Why space experiments?
Space provides a unique environment for performing astrobiology and astrochemistry experiments. Ground-based research is useful for studying the impact of environmental factors on the origin and evolution of life on Earth, and typically provides access to standardized reproducible conditions allowing quick repetitions of experiments, larger samples sizes, higher sample numbers, precise control of physicochemical parameters and an increase in the variety and resolution of analytical techniques at typically lower cost, when compared to space-based experiments. However, ground-based research can currently only be used for assessing single (or a limited sub-set of) space-based environmental factors, and as such provides only limited information on the combined influence of these factors. Experiments performed in space allow the study of effects induced by microgravity, by the wide spectrum of photons and energetically charged particles, as well as their combined effects on samples to be studied. To gather a complete and robust picture of influence of the space environment, a complementary approach must be utilized, exploiting the strengths of both in situ experimentation and ground-based research.
Within the context of searching for signs of life, the rationale for missions with the aim of visiting other celestial bodies (e.g., Mars) is mostly self-evident; however, remote-sensing platforms must also be tested and implemented. In addition, space-based experiments that focus on cellular life cycles, adaptation, biomineralization and fossilization processes must often be complemented by diverse ground-based experiments.
Commonalities and properties of existing and planned platforms have to be identified to better define the experimental requirements and limitations of specific space platforms, and their suitability for astrobiology and astrochemistry experiments must be assessed. With this assessment, it is possible to decide how best to utilize space experiments to address key astrobiology and astrochemistry topics. Figure 2 illustrates potential locations for a number of space platforms, their distance from Earth and the potential range of mission durations. The distance from Earth and the mission duration give an initial indication of the possibilities of these platforms and are important characteristics for various astrobiology and astrochemistry experiments. For example, distance and duration are correlated with the type and amount of radiation that targets would receive.
Space exposure experiments require suitable platforms for providing levels of radiation and microgravity. Platform location dictates mission duration, radiation exposure, the potential for sample return and the necessity of in situ measurements. As the distance from Earth increases, different radiation environments become available at the cost of increasingly challenging sample return.
Why experiments in specific orbits/locations?
Space-based experiments in certain low Earth orbits (LEOs), or on the Moon and Mars, allow access to higher fluxes of high-energy photons, galactic cosmic rays and solar energetic particles compared to the terrestrial environment. Specific locations, however, can have vastly different radiation levels. For example, the Moon receives a very high radiation dose whereas the level on Mars is lower due to its thin atmosphere. To constrain radiation-driven processes and examine their effects on biology, simultaneous ground-based and space experiments are needed. An advantage of space experiments, especially in the field of astrochemistry, is that platforms can be designed and operated far from sources of terrestrial or artificial contamination (e.g., atmospheric pollution, outgassing events from larger platforms, vibrations and electromagnetic interferences). Similarly, remote-sensing methods that use telescope optics to collect spectral data in the near-infrared (IR) to radio wavelength ranges, as well as visible and UV, function most optimally beyond the Earth’s atmosphere.
Within the context of finding signs of life in our own solar system, it is clear that sending probes to planets and moons of interest is the most efficient way to search for signatures of extant or extinct life. The nature of these missions is highly dependent on the environment of the world under investigation. For example, in the case of Europa, current projects are solely orbital, relying on remote-sensing techniques as well as encountering ejecta from the surface of the moon itself36,37, even though mission proposals for in situ investigations are discussed. On the other hand, previous, current and future missions to Mars and Titan include significant landing modules to study the surface directly38,39. A special case, Saturn’s moon Enceladus ejects ice particles from its subsurface ocean into space via south polar “cryovolcanoes”, providing fly-by missions the opportunity to examine recently frozen water for life’s signatures40.
How long would the mission duration need to be?
In order for large-scale space-based facilities (e.g., the ISS, the James Webb Space Telescope, or the planned PLATO spacecraft41) to make fiscal sense, their lifetimes must be typically on the order of decades. However, the advent of SmallSats and CubeSats (e.g., O/OREOS42, SpectroCube7, IR-COASTER5, BioSentinel8,9) are currently challenging this assumption. Short-term exposure experiments (e.g., BIOPAN43) should be used as predecessors or viability assessments for long-term exposure experiments, and small-scale missions (e.g., Twinkle44, CUTE45) should be used to support multi-decade lifespan spacecraft. This implies that miniaturization of existing technology is of the utmost importance.
