For people interested in seeing Mars up close, Aug. 26 and 27 present an opportunity rarer than once in a lifetime. On those dates, the Red Planet will be the closest it has been to Earth in 50,000 to 70,000 years, depending on the computer model. The enthusiasm, stoked by Saturday’s scheduled launch of a second land rover to Mars, has sparked telescope sales at shops such as Photon Instruments in Mesa. Elsewhere in the Valley and the state, stores and observatories are gearing up for what is called the Mars opposition.