With back-to-back rocket launchings next month, NASA hopes to revive its Mars exploration program — and burnish its battered image — with an ambitious $800 million mission to figure out what happened to the water that scientists think once scoured the planet’s surface. If all goes as planned, two identical spacecraft will slam into the Martian atmosphere next January after seven-month voyages, bounce like giant beach balls across the frozen surface and then unfold like flower blossoms to reveal a pair of “monster truck” robot geologists.