Why has the Mars unmanned aircraft that should stop in 90 days keep moving for 10 years?

NASA is "Mars mission mission"for"Spirit"(MER-A) and"Opportunities"(MER-B) to the Mars to conduct surveys. Two aircraft that landed on Mars were expected to stop operating in 90 days, but opportunities continue to send photos to the earth even after 3560 days have passed. Why have the prospecting machine been able to run for 10 years, far exceeding the initial forecast? That is summarized.

Why This Mars Rover Has Lasted 3,560 Days Longer Than Expected - SFGate
http://www.sfgate.com/technology/businessinsider/article/Why-This-Mars-Rover-Has-Lasted-3-560-Days-Longer-5173078.php


Mars Exploration Program: Spirit & Opportunity 10-Year Highlights
http://mars.nasa.gov/mer10/

Two unmanned spacecraft "Spirit" and "Opportunity" sent out by "Mars Exploration Mission" on July 7, 2003 have landed on Mars by January 24, 2004. The mission was supposed to be 90 days, but Spirit has been operating for about 6 years until 2010, and 10 years have passed since the opportunity has continued to send Mars data to Earth as of 2014.


The ground of Mars is covered with sharp rocks and steep hills, tornadoes containing a lot of dust blowing and ruining the surface. The temperature of Mars rises to 80 degrees Fahrenheit in the summer (26 degrees Celsius), and in winter it is a harsh environment where it falls to minus 199 degrees Fahrenheit (minus 128 degrees Celsius).NASA websiteAccording to the spacecraft, it is as big as a golf cart, and six wheels that can overcome the rugged groundRocker · Bogey"It is designed not to roll by the suspension system called. Also, it is possible to operate even in severe cold conditions with built-in heater and temperature sensing device in the body.


The scientific team thought that the wind and dust that blows on Mars would cover the solar panels, which are power sources, in the course of a few months, but because the strong wind blows away the dust accumulated on the surface of the panel, the power We are able to maintain that. He is the manager of the mission explorer missionJohn CrowMr. said, "Technical design that can operate in harsh environments and longevity not possible to imagine due to fortune exceeding human knowledge was made possible."


The most important purpose given to the two explorers was to investigate the evidence of the existence of water in the environmental conditions of early Mars and the possibility of the existence of life. As a result of the survey, we succeeded in discovering the existence of a lake and evidence that a river was flowing on ancient Mars. Furthermore, by recent opportunities' activities,Endeavor Crater"We have demonstrated the possibility of small amounts of clay minerals on Mars and successfully photographed strange jelly donut shaped rocks over 13 days.


A spacecraft with many unexpected discoveries has reached about 39 km in travel distance to date and continues to send 170,000 photographic images to the earth, according to Mr. Cullas " Two of the instruments are out of order, and robot arms are involved in "arthritis", and the aging of the aircraft is progressing. "