You may recall that the rover got stuck in 2009 on the edge of a small crater, and when controllers on earth couldn't free it after months of trying, they knew the clock was ticking.
Among other things, they couldn't move the rover to a sun-facing slope for the six-month-long Martian winter, so that its solar panels could gather at least enough energy to run heaters and the rover's radio system. The solar panels are considered essentially useless unless the sun shines almost directly down on them.
After the Martian winter ended, engineers tried repeatedly to get the rover's computer to respond to signals. They sent hailing signals once a week. No joy. They said they would make one more try overnight and then stop.
"We're all taking a realistic look at the situation," said David Lavery, NASA'S program executive for solar system exploration. "We drove it basically until its wheels came off."
Spirit's twin, Opportunity, is still driving slowly on the opposite side of the planet, making a forced march toward a large crater called Endeavour. It landed a month after Spirit arrived.
A reminder: NASA's gotten its money's worth out of the rovers. When Spirit landed, NASA (perhaps playing down expectations), said it planned a mission that would last 90 "sols," or Martian days. Today was sol number 2,537.
"Yes, there's a sadness that we have to say goodbye to Spirit, but we also remember what a massive overwhelming success it was," Callas said in a teleconference with reporters.
Lavery was asked if there would be an informal "funeral" for the rover.
"I think it's more in the spirit of an Irish wake," he said.