We did that with Cassini on Saturn, and we intend to do that with Juno on Jupiter. But, on the other hand, while measures are taken to ensure that spacecraft h

We did that with Cassini on Saturn, and we intend to do that with Juno on Jupiter. But, on the other hand, while measures are taken to ensure that spacecraft headed to other bodies are not contaminated, we've also found that life still often manages to find a way. So I thibk the various landings on Mars, and the lander on Titan (plus the quadcopter heading there later this decade) are somewhat likely to have brought along some extremely resilient (but dormant) life anyway.