I. Accessible spacecraft debris currently on the Moon provides an ideal case study for
unanswered astrobiology and planetary protection questions
All spacecraft sent to the Moon have carried viable microorganisms with them, from contamination
associated with assembly, test and launch activities on Earth. Since September 1959, approximately 60
missions have delivered upwards of 80 spacecraft, boosters, payloads, rovers, and other structures to
the surface of the Moon. Historical measurements and recent mathematical models predict that even
after the effect of space exposure and, in some cases, high-velocity impact, most deliver a bioburden
of thousands to millions of cells each to the Moon (1, 2). While it is widely assumed that no unsheltered
life can indefinitely withstand the harsh physical conditions of the Moon, it is worth examining
whether any of the recently delivered Earth biomass remains viable, given that survivors have been
reported from multiple space biology exposure studies (3, 4).
Because the Moon’s surface environment is
sterilizing to life and the Moon itself is not
considered a body that will house any
indigenous life forms, NASA policy has not
required the elimination or even the
measurement of microbial contamination on
lunar space equipment prior to launch (5, 6).
Consequently, contaminated hardware
remaining on the moon provides an ideal case
study on the outcome of microbial dispersal on
pristine worlds. Upcoming Artemis program
missions could enable sampling and
investigation of this hardware, offering the
first ever impact survival data for refining
planetary protection protocols (PP) for solar
system exploration, as well as unique insight
for the field of astrobiology. We propose a
two-phased mission where astronauts on the
Moon would recover debris from previously
crashed spacecraft for return to Earth;
subsequently, samples could be analyzed in
terrestrial laboratories for viable life and
biosignatures (Fig. 1). This sample return
mission would be preceded by an imaging
mission to obtain high-resolution data on the target sampling site near the Lunar South Pole. We
propose the name of SOTERIA for this PP mission, in honor of the Greek goddess of safety and
delivery from harm.
Under COSPAR regulations, SOTERIA is a Category V “unrestricted Earth return” mission. Since
the Moon can host no indigenous life, samples may be returned to Earth without concern for back
contamination, and potential forward contamination from Earth outward must be mitigated with a
“documentation-only” PP plan corresponding to the Moon’s Category II designation. This applies to
the outbound leg only and must include pre- and post- launch reports, a post- encounter report, an
end-of-mission report, and inventory of all organic compounds onboard in quantities over 1 kg (7),
with the goal of preventing contamination of a body other than the Moon that could potentially host
indigenous life (i.e., Mars), should an unintentional impact occur (8). Although indigenous life cannot
Figure 1. Proposed mission architecture.
exist on the Moon, viable terrestrial contaminants could; studying the survival potential of these
contaminant organisms will help predict if and how microbes could survive on other astrobiologically-
relevant bodies, including Mars and Icy moons. Additionally, current COSPAR bioburden regulations
for Category III-IV missions apply to viable aerobic spore-formers only, but SOTERIA will elucidate
whether other types of microbes could survive impact, which could potentially widen the definition
of “bioburden” for future missions (9). Finally, if SOTERIA does not find viable microbes on the
lunar surface but does find dead ones or cellular debris, this could influence contamination control
planning for future life detection investigations targeting non-viable organic and biological matter (10).
As one of the three main themes of the 2013-2022 Planetary Science Decadal Survey, the Planetary
Habitats theme sought to define the conditions required for life and to understand the processes that
allow life to arise and persist in the universe. While much progress has been made, many questions
remain unanswered, and addressing them must constitute continuing goals in the coming decade. This
theme requires sending spacecraft to Mars and other planetary bodies without contaminating them
with terrestrial life, and assessing with certainty the provenance of any detected biosignatures.
SOTERIA directly addresses these high priority areas in two key aspects, by 1) providing data on the
survival of spacecraft-associated organisms in extraterrestrial conditions, directly informing questions
of forward contamination, and (2) serving as an opportunity to perform sample collection and analysis
for life detection in real planetary field conditions under the necessarily high levels of sterility and
contamination control that will be required at other targets in the search for extraterrestrial life.
