Two decades after the twin Viking orbiters wowed scientists with images of Mars' gargantuan volcanoes and canyons, a new robotic envoy is ready to lift the veil on the red planet's mysterious past.
Following a one-year delay, NASA's Mars Global Surveyor began mapping the dusty, frozen planet this month with a camera keen enough to detect objects the size of a Volkswagen beetle.
Swooping from pole to pole in a 240-mile-high orbit, Surveyor also packs an array of instruments that will search for prime landing sites for future landers and may explain what turned Mars' once-warm, water-harboring surface into a frigid, bone-dry realm of shifting dunes.
At the Jet Propulsion Laboratory in Pasadena, Calif., anticipation is running high among mission scientists, most of whom have worked 15 years to put the orbiter in its lofty perch above Mars.
"This mission will tell us more about Mars than all of the previous missions combined. It's been a long time coming and of course we're very eager," said Glenn E. Cunningham, deputy director of the Mars Exploration Program at JPL.
"This sets the stage for everything that follows," he added.
Launched in November 1996, Mars Global Surveyor reached Martian orbit in September 1997, two months after Mars Pathfinder landed on the surface and deployed a rover, grabbing headlines worldwide. But to most planetary scientists, it was the arrival of Surveyor -- not Pathfinder -- that had been eagerly awaited.
For a full 687-day Martian year, Surveyor would scan Mars as it turned underneath, keeping watch over its shifting weather patterns and waxing and waning polar caps. The 2,300-pound orbiter would snap wide- and narrow-angle photographs for a global mosaic at least 20 times sharper than the one produced by the Viking orbiters in the late 1970s.
Meanwhile, the non-optical instruments would peer down, shedding light on Mars' Earth-like past by collecting information about its weak magnetic field and the distribution of its surface minerals.
And there was always the chance of stumbling across the unexpected -- extinct hot springs or lingering volcanic hot spots.
As it turned out, Mars wasn't giving up its secrets so easily.
Surveyor arrived in a highly elliptical orbit that would be trimmed to a circular, two-hour mapping orbit by skimming it across Mars' upper atmosphere -- a process called aerobraking. But a solar panel began wobbling shortly after aerobraking began, forcing mission controllers to pull the spacecraft out of the atmosphere.
They soon learned the solar panel had been damaged during its deployment. To reduce the risk of the panel shearing off, Surveyor would have to be slowly eased into its circular orbit, delaying the start of the mapping mission from March 1998 until March 1999.
Surveyor scientists were disappointed but relieved the probe hadn't met the same fate of its bigger, more expensive predecessor.
The $1 billion Mars Observer was only three days away from reaching Mars when it suddenly fell silent in August 1993, leaving Earth-bound scientists under a cloud of gloom.
The mission, loaded with more instruments than Mars Global Surveyor, had been the most ambitious ever sent to Mars. Investigators concluded that a broken fuel line probably sent it spinning into space.
"It was a crushing blow to these guys. It really was," Cunningham recalled. "Some of these people's whole careers were based on that mission. We had psychological advisers come in to help. Everyone had been so fired up about the mission -- it was a big psychological blow."
After that costly disaster, NASA raced to construct a cheaper follow-up mission. Plans for the $250 million Surveyor, made from Mars Observer's spare parts, soon emerged.
Though less sophisticated than Mars Observer, Surveyor is expected to accomplish most of its mission goals.
Even before mapping began March 9, its camera had used the long aerobraking mission to snap thousands of photographs, some taken from as close as 100 miles -- 140 miles closer than the eventual mapping orbit.
Those images showed extensive layering in the walls of the 2,400-mile-long Valles Marineris canyon. Scientists believe that's evidence volcanoes spewed 10 times more lava onto Mars during its first 3 billion years than had been previously believed.
There were also signs of eruptions as recent as 40 million years ago and tantalizing photos of the rust-colored planet's meandering river valleys and dune fields.
And when Surveyor passed over the spot where Viking I photographed a gape-mouthed "Mars Face" some earthlings saw as evidence of a Martian civilization, its superior optics revealed it as nothing but a bumpy hill that had fallen under the spell of late afternoon shadows.
Now that mapping is under way, Surveyor's camera is unmasking more compelling surface features.
The Viking orbiters snapped about 55,000 images, most showing objects only as small as 150 feet. But Surveyor can see objects smaller than 10 feet across, said Michael Malin, the mission's top camera scientist.
"We're seeing individual rocks the size of cars, something we've never seen before. The most exciting thing is right now we're seeing things we can't explain, geological landforms that challenge us," said Malin.
He and his team are comparing the newly acquired images to the best set of Viking photos to see whether anything has changed -- a landslide, a missing sand dune, or a new meteor impact crater.
Meanwhile, Surveyor's laser altimeter is measuring the highs and lows of Mars' polar caps, volcanoes and gorges; its thermal emission spectrometer is mapping the planet's surface minerals. That map may show shorelines of ancient lakes or evidence of extinct hot springs where Martian life -- if it ever existed -- could have thrived.
Or maybe it will answer the main question about Mars: what happened to all the water scientists believe carved its gorges and valleys. Is it frozen underground, or did it evaporate into space?
"It's absolutely the big mystery and it's closely connected with the question of life. Where did all the water go?" said Dr. Arden Albee, the Surveyor's project scientist at the California Institute of Technology in Pasadena, Calif.
But Surveyor has one final test to pass before it can live up to its promise. The orbiter's high-gain antenna, mounted on the end of a 6-foot boom, remains in its stowed position, awaiting a signal from Earth to deploy it.
Until then, Surveyor must collect data on its solid state recorder and then turn back toward Earth to send that information back.
Mission scientists have delayed releasing the spring-activated boom because it's fitted with the same type of damper -- or cushioning device -- that apparently failed when Surveyor's solar panels were deployed.
Tests on system replicas at JPL suggest that chances the boom might snap off are slim. Mission controllers intend to send the deployment commands March 29, but are using the first three weeks of mapping to photograph Mars' most interesting areas -- just in case.