist that captivated the world with its antics on Mars before getting stuck in a sand trap, is about to meet its end after six productive years.

Spirit has been incommunicado for more than a year despite daily calls by NASA. The cause of Spirit's silence may never be known, but it's likely the bitter Martian winter damaged its electronics, preventing the six-wheel rover from waking up.

The space agency tried every trick to listen for Spirit to no avail. Project manager John Callas of NASA's Jet Propulsion Laboratory said the last commands will be sent up today. Though orbiting spacecraft will continue to listen through the end of May, chances are slim that Spirit will respond.

"Spirit went into a deep sleep," said Callas, who said the plucky rover will be remembered for demystifying Mars to the masses.

When the rover team gets together this summer, David Lavery of NASA headquarters said the mood would likely be that of an Irish wake rather than funeral.

"We drove it until its wheels came off," he said. "We never expected that that would be the way that we'd finish up with this project."

The solar-powered Spirit and its twin Opportunity parachuted to opposite ends of the Martian southern hemisphere in January 2004 for what was supposed to be a three-month mission.

The golf cart-size rovers were an instant hit with the public which followed the rovers' every move as they rolled across the Martian plains and stopped to drill into rocks.

Their greatest achievement was uncovering geologic evidence that Mars, now dry and dusty, was far more tropical billions of years ago. The red planet was toastier and wetter, conditions that suggest the ancient environment could have been favorable for microbial life.

As far as sibling rivalry went, Opportunity was the overachiever while Spirit was every bit the drama queen underdog.

Soon after landing, Spirit went into critical condition and sent nonsense data back to Earth. Engineers had to nurse it back from the brink of death.

Unlike Opportunity, which landed in an ancient lakebed awash with water-forming minerals, Spirit plopped into a Connecticut-sized crater named Gusev that contained limited hints of past water.

Spirit had no choice but to trek toward the hills to make discoveries. It managed to shine despite having a rocky start on Mars.

In 2005, Spirit scaled a mountain the height of the Statute of Liberty. It also was the first to record Martian dust devils as they formed, which NASA later made into movie clips.

As the years passed, Spirit began showing signs of aging.

In 2006, one of its front wheels stopped spinning, forcing the rover to drive backward and drag its lame wheel. More recently, it suffered bouts of amnesia by failing to record data to its flash memory.

Spirit survived three Martian winters, but the hardy rover was no match for the latest cold.

A formal farewell is planned at NASA headquarters after the Memorial Day holiday and will be televised on NASA TV.

The mission's deputy investigator, Ray Arvidson of Washington University in St. Louis, said he will remember Spirit as a fighter.

"It wouldn't quit just like the little engine that chugged up the hill," said Arvidson, referring to the children's bedtime story.

Spirit trekked 4.8 miles since landing. Its healthy twin Opportunity has logged 18.5 miles so far - surpassing the distance of a half marathon - and poked into three craters.

Opportunity has been trundling toward a large crater named Endeavour. It is currently less than 3 miles away, and if all goes as planned it should reach the rim later this year.

Spirit is the second Mars spacecraft in three years to stop working. In 2008, NASA bid adieu to the Phoenix lander after five months of studying a Martian arctic plain.

Opportunity could soon get some company on the Martian surface. NASA later this year is expected to launch a megarover the size of a Mini Cooper that will land at a still-to-be-determined spot on Mars in summer 2012.