II. Lunar debris was contaminated with terrestrial microbes – any survivors?
Lunar landers arrive with a significant microbial bioburden
Spacecraft leaving Earth carry microbiological contaminants onboard, including vehicles intentionally
and inadvertently impacted into the Moon’s surface. Recent PCR-based studies examining the
abundance and diversity of microbial populations in NASA spacecraft assembly clean rooms have
consistently found evidence of viable bacteria, archaea, eukaryotes, and viruses in these environments
(11, 12). And in earlier decades, regular surveys were conducted on six Surveyor and five Lunar Orbiter
vehicles between the years 1959 and 1973, enumerating both vegetative cells and spores (1). From
these and other studies, Schuerger et al. (2) estimated that lunar vehicles carry an average bioburden
at launch of ~3.89 x 106 viable spores and vegetative cells per kg of spacecraft dry mass.
Little is known about the effect of high-velocity impact forces (i.e. crashed spacecraft) on
The violent forces involved in crash landing on the Moon surface and their possible effect on the
spacecraft's microbial population must be considered, since other solar system missions use impacts
for “end of life” operations. Moreover, planetary exploration mission failures can also result in
uncontrolled high velocity impacts (e.g., the NASA Mars Polar Lander; the ISRO Chandraayan-2
Vikram lander). Fundamentally, how impact forces reduce spacecraft bioburden is unknown, and the
Moon can be used as the first real case study to inform survival outcomes for impacts at other solar
system destinations. More research is needed into how kinetic energy and heat dissipate in the process
of a spacecraft structure crashing, collapsing and breaking apart (2). Much of our knowledge of
microbial impact survival thus far is based on laboratory simulations aimed at constraining the
parameters around the potential of lithopanspermia (13), with a focus on spores of Bacillus subtilis,
given evidence of spores' stress resilience. Individual, unprotected B. subtilis spores have shown
survival through high velocity impacts up to 229 ± 28 m/s (14). Within spacecraft, survivability might
be increased due to cushioning; estimates for Bacillus sp. spore survival within rocks at impact velocities
of 1-5 km·s-1 have yielded survival rates between 0.1% and <10-6 of the starting spore densities (13–
15). Therefore, microbial spores might be able to withstand substantial impact forces in many cases,
but only a study of actual spacecraft samples could provide insight into the integrated effects of the
crash dynamics of those complex structures and the biocidal environment on the lunar surface.
Biocidal conditions on the Moon reduce bioburden, but not instantaneously
The Moon possesses no substantial atmosphere, nor a planetary magnetic field, so the surface is
exposed to the full influx of galactic and solar radiation. Moreover, lunar surface conditions are
extremely harsh, with high doses of ultraviolet (UV) irradiation (26.8 W·m-2 UVC/UVB), wide
temperature extremes (‒171°C to 140°C), and low pressure (10-10 Pa). Diverse experiments have tested
the effects of each of these stressors individually on microbial viability, again often with focus on
spore-forming organisms as the most likely source of forward contamination.
