Interplanetary contamination

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Interplanetary contamination refers to biological contamination of a planetary body by a space probe or spacecraft, either deliberate or unintentional.

There are two types of interplanetary contamination:

• Forward contamination is the transfer of life and other forms of contamination from Earth to another celestial body.
• Back contamination is the introduction of extraterrestrial organisms and other forms of contamination into Earth's biosphere. It also covers infection of humans and human habitats in space and on other celestial bodies by extraterrestrial organisms, if such habitats exist.

The main focus is on microbial life and on potentially invasive species. Non-biological forms of contamination have also been considered, including contamination of sensitive deposits (such as lunar polar ice deposits) of scientific interest. In the case of back contamination, multicellular life is thought unlikely but has not been ruled out. In the case of forward contamination, contamination by multicellular life (e.g. lichens) is unlikely to occur for robotic missions, but it becomes a consideration in crewed missions to Mars.

Current space missions are governed by the Outer Space Treaty and the COSPAR guidelines for planetary protection. Forward contamination is prevented primarily by sterilizing the spacecraft. In the case of sample-return missions (back contamination) the aim of the mission is to return extraterrestrial samples to Earth, and sterilization of the samples would make them of much less interest. So, back contamination would be prevented mainly by containment, and breaking the chain of contact between the planet of origin and Earth. It would also require quarantine procedures for the materials and for anyone who comes into contact with them.

Overview

Most of the Solar System appears hostile to life as we know it. No extraterrestrial life has ever been discovered, but there are several locations outside Earth where microbial life could possibly exist, have existed, or thrive if introduced. If extraterrestrial life exists, it may be vulnerable to interplanetary contamination by foreign microorganisms. Some extremophiles may be able to survive space travel to another planet, and foreign life could possibly be introduced by spacecraft from Earth and transform the location from its current pristine state. This poses scientific and ethical concerns.

Locations within the Solar System where life might exist today include the oceans of liquid water beneath the icy surface of Europa, and Enceladus, and Titan (its surface has oceans of liquid ethane / methane, but it may also have liquid water below the surface and ice volcanoes).

There are multiple consequences for both forward- and back-contamination. If a planet becomes contaminated with Earth life, it might then be difficult to tell whether any lifeforms discovered originated there or came from Earth. Furthermore, the organic chemicals produced by the introduced life would confuse sensitive searches for biosignatures of living or ancient native life. The same applies to other more complex biosignatures. Life on other planets could have a common origin with Earth life, since in the early Solar System there was much exchange of material between the planets which could have transferred life as well. If so, it might be based on nucleic acids too (RNA or DNA).

This is true of species detected in spacecraft cleanrooms. The majority of the species isolated are not well understood or characterized and cannot be cultured in labs, and are known only from DNA fragments obtained with swabs. On a contaminated planet, it might be difficult to distinguish the DNA of extraterrestrial life from the DNA of life brought to the planet by the exploring. Most species of microorganism on Earth are not yet well understood or DNA sequenced. This particularly applies to the unculturable archaea, and so are difficult to study. This can be either because they depend on the presence of other microorganisms, or are slow growing, or depend on other conditions not yet understood. In typical habitats, 99% of microorganisms are not culturable. Introduced Earth life could contaminate resources of value for future human missions, such as water.

Invasive species could outcompete native life or consume it, if there is life on the planet. One argument against this is that the native life would be more adapted to the conditions there. However, the experience on Earth shows that species moved from one continent to another may be able to outcompete the native life adapted to that continent. Additionally, evolutionary processes on Earth might have developed biological pathways different from extraterrestrial organisms, and so may be able to out-compete it. The same is also possible the other way around for back contamination introduced to Earth's biosphere.

In addition of science concerns, ethical or moral issues have also been raised on accidental and intentional interplanetary transport of life.

Evidence for possible habitats outside Earth

Enceladus and Europa show the best evidence for current habitats, mainly due to the possibility of their hosting liquid water and organic compounds.

Mars

There is ample evidence to suggest that Mars once offered habitable conditions for microbial life. It is therefore possible that microbial life may have existed on Mars, although no evidence has been found.

