What a long, short trip it's been.
And what a triumph, scientists say of the epic yet tiny travels of the NASA's two Mars robotic rovers as they ferret out geological secrets of the neighbor that has for so long obsessed earthlings.
Opportunity has trundled 7 miles on the Red Planet's surface, while Spirit has racked up just 4.4 miles. Yet those are fantastic ramblings through unforgiving terrain, ferocious dust storms, and wild temperature fluctuations -- especially given that the rovers were originally meant to carry out exacting explorations measured in meters or yards, not miles.
Each rover landed on Mars 3 1/2 years ago with a "warranty life" of just 90 days. Spirit, which touched down three weeks before its twin, is now at 1,248 days, while Opportunity is humming along at 1,228 days. And those are Martian days, called "sols," about 40 minutes longer than Earth's.
"The mission has accomplished scientific objectives far beyond anything expected," said Richard P. Binzel, professor of planetary sciences at the Massachusetts Institute of Technology.
"Plus, it's been a thrill," said Binzel, who has no affiliation with the NASA mission. "The rovers have allowed the whole world to take a road trip on Mars. It's been amazing, and the ride isn't over. The rovers remain on a roll."
In their tightly choreographed forays, Spirit and Opportunity have taken exquisite photographs, performed light wave analysis to identify rock and sediments, measured soil and air temperatures, and provided magnifications that have helped scientists gain a deeper understanding of the geological composition and history of Mars.
"The rovers have served as robotic field geologists," said Steven W. Squyres, professor of astronomy at Cornell University and principal investigator for the rover mission. "They've gone to work with cameras, spectrometers, and [rock] grinders. It's the next best thing to having humans there."
In their key discovery, the one that makes the whole mission worthwhile in the minds of many scientists, the solar-powered rovers have found, in Binzel's words, "on-the-ground evidence that Mars had a wet early history."
Water was once present and perhaps even abundant on Mars. Opportunity and Spirit, working on opposite sides of the planet, have both found signs of past surface and ground water in scrutiny of rock and soil. That means life was -- and is -- at least theoretically possible on Mars, though neither rover has found traces of it.
Proof of such life -- perhaps in the form of primitive micro-organisms, fossilized or frozen -- may be found by the next generation of Mars voyages. These new missions have won funding at least partly on the strength of the rovers' success.
Next month, for example, the Phoenix mission is set to lift off from Cape Canaveral Air Force Station in Florida. The lander vehicle, equipped with a scooper arm and tiny chemical laboratory, is designed to dig for preserved organic material from ice near the Martian north pole. "The [bare rock and soil] surface of the planet has been bleached clean by ultraviolet radiation and superoxides," said Chris McKay, a NASA planetary scientist who will serve as biological interpreter for the mission. "To find microbes, frozen or dead, we need to go to the ice."
An even more ambitious mission -- dubbed the Mars Science Laboratory -- is scheduled to launch in 2009. The plutonium-powered robot vehicle is basically a heftier version of the rovers, equipped with an array of more sophisticated instruments and scanners. It will analyze samples dug from the Martian soil and drilled from rocks, again with the main purpose of scouting out a sign of past or present life.
As the Mars planners plan and the Mars dreamers dream, Spirit and Opportunity -- in defiance of all odds and expectations -- continue their slow-motion peregrinations of the fourth planet from the sun. After traveling millions of miles from Earth in separate space vessels, the rovers landed on Jan. 4 and Jan. 25, 2004, according to NASA.
The rover mission has cost more than $900 million so far, and is continuing at a cost of roughly $22 million a year. The mission is in its fourth extension, and NASA officials are seeking to extend operations at least through the end of October 2008.
Spirit is currently examining a series of hills within the immense Gusev Crater. One of its six wheels is broken, so the robotic vehicle's movement is a bit creaky as its cameras and other sensors scrutinize a silica-rich outcropping that may have been formed by water. As long as it remains on relatively flat terrain, the broken wheel is not a serious hindrance to mobility.
Opportunity, meanwhile, is poised for a perilous descent into Victoria Crater, a 200-foot deep, half-mile-wide hole that most likely was formed by the impact of a meteor billions of years ago. The descent is considered high risk because the slope is steep enough that a broken wheel or other mechanical failure might leave Opportunity stranded forever in the pit.
"We're definitely not planning a 'Thelma and Louise,' " said Squyres, referring to the movie that ends with the lead characters hurtling off a precipice toward certain doom. "The hope is to have Opportunity have a look around and then get back to business on the Meridiani Planum," the plain that is the vehicle's area of operation.
The adventurous descent is worth it, scientists said, because rock strata within the crater could offer extraordinary insights into the planet's past. And the time is right because the clock is ticking down on the rover vehicles -- sooner or later they will succumb to wear and tear in the brutal Martian environment.
"It's a calculated risk, worth taking because this mission has already far exceeded its original goals," said S. Alan Stern, associate administrator for NASA's space mission directorate. "The potential rewards outweigh the risks."
Scientists are especially intrigued by a "bathtub ring" of bright rocks that circles the crater about 10 feet below the rim. They believe it might contain a chemical signature of Mars's ancient climate -- at some point in its past, the planet may have had a richer atmosphere.
"The crater is like a window back into the ancient Martian past," said Squyres. "These layers of rock are a storybook of time."
For now, Opportunity is hunkered on the edge of Victoria Crater, sitting out a mammoth dust storm. Since the dust blocks sunlight, the robotic vehicle needs to conserve power. When the storm has passed, Opportunity's solar collecting panels, which provide power to the vehicle, should be almost like new -- scrubbed clean by the winds that serve as a sort of "Martian car wash," as Squyres put it.
"These vehicles aren't as frisky as they once were, but they've proven to be amazingly durable," said Squyres.
"They're holding up to the stress and strains better than some scientists. It's been a long ride, but there's no thrill like the thrill of discovery."
Information on NASA's Mars Rovers mission can be found at marsrovers.nasa.gov/mission/.