Every 26 months or so, Earth uses its inside track around the sun to lap its slower-circling neighbor Mars. At that point, the two planets are at their closest, and the amount of energy required to fly from one to the other is tantalizingly modest by interplanetary standards.

Since 1996, NASA has taken advantage of every one of those biennial opportunities to launch spacecraft to Mars, and last week was no exception. On Friday, with Earth coming up fast on the Red Planet's heels, NASA launched the Mars Reconnaissance Orbiter (MRO), the latest and most sophisticated package of scientific instruments ever tossed toward the war-god planet.

The MRO is designed to orbit Mars for as long as eight years. Its prime mission is to help determine whether life has ever existed there and whether, perchance, any kind of life resides there today in some of the planet's less hostile niches.

It will also look for landing spots for future spacecraft -- including a very special lander that NASA hopes will someday carry life-forms of its own, namely earthlings.

The SUV-size spacecraft rode atop a 19-story, candlestick-thin Atlas-Centaur rocket that crackled skyward at 7:43 a.m. Eastern time. (Speaking of SUVs, the Atlas guzzled 200 tons of fuel and oxygen in just over four minutes.)

About an hour later, already halfway around Earth and more than 100 miles high, the two-ton MRO blasted free of its launch vehicle and, like an awakening bat, unfolded its two 20-foot-long solar panels. They will power the craft during its seven-month, 310 million-mile journey to Mars, and for years to follow.

As it approaches its destination in early March, a 30-minute rocket burn will slow the $450 million craft, allowing it to be captured by Mars's gravitational field. Once snagged into orbit -- where it will join the three other satellites now circling the planet -- the craft will begin a series of "aerobraking" maneuvers, dipping briefly and repeatedly into the thin Martian atmosphere like a skipping stone.

That will gradually slow the craft, dropping it into lower orbits. By November 2006, the MRO will be the lowest orbiting satellite ever sent to Mars, regularly passing within 158 miles of the surface.

From that radically low vantage, MRO will begin its two-year scientific mission -- one that scientists expect will teach them a lot about our own planet by providing new insights into how Earth was formed, how life may have evolved here and how natural forces can contribute to climate change.

One thing is central to all those issues, both on Earth and on Mars: water. So it should come as no surprise that NASA's chief scientist for Mars exploration, Michael Meyer, sounded a little like Deep Throat last week when he summarized MRO's MO: "Follow the water," he told a Washington Post reporter, speaking by phone from an undisclosed location that probably was not a parking garage.

Recent Mars missions have offered compelling evidence that Mars once had substantial amounts of water, and there are clues that it may yet have large reservoirs, albeit underground or beneath frozen caps. Those clues, in conjunction with recent revelations that some earthbound organisms can thrive in environments far more inhospitable than science had thought capable of supporting life, lead many scientists to suspect that the Red Planet may be home to at least some relatively simple microbes.

To look for the water that would support that life, or for evidence of its existence in the past, the MRO carries six instruments.

* A high-resolution camera, the most powerful ever built for imaging another planet, will look at landforms such as gullies, dunes and boulders as small as three feet in diameter. It will help scientists determine whether surface channels were cut by wind, liquid water, glacial ice or lava.

* An imaging spectrometer, which can differentiate among 544 wavelengths of energy reflected from the ground, will be able to identify many minerals on the ground, each of which has its own reflective "fingerprint." Scientists will look especially for minerals typically formed by water-related processes, including clays, carbonates and salts, whose presence could indicate that ponds or hot springs were once there.

* A context camera will take panoramic black-and-white images of the planet's surface in 19-mile swaths to identify features worthy of closer inspection by other instruments and to answer questions about how the Martian surface has evolved over geologic time.

* A Mars color imager, whose extremely wide-angle lens can shoot pictures stretching from one horizon to the other, will produce daily weather maps of the entire planet and track seasonal changes on the ground and in the Martian atmosphere.

* A Mars "climate sounder" will use a pair of infrared telescopes to track how water vapor, ice and dust in the atmosphere move and vary from day to night and through the seasons. Instead of simply looking straight down, as previous Mars orbiters have, it also will look sideways to the Martian horizon, to analyze three-mile-thick layers of the atmosphere from ground level up to an altitude of 50 miles. Among other things, that layer-cake view will allow a detailed analysis of how gases such as carbon dioxide are exchanged between the atmosphere and the planet's ice caps, providing data crucial to understanding climate on both Mars and Earth.

* A subsurface radar device will beam radio waves as deep as half a mile beneath the Martian surface, then analyze the reflected energy to distinguish layers of rock, water and ice. With its ability to resolve layers of material just 33 feet thick, it may finally answer the question of whether the surface ice detected by previous spacecraft is just the tip of a Martian iceberg.

Information from these instruments will be transmitted to an Earth-based network of giant receiving antennas in California, Spain and Australia at speeds 10 times what was possible in previous Mars missions. A total of at least 26 terabits of information is expected -- more than every other Mars mission combined.

Among the images that may turn up: A picture of the Mars Polar Lander, which lost contact with Earth as it tried to make a soft landing in 1999.

Any clues as to why that crash occurred -- an image revealing whether the parachute opened, for example -- could be useful, scientists said, especially because the design of the Phoenix Mars Scout lander, scheduled to launch in 2007, is closely derived from that of the ill-fated polar lander.