Contamination control in space
Containment a key issue as space agencies move closer to collecting Mars samples
By Bruce Flickinger
As US and European space agencies enter the next phase of Mars exploration, during which surface and subsurface samples will be collected for analysis in spacecraft-borne laboratories and, eventually, in a laboratory on Earth, scientists are grappling with the attendant issues of contamination control. Also a concern is the engineering challenges involved with launching a craft from Mars and returning it to Earth-a critical precursor to future manned missions to the red planet.
Shown here is an atrist's rendering of the Mars Sample Return ascent module lifting off from Mars. Image courtesy of ESA.
Based on knowledge already gained about Mars, scientists with the International Mars Exploration Working Group (IMEWG), and NASA’s Mars Exploration Program Analysis Group (MEPAG) and Organic Contaminants Science Steering Group (OCSSG), are beginning to formulate strategies for collecting, handling, containing, and analyzing Martian samples. Most recently, the European Space Agency (ESA) announced in April that its Aurora Mars exploration program entered into a 12-month Mars Sample Return (MSR) Phase A2 Systems Study. The program involves a number of technical studies and hardware testing projects addressing, among other items, the issue of a sample container and the biological containment of a Mars sample.
“We intend to send a very clean spacecraft to Mars,” says Gerhard Kminek, PhD, planetary protection officer with the ESA’s Human Spaceflight, Microgravity, and Exploration Directorate, based in Holland, The Netherlands. “All components that will contact any samples will be sterilized to a level far beyond what we have in the most stringent transplant surgery operations, and bidirectional biobarriers will protect the samples from the terrestrial environment and vice versa.”
The Aurora sample-return mission is envisioned in two stages. The first consists of a Mars orbiter and an Earth return capsule; the second carries the surface lander and the Mars ascent vehicle, which will launch the sample into Mars orbit for retrieval and return to Earth. The mission calls for five spacecraft: an Earth/Mars transfer stage, a Mars orbiter, a descent module, an ascent module, and an Earth re-entry vehicle.
The lander will carry a surface rover with a miniature drill to collect samples from underneath the upper layer of soil; this region is expected to be completely sterile due to the high level of radiation on the planet. A soil sample of 500 grams is being considered, in line with the recommendations of the IMEWG.
Here, the major problem with regard to maintaining sample integrity is temperature. “A sample retrieved from the Martian subsurface is at approximately 200° K (roughly -99° F/ -73° C),” Kminek says. “Ideally, the sample temperature should never deviate much beyond this temperature, but this is technically and financially very demanding. The temperature will rise much higher during cruise and especially during Earth entry.” Both passive and active systems are being explored to address this issue, “but they are either heavy or require a large volume, both of which affect the design of the orbiter and launcher systems,” he notes. Current discussions are looking at 25° C as an acceptable temperature ceiling for holding samples.
Once samples of Martian soil have been collected, they will be loaded onto the Mars ascent vehicle, which then will be launched into orbit to rendezvous with the Earth re-entry vehicle. The re-entry vehicle will return to Earth on a ballistic trajectory, and samples will be recovered and isolated in a “curation facility” to allow scientists to analyze them safely.
This containment facility will be Biosafety Level 4, and will have to function as “both a cleanroom and a biological containment facility. Such a facility does not exist as of today,” Kminek notes. Both ESA and NASA have completed feasibility studies on such a proposition, “and we are now entering the next phase of defining functional requirements before going into a design phase.”
Missions of such scope are at least a decade away. Taking more-advanced onboard laboratories to Mars needs to occur before samples are brought back to Earth. NASA, while declining to comment specifically on a Mars sample-return mission, has identified several steps along this critical path: the 2009 Mars Science Laboratory mission, a lander that will search for organic molecules using an onboard laboratory with sample preparation and processing capabilities; the Phoenix Scout Mission, which is designed to land a mass spectrometer on an ice-rich region of the planet and analyze near- and sub-surface samples; and the Next Decade Astrobiology Mission, which will include extinct and extant life-detection experiments as elements of its payload.
Because these missions do not involve direct human contact, the larger concern is “forward contamination”-organics and other materials introduced by onboard analytical systems that could compromise the integrity of the Martian samples. Astrobiology, in particular, calls for a focus on higher-quality measurements, which, in turn, mandates stringent control of the terrestrial bioload carried by the equipment. Indeed, NASA’s OCSSG has called this “critical to mission success.”
In a 2004 OCSSG white paper titled “Science Priorities Related to the Organic Contamination of Martian Landers,” OCSSG shifted the discussion from the Martian environment contaminating equipment and measurement systems on Mars rover missions to a new definition of “clean” (with regard to molecular organic contaminants) that takes the perspective of the sample as it is delivered to the instruments. “For astrobiology-related landers, [contamination] issues can lie at the heart of their science logic, and can make the difference between the eventual results being definitive or merely being suggestive,” the authors wrote.
The OCSSG distinguishes two issues. First, until organic carbon is definitively discovered on Mars, the highest priority is preventing any sample from being exposed to Earth-sourced organic contaminants. Second, once Mars-sourced organic material has been proved, it will be critical to minimize cross-contamination of Mars-sourced organic material. Based on current understanding of the Martian environment, the following were judged as the biggest “worries” in terms of cross- and forward-contamination: benzene and more complex aromatics; organic molecules with carbonyl or hydroxyl groups; non-aromatic hydrocarbons; amino acids, amines and amides; and DNA.
Kminek says dust management is one overarching concern as our interactions with the Martian habitat become more intimate. A ubiquitous presence in the Martian milieu, dust is a problem in and of itself, independent from its specific chemistry. “The dust on Mars is made partly by impact erosion and not by water or wind erosion, so the particles are highly irregular and sharp, and more readily stick to surfaces,” Kminek says. “The particles also are charged due to the effect of low humidity and high UV radiation, and, therefore, are electrostatically sticky. Essentially, dust will get everywhere-filters, air locks, instruments, etc.”
Beyond its physical presence, dust will be the primary carrier of Martian chemistry into controlled environments containing instruments and scientists. “Mars has a reactive chemistry; we know the environment contains a lot of salts and sulphur compounds, both of which can easily create problems in a pressurized man-rated habitat,” which typically has high humidity levels, Kminek says. “In the dry Martian environment, these compounds are benign, but in contact with water or high humidity, they can react quite fast and can pose a threat to people and hardware.”
Similarly, if there is any biology on Mars, Kminek notes, it would come into any controlled habitat primarily with dust.
“We have to assume that all Martian samples are hazardous until proven otherwise, and they will be treated that way,” Kminek adds. “A biohazard assessment protocol will evaluate the nature of the samples before they can be released to the scientific community, either with or without being sterilized beforehand.