Derelict rocket stages that propelled four spacecraft toward the edges of our solar system and beyond are likely carrying Earthly bacteria out 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.

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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 put there by the hands and breath of the engineers who built the upper stages.

Surviving space

The microscopic interstellar travelers are now racing out of the solar system at speeds of between 7 and 11 miles per second, but are they still alive?

"Survival is more likely than any 'thriving,'" said Mark Burchell of the University of Kent in England.

Cold temperatures would likely plunge a microbe into a hibernation-like condition called a spore state.

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."

Belts of radiation might have sterilized the bacteria when each upper stage reached Jupiter.

The Pioneer 10 upper stage passed closest to the giant planet and endured many times the radiation level lethal for humans as it plunged through Jupiter's radiation belts. But perhaps some bacteria survived.

"Some bacteria are quite a bit hardier than humans, so this probably wasn't enough to kill shielded bacteria on the inside of the upper stage," Rummel said.

Low odds

The Voyager 1 upper stage has been in space for almost 30 years. Its diminutive cargo have many billions of years of travel ahead.

In 40,000 years, this wayward 185-pound (84-kilogram) lump of metal will pass by the star AC+79 3888 at a distance of 1.64 light-years.

AC+79 3888 is a dwarf star and its feeble output is unlikely to revive even the liveliest bacteria. Far more time must pass before any of the upper stages will encounter a star with an environment suitable for Earthly organisms.

"Exiting the solar system is not actually the problem," Burchell told SPACE.com. "The problem is that you then have to enter another solar system and be captured by a planet. The chances of this are just vanishingly small."

Given the sheer expanse of time that lies ahead of the four discarded rockets, at least one is likely to eventually encounter a planet.

But even if that planet's environment is conducive to life, the long dormant bacteria will not just gently plop into some exotic ocean. No soft landing can be expected.

"Getting to another solar system is one thing, but stopping there in a non-destructive way is quite something else again," Rummel said.