It is thought that many bacterial spores (endospores) from Earth were transported on Mars spacecraft. Some may be protected within Martian rovers and landers on the shallow surface of the planet. In that sense, Mars may have already been interplanetarily contaminated.

Certain lichens from the arctic permafrost are able to photosynthesize and grow in the absence of any liquid water, simply by using the humidity from the atmosphere. They are also highly tolerant of UV radiation, using melanin and other more specialized chemicals to protect their cells.

Although numerous studies point to resistance to some of Mars conditions, they do so separately, and none has considered the full range of Martian surface conditions, including temperature, pressure, atmospheric composition, radiation, humidity, oxidizing regolith, and others, all at the same time and in combination. Laboratory simulations show that whenever multiple lethal factors are combined, the survival rates plummet quickly.

Other studies have suggested the potential for life to survive using deliquescing salts. These, similarly to the lichens, use the humidity of the atmosphere. If the mixture of salts is right, the organisms may obtain liquid water at times of high atmospheric humidity, with salts capture enough to be capable of supporting life.

Research published in July 2017 shows that when irradiated with a simulated Martian UV flux, perchlorates become even more lethal to bacteria (bactericide effect). Even dormant spores lost viability within minutes. In addition, two other compounds of the Martian surface, iron oxides and hydrogen peroxide, act in synergy with irradiated perchlorates to cause a 10.8-fold increase in cell death when compared to cells exposed to UV radiation after 60 seconds of exposure. It was also found that abraded silicates (quartz and basalt) lead to the formation of toxic reactive oxygen species. The researchers concluded that "the surface of Mars is lethal to vegetative cells and renders much of the surface and near-surface regions uninhabitable." This research demonstrates that the present-day surface is more uninhabitable than previously thought, and reinforces the notion to inspect at least a few meters into the ground to ensure the levels of radiation would be relatively low.

Enceladus

The Cassini spacecraft directly sampled the plumes escaping from Enceladus. Measured data indicates that these geysers are made primarily of salt rich particles with an 'ocean-like' composition, which is thought to originate from a subsurface ocean of liquid saltwater, rather than from the moon's icy surface. Data from the geyser flythroughs also indicate the presence of organic chemicals in the plumes. Heat scans of Enceladus' surface also indicate warmer temperatures around the fissures where the geysers originate from, with temperatures reaching -93 °C (-135 °F), which is 115 °C (207 °F) warmer than the surrounding surface regions.

Europa

Europa has much indirect evidence for its sub-surface ocean. Models of how Europa is affected by tidal heating require a subsurface layer of liquid water in order to accurately reproduce the linear fracturing of the surface. Indeed, observations by the Galileo spacecraft of how Europa's magnetic field interacts with Jupiter's field strengthens the case for a liquid, rather than solid, layer; an electrically conductive fluid deep within Europa would explain these results. Observations from the Hubble Space Telescope in December 2012 appear to show an ice plume spouting from Europa's surface, which would immensely strengthen the case for a liquid subsurface ocean. As was the case for Enceladus, vapour geysers would allow for easy sampling of the liquid layer. Unfortunately, there appears to be little evidence that geysering is a frequent event on Europa due to the lack of water in the space near Europa.

Planetary protection

Forward contamination is prevented by sterilizing space probes sent to sensitive areas of the Solar System. Missions are classified depending on whether their destinations are of interest for the search for life, and whether there is any chance that Earth life could reproduce there.

NASA made these policies official with the issuing of Management Manual NMI-4-4-1, NASA Unmanned Spacecraft Decontamination Policy on September 9, 1963. Prior to NMI-4-4-1 the same sterilization requirements were required on all outgoing spacecraft regardless of their target. Difficulties in the sterilization of Ranger probes sent to the Moon are the primary reasons for NASA's change to a target-by-target basis in assessing the likelihood forward contamination.

Some destinations such as Mercury need no precautions at all. Others such as the Moon require documentation but nothing more, while destinations such as Mars require sterilization of the rovers sent there. For the details, see Planetary protection.

