So you want to explore the Solar System? You’re going to need to clean your spacecraft first. Any missions that NASA sends to other worlds nearby have to adhere to something called planetary protection. It’s the rule of preventing "harmful contamination" of the places we explore in the Solar System. That means making sure exploring spacecraft don’t spread too many harmful Earth’s organisms to all of our planetary neighbors.
Planetary protection was initially created in service of humanity’s oldest question: are we alone in the Universe? Back in the 1950s, people interested in exploring space recognized that biological contamination could pose a threat to finding extraterrestrial life elsewhere in our Solar System. Life exists everywhere here on Earth, so Earth’s organisms will likely be on any vehicles we send out into space. And that could make it difficult for spacecraft to distinguish between an alien life form on another planet — or a stowaway microbe from Earth. "We would find it very difficult to identify Mars life if we already contaminated the planet with Earth life," says Catharine Conley, NASA’s Planetary Protection officer.
"We would find it very difficult to identify Mars life if we already contaminated the planet with Earth life."
This dilemma led to the formation of planetary protection. Less contamination preserves the integrity of the Solar System’s planets and protects them from any damage that Earth’s organisms might cause. And it means that missions looking for life on other worlds will avoid misclassifying Earth life as ET.
NASA has different standards for planetary protection depending on where you want to go in the Solar System, though. If you want to go to places like the Moon or asteroids, the planetary protection rules for your spacecraft aren’t that strict. But for special places like Mars and Europa — where conditions may allow Earth life to grow — your spacecraft will need to go through some sterilization procedures. That way, you won’t inadvertently start any interplanetary microbe colonies.