As a flotilla of Mars-bound probes nears their target, scientists and engineers have begun work on the Mars Phoenix lander, the flagship spacecraft for NASA's Scout line of innovative and econo-class Red Planet explorers.

A follow-on lander was being tested to fly as part of the 2001 Mars Surveyor program, but this work was halted after the failure. That gear was stored in a clean room at Lockheed Martin in Denver in 2000. Phoenix, in effect, was created from the embers of previous missions.

In August of this year, NASA picked Phoenix over other competitive concepts, giving it the go-ahead as the first of a hoped for line of relatively low-cost Mars Scout missions. It will be launched in August of 2007, headed for a still to be selected high northern latitude locale on Mars. Once on duty, the robot is to characterize the landing zone's cache of ice, soil, rock, as well as study the local atmosphere using state-of-the-art methods.

A rich and diverse scientific payload has been chosen from the instruments selected through the NASA review process for the Mars Polar Lander and the shelved Mars Surveyor 2001 mission. The Scout spacecraft and science payload is well matched to carry out an up-close inspection of Martian arctic soils for clues to geologic history and potential as a habitat for biology.

The Phoenix lander brings to that distant world an acronym-rich set of top-notch science gear:

• Mars Descent Imager (MARDI): Acquires numbers of images of the geology surrounding the landing site prior to touchdown
• Stereo Imager (SSI): Surveys the scene at the landing site, and looks in on digging operations from its location several feet above the action
• Robot Arm (RA) and Camera (RAC): Digs and acquires samples for analysis while using up-close camera to inspect specimens.
• Thermal Evolved Gas Analyzer (TEGA): Heats and makes assessment of soil samples taken from various subsurface depths
• Microscopy, Electrochemistry, and Conductivity Analyzer (MECA): Makes microscopic and chemical analyses of soil particles
• Meteorology Suite (MET): Provides data for characterizing Mars' present climate and weather processes at the landing site

"We've ramped up and things are underway," explained Peter Smith of the University of Arizona's Lunar and Planetary Laboratory in Tucson and head of the Phoenix mission. NASA's Jet Propulsion Laboratory in Pasadena is managing the Phoenix project. Lockheed Martin in Denver is building and testing the spacecraft. In addition, the Canadian Space Agency is contributing a meteorological package to study polar climate.

The $325 million total mission cost cap award to the University of Arizona (UA) is more than six times larger than any other single research grant in UA's history. The Phoenix mission Science Operations Center will be located in Tucson. Two Phoenix instruments will be built at the UA, so about $50 million will remain at the university.

"It's true that we have a head start in terms of having hardware done. However, this is the same spacecraft that crashed in 1999. So in order to make sure we've got a spacecraft that can land safely, we need to do a lot of extra testing not normally done on a program like this," Smith told SPACE.com.

There are some extra-added attractions that might be carried by Phoenix, Smith said, such as magnets to help understand properties of soil and dust. Also a microphone may be toted along to Mars. While there might not be much to hear, the craft's robotic arm should make noise during digging operations, he said.

That powerful robotic arm will muscle into Mars' surface, plowing down into the soil for some 3.3 feet (one-meter). The trench-digging could reveal a habitable zone that may exist in the ice-soil boundary - hitting, in a very real sense, "biological paydirt." The Phoenix will explore the biologic potential of the ice-soil boundary. In this regard, microbial colonies can survive in a dormant state for eons.

According to the now-orbiting Mars Odyssey, there may be as much as 80 percent water ice by volume within 20 inches (a half-meter) beneath the surface at Mars' north polar region. Part of the strong-arm tactics for Mars is imaging the big dig with a camera mounted on the arm itself.

"We have a plan to make sure that the forearm of the robotic arm, from the elbow joint down to the scoop, is totally sterile. This is important because we're trying to measure organics. If we bring them with us, we contaminate our measurements. Of course, when we deliver samples to our analysis instruments, we want to make sure that any part that touches the soil is sterile also," Smith said.

William Boynton, a co-investigator on the Phoenix mission at the University of Arizona is building one of the spacecraft's major instruments: the Thermal and Evolved-Gas Analyzer (TEGA).

Once on Mars, the Phoenix robotic arm will scoop up soil samples and deliver them for heating into TEGA's tiny ovens. Scientists can then measure how much water vapor and carbon dioxide gas are given off, how much water ice the samples contain, and what minerals are present that may have formed during a wetter, warmer climate on Mars. TEGA will also measure any organic volatiles.

The TEGA instrument, flown on the failed Mars Polar Lander, has been improved. Better control of the amount of heat added to the samples is now possible, Boynton said. Also the unit is now outfitted with a mass spectrometer, able to detect a wider range of compounds and carry out certain high precision measurements contrasted to the older TEGA lost to Mars nearly four years ago.

"We are now sensitive to the detection of any organic compounds. So far organic compounds have evadeddetection on Mars, but we may find them buried in the ice," Boynton said.

TEGA fits in well with the Mars exploration strategy, Boynton said. It provides a ground truth for the Mars Odyssey results that showed the presence of very large quantities of ice buried beneath the surface. Mars exploration strategy is all about following the water, and this the best, maybe only, readily accessible significant amount of water, he said.

"In addition, the ability to detect organic compounds will be very important if, indeed, we get a positive result. If so, it will be the place to go for future missions to look for evidence of past life," Boynton said.

