And there is Europa, probably the most likely source of extra-terrestrial life in our solar system. NASA has plans to send an orbiter and then a lander to search for signs of life in Europa's planet-wide ocean. What is being done to protect Europan life?

How can we seek out life in the solar system without harming it? Can robotic probes built on Earth be made clean enough to search for life on other planets without contaminating it? If we bring samples of alien life back to Earth, how do we prevent them from contaminating Earth's biosphere?

"Planetary protection" is the prevention of "cross contamination." That is, preventing life from getting from one planet to another and causing harm. It's an important factor in space exploration that the public is barely aware of, but one that NASA spends a lot of time working on.

Dr. Karen Buxbaum, a supervisor of the Jet Propulsion Lab's (JPL) Planetary Protection Technologies Group says, "There's a certain amount of responsibility that we have as an agency that's doing exploration to not be sort of reckless in dumping stuff in other parts of the solar system."

NASA divides planetary protection concerns into two categories; forward and backward contamination.

Backward contamination is the type of thing that books and movies like H.G. Wells' "War of the Worlds" and Michael Crichton's "The Andromeda Strain" have made popular. It is the contamination of Earth life by alien spores, microbes or organisms.

Science fiction has put the fear of contamination by alien life in our minds. But, what about the reverse? Could our space probes be "infecting" other worlds with Earth life?

It turns out that NASA is working to protect life on other worlds from Earth life, what the space agency calls forward contamination. Buxbaum defines it this way; "Forward contamination refers to contamination of other solar system bodies with biological material from the Earth." But, this concern for alien life remains largely unknown to the American public.

Should we care if we spread Earth life to other planets in our solar system, or anywhere else? NASA cares and that's why the agency has spent over 30 (1967-2001) years and countless dollars trying to prevent cross contamination.

Protecting life on other planets is important business for NASA. It is crucial to the exploration of the solar system. So much so that NASA has created an entire Planetary Protection branch. Dr. John Rummel, NASA's Planetary Protection Officer, works to protect life on Earth and life elsewhere based on NASA's planetary protection policy.

"The policy is actually based on the desire to preserve extraterrestrial environments for the science opportunities that are there," says Rummel.

In other words, if we bring Earth life with us to another planet, there is the chance that we may kill or harm indigenous life. Or, we may make it harder to determine if life ever existed there. We may mistake Earth life for alien life.

"It's in nobody's best interest to obscure that by contamination with Earth organisms," says Rummel. "Nor would you want to discover a wonderful new life form and know that you've killed it � Essentially we can meet ethical considerations by the desire to preserve science."

Rummel must approve every NASA space probe before launch. "I often imagine myself strapped to a booster somewhere," Rummel says in a comic voice," 'Now, you won't launch this unless you get my signature.'"

The search for life beyond the Earth has lead to the new science of Astrobiology. Through a combination of many physical and life sciences, astrobiologists seek out life elsewhere in the solar system and the universe. It's important to know where life might be in order to understand where it must be protected. Scientists are only now starting to understand the so-called "habitable zone," the range of environments where life can exist.

Rummel ties astrobiology to planetary protection saying, "The idea of astrobiology � [is] to study the origin, evolution, and distribution of life in the universe. And its extremely complementary on one level with planetary protection, in that by preserving the environments in outer space, you give yourself the potential to be able to discover more about them."

On Earth, where there is water, there is life. But life doesn't need water to survive. In the past decade, scientists have discovered "extremophiles", organisms that live in the limits of the Earth's environment. Scientists have found life near hydrothermal vents at the bottom of the ocean, deep inside solid rock, and even at the core of nuclear reactors.

"One of the things that's changed in biology," Rummel says, "Is we've found life in extreme environments on Earth, that are completely different from anything you or I would be comfortable living in. Nevertheless, there would be ample opportunity to have life there. I don't want to live in a boiling pool in the middle of Yellowstone Park, but there are microbes that just love it."

Astronomers have found all the necessary ingredients for life (water, carbon, hydrogen, oxygen and nitrogen) inside clouds of gas and dust floating in deep space. At last count, our solar system has one star, the Sun, 9 planets with 68 moons, and thousands of comets and asteroids. It's quite possible that life arose in at least one of these places.

Detecting life is difficult, and scientists must be careful not to confuse Earth life with alien life. This would risk ruining future life detection experiments. Karen Buxbaum says, "Confusing the scientific results is a threat to the program."

In the near future, NASA plans to use astrobiology to search for life on Mars again. JPL scientist Dr. Roger Kern is planning for such a mission. "What we anticipate will happen with the first landers on Mars is there will be life detection experiments done in-situ, at the site," says Kern. "And those experiments are probably not going to be looking for life, per se, but will be looking for molecules associated with life. So we want to remove as much [Earth life] as possible."

Kern continues, "Where as once NASA was only concerned with sterilizing spacecraft and making sure that the spacecraft couldn't shed a live organism, now we have an interest in seeing to it that it doesn't shed a dead organism as well � it kind of takes you into a new definition of clean."

Even with super clean spacecraft, some microbes will always get by. Dr. Rummel says that the current planetary protection plan includes, "An inventory of organic constituents that might be delivered to another body. So that if you happen to go back there and find these things you know that you brought them."

In preparing a spacecraft for launch, technicians take samples of any microbes, spores, or cells on the spacecraft's surfaces. They work to reduce the number of contaminants to as low as possible, cleaning several times if needed.