The study of photochemical processes and reaction pathways typically requires several months of radiation exposure in, for example, LEO to accrue a total radiation dose that produce measurable effects, leading to overall mission times on the order of one year. Similarly, radiation-biological effects on some extremophilic microorganisms require months of exposure to accumulate. However, this must be assessed based on the tolerances of the organism under investigation, as well as the specific location of the experimental platform. Finally, to assess the long-term, cumulative effects and adaptations to space radiation, radiotolerant and extremophilic organisms (and organisms with resistant forms, e.g., spores) should be exposed for long durations. The importance of time taken to reach a destination becomes even more important for missions further afield, such as to Mars or the moons of Jupiter and Saturn. In these cases, there is a minimum mission duration ranging from months to a decade or more with current propulsion technologies.
What mode of operation is required?
In the past, space-exposure platforms have relied on sample-return experiments (e.g., the Long-Duration Exposure Facility46, EURECA, EXPOSE-E, EXPOSE-R, EXPOSE-R247,48,49,50), and while such methods provided access to the LEO environment, the lack of time-resolved data was a major limit to the conclusions that could be drawn. The collection of data during space missions is highly desirable for future space experiments. Such in situ analyses are an important way of adding redundancy and reducing the risks of space missions, while at the same time providing a more detailed, comprehensive data set compared to experiments relying solely on pre/post-flight analysis.
When designing future space facilities, organic compounds of prebiotic interest, cellular and molecular biosignatures, as well as both the fossilized remains and live microorganisms should be studied under plausible space and planetary conditions, with variable but known and controlled radiation, pressure, and temperature parameters51. To investigate a wide range of environmental parameters, space-based experimental facilities should implement dynamic humidity levels, wet/dry cycling and freeze/thaw cycles with the possibility of real-time analyses to follow any changes encountered. New facilities should allow for in situ thermal control and the possibility to simulate cool planets (e.g., N2 cooling cycles), icy moons, comets, and interstellar-medium conditions (e.g., He cooling).
For experiments involving living organisms, multiple generations of live, metabolically active organisms should be exposed via small payloads that implement fine temperature control, relative humidity, pressure, pH, atmospheric composition, nutrient/reagent supply, and the removal of waste products (liquids and gases). Bioreactors and microevolution chambers require further development and optimization, for instance microfluidic systems can implement fine control of a variety of environmental parameters. Microwells, each with independent fluidic inlets and outlets, can be utilized for a large number of low-volume microbial growth experiments, operated in parallel52. Experiments with living systems (such as NASA’s BRIC53, BioCell Habitat54) require automatic assay, often including subsampling at regular intervals, in situ telemonitoring (observing the appropriate functions, e.g., metabolism, genetic transcription and translation, self-repair mechanisms and quantification of adaptions), and the capacity for adjustments to be made via telecommand.
Although in situ analysis is currently the most effective means to acquire data from interplanetary probes, sample return or (ex situ) lab analogs are highly informative and complementary to such in situ analyses. To learn the most from in situ biological experiments conducted in space or planetary environments, following exposure, it is highly desirable to preserve and return samples to Earth for in-depth, laboratory-based studies. Of particular interest is the genomic, proteomic, transcriptomic & metabolomic influence of the space environment. This will require standardization of experimental protocols for selected, well-studied model organisms of interest, allowing gathered in space to be compared between experiments data.
In general, analytical techniques should have a dual function: on the one hand, to give extensive information on the processes at work, and on the other hand, to allow comparison with astronomical data and data from space missions38,55. In addition to experiments focusing on the exposure of samples to the space environment, methods must be designed to process and handle samples returned from space missions, with particular emphasis on planetary protection and life detection. In this regard, space platforms with frequent access (e.g., ISS) are ideal to test sample-return scenarios for interplanetary missions (e.g., Mars).
Which (in situ) analyses are foreseen?
While several biological methods and technologies have recently been adapted to space conditions with operation by human crew (e.g., DNA extractions56, the FLUMIAS live cell imaging microscope57, and RT-PCR instruments58), to fully understand the scope and details of the impacts of extended durations in the space environment upon terrestrial organisms, it is imperative to continue advancing space-compatible cellular analytical techniques, such as qPCR58, high-throughput sequencing, fluorescence-activated cell sorting59 or sub-cellular microscopic techniques. In addition to studies of monocultures of extremophilic microbes, biological interactions, such as biofilms, symbionts or microbial communities may result in increased resistance to the environmental stressors of space. As such, focus should be placed on understanding how a given biological interaction influences survivability and adaptation, along with the identification of keystone species that are particularly influential. For a detailed analysis of such community samples, both in situ and postexposure analyses (with instrumentation not available for in-flight measurements) are typically required.
