Whether it's Martians in War of the Worlds or a telekinetic visitor in E.T. the Extra-Terrestrial, science fiction is filled with aliens making their way to Earth.

In science fact, though, there's greater concern that Earth-dwellers—specifically bacteria and microorganisms—could arrive at extraterrestrial destinations.

As NASA sends rovers to Mars, plans a trip to Jupiter's icy moon Europa, and looks for an ocean on Saturn's moon Enceladus, the hope is to find life-forms on those interplanetary bodies. To ensure that doesn't include forms that originated on Earth—and that the new environment isn't compromised the way Earth ecosystems can be by invasive species or infectious diseases—NASA is now thoroughly cleaning its space-bound vessels.

The vacuum and radiation of space, plus the heat given off