Thermal inactivation of microorganisms can be modeled as an exponential decline in viability with
rate dependent on temperature. A temperature of 100°C can incur a >−6 log10 reduction in the viability
of B. subtilis spores in fewer than two Earth days, whereas at 70°C the reduction is only between -3
and -4 logs over 56 days (1, 2). The temperatures experienced by spores or cells on lunar spacecraft
debris depend on both geographic location and the location of the organisms within the debris
(hardware can offer some thermal shielding). Most experiments testing the effects of space vacuum
(10-14 Pa) lasting <100 days have found survival of at least 30% (4, 16–20); survival is 10-25% after
327 days, and 1-2% after 6 years, if shielded from UV
radiation (21, 22). However, laboratory experiments
have found a synergistic interaction between low
pressure and high temperature; i.e., spores incubated
for 8 hours in the combined conditions of 100°C and
<10-4 Pa saw increased lethality of −6 logs relative to
each effect alone (2). Of all relevant biocidal factors,
solar UV radiation is likely to be the most potent on
exposed surfaces. As with temperature, UV exposure
leads to an exponential decline over time in viability
of spores of several Bacillus strains, with a flux of 9.78
W·m-2·min-1 (lower than that at the Moon's equator)
resulting in a decrease of −6 logs in less than 3 hours
(23). Like temperature, surface UV flux varies with
latitude and longitude, and hardware shielding can
substantially reduce lethality (21, 22, 24). Ionizing
radiation is another biocidal factor; however, cosmic
rays, solar wind particles, and solar particle events
have relatively low penetration depth and are
therefore relevant only to surface-associated cells and
spores, not those within spacecraft interiors (2).
The Lunar Microbial Survival model predicts the highest likelihood of surviving microbiota
on recently delivered spacecraft debris located near the Lunar South Pole
The above information about spacecraft bioburden at launch and the factors affecting microbial
survival were used by Schuerger and colleagues in the Lunar Microbial Survival (LMS) model (2) to
predict the abundance of viable microbial life that might still be present on the debris from the 54
Figure 2. The LMS model (2) predicts bacterial spores
might remain in the interiors of recently landed/crashed
spacecraft, particularly at polar latitudes where Artemis
and CLPS missions are planned. Here, each dot
represents one spacecraft with x- and y-coordinates
denoting its longitude and latitude, respectively; color
indicates the predicted bioburden (total number of
viable spores) on deep internal surfaces as of 2019.
missions since 1959 that have landed or crashed vehicles or components on the Moon. Outcomes
were calculated separately for external surfaces (exposed to all effects including all ionizing radiation),
shallow internal surfaces (in thermal contact to the exterior but shielded from other effects), and deep
internal surfaces (shielded from all effects except low pressure) (Fig. 2) (2). The LMS model predicts
at least a ‒231 log10 reduction in viable bioburden for external spacecraft surfaces per lunation at the
equator, but only ‒0.02 logs for deep internal surfaces. At the South Pole, a similar low rate would be
expected for permanently shadowed craters, while sun-exposed crater rims at the lunar poles had an
inactivation rate predicted at ‒4 logs per lunation.
At these rates, it was predicted that the 10 most
recently landed lunar spacecraft might all still carry viable microorganisms as of January 2019
(2). The highest bioburdens were predicted to be on the deep internal surfaces of spacecraft near the
Lunar South Pole (Fig. 2); of these, the Chandrayaan-1 Moon impact probe is remarkably close to the
locations being discussed for a NASA Artemis missions (see below).
We therefore propose that
some of the Chandrayaan-1 probe debris be collected for return to Earth, to test LMS model
predictions and assess the likelihood of forward contamination
III. A Lunar South Pole debris retrieval mission should be prioritized in the coming decade
The Moon is not a protected exploration target in PP, but its accessibility and the plans for human
missions on the surface offer an opportunity to determine whether spacecraft carrying bioburden can
result in viable forward contamination; or, alternatively, if impact forces and subsequent radiation
effects can sterilize contaminants. Such knowledge could refine PP approaches for spacecraft heading
to other solar system targets, sometimes resulting in hard impacts (purposeful or accidental).
There is precedent for this proposed mission architecture.