In order for landed missions, such as the Mars surface rovers (mars.nasa.gov/msl/, mars.nasa.gov/mars2020/) to aqcuire evidence that could point to (extant or extinct) extraterrestrial life and to more generally understand the organic chemical history of other bodies in our solar system, they require sophisiticated in situ measurement and data analysis capabilities. Samples such as rock cores can be examined in both a geological context and in the search for organic molecules, having the potential to provide information on the decay of biomarker molecules or life cycle processes. To obtain such information, landers and rovers use a range of observational techniques (surface imaging via radar, cameras and microscopes) in combination with various in situ spectroscopic and spectrometric methods. Gas chromatography mass spectrometry (GC-MS) is a cornerstone in situ analytical technique for landers60 and is currently the only way to detect enantiomeric excess of chiral molecules in situ61. Prominent examples of its use on Mars include the Viking missions of the mid 1970s62 and the Mars Science Lander63 that continues to operate on the Martian surface. Sample mapping and composition analysis is commonly performed using a variety of spectroscopic techniques, including UV–vis absorption and UV–vis fluorescence measurements, transmission and reflection Fourier-transform IR microscopy, Raman, Mössbauer, X-ray diffraction and X-ray fluorescence, and laser-induced breakdown spectroscopy7,42,64,65,66. Other useful techniques for analyzing both organic and inorganic species, currently being miniaturized for in situ use include laser ablation and laser ablation ionization mass spectrometry67,68.
The current technologies outlined above are being utilized for specific selection of candidate samples, which can later be returned to Earth for more detailed study (e.g., the current Mars Sample Return campaign69). However, looking to the future, more advanced in situ techniques could be miniaturized and implemented, potentially alleviating the need for sample return and thus reducing overall mission complexity and cost. For example, in situ MS is a very relevant analytical technique, recently used by the Mars Science Laboratory70 to analyze gases and sublimated species released by thermal means. However, heating a sample to high temperatures may release volatile organic species that can trigger chemical reactions or the degradation of potential biomarkers. As such, new technologies are being developed to extract soluble organics from solid (irradiated) samples at mild temperatures using solvent-based techniques, without degradation71; following such extraction, various high-resolution MS and tandem-MS techniques can be employed to understand the nature and provenance of the organic signatures by measuring structural information as well as the extent of the “decay” (alteration over time) of molecular structures.
Which platform would be best suited?
The launch and maintenance of large-scale space platforms (such as space-based telescopes or manned platforms) require huge, dedicated, often multinational, space agency missions. The ISS remains an important exposure platform for both short- and long-term experiments, with the possibility for sample return. Furthermore, the ISS can be utilized as a test platform for future developments and the technological heritage from the ISS can be re-utilized on other platforms. The Lunar Gateway is progressing toward hosting such experiments in the Moon’s vicinity in a matter of years; nonetheless, nanosatellites, CubeSats, and SmallSats are becoming increasingly robust and readily available. They have proved capable of providing complementary information and thus are opening the field of study in this regard (e.g., Pandora72). SmallSats allow for studies ranging from the time-dependent alteration of molecules exposed to particle and electromagnetic radiation, to mimicking conditions on small bodies, to studies of the impact of the space environment on organics in meteorites, and they are showing that astrochemistry exposure experiments can be done outside of traditional platforms such as the ISS. These platforms are potentially also well suited for space biology experiments that expose living organisms over multiple generations to microgravity in combination with levels and distributions of energetic particle radiation only available beyond LEO. To execute such studies effectively the experimental durations aboard these small platforms need to be extended to (many) months to accumulate total radiation dosage with measurable biological effects. As mentioned previously, the continued development of highly sensitive and sophisticated autonomous bioanalytical systems with potential to measure genetic parameters, -omics, and other key biological properties is required.
Nevertheless, life-detection experiments requiring surface landers continue to require costly, dedicated missions. For a lander to make an unambiguous set of measurements that either support or refute a finding of the presence of life on another world, multiple complementary and synergistic analytical methods will most likely be required. As such, larger-scale platforms (relative to CubeSats and SmallSats) are required in this scenario, first to allow for landing, and second to house the required suite of sophisticated analytical tools that are typically too bulky for small platforms. A drawback of SmallSats is the lack of sample-return capabilities in most cases, which is necessary to investigate a variety of cellular effects on ground with a suite of sophisticated instruments not (yet) available in and beyond LEO.
