Planetary Quarantine Program

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NASA started a planetary quarantine program in the late 1950s at the beginning of the U.S. civilian space program. This quarantine program, conducted with international cooperation, was intended to prevent, or at least minimize, the possibility of contamination of alien worlds by early space probes. At that time, scientists were very concerned about the issue of forward contamination. In this type of extraterrestrial contamination, terrestrial microorganisms, "hitchhiking" on initial planetary probes and landers, could spread throughout another world, destroying any native life-forms, life precursors, or perhaps even remnants of past life-forms. If forward contamination occurred, it would compromise future scientific attempts to search for and identify extraterrestrial life-forms that had arisen independently of the Earth's biosphere. At the start of the 21st century, the concern about forward contamination remains high for missions to Mars and Europa.

To address potential extraterrestrial contamination problems, NASA scientists and engineers developed a rigorous planetary quarantine protocol. This protocol required that outbound unmanned planetary missions

Wearing proper protective clothing (to minimize the possibility of forward contamination), aerospace technicians inspect NASA's Mars exploration rover Opportunity in the Payload Hazardous Servicing Facility prior to launch from Cape Canaveral, Florida, on July 7, 2003. Opportunity successfully landed on Mars on January 25, 2004, and then began to explore the surface of the Red Planet in the Terra Meridiani region. (NASA/JPL)

Wearing proper protective clothing (to minimize the possibility of forward contamination), aerospace technicians inspect NASA's Mars exploration rover Opportunity in the Payload Hazardous Servicing Facility prior to launch from Cape Canaveral, Florida, on July 7, 2003. Opportunity successfully landed on Mars on January 25, 2004, and then began to explore the surface of the Red Planet in the Terra Meridiani region. (NASA/JPL)

be designed and configured to minimize the probability of alien-world contamination by terrestrial life-forms. As a design goal, these spacecraft and probes had a probability of 1 in 1,000 (1 x 10-3) or less that they could contaminate the target celestial body with terrestrial microorganisms. Decontamination, physical isolation (for example, prelaunch quarantine), and spacecraft design techniques have all been used to support adherence to this protocol.

One simplified formula for describing the probability of planetary contamination is:

where

P(c) is the probability of contamination of the target celestial body by terrestrial microorganisms, m is the microorganism burden,

P(r) is the probability of release of the terrestrial microorganisms from the spacecraft hardware,

P(g) is the probability of microorganism growth after release on a particular planet or celestial object.

As previously stated, P(c) had a design goal value of less than or equal to 1 in 1,000. A value for the microorganism burden (m) was established by sampling an assembled spacecraft or probe. Then, through laboratory experiments, scientists determined how much this microorganism burden was reduced by subsequent sterilization and decontamination treatments. A value for P(r) was obtained by placing duplicate spacecraft components in simulated planetary environments. Unfortunately, establishing a numerical value for P(g) was a bit more tricky. The technical intuition of knowledgeable exobiologists and some educated "guessing" were blended together to create an estimate for how well terrestrial microorganisms might thrive on alien worlds that had not yet been visited. Of course, today, as scientists keep learning more about the environments on other worlds in humans' solar system, they can keep refining their estimates for P(g). Just how well terrestrial life-forms survive or possibly even grow on the Moon, Mars, Venus, Europa, Titan, and a variety of other interesting celestial bodies is the subject of future in-situ (on-site) laboratory experiments that will be performed by exobiologists or their robot spacecraft surrogates.

As a point of aerospace history, the early U.S. Mars fyby missions (for example, Mariner 4, launched on November 28, 1964, and Mariner 6, launched on February 24, 1969) had P(c) values ranging from 4.5 x 10-5 to 3.0 x 10-5. These missions achieved successful fybys of the Red Planet on July 14, 1965, and July 31, 1969, respectively. Postflight calculations

Astronaut Charles Conrad, Jr., retrieves some equipment from the Surveyor 3 robot spacecraft during the Apollo 12 lunar landing mission in November 1969. Astronaut Alan L. Bean took this picture, and the lunar module Intrepid appears on the horizon in the right background. The Surveyor 3 spacecraft made a soft landing on the Moon on April 19, 1967. The Apollo astronauts brought back some of the equipment from Surveyor 3 so that NASA scientists, concerned with the issue of extraterrestrial contamination, could study the survival of hitchhiking terrestrial microorganisms after exposure to the harsh lunar environment for more than two and one-half years. The results were inconclusive. (NASA)

indicated that there was no probability of planetary contamination as a result of these successful precursor missions.

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