In A Nutshell
NASA has a division called the Office of Planetary Protection which works kind of like a reverse Men in Black. Part of their mission is to ensure that Earth does not biologically contaminate the rest of the universe with our pathogens. One of their key tasks is building spacecraft components in “clean rooms” to make sure that each craft is as sterile as possible.
The Whole Bushel
When the Apollo 11 astronauts returned from the first walk on the moon, the men were not immediately subjected to ticker tape parades and television appearances. Instead, they were kept in a converted Airstream trailer for 21 days under tight quarantine on the off chance that they may have brought back some contaminants from the lunar surface. Today, we understand that the moon is devoid of life, but at the time it was a valid concern. The human immune system would be practically helpless against alien viruses, much in the way that Native Americans were decimated by smallpox carried by Europeans.
Conversely, NASA understands that humanity has its own host of nasty sluggers. To this end, they developed the Planetary Quarantine Program, which would later become the Office of Planetary Protection. This agency works as sort of a reverse Men in Black, aiming to preserve not only life on Earth, but any potential life throughout the universe. Each mission is different and the requirements for each are carefully tailored: Protocol for the Mars Rover, Curiosity, might be different than that of a lunar module.
A single human body teems with bacteria—there are more bacteria on your body than there are human beings on the planet and at that rate, it might seem impossible to fully shield the galaxy from our germs. But they make every effort. Spacecraft components are built in “clean rooms” whose filters and air flow systems leave them practically void of contaminants. Workers are forced to dress in masks, gloves, and protective suits. Then, the spacecraft are sterilized. This was formerly accomplished by sheathing them in a ceramic shell and then baking them for 30 hours. Today, they are moving toward sterilization with freezing vapor phase hydrogen peroxide. Often, other NASA personnel are not particularly enamored of Planetary Protection Officers, because their demanding standards make things much more difficult.
Given the inhospitable nature of the rest of the known solar system, it seems unlikely these measures are necessary. On Venus, for instance, the average temperature is a balmy 485 degrees Celsius (900 °F). By comparison, Uranus clocks in around -240 degrees Celsius (-407 °F), not too far from the theoretical “absolute zero.” Coupled with raging storms, atmospheres of ammonia, sulfuric acid, carbon dioxide, and the like, it would seem that even the hardiest organism would have trouble eking out a living.
However, there are creatures called extremophiles, such as the tardigrade (or water bear), that can endure conditions that would kill other animals in mere moments. The tardigrade in particular has been shown to be capable of withstanding the vacuum and solar radiation of outer space. Other animals are capable of living at the bottom of the Mariana Trench, the deepest part of the ocean, nearly 11 kilometers (7 mi) below the surface, where the pressure is more than 1,000 times that of sea level, and sunlight does not penetrate.
After innumerable journeys into the cosmos, we seem no closer to finding life, but with commercial space travel becoming increasingly likely, the importance of maintaining a barrier between terra firma and that which lurks among the stars is greater than ever.