In the ongoing quest for extraterrestrial life, imagine the
day scientists discover life on another planet. Then imagine finding out that
those life forms actually originated on Earth.
With every spacecraft that leaves Earth, millions of
microbes try to hitch a ride
into outer space. NASA hopes to launch a Mars Sample Return Mission in the
future, and preventing cross-contamination of Mars and the Earth in such a
mission would be a top priority. A paper published in the September issue of Trends
in Microbiology takes a look at NASA's current planetary protection
policies and procedures.
"If terrestrial bugs make
it to Mars can they survive and grow, or would they be able to thrive in
extreme environments?" asks lead author of the paper Wayne Nicholson, who is a
professor at the Department of Microbiology & Cell Science at the University of Florida, Kennedy Space Center. "So far, our results show that they wouldn't
be able to grow."
Even so, NASA's planetary protection program is working to
improve procedures to make sure hitchhiker microbes traveling on robotic or
crewed vehicles don't contaminate
new worlds. Of primary concern is "back contamination," or the transferring
of extraterrestrial life forms back to Earth where they could harm the
biosphere. "Forward contamination" is when microbes from the Earth get
transported on outbound spacecraft to other planetary bodies, potentially
colonizing those environments.
"The highest priority is to prevent contamination of the
Earth," says Catharine Conley, NASA's planetary protection officer. "We have
never brought samples back from places where we think (life could exist). So
from a practical point, we are concerned with forward contamination."
Cautious From the Start
Back contamination was the main concern when the planetary
protection program first began. After the Soviet Union commenced space
exploration with the launching of the first Earth satellite Sputnik in 1957,
the International Council for Science established the Committee on Space
Research (COSPAR) to promote international space research. One of its
responsibilities was the planetary protection program.
The 1967 Outer Space Treaty formally outlined global guidelines
for any nation engaging in space exploration. It stated that any activities in
outer space, including the moon and other planetary bodies, must be carried out
in a manner that avoids
biological contamination.
Though the scientific community is expected to act
responsibly in exploring new frontiers, Larry Esposito, professor of
astrophysical and planetary sciences at the University of Colorado, Boulder, feels that back contamination is ultimately more important.
"The greatest dangers are back contamination even though the
probability might be lower," says Esposito. "The fact is that we're living on
the Earth and determining our livelihoods, and we're part of the ecological
(system), which could potentially be affected by back contamination."
In recent times, forward contamination also has become a
priority, particularly with the exploration of Mars and the potential for
finding life there.
In March of this year, astronauts on the International Space
Station conducted the LOCAD-PTS Exploration experiment, the first test of
planetary protection technology. This involved swabbing the astronauts' gloves
with a high-tech Q-tip and testing that Q-tip for microbes. The astronauts
found almost no bacteria but they found glucan, a marker for fungi, at 15
sites.
No Ticket to Ride
The protection procedures are determined by two factors: the
type of mission (flyby, orbiter, lander or rover) and how likely the
destination is to harbor life. A mission can fall into one of five categories,
with Category V consisting of "return to Earth" missions. Once the mission is
categorized, NASA uses a variety of procedures to reduce
microbial material on spacecrafts. All procedures take place in clean
rooms, designed with special air-flow systems to filter out contaminants.
Prior to cleaning, scientists sample the number of spores on
the spacecraft. Sterile cotton swabs are used to sample areas on the spacecraft
to obtain microbial samples. The collected samples are incubated to cultivate
spore colonies, and using complex algorithms, scientists then calculate the
total level of contamination.
NASA's current method for spacecraft sterilization is a
process called dry heat microbial reduction. It involves cleaning the
spacecraft and then baking it in an oven at 233 degrees Fahrenheit (112 degrees
Celsius) for 30 hours. It's the same process that was used on the Viking Mars
landers in 1970s. Conley says the cleaning reduces the initial 300 spores per
square meter to 0.03 spores per square meter. Statistically, since 0.03 doesn't
consist of a whole spore, the spacecraft is deemed safe from microbial
contamination.
NASA expects future missions to require more stringent
cleaning measures and is considering two particular methods for use in the near
future.
One method - the Limulus Amebocyte Lysate (LAL) assay -
searches for the presence of microbial cell wall materials. A method originally
used in the pharmaceutical industry, this has the advantage of being highly
sensitive, fast and reactive to both live and dead organisms.
The second method - Adenosine Triphosphate (ATP) assay -
detects ATP, which is an essential energy source generated by all living cells.
This method, too, detects both live and dead cells so it is a good indication
of biological contamination.
Proof of Life
Even with the rigorous cleaning procedures currently in
place, millions of microbes still get launched from Earth. How can scientists
be certain that any life found on another planetary body is not a result
of contamination?
"When searching for life, we don't want to have a false
positive," explains Esposito. "That would confuse identification of life on
another planet or location."
For identification of potential alien life, NASA has an
inventory of samples collected on previous missions (like Viking) to which new
life forms can be compared. Conley says current DNA-based testing methods allow
scientists to rule out the possibility that extraterrestrial life forms did not
originate on Earth. This means that scientists expect alien life to have DNA different
from any known organism on Earth (if they have DNA at all).
As astrobiologists continue the search for life in other
worlds, safeguarding the environment here on Earth and elsewhere in the
universe remains a key priority.
"We're concerned about life everywhere, and we don't want to
pollute or invade other locations with Earth life," says Esposito. "In the same
way, we don't want to infect life on Earth with species from other planets."
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