During the Apollo 12 mission, astronauts retrieved a portion
of the Surveyor III’s camera during a surface EVA in
November 1969 (Fig. 3) (25). Back on Earth, Mitchell and
Ellis (1971) reported that a single pure culture of Streptomyces
mitis was recovered from circuit board foam insulation that
was deeply embedded within the camera body. These results
suggested that the S. mitis cells had survived launch, space
travel, and 2.5 years of the harsh lunar conditions described
above (26). However, photographic and video evidence,
recently re-analyzed, show researchers very clearly handling
the lunar hardware without following currently acceptable
sterility procedures (27). Additional observations furthered
skepticism about the survival claim, including the fact that
concurrent analysis on wire cables of the Surveyor III were
unable to isolate any surviving microbial species, and that S.
mitis was among the organisms isolated from the crew during
microbial testing (27, 28). Similar analysis should be repeated
using debris retrieved from other landing/impact sites. With more modern, stringent, and sensitive
microbiological methods, assessment of recovered microbial cells and spores from lunar spacecraft
debris could be achieved with a lower chance of false positives or negatives compared to the effort by
Mitchell and Ellis (1971).
Recent image analysis identifies Chandrayaan-1 Moon Impact Probe as an optimal target
near the Lunar South Pole
Figure 3. Apollo 12 astronauts visit the
Surveyor III lander in 1969. From (25).
India’s Moon Impact Probe, part of the Chandrayaan-1 mission to the Moon, was launched from
Sriharikota, India on 22 October 2008, released by the orbiter, and braked to descend to the surface
on 14 November 2008, impacting near the south pole while imaging a swath down the 14° E meridian
from the equator to the pole. The exact coordinates of impact have been uncertain until recently.
Initial attempts to locate the impact site involved matching MIP descent images (29) with the best
available map of the time, the Clementine global mosaic, and following them down to the impact site;
locating the final images taken near the pole was challenging. The most specific published impact
location was 89.76° S, 39.40° W (320.60° E), reported by ISRO to the United Nations in 2009 (30).
More recent, high-resolution polar
mosaics have been produced from
Lunar Reconnaissance Orbiter Narrow
Angle Camera (NAC) images; these
enable unambiguous location of the last
few MIP images. Figure 4A shows the
MIP groundtrack with frames on a base
map constructed from LRO data. The
last few MIP frames were taken from
ISRO (2011) and those preceding it
were taken from ISRO video sequences.
The final frames pass west of the
previous published location and the
pole, ending with frame 3105 (Figure
4B) on the eastern wall of a crater
earthwards of the high ridge connecting
the Shackleton and De Gerlache rims
(“Connecting Ridge”). Although 3110
images were taken, frame 3105 is the last
one presently known. The dataset is not
part of ISRO’s formal lunar data release.
Using these resources, the LRO
topography, MIP image locations and
estimated spacecraft altitudes based on
the footprint dimensions of the images
(31), an impact site can be estimated.
The impact apparently occurred at
89.44° +/- 0.01° S, 130.3° +/- 4.0° W
(229.7° +/- 4.0° E). The longitude uncertainty is largely along track and dependent on the spacecraft
altitude. This location is on the Earth-facing flank of the Connecting Ridge, one of the most
illuminated points on the Moon and one considered for an early NASA Artemis Program landing, for
example, by Coan and colleagues (32)). This MIP impact location is 1500 m from Coan’s proposed
In addition to the Chandrayaan-1 MIP, we have identified three other high-priority target locations
near the South Pole: the NASA Lunar Prospector orbiter (crashed 31 January 1999 at 87.5° S/42.3°
E), the NASA LCROSS shepherding spacecraft and Centaur upper stage (crashed 9 October 2009 at
84.7° S/311° E), and the ISRO Chandrayaan-2 Vikram lander (crashed 6 September 2019 at 70.9 °S/
Figure 4. Descent track and probable landing site of the
Chandrayaan-1 Moon Impact Probe, inferred from descent images.
22.8°E). These sites might serve as alternatives or complements to the Chandrayaan-1 site. The NASA
spacecraft would avoid the requirement, posed by the Chandrayaan craft, for diplomatic negotiation
with India to obtain cooperation on crash site identification and permission for use of the debris.
However, as they lie within permanently shadowed regions, they are more difficult to access.
