In the summer of 1957, the Earth stood witness as a meteorite cratered in rural Pennsylvania, bringing with it a people-eating plague never seen: an alien amoeba with the taste for human flesh. While we had Steve McQueen around for the first invasion, humanity is now defended against microbial marauders from outer space by NASA and its international counterparts.

Biological contamination goes both ways, mind you. Just as important as keeping extraterrestrial organisms from reaching the surface (aka "backward contamination") is ensuring that our planetary probes carry as few microbial hitchhikers from Earth as possible ("forward contamination"). To that end, in 1958, the U.S. National Academy of Sciences (NAS) issued a decree urging "that scientists plan lunar and planetary studies with great care and deep concern so that initial operations do not compromise and make impossible forever after critical scientific experiments."

The following year, the newly formed Committee on Space Research (COSPAR) argued "that all practical steps should be taken to ensure that Mars be not biologically contaminated" until an exhaustive search for life on the planet had been undertaken. These recommendations became law in 1967 when the US, the USSR and the UK all signed onto the United Nations Outer Space Treaty.

"Part of our thinking about planetary protection is that we want to make sure that we safeguard to any future human exploration," Dr. Lucianne Walkowicz, an astronomer at Adler Planetarium and the Astrobiology Chair at the Library of Congress, told Engadget. "When we bring spacecraft to other worlds (or eventually human beings), we want to make sure that we understand that environment. That means being relatively cautious about contaminating it."

However, not every extraterrestrial target of human interest requires the same degree of caution. Places like the Sun or Mercury, which are almost assuredly devoid of biological organisms, don't require the same level of protection as, say, Mars or the Moon, which are just heavily irradiated and desperately cold. In fact, COSPAR has developed a 5-category system which space agencies must abide by when they're developing their planetary probes:

- Category I covers places with little chance of finding even basic forms of life, like Mercury.

- Category II includes places that might be explored for the origins of life but where the chances of contamination by Earthly microbes is remote. Think Venus or the Moon.

- Category III regulates flyby and orbiter missions where the chances of contamination are moderate, like Mars or Europa. This is why Cassini was thrown into Saturn: we couldn't have it falling into Enceladus or Titan.

- Category IV regulates lander or probe missions to the same places as category III, though it is further divided into a series of subclasses based on specific regions of the planet's surface and what the lander is actually looking for.

- Category V is what happens if there's a good chance we'll pick up a Blob in space. It demands "absolute prohibition of destructive impact upon return, containment of all returned hardware which directly contacted the target body,       and containment of any unsterilized sample returned to Earth."

"I think they're good for us as a working framework," Walkowicz said. "They certainly have served us well in the history of exploration and our solar system thus far."

It's extremely important that space agencies understand the categorical protection requirements of their mission, explains Dr. John Rummel, Senior Scientist at the SETI Institute and former NASA Planetary Protection Officer. "If you tell someone at the last minute they going to do something they had never been planning on, well, they may have to re-engineer entire spacecraft," he told Engadget. "If, on the other hand, they anticipate these requirements from the beginning... then it's not that big of a deal."