When NASA’s Phoenix Mars Lander rocketed into space Saturday, it went, like all missions, with the assurance that as few Earth microbes as possible tagged along.

Hitchhiking microbes could impair the experiments, or worse – an errant microbe could contaminate the planet.

Keeping the spacecraft sterile was the job of an obscure but crucial part of NASA known as the Planetary Protection unit.

The main Planetary Protection research center is at the Jet Propulsion Laboratory in La Canada Flintridge, Calif., where Jason Kastner supervises a small team of scientists working in labs that smell of bacterial cultures.

Kastner, an exacting, bespectacled mathematician, is in charge of nitpicking about the exact number of bacteria NASA inadvertently shoots into space.

Phoenix had to be even cleaner than most missions, going a step beyond Planetary Protection’s sterilizing procedures. Merely killing all the bacteria wasn’t good enough for this trip to Mars’ north pole in search for signs of water and organic material.

Phoenix scientists had to scrape every bit of organic material from the craft. Even dead bacteria could contaminate the experiments, making it impossible for scientists to distinguish between organics that came from Earth or Mars.

Since a 1967 United Nations treaty, Planetary Protection has sterilized every NASA mission. Its motto is “All of the planets, all of the time.”

NASA’s official worrywarts are anxious not only about microorganisms contaminating other celestial bodies but also about organisms reaching Earth.

“If we’re bringing samples back to Earth, we don’t want to bring back something like the Andromeda Strain,” said Kastner’s boss, Cassie Conley, NASA’s planetary protection officer in Washington, referring to the Michael Crichton novel in which a space pathogen causes a terrestrial outbreak.