The Mars Exploration Rovers Spirit and Opportunity have exceeded NASA’s expectations by such a large margin that 2005 was just more icing on the cake as both rovers continue to return invaluable insights into Mars’ past.

Each spacecraft was expected to last 90 days. On Nov. 21, Spirit completed one martian year — some two Earth years — on Mars. Opportunity, reached that same milestone Dec. 12. Both found what they were sent to probe: signs of past water on Mars.

Spirit has now wheeled itself over 3 miles since landing within Gusev Crater. Opportunity’s odometer reads over 4 miles following its touchdown in Meridiani Planum on the opposite side of the planet. A high point in Spirit’s exploration was a yearlong climb through the Columbia Hills , the first robot explorer to ascend a mountain on another planet.

In late September, the rover reached the top of Husband Hill — a spot nearly as tall as the Statue of Liberty from the floor of Gusev Crater. From that vantage point, Spirit produced a sweeping panorama of the surrounding martian territory — including a view of miniature tornados in the form of dust devils that raced across Mars’ surface.

From that same scenic spot, Spirit also snagged images of the two Martian moons, Phobos and Deimos. Other nighttime duties included charting meteor showers skirting across the red planet’s sky.

Spirit, also found rocks in the Columbia Hills that were either formed in, or were altered by, water. Furthermore, rover scientists discerned that the hills hold the highest sulfur content ever found on Mars: sulfate salts, deposited by water. Spirit spent over a month exploring the summit region, then began a descent to additional scientific targets.

Early in the year, Opportunity, wrapped up surveying its own heat shield. The broken and twisted remains of the hardware were examined by the robot’s microscopic imager — the first time engineers could examine a heat shield after it had entered another planet’s atmosphere.

During its heat shield survey work, Opportunity came across an iron meteorite, the first meteorite of any type ever identified on another planet. The basketball-sized object was inspected by the rover, including use of onboard spectrometers, determining that the meteorite was mostly made of iron and nickel.

Scientists studying the bounty of data from Opportunity now believe that life at Meridiani Planum would have likely had a tough time getting started eons ago. Conditions in this region of Mars were strongly acidic, oxidizing and sometimes wet. Those circumstances probably posed drastic challenges to the potential origin of martian life.

Leonard David has been reporting on space activities for nearly 50 years. He is the 2010 winner of the prestigious National Space Club Press Award and recently co-authored with Apollo 11’s Buzz Aldrin the book “Mission to Mars — My Vision for Space...