Astrobiology Magazine: What exactly does a planetary protection officer do?

Cassie Conley: For all missions in that NASA heads, I am responsible for making sure that those missions adhere to planetary protection requirements.

The requirements are dictated by the 1967 International Outer Space Treaty, which requires that any spacecraft exploring the moon or other solar system bodies must be clean enough to prevent contamination that would affect future science, specifically in regard to biological scientific investigation.

So we don't care about pieces of dirt that are not organic carbon or biological material, but we have to guarantee that a spacecraft will not be contaminating wherever it goes, such that somebody going there later wouldn't be able to do they science they want to do.

That treaty simply says, in Article IX, that every space agency must protect the places they go for future science. Prior to that, the International Council of Scientific Unions created the Committee on Space Research (COSPAR), which has a subcommittee called the Panel on Planetary Protection.

AM: What do you actually do?

CC: Mostly I sit at my desk and answer e-mails. But what I actually do is monitor activities for keeping spacecraft clean for every project that's leaving the Earth-moon system. There are people within each project who are constantly assessing the spacecraft to make sure that it's staying clean as it's being assembled.

And then I have a group of people who, as the spacecraft is in its final assembly phase, go and confirm that project's assessments are correct, that the number of contaminating bacteria that they have measured is what we also measure. So it's a system of checks and balances to make sure that the spacecraft really is clean.

AM: What does "clean" mean?

CC: For Mars, "clean" in terms of spacecraft surfaces, regarding biological contamination, is that there should be fewer than 300 heat-resistant bacterial spores per square meter of spacecraft surface. There's an additional requirement for internal bacterial spores, inside a circuit board or inside the glue that's been used to attach two things together.

But for surfaces, which are what you worry about for spacecraft that haven't crashed, it should be fewer than 300 per square meter, if you're going to a place on Mars that isn't given special protection. If you want to go to a place where there might be liquid water on Mars - a "special region," as it's called - it should be reduced by an additional 4 orders of magnitude, by some kind of treatment like baking in a dry-heat oven.

AM: What's that second level called?

CC: That second level is called "Viking post-sterilization levels." The first level is called "Viking pre-sterilization levels." It's a little bit awkward when you're talking with your international partners, who may not be aware of how Viking was cleaned, so I always try to mention the numbers instead of referring to Viking.

The Viking landers were sterilized. They were treated to a fewer-than-300-spores-per-square-meter surface contamination level, and then packed up in their launch mode in basically a giant casserole dish, and baked for several days at not quite 125 degrees Celsius (257 degrees Fahrenheit), to obtain the 4 orders of magnitude additional reduction.

AM: How do you know that you've been successful?

CC: Obviously, after you've done your 4 orders of magnitude baking, you don't want to unpack the thing and test it again, because whatever you swab it with could be dirtier than the spacecraft itself. So you test sample materials, pieces of the same materials that are assembled into the spacecraft. You make very sure, by doing assays beforehand and tests on sample material, that what you think is going to happen when you bake the spacecraft really is going to happen.

AM: Were the Spirit and Opportunity rovers sterilized the way the Viking landers were?

CC: No. The rovers were cleaned to the standard of 300 spores per square meter. We did not expect them to go to special regions so they were not baked. The idea of special regions was still in flux at that point, but because the rovers were intended to go places where we were fairly confident that life would not survive on the surface, we didn't require them to be sterile.

COSPAR has an international policy that limits the requirement for sterilization to places that are special, where there might be liquid water. It's extremely difficult to build a spacecraft that can tolerate several days of baking.

The cleaning procedure is something that a lot more materials can withstand than the baking, so in order to allow missions to go to Mars without such a stressful treatment, it was decided in the early 90s that, depending on what you were doing, it was okay to just do the Viking pre-sterilization levels of 300 per square meter.

AM: What's the requirement for the upcoming Phoenix mission?

CC: Phoenix is going to a place where there is ice beneath the surface. It will not be going to a place where there is ice on the surface. And Phoenix as a lander is a fairly light spacecraft. It doesn't have a lot of big heavy massive things in it. It also doesn't have any thermonuclear generators, so it will not be producing its own heat; it runs on solar panels.

So based on calculations that were done by the project to document all this to the appropriate level of confidence, the spacecraft itself is not being required to be sterilized because the martian surface at its landing site is not considered to be a special region.

It does, however, have a robot arm that will be digging beneath the surface, and that entire robot arm just recently underwent its final baking treatment. So the robot arm that is digging to go to the special region is required to be sterilized, but the rest of the spacecraft meets the pre-sterilization requirement of 300 spores per square meter.

AM: Something's still bothering me about this, though. When you go to all this trouble to sterilize a spacecraft, aren't the one or two spores that survive, the resistant ones, the ones you're most worried about?

CC: They could be. But, if you start out with 300 spores per square meter, and you reduce that by 4 orders of magnitude, that means you have .03 spores per square meter when you end up, and .03 of a spore isn't really going to be able to grow.

AM: Okay, but that's a statistical argument. Really what you have is something on the order of one spore for every 30 square meters of surface area. It's not really .03 non-viable spores per square meter, it's 1 or 2 viable spores on the entire spacecraft.

CC: Yes, that's one way to look at it, but a lot of the methods that are used to achieve 4 orders of magnitude reduction actually do a considerably greater amount of reduction.

Essentially, it's as good as we can get.

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