Humanity has sent around 30 spacecraft and landers to the Red Planet since the space age began. Now, we know which microbes might have survived the trip, says geneticist Christopher Mason. Nasa and its engineers in the Jet Propulsion Laboratory (JPL) have precise and thorough protocols to minimise the number of organisms that might inadvertently hitchhike on a space mission. Internationally agreed standards guide how rigorous these protocols should be and Nasa meets, and in some cases, exceeds them. Yet, two recent studies highlight how some organisms might survive the cleaning process and also the trip to Mars, and also how fast microbial species can evolve while in space.