The Mars Pathfinder mission was supposed to prove the viability of unmanned exploration of the red planet. Comprised of a lander module and a rover module, the objective was to make it through Mars' atmosphere, land safely on the surface, and set free a robot to roam the planet. All this would be accomplished in an efficient and cost-effective way. The rest -- and there was quite a bit more -- was gravy.
Pathfinder left Earth in December 1996, traveled the 309 million miles to Mars, and landed in July 1997 [source: Space Today]. No previous spacecraft had landed on a planet without first orbiting it. The landing gear consisted of a parachute and a series of airbags; the module landed on a bed of rocks unharmed, and the rover took off. The mission was a success. Not only did Mars Pathfinder return a total of 2.3 billion bits of data back to NASA (more than 17,000 photos among that mass of information), but it outlived its projected life [source: NASA]. The lander was supposed to remain in working order, recording data and images, for about three months; it kept sending information for a year. The rover had a projected lifespan of several days. It ended up roaming Mars for a month [sources: Space Today, NASA].
Among other things, the Mars Pathfinder taught us that Mars was probably once warm, wet, and far more friendly, in human-survival terms, than it is now.
Mars Pathfinder taught us that Mars exploration is possible. Someday, NASA may even get a human there. But without the next achievement on our list, astronauts wouldn't be going anywhere at all.