When the Apollo 11 astronauts landed on Earth's surface after humankind's first trip to the Moon, their journey wasn't over yet. Concerned about the possibility of lunar pathogens infecting the astronauts, NASA scientists quarantined the crew members for 21 days. Now, as humans prepare to return to the Moon decades later, we're faced with a different kind of planetary protection issue: preventing the spread of our own germs to other worlds.