For any scientific project whose objective is the exploration of the surface of Mars, the riskiest phase of the mission is the short time period between entry into the Martian atmosphere and touchdown on the surface. Many different techniques for implementing this part of the mission are theoretically feasible. Most recent missions, however, have employed similar approaches, using an aeroshell with an ablative heat shield during hypersonic atmospheric entry, one or more parachutes to reduce the velocity with respect to the ground after the aeroshell has been jettisoned and a small rocket propulsion system for terminal guidance and control after the attachment to the parachute has been severed. Each of these separate deceleration systems must work properly, of course, if the landing capsule, which is nested inside the package descending through the Martian atmosphere, is to have a chance to implement its scientific mission.

Mars offers a particularly difficult problem to the decelerator designer because it has a very thin atmosphere. Its surface pressure at the equivalent of mean sea level averages less than 1 percent of the same pressure on the Earth. Thus there is simply not enough air to slow the spacecraft easily. The combination of deceleration techniques, taken together, have only five or six minutes to make certain the vehicle velocity is reduced from the 10,000 miles per hour at entry to 25 miles (40 kilometers) per hour (for an airbag or other robust landing system) or the 10 miles (16 kilometers) per hour level required by a soft lander like Viking.

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Finding firm footing

Unfortunately, our lack of detailed knowledge of the characteristics of the Martian surface on the scale of a landing vehicle makes it impossible, no matter how well designed the landing capsule is, to guarantee a successful landing. What we do know about the surface of Mars comes from the images transmitted by the two Vikings and Pathfinder -- which show detail of just three specific locations on Mars -- and from remote observations, mostly taken from spacecraft orbiting our neighbor planet.

It is from these global observations that scientists develop models describing the large-scale processes at work on Mars. Based upon the observations and the models, the scientists also try to infer information about the nature of the surface on a much smaller scale. However, the resolution that can be obtained from orbital vehicles is simply not sufficient to allow foolproof extrapolation down to the scale of a landing vehicle. Geologists and other planetary scientists estimate what a proposed landing site area will look like on the scale of our backyards on Earth, but these estimations are at best only educated guesses.

To be able to extrapolate consistently and accurately from remote sensing data to surface characteristics the size of a landing vehicle requires a thorough understanding of the complex processes at work, and that understanding is still decades away for a planet as varied and puzzling as Mars.

Engineers can design a spacecraft that can tolerate small- to medium-sized rocks and reasonable slopes of 20 degrees or so. But it is virtually impossible to land safely in a region riddled with large and/or sharp boulders or crisscrossed by narrow, steep chasms. Can the scientists looking at orbital pictures and other remote sensing data state, unequivocally, that a possible landing site area is free of large boulders or chasms? They cannot. At least not yet.

In short, until we have many, many more landings on Mars, or some kind of survey vehicle that is able to compile a catalogue relating observations from orbit to the characteristics of the surface at a small scale, landing on Mars will continue to be fraught with uncertainty.

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Storm clouds of Mars

Compounding the problem of landing safely on Mars are two other lesser, but related issues. The Martian atmosphere is very dynamic. Complex weather systems move around the planet as a function of the seasons just as they do on Earth. Global dust storms, in which raging winds lift dust from the surface to altitudes of 30,000 to 40,000 feet (9,150 to 12,200 meters), can completely obscure the surface. The first Martian orbiter, Mariner 9 in 1971, arrived when such a dust storm circled the Red Planet. These storms contain winds well outside the tolerance range of any landing system. The Mars Global Surveyor and other orbiting spacecraft have also observed regional storms with wind velocities that would wreak havoc with a vehicle trying to land on Mars.

Scientists believe they understand, in principle, the mechanisms creating these Martian storms, and can therefore describe the most likely times and places for storm occurrence. But weather forecasting on Mars is not yet a precise science, and even a moderate unscheduled storm during the descent of a landing vehicle would be an untoward event that would push the spacecraft design margins to the limit.

Hitting the target

Navigational inaccuracies make both the unknown surface detail and weather problems that much more difficult.

Until such time as Mars-based tracking data is available, either from satellites permanently in orbit around Mars or fast-processed optical data relating the incoming spacecraft to the planet or its moons, our knowledge of where a Mars-bound spacecraft is located will continue to have significant uncertainties. These possible errors map into variations in the entry flight path angle of the spacecraft containing the lander, and then into a scatter around the desired landing site. Thus the landing system must be designed to accommodate a safe touchdown anywhere in a wide region that may, today, be a hundred miles across.

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There are basically two trajectory modes possible for accomplishing a landing on Mars. In the direct mode, used by Pathfinder and the two Mars Exploration Rover (MER) spacecraft to be flown in 2003, the entry and landing vehicles are essentially launched from Earth on to a trajectory that will eventually impact Mars. For the direct mode, therefore, the time of arrival and landing at Mars is tightly constrained as soon as launch occurs.

One way to vary the landing time -- thereby avoiding any temporary dust storms around the landing site -- would be to first place the lander and its carrier vehicle in orbit around Mars. This orbital mode, which was employed by Viking, not only allows the operations team to select the landing time, but also permits the navigators to obtain a good fix on the location of the spacecraft with respect to Mars. However, entry from orbit requires a propulsion and/or aerocapture system to insert the spacecraft into Mars orbit, and this orbit insertion requirement exacerbates the already difficult problem of finding a launch vehicle powerful enough to launch the entire system toward Mars.

Human ingenuity...and human error

New technologies now under development should reduce the risk of landing on Mars by the end of this decade.

Landing vehicles of the future will be equipped with hazard avoidance sensors and algorithms, for example, which will steer the spacecraft away from large boulders and chasms during the terminal descent. Precision landing, guaranteeing a touchdown within only a few miles of a specified landmark on the Martian surface, will be enabled by improved navigation techniques. These techniques, including aeromaneuvering during descent and the processing of data transmitted between the spacecraft approaching Mars and a permanent telecommunications orbiter around the Red Planet, should be available prior to the first Mars Sample Return mission, currently scheduled for launch in 2011.

Precision landing will also significantly enhance the scientific return of a Mars surface mission. At present, Mars landing site specialists are constrained to select sites that are predicted to be benign (that is, comparatively free of landing hazards) over a large region, since the "footprint" of possible landings is a hundred miles or so across. Such sites may or may not be of prime scientific interest. In the future, the scientists will be able to choose more intriguing Martian landing sites without having to worry about possibly dangerous surface conditions tens of miles away from the selected target.

The challenge of landing safely on Mars excites the engineering imagination. How should the system design accommodate the range of uncertainties in the Martian surface characteristics and the Martian atmosphere? No matter what the engineers do, they cannot be 100-percent certain that the mission will succeed.

Mars itself is the culprit. Our current knowledge of our neighbor planet is simply not good enough to preclude possible failures caused by the environment.

Therefore, to maximize the probability of achieving successful robotic landings on Mars during the next decade, engineers must redouble their efforts to remove, as far as possible, the element of "human error" from the reliability equation.

Cautious, prudent engineering practices dedicated to mission success, including vigilant risk management and extensive testing of both the space hardware and the operations system, are essential if near-term Mars surface missions are going to have a high probability of landing safely.