Just as the lure of Mars grows stronger, with scientists poring over tantalizing new evidence of an ancient ocean and fresh views of layered canyons, sculpted polar ice caps and swirling dust devils, missions to the Red Planet are in disarray. The back-to-back failures of the Mars Polar Lander and Mars Climate Orbiter late last year have NASA rethinking what kind of spacecraft it will send to the planet most like our own.