Derelictrocket stages that propelled four spacecraft toward the edges of our solarsystem and beyond are likely carrying Earthly bacteria out into the galaxy.
Thefour 'STAR' upper rocket stages, also known as kick motors, are responsible forbooting Voyager1, Voyager 2 and Pioneer 10 to the solar system's fringes, as well as sendingNASA's New Horizons spacecraft on a path to Pluto. The rocket stages arethemselves on course to move beyond the Sun's influence into interstellar space.
Althoughtheir roles were vital to their respective missions, the upper stages were notlavished 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 onebig directive: "Their requirement was not to hit any of the planets in oursolar system," a caution necessary since the rocket stages would almostcertainly play host to large numbers of Earthly bacteria.
These bacteriawould have been emplaced by the hands and breath of the engineers who built theupper stages.
The microscopic interstellartravellers are now racing out of the solar system at speeds of between 7and 11 miles per second, but are they still alive?
"Survival is more likely than any 'thriving,'" said MarkBurchell of the University of Kent in the UK. Cold temperatures would likelyplunge a microbe into a hibernation-like condition called a spore state.
Bacteria have been revived on Earth after millions of years ofdormancy and experiments involving the exposure of bacteria and lichens tospace have revealed just how tough these simple organisms are.
So how long might a microbe last inspace, clinging to a rocket? "There is still debate," Burchellsaid. "1,000 years? 100,000 years? We don't know."
Belts of radiation might have sterilized the bacteria when eachupper stage reached Jupiter. The Pioneer 10 upper stage passed closest to thegiant planet and endured many times the radiation level lethal for humans as itplunged through Jupiter's radiation belts. But perhaps some survived.
"Some bacteria are quite a bit hardier than humans, so thisprobably wasn't enough to kill shielded bacteria on the inside of the upperstage," Rummel said.
The Voyager 1 upper stage has been in space for almost 30 years. Itsdiminutive cargo have many billions of years of travel ahead.
In 40,000 years, this wayward 185-pound (84 kilogram) lump ofmetal 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 torevive even the liveliest bacteria. Far more time must pass before any of theupper stages will encounter a star with an environment suitable for Earthlyorganisms.
"Exiting the solar system is not actually the problem,"Burchell told SPACE.com. "The problem is that you then have toenter another solar system and be captured by a planet. The chances ofthis are just vanishingly small."
Given the sheer expanse of time that lies ahead of the fourdiscarded rockets, at least one is likely to eventually encounter a planet. Buteven if that planet's environment is conducive to life, the long dormantbacteria will not just gently plop into some exotic ocean. No soft landing canbe expected.
"Getting to anothersolar system is one thing, but stopping there in a non-destructive way isquite something else again," Rummel said.
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