SpaceX's launch of a Tesla Roadster into deep space may have got 10 out of 10 for style and PR, but it looks as if it got one out of 10 for cleanliness. According to scientists at Purdue University, the electric sports car and its plastic mannequin driver may be the dirtiest unmanned objects ever launched into orbit – carrying an unprecedented amount of bacteria that could one day contaminate Mars.

Anyone who's ever bought a second-hand car knows that they're likely to find unpleasant surprises under the seats or tucked in the glove box. In fact, if you were to commission a forensic analysis of the average used vehicle with its collection of skin flakes and dried mucus, you'd probably never get into another one without a hazmat suit on.

Not surprisingly, spacecraft tend to be a bit more hygienic. International agreements require any unmanned probe that is likely to land on another planet, moon, asteroid, or other celestial body meet rigorous sterilization standards inside and out to minimize the chance of terrestrial microbes ending up colonizing another world. At the minimum, this would complicate the job of scientists seeking alien life, but it could have worse effects.

"If there is an indigenous Mars biota, it's at risk of being contaminated by terrestrial life," says Jay Melosh, a professor of earth, atmospheric and planetary sciences at Purdue. "Would Earth's organisms be better adapted, take over Mars and contaminate it so we don't know what indigenous Mars was like, or would they be not as well adapted as the Martian organisms? We don't know."

Legally, this requirement doesn't apply to spacecraft that won't land on other planets, like those that orbit the Earth or the Sun, but space engineering is already a pretty scrubbed enterprise with satellites being assembled in special clean rooms to keep delicate equipment from being contaminated by dust, dirt, and other undesirables. Ultimately, the main difference between a lander and a non-lander is that the former are built to withstand being steamed in a giant autoclave designed to kill off any microscopic hitchhikers.

By comparison, the Tesla Roadster is extremely dirty. It's an ordinary production car that was driven personally be SpaceX founder Elon Musk on public roads and wasn't specially cleaned before the launch – not that that would have helped much.

"Even if they radiated the outside, the engine would be dirty," says Melosh. "Cars aren't assembled clean. And even then, there's a big difference between clean and sterile."

Of course, the Roadster is in a heliocentric orbit that will likely keep it circling the Sun for tens of millions of years until the gravitational influence of Earth, Mars, and Jupiter disturb it. And it's now much more sterile than it was. Every moment until it eventually burns up in the atmosphere of the Earth or Mars or plunges into the Sun, it's being pelted with micrometeoroids and bombarded by hard ultraviolet and cosmic rays as it bakes in temperatures of up to 260° F (127° C).

Such conditions will kill any bacteria on the car's surface, and as the plastic body disintegrates under this cosmic onslaught, there will be fewer places to hide. However, some places inside the frame and drivetrain will still provide tiny protective havens where microbes can freeze dry and go dormant for thousands or perhaps millions of years.

The fear is that there is a remote chance that fragments of the Roadster might land on Mars in the distant future, though it's more likely to burn up completely in the thin Martian atmosphere. But Alina Alexeenko, a professor of aeronautics and astronautics at Purdue sees this as a glass half empty/half full situation.

"The load of bacteria on the Tesla could be considered a biothreat, or a backup copy of life on Earth," she says.

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