SOTERIA Phase 1: Image
Before sending astronauts to recover samples, a precursor imaging mission is required to map out the
Chandrayaan-1 debris field and potentially also the Lunar Prospector, LCROSS, and Chandrayaan-2
impact sites. This mission could be carried out by lunar orbiters with high-resolution cameras, and/or
future Commercial Lunar Payload Services (CLPS) landers.
SOTERIA Phase 2: Gather
SOTERIA's second phase would consist of collecting debris from the impact site and returning it to
Earth for microbiological analysis. Only a few kilograms of material would be required, and the
required crew training would be minimal. Unlike other surface science investigations, an untrained eye
could likely identify metallic spacecraft debris amongst lunar rocks/regolith, and crewmembers could
place samples in pre-sterilized containers, eliminating additional steps for maintaining aseptic
conditions. If lack of a Lunar Terrain Vehicle (LTV) precludes execution during the first Artemis
mission, this phase could be carried out on future crewed missions. Moreover, if the LTV can be
remotely operated and equipped with a motorized arm, the LTV could recover and bag the debris and
transport it to a crew-accessible location for pickup by the next crewed mission. An added advantage
of this procedure could be the lowered possibility of contamination by crew handling.
Earth-based analyses should be diverse
Returned spacecraft debris samples could undergo a suite of microbiological assays in modern
laboratories. We propose analysis for enumeration of culturable organisms, detection of non-
culturable viable organisms, and analysis of biosignatures that would pose a contamination risk to
detection of novel life on other planets (Table 1).
Table 1. Recommended assays to be carried out on returned lunar samples
Enumeration of culturable
spores & cells
Swab- or wipe-rinse sampling &
culturing in Trypticase Soy Agar
Procedures from NASA Handbook for the
Microbial Examination of Space Hardware. (33)
of all viable organisms,
including novel species,
Viability PCR: treatment with
propidium monoazide to inactivate
nonviable DNA, followed by 16S
rRNA gene amplicon or
Verified by NASA Planetary Protection
researchers to distinguish between viable and non-
viable organisms in spacecraft assembly
cleanrooms (11, 12, 34). Untargeted assay.
based identification of
Verified by NASA Planetary Protection
researchers for identification of low-abundance
microorganisms in spacecraft assembly
cleanrooms, with higher sensitivity than PCR. (35)
Visual observation of
intact microbial cells
Scanning electron microscopy
Previously used for analysis of biogenic signatures
on meteorites, e.g. (36)
Detection of biogenic
Deep UV resonance Raman and
Similar to in situ analysis by SHERLOC on Mars
Perseverance rover for biosignature detection
Identification of biogenic
organic compounds (e.g.,
lipids, amino acids)
Organic extraction followed by gas
Standard laboratory methods for analysis of
biological compounds, as in (37)
Controlled exposure experiments could complement the recovery of debris collection
Because many factors of the initial bioburden of the debris are unknown, a complementary exposure
mission experiment could carry a known density of microbial cells, including several different species
within diverse materials and shaped objects, to the Moon for controlled exposures. This type of
experiment would help with interpreting results from any recovered debris, and validation of the LMS
model. The science payloads could be simply deployed during Artemis landings and collected at a later
time point for concurrent laboratory analysis on Earth alongside debris assessments.
Even a negative result would be valuable
Data gathered from genuine space artifacts would fill a unique and important knowledge gap in
astrobiology, planetary exploration and space biology. The proposed SOTERIA mission takes
advantage of a natural experiment: hardware from previous missions readily accessible on the Moon.
The feasibility of this mission, particularly its proximity to planned Artemis landing sites near the
Lunar South Pole, makes it especially promising.
Recovering live microorganisms is not the goal
of this mission concept; a negative result, in which even the most sensitive microbiological
assays fail to detect viable cells, would also be valuable to the planetary science community
by helping to place current COSPAR PP regulations and procedures in perspective with data
derived from another world for the first time in the history of spaceflight.
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