It was supposed to roam the surface of Mars for only three months and cover a distance of just a few hundred yards. Instead, NASA's Spirit rover traveled nearly five miles over five years, finding geological evidence that the Red Planet had once been warm and wet enough to have harbored life. Even after it got hopelessly stuck in the powdery soil of Gusev crater, the rover continued to make discoveries and beamed them to scientists millions of miles away.
But engineers at the Jet Propulsion Laboratory in La Canada Flintridge sent their final commands to Spirit early Wednesday morning, more than a year after the rover made its last transmission to Earth.
After seven years on Mars, the craft had exhausted its batteries, and its electronic innards had probably been destroyed during the long Martian winter, with temperatures sinking to about 150 degrees below zero.
"It's very sad," said Diana Blaney, a geologist and spectroscopist who has been examining the pictures sent by Spirit and its twin rover, Opportunity, since they arrived on Mars. "I honestly thought up until March that Spirit was going to pull through."
Opportunity continues to soldier on on the opposite side of the planet.
Scientists at JPL mourned the loss of Spirit because they, like many people around the world, had come to think of the two explorers almost as living beings, said John Callas, project manager for the rovers. But they also celebrated its achievements.
"It's impossible to think about Mars before the rovers — they've so revolutionized our understanding of this planet," Callas said. "The Red Planet is no longer this distant alien world. It's now a familiar place, and Spirit has given us that."
The launch of the two rovers in 2003 came at a difficult time for planetary exploration in general and for JPL in particular. The lab's previous two Mars missions, the Mars Climate Orbiter and the Mars Polar Lander, had both failed in 1999. The loss of the Mars Climate Orbiter, which disintegrated as it plunged into the atmosphere, was particularly embarrassing because a contractor had programmed it in English units of measurement rather than the required metric units.
Britain's Beagle 2 lander, scheduled to land Christmas Eve 2003, disappeared without a trace during its descent. That same month, Japan aborted the mission of its Nozomi Mars orbiter because of problems with its navigation system and other equipment.
A lot of hopes were therefore riding on Spirit when it made its fiery descent through the thin Martian atmosphere on Jan. 3, 2004. When the JPL team acquired a signal from the craft at 8:51 that evening, then-NASA Administrator Sean O'Keefe proclaimed, "We're back. This is a big night for NASA."
Planning for the Mars Exploration Rovers had begun more than a decade earlier. JPL's initial idea was to investigate what the planet's geology had been like about 3.5 billion to 4 billion years ago, when life was beginning to develop on Earth. A crucial part of the proposal was to make two rovers and deploy in different places.
"The cost is not twice as much as one because a lot of the expense is in the design and validation," Callas said. "You get twice the bang at less than twice the cost."
The initial price tag for the twin-rover mission was $820 million.
Spirit and Opportunity were equipped with a variety of onboard instruments to study rocks on Mars.
Each rover has "eyes" — sharp color cameras that capture images for scientists to study (as well as navigation imagers that help it avoid hazards while driving from place to place). They each have a robotic arm that contains a microscopic imager that is the equivalent of a geologist's hand lens, a rock abrasion tool to help see material just beneath the surface, and spectrometers to examine the composition of the Martian surface.
"Spirit and Opportunity are basically geological explorers," said Ray Arvidson, a professor of Earth and planetary sciences at Washington University in St. Louis who serves as deputy principal investigator for the rovers' scientific payload.
At 384 pounds apiece, each was more than 15 times the weight of Sojourner, the toy-like rover that had been carried to Mars' surface by the Pathfinder mission in 1997. That required a completely new way of doing things.
Seven months into its journey, Spirit entered the Martian atmosphere at 12,000 mph. Thus began a complex descent-and-entry sequence Callas described as "six minutes of terror."
After friction slowed its descent to less than 1,000 mph in just a few minutes, the lander deployed a parachute and reduced its speed to 200 mph. Just before the lander reached the surface, retro rockets slowed it almost to a stop. Seventeen-foot-high airbags inflated and the craft bounced to a halt in massive Gusev crater.