Sojourner taking an Alpha Proton X-ray Spectrometer(APXS) measurement of Yogi.

On Friday, July 4, 1997, American flags dressed the nation in a giant Independence Day celebration. It was National Hot Dog Month, and an estimated 155 million hot dogs hit the grill that weekend alone. Space must have been on moviegoers minds, as the alien flick "Men in Black" took in a whopping $84 million during its holiday opening.

How appropriate then that 192 million kilometers (119 million miles) away from Earth, there was even more to celebrate: NASA's Mars Pathfinder mission had completed its seven-month journey by bouncing to a landing on Mars and opening up a whole new world of Mars exploration. The landing was a tremendous event at JPL, where mission controllers cheered, clapped and even shed tears over their success.

Now known as the Sagan Memorial Station, the Mars Pathfinder lander, along with the moving vehicle Sojourner, brought Americans together to share new and exciting information about our mysterious neighboring planet. Initially sent to demonstrate new and inexpensive technology, the spacecraft ended up literally "finding a path" for a parade of Mars missions that will follow in the next decade.

Making a Big Entrance

Sojourner, after the placing its APXS in the dark soil surrounding the rock named Lamb.

"The dominant thing that Pathfinder was about was an entry, descent and landing demonstration and a development of technology that could be used," said Mars Pathfinder Project Scientist Matt Golombek. "That's really come through here, now, in everything, so that's the big deal."

Pathfinder's robust, inexpensive airbag landing system used a ballistic approach to land on Mars. Without orbiting first, the space probe simply dropped everything and landed. In 2003, when the first of two Mars Exploration Rovers makes the same journey, it will be equipped with an updated version of this revolutionary landing mechanism. Instead of deflating upon initial impact like a car's airbag system would, the spacecraft's sealed airbags stay inflated for every hit. They then bounce about 1 kilometer (.62 miles) across Mars' rocky surface like a superball in slow motion, eventually losing energy through friction.

Engineers have changed a lot about the landing system for the Mars Exploration Rover, but the overall architecture is basically the same. The new lander is about 150 kilograms (402 pounds) heavier than Pathfinder's, and its larger structure allows for a bigger rover. The airbags now have more abrasion resistance, additional layers and back-up equipment. Still, even though the landing system was successful on Pathfinder, it isn't guaranteed a repeat performance.

"The more we learn about Mars, the harder it seems to get there," said Rob Manning, flight systems chief engineer on Mars Pathfinder and systems engineering manager for the Mars Exploration Rover. "We can, however, make the odds very much in our favor."