But those miserly 1.5 kilowatt hours a day will have to go a long way to enable NASA to extract all the science it hopes to from the estimated $203 million mission.

That amount will have to suffice for everything on the polar lander during its 60- to 90-day mission, including its robotic arm, meteorological and other science instruments, cameras and ability to relay all collected data back to Earth.

Indeed, if there is one issue that worries NASA most when planning a mission to land on Mars, its power.

"Ultimately, its always power," said Dias, who worked on Mars Pathfinder and now on the 2001, 03 and 05 missions to Mars, but not Polar Lander.

"Youre so used to just flipping on the switch in the hall, the average person doesnt realize how precious it is."

The Polar Lander uses its solar arrays to either power its various components directly or to charge up its battery, which sustains it during periods of little or no sun.

Since the Mars Polar Lander will arrive in the southern hemisphere in spring, it will bask in the sun -- albeit a sun that is low in the sky -- 24 hours a day.

As a consequence, the landers solar arrays can provide on average only about 61 watts of power over the course of the mission, said Ben Clark, spacecraft manufacturer Lockheed Martin Astronautics chief scientist for the mission. If needed, however, the spacecraft can draw as much as 180 watts from both the arrays and the battery at once.

The largest chunk, about half, of the daily amount of energy generated and stored in the spacecraft's battery goes to keeping it and its electronics warm.

"Managing power is just the biggest deal ever," said Donna Shirley, former manager of JPLs Mars Exploration Program. "In fact, that is what killed Pathfinder."

It is likely to kill Polar Lander, too, because no matter how thrifty a martian spacecraft is, it still has to deal with Mars.

Temperatures on the surface of the planet vary greatly during each Martian day, or sol, often plummeting at night to minus 126 Fahrenheit (minus 88 Celsius).

"Its like tossing your portable computer into the deep freezer and telling it, "Ill be back tomorrow," Clark said.

The colder it is at "night," (that is, when the sun is lowest in the south-polar sky) the more electricity is needed to keep the spacecrafts components above a temperature of minus 40 Fahrenheit (minus 40 Celsius). However, colder nights come when the days are colder as well, with less available sunlight.

So as the batteries are drained more and more at night, they are recharged less and less during the day.

"Eventually you freeze to death because you get the double whammy," Clark said.

By missions end, Polar Lander will be able to generate on a daily basis only about half of the electricity it will have at the start of the mission.

Exactly how much electricity the Polar Lander can generate, however, wont be known until it gets to Mars. The efficiency of its solar arrays depends on a variety of factors: the slope on which the spacecraft lands, the ambient temperature, and the amount of dust that will eventually settle on the solar arrays. On Pathfinder, engineers found the steady accumulation of dust lessened the amount of electricity its solar arrays could generate by 0.3 percent each day.

To save on power, engineers design ever-more efficient components and instruments. When it comes to communications, enormous power savings can be achieved, especially in dealing with how the spacecraft relays its data to Earth.

For example, it will require about 60 watts for the Polar Lander to transmit directly to Earth. Thats about four times what would have been needed if it could have used the ill-fated Mars Climate Orbiter as a relay satellite.

Rather than rely solely on the "green" power of the sun, past Mars missions -- including the two Viking landers -- have used nuclear material to produce electricity and heat. Of the two landers, which both set down in 1976, one remained operational until 1980, the other 1983.

But in recent years, NASAs use of radioisotopes on spacecraft has been the subject of controversy. Activists have protested, fearing that a launch accident would release toxic plutonium into the atmosphere. Radioisotopes were strictly ruled out on the Mars Polar Lander mission, Clark said.

Ultimately, the lander's operational lifetime will depend on how well it withstands increasingly harsh conditions in the martian south polar region with the arrival of fall in the southern hemisphere in early 2000. Near the end of the mission, daily temperatures are expected to range between -22 Fahrenheit (-30 Celsius) and -130 F (-90 C). There is a slim chance that even if the lander becomes inoperative during the martian fall, it may survive the martian winter and be revived in 2001.