Back contamination would be prevented by containment or quarantine. However, there have been no sample-returns thought to have any possibility of a back contamination risk since the Apollo missions. The Apollo regulations have been rescinded and new regulations have yet to be developed, see Precautions suggested for sample-return

Crewed spacecraft

Crewed spacecraft are of particular concern for interplanetary contamination because of the impossibility to sterilize a human to the same level as a robotic spacecraft. Therefore, the chance of forward contamination is higher than for a robotic mission. Humans are typically host to a hundred trillion microorganisms in ten thousand species in the human microbiome which cannot be removed while preserving the life of the human. Containment seems the only option, but effective containment to the same standard as a robotic rover appears difficult to achieve with present-day technology. In particular, adequate containment in the event of a hard landing is a major challenge.

Human explorers may be potential carriers back to Earth of microorganisms acquired on Mars, if such microorganisms exist. Another issue is the contamination of the water supply by Earth microorganisms shed by humans in their stools, skin and breath, which could have a direct effect on the long-term human colonization of Mars.

The Moon as a testbed

The Moon has been suggested as a testbed for new technology to protect sites in the Solar System, and astronauts, from forward and back contamination. Currently, the Moon has no contamination restrictions because it is considered to be "not of interest" for prebiotic chemistry and origins of life.  Analysis of the contamination left by the Apollo program astronauts could also yield useful ground truth for planetary protection models.

Non-contaminating exploration methods

Telerobotics exploration on Mars and Earth

One of the most reliable ways to reduce the risk of forward and back contamination during visits to extraterrestrial bodies is to use only Robotic spacecraft. Humans in close orbit around the target planet could control equipment on the surface in real time via telepresence, so bringing many of the benefits of a surface mission, without its associated increased forward and back contamination risks.

Back contamination issues

Since the Moon is now generally considered to be free from life, the most likely source of contamination would be from Mars during either a Mars sample-return mission or as a result of a crewed mission to Mars. The possibility of new human pathogens, or environmental disruption due to back contamination, is considered to be of extremely low probability but cannot yet be ruled out.
There are no immediate plans for a Mars sample-return, but it remains a high priority for NASA and the ESA because of its great potential biological and geological interest. The European Space Foundation report cites many advantages of a Mars sample-return. In particular, it would permit extensive analyses on Earth, without the size and weight constraints for instruments sent to Mars on rovers. These analyses could also be carried out without the communication delays for experiments carried out by Martian rovers. It would also make it possible to repeat experiments in multiple laboratories with different instruments to confirm key results.

Carl Sagan was first to publicise back contamination issues that might follow from a Mars sample-return. In Cosmic Connection (1973) he wrote:

Precisely because Mars is an environment of great potential biological interest, it is possible that on Mars there are pathogens, organisms which, if transported to the terrestrial environment, might do enormous biological damage.

Later in Cosmos (1980) Carl Sagan wrote:

Perhaps Martian samples can be safely returned to Earth. But I would want to be very sure before considering a returned-sample mission.

NASA and ESA views are similar. The findings were that with present-day technology, Martian samples can be safely returned to Earth provided the right precautions are taken.

Suggested precautions for sample-returns

NASA has already had experience with returning samples thought to represent a low back contamination risk when samples were returned for the first time by Apollo 11. At the time, it was thought that there was a low probability of life on the Moon, so the requirements were not very stringent. The precautions taken then were inadequate by current standards, however. The regulations used then have been rescinded, and new regulations and approaches for a sample-return would be needed.

Chain of contact

A sample-return mission would be designed to break the chain of contact between Mars and the exterior of the sample container, for instance, by sealing the returned container inside another larger container in the vacuum of space before it returns to Earth. In order to eliminate the risk of parachute failure, the capsule could fall at terminal velocity and the impact would be cushioned by the capsule's thermal protection system. The sample container would be designed to withstand the force of the impact.

