“We are still having ongoing discussions,” Dr. Conley said. “It depends on what the results of the calculations are.”

This is not just a fastidious whim of NASA, but an international agreement. The Outer Space Treaty of 1967 dictates that nations should take care when exploring other planets “to avoid their harmful contamination.”

The Committee on Space Research, part of the International Council of Science, develops planetary protection policies that Dr. Conley is responsible for carrying out. For most missions, like the Cassini orbiter at Saturn, the requirements are fairly simple — do not crash into a body where life might exist, and when done, dispose of the spacecraft. (Cassini will be sent on a death dive into Saturn, where heat and pressure will obliterate it and any remaining microbes.)

Similar care will be taken studying Europa, a moon of Jupiter, and Enceladus, a moon of Saturn, known to have oceans beneath the surface. But it will be many years before a lander sets down there. For now, the main concern is Mars.

“We’re treading new ground,” John M. Grunsfeld, NASA’s associate administrator for science, said of the discussions of Curiosity. “The issue of planetary protection has gone very much from one where we’re just trying to be careful to one that has very real, near-term consideration.”

With the two Viking landers in 1976 — NASA’s first and so far only attempts at detecting life on another planet — the agency took extraordinary precautions sterilizing the spacecraft, first cleaning it to fewer than 300 heat-resistant bacterial spores per square meter. Then it was packed up and baked for several days, reducing the number of spores by a factor of 10,000.