Smith cautioned that finding a viable habitat for life subsurface doesn't mean there's anything living in the habitat.

"It's the first step in reinvestigating the life issue on Mars. We're going there to assess the habitability of this zone," Smith added. "As we dig under the surface, down deep, we'll be opening up a whole new region that nobody knows anything about at this point."

There is only so much that can be done from orbiting the planet, looking down on Mars with various instruments. "At some point you've got to go down there…get your hands in the dirt, get a little muddy and find out the truth," Smith said.

"Our real science happens beneath the surface," Smith explained. A "sweet spot" for Phoenix is soft-landing at a place where the Mars Odyssey orbiter sees a super high abundance of water ice near the surface, almost as much as in the polar cap.

The Phoenix project team has started looking at prospective landing spots. To date, however, images of that area are few in number. What perils await the robotic lander are not fully known at present. "It's pretty much unexplored territory," Smith noted.

Smith said that NASA's Mars Reconnaissance Orbiter (MRO), outfitted with a super-powerful camera, can help scope out a projected Phoenix landing area for literal pitfalls. A stream of MRO data from the red planet doesn't begin until the fall of 2006, about six months before Phoenix is headed for Mars.

"I think of MRO as our hazard avoidance system. If they start seeing hazards at our primary landing site, then we go to the backups," Smith said.

Touching down on Mars is tricky business.

For one, Phoenix is to aeromaneuver its way toward the selected landing zone. Borrowing from the olden days of NASA's Gemini and Apollo spacecraft designs, the Mars lander is to perform an aerial balancing act.

"What you do is build in a center of gravity offset in the entry system. That allows us to fly with the center of gravity of the vehicle through the velocity vector," said Ed Sedivy, Program Manager for the Phoenix at Lockheed Martin Space Systems in Denver. That means tipping the vehicle either up or down to fly through the center of gravity, thereby creating either lift or drag.

Due to the distance between Earth and Mars, and lag time in communications, this aeromaneuvering will be done by go-it-alone software smarts built into spacecraft, Sedivy told SPACE.com. A 12.4-mile (20-kilometer), hopefully hazard-free, bull's-eye is the target, he said.

Phoenix will be equipped with a robust communications system for use during the critical entry, descent, and landing phases. The spacecraft is to have direct radio link with Earth, as well as through a Mars orbiting craft, whether via the now-orbiting Odyssey spacecraft or the yet-to-be launched Mars Reconnaissance Orbiter remains to be seen.

There is no follow-the-bouncing airbags for Phoenix like those onboard NASA's Spirit and Opportunity Mars rovers, set to hit and run across the planet this coming January.

A dozen descent engines are to provide a gentle touchdown for the craft, making it the first soft-lander since the dual Viking robots set down on the planet in 1976, Sedivy said.

Hazard avoidance hardware for Phoenix is under discussion. A NASA review board, however, has recommended that the project "descope" the idea of such a system. Lockheed Martin engineers have put forward a plan to plug into the lander a "steer clear" sensor and software package.

Basically, Phoenix could then treat Mars like a planetary battleground. It would be on the lookout for craters, big boulders, and other nasty landscape. This high-tech aptitude draws from the company's tactical hardware experience that has already proven itself in the field, Sedivy said.

Nevertheless, still to be weighed is the risk to the overall project of using hazard avoidance gear. Could it cause more problems than what it's trying to circumvent? A go/no-go decision on including the hazard avoidance is forthcoming.

In putting together and checking out Phoenix pre-flight, Lockheed Martin engineers are driving down risk wherever possible. The experience of losing the Mars Polar Lander had an intense impact on the company. Several review boards looking into that failure criticized the firm for software issues, inadequate testing and other technical and management glitches. Those problems likely led to the Mars Polar Lander shutting off its engines far too early, high above Mars. That meant a crash down instead of a safe and serene touchdown.

In regards to Phoenix, "the bottom line is that failure is unacceptable," Sedivy said.To back that up, the company is building a "form, fit, and function" version of the Phoenix, outfitted with a set of titanium landing legs. Using specially devised ground equipment, the spacecraft's propulsion system, avionics, guidance and control software, deployment of landing legs, and other items will all be repeatedly wrung out prior to launch time.

"We've pretty much got a matrix of diabolical failures that we can throw at it," Sedivy said.

Furthermore, a Payload Interoperability Testbed -- better known as the PIT -- is being fabricated. Once installed at the University of Arizona in Tucson, engineering versions of the Phoenix science payloads can be tried out too.

"We'll try to break the system. When you learn where it is most vulnerable, you go fix those vulnerabilities and make yourself as bulletproof as possible," Sedivy concluded.

"Scientifically, any mission that offers access to the 'water of Mars' -- in whatever state it may be in -- is of major scientific import," said James Garvin, NASA Lead Scientist for Mars Exploration in the space agency's Office of Space Science in Washington, D.C.

"Phoenix will target what could be an ice-rich terrain and explore the water ice in the soil with a set of measurements that will prepare the way for next decade missions to intensify our search for 'the stuff of life' and for signs of 'habitats'", Garvin pointed out.

Garvin said that Phoenix offers a new opportunity to acquire data "directly germane to future planning and preparations for eventual human exploration."

Scientifically, Phoenix is a winner, with few weaknesses, Garvin said, "other than the usual agonies of landing sophisticated laboratories on the surface of Mars."