Receiving facility

Working inside a BSL-4 laboratory with air hoses providing positive air pressure to their suits

To receive, analyze and curate extraterrestrial soil samples, NASA has proposed to build a biohazard containment facility, tentatively known as the Mars Sample Return Receiving Facility (MSRRF). This future facility must be rated biohazard level 4 (BSL-4). While existing BSL-4 facilities deal primarily with fairly well-known organisms, a BSL-4 facility focused on extraterrestrial samples must pre-plan the systems carefully while being mindful that there will be unforeseen issues during sample evaluation and curation that will require independent thinking and solutions.

The facility's systems must be able to contain unknown biohazards, as the sizes of any putative Martian microorganisms are unknown. In consideration of this, additional requirements were proposed. Ideally it should filter particles of 0.01 µm or larger, and release of a particle 0.05 µm or larger is unacceptable under any circumstance.

The reason for this extremely small size limit of 0.01 µm is for consideration of gene transfer agents (GTAs) which are virus-like particles that are produced by some microorganisms that package random segments of DNA capable of horizontal gene transfer. These randomly incorporate segments of the host genome and can transfer them to other evolutionarily distant hosts, and do that without killing the new host. In this way many archaea and bacteria can swap DNA with each other. This raises the possibility that Martian life, if it has a common origin with Earth life in the distant past, could swap DNA with Earth microorganisms in the same way. In one experiment reported in 2010, researchers left GTAs (DNA conferring antibiotic resistance) and marine bacteria overnight in natural conditions and found that by the next day up to 47% of the bacteria had incorporated the genetic material from the GTAs. Another reason for the 0.05 µm limit is because of the discovery of ultramicrobacteria as small as 0.2 µm across.

The BSL-4 containment facility must also double as a cleanroom to preserve the science value of the samples. A challenge is that, while it is relatively easy to simply contain the samples once returned to Earth, researchers would also want to remove parts of the sample and perform analyses. During all these handling procedures, the samples would need to be protected from Earthly contamination. A cleanroom is normally kept at a higher pressure than the external environment to keep contaminants out, while a biohazard laboratory is kept at a lower pressure to keep the biohazards in. This would require to compartmentalize the specialized rooms in order to combine these in a single building. Solutions suggested include a triple walled containment facility, and one of the suggestions include extensive robotic handling of the samples.

The facility would be expected to take 7 to 10 years from design to completion, and an additional two years is recommended for the staff to become accustomed to the facilities.

Dissenting views on back contamination

Robert Zubrin, from the Mars Society, maintains that the risk of back contamination is negligible. He supports this using an argument based on the possibility of transfer of life from Earth to Mars on meteorites.

Legal process of approval for Mars sample-return

Margaret Race has examined in detail the legal process of approval for a MSR. She found that under the National Environmental Policy Act (NEPA) (which did not exist in the Apollo era) a formal environment impact statement is likely to be required, and public hearings during which all the issues would be aired openly. This process is likely to take up to several years to complete.

During this process, she found, the full range of worst accident scenarios, impact, and project alternatives would be played out in the public arena. Other agencies such as the Environment Protection Agency, Occupational Health and Safety Administration, etc., may also get involved in the decision making process.

The laws on quarantine will also need to be clarified as the regulations for the Apollo program were rescinded. In the Apollo era, NASA delayed announcement of its quarantine regulations until the day Apollo was launched, so bypassing the requirement for public debate - something that would be unlikely to be tolerated today.

It is also probable that the presidential directive NSC-25 will apply which requires a review of large scale alleged effects on the environment and is carried out subsequent to the other domestic reviews and through a long process, leads eventually to presidential approval of the launch.

Then apart from those domestic legal hurdles, there are numerous international regulations and treaties to be negotiated in the case of a Mars sample-return, especially those relating to environmental protection and health. She concluded that the public of necessity has a significant role to play in the development of the policies governing Mars sample-return.

Alternatives to sample-returns

Several exobiologists have suggested that a Mars sample-return is not necessary at this stage, and that it is better to focus more on in situ studies on the surface first. Although it is not their main motivation, this approach of course also eliminates back contamination risks.

Some of these exobiologists advocate more in situ studies followed by a sample-return in the near future. Others go as far as to advocate in situ study instead of a sample-return at the present state of understanding of Mars.

Their reasoning is that life on Mars is likely to be hard to