Front and center is the flight spare for the first Mars rover, Sojourner, which landed on Mars in 1997 as part of the Mars Pathfinder Project. On the left is a Mars Exploration Rover Project test rover that is a working sibling to Spirit and Opportunity, which landed on Mars in 2004. On the right is a Mars Science Laboratory test rover the size of that project's Mars rover, Curiosity.
In the 20 years since Pathfinder's touchdown, eight other NASA landers and orbiters have arrived successfully, and not a day has passed without the United States having at least one active robot on Mars or in orbit around Mars.
As the Mars Pathfinder spacecraft approached its destination on July 4, 1997, no NASA mission had successfully reached the Red Planet in more than 20 years.
The momentum propelled by Pathfinder's success is still growing. Five NASA robots and three from other nations are currently examining Mars. The two decades since Pathfinder's landing have taken us about halfway from the first Mars rover to the first astronaut boot-print on Mars, proposed for the 2030s.
Pathfinder's rover, named Sojourner for the civil-rights crusader Sojourner Truth, became the best-known example of the many new technologies developed for the mission. Though Sojourner was only the size of a microwave oven, its six-wheel mobility system and its portable instrument for checking the composition of rocks and soil were the foundation for the expanded size and capabilities of later Mars rovers.
NASA planned Pathfinder primarily as a technology demonstration mission, but it also harvested new knowledge about Mars, from the planet's iron core to its atmosphere, and from its wetter and warmer past to its arid modern climate.
"We needed to invent or re-invent 25 technologies for this mission in less than three years, and we knew that if we blew the cost cap, the mission would be cancelled," said JPL's Brian Muirhead, flight system manager and deputy project manager for Pathfinder. Crucial new technologies included an advanced onboard computer, the rover and its deployment system, solid-fuel rockets for deceleration, and airbags inflating just before touchdown to cushion the impact of landing. NASA re-used most of the Pathfinder technologies to carry out the Mars Exploration Rover Project, which landed Spirit and Opportunity on Mars in 2004.
The lander and rover operated for three months -- triple the planned mission for the lander and 12 times the rover's planned mission of one week. This longevity enabled Pathfinder to overlap the Sept. 12, 1997, arrival of NASA's Mars Global Surveyor orbiter. That orbiter, in turn, operated at Mars for more than nine years, overlapping with arrivals of two later orbiters -- Mars Odyssey in 2001 and Mars Reconnaissance Orbiter in 2006, which are both still active -- and the 2004 landings of two rovers, one of which -- Opportunity -- is still active. Subsequent successful NASA missions of the post-Pathfinder era have been the Phoenix lander, Curiosity rover and MAVEN orbiter.
Examples of what has been learned since the day Pathfinder landed include:
-- Early Mars offered watery environmental conditions with all the chemical ingredients needed for life and with a chemical energy source of a type used by many microbes on Earth.
-- Diverse types of wet environments that may have been favorable for microbes existed in many locations on early Mars, including lakes, rivers, hot springs and underground water.
-- Mars lost much of its original atmosphere early in the planet's history.
-- Modern Mars holds vast reservoirs of frozen water accessible underground at middle and high latitudes, a valuable resource for future human explorers. Many fresh meteor impacts, documented by before-and-after images, have revealed ice an arm's reach or so beneath the surface.
-- Active processes on modern Mars include avalanches, dust storms, fresh gullies, frozen carbon-dioxide snow, sand-dune movement, geysers from seasonal thawing of carbon dioxide, meteor impacts, and streaks that extend seasonally down some slopes and may be related to possible presence of brine.
-- Layers in the polar caps record climate changes on the scale of thousands to millions of years related to cyclical changes in the planet's tilt. There is enough carbon-dioxide ice buried at the poles that, if released, could more than double the planet's present atmosphere.