Living metabolically active organisms, cellular processes or community composition may be directly influenced by non-Earth gravity (either micro- or hypergravity). Direct, in situ investigations (using exposure platforms) will solve many issues associated with simulated gravity experiments73. Similar experiments with high fluxes of galactic cosmic rays and solar particles are required, especially in preparation for human exploration. A main focus of space platforms is the combined influence of microgravity and varying space-radiation conditions. These can be compared against laboratory facilities (clinostat, simulated solar radiation, gamma radiation sources, heavy ion accelerators, electron beam facilities, X-ray sources, etc). Additionally, new space facilities (such as the Lunar Orbital Gateway) will provide a novel environment in which the establishment of a new microbiome can be studied. While a similar capability for a more limited class of experiments is in principle also feasible with SmallSats or CubeSats, the Lunar Orbital Gateway will be distinct, given the limited human presence and potential for long-term monitoring. A “clean” and isolated environment such as this is unique, and thus monitoring of the microbiome over time could provide valuable insights and important information for future habitats on the Moon or Mars.
In addition to focusing on changes induced by the space environment, experiments to determine the transformative effects from the process of re-entry have also been performed6,74,75. Samples such as Martian sedimentary rocks containing organic material have been placed in the heat shield of craft returning from LEO, subjecting them to extreme temperatures and a high-velocity plasma environment that is incredibly challenging to replicate ex situ. The survivability of organisms and/or the degradation of biomolecules should be assessed under the extreme pressure and temperature conditions of atmospheric re-entry and surface impact. Such experiments should be conducted on platforms (e.g., STONE76,77), re-entry nanosatellites, or as external additions on larger returning spacecraft. The potentially protective influence of rocky body-associated minerals must also be accounted for. With regard to both forward (contamination from Earth carried to other bodies) and reverse (extraterrestrial organisms brought back to Earth) planetary protection, both internal and external biocontamination must be assessed at the molecular level; a process that is also mandatory for search-for-life missions in order to eliminate serious risk of false positives.
Recommended space-exposure payloads: short and medium term
The key questions in each of the topics presented in the introduction of this perspective can be addressed by specific space experiments on either multi-experiment space-exposure facilities or by means of tailored space platforms. Short (next 3 years) and medium (next 5 years) term recommendations for experiments in the key area A, “understanding the origins of life” include the design and implementation of experiments with active analytical capabilities (e.g., in situ spectroscopy and mass spectrometry) and active environmental control. In particular, platforms capable of maintaining sample temperatures well below 0 °C, ideally even at temperatures as low as <100 K, are required for a next generation of astrochemistry experiments and investigations of ice-organics mixtures, icy-moon, and interstellar-medium conditions. A further recommendation is to perform such experiments in locations with minimal terrestrial pollution, for example avoiding outgassing events from larger facilities such as the ISS. In addition to exposure experiments, platforms designed for re-entry into the Earth’s atmosphere are recommended to advance our understanding of meteoritic impact processes. Such platforms should be capable of carrying and analyzing samples either in situ or after hardware retrieval.
In key area B, “understanding habitability and the limits of life”, recommended experiments and platforms should focus on the impact of the space environment on living (micro)organisms, in particular, the combination of radiation and microgravity. Paired with in situ analytical capabilities, these experiments should study the response and adaptation of living systems to multiple stresses that can be monitored directly in space. This will likely require microfluidic and liquid-handling systems that function reliably in space. In addition to in situ data, sample return for in-detail analysis after the space-exposure phase is highly desirable. Re-entry platforms are recommended for the study of impact scenarios and how they affect either actively-growing or dormant living organisms.
With the focus of key area C, “understanding the signs of life”, being on detectability and identification of potential biosignatures, experiments in this area must be capable of simulating space and planetary conditions, including the respective radiation environment (electromagnetic and particle radiation). This can be achieved by designing and implementing space-exposure platforms that can access specific radiation environments, e.g., low- or highly elliptical orbits around Earth, the moon or interplanetary platforms. In situ analysis will be a key tool for such experiments investigating the stability or alteration of specific biosignatures (in the solid or gas phase) under conditions mimicking space and (exo)planetary conditions.
