How we spread life into the galaxy

The four 'STAR' upper rocket stages, also known as kick motors, are responsible for booting Voyager 1, Voyager 2 and Pioneer 10 to the solar system's fringes, as well as sending NASA's New Horizons spacecraft on a path to Pluto. The rocket stages are themselves on course to move beyond the Sun's influence into interstellar space.

Although their roles were vital to their respective missions, the upper stages were not lavished with the same attention as the spacecraft they parted company with.

"The upper stages were not required to be sterilized," said John Rummel, senior scientist for astrobiology at NASA. There was just one big directive: "Their requirement was not to hit any of the planets in our solar system," a caution necessary since the rocket stages would almost certainly play host to large numbers of Earthly bacteria.

These bacteria would have been emplaced by the hands and breath of the engineers who built the upper stages.

Bacteria have been revived on Earth after millions of years of dormancy and experiments involving the exposure of bacteria and lichens to space have revealed just how tough these simple organisms are. So how long might a microbe last in space, clinging to a rocket? "There is still debate," Burchell said. "1,000 years? 100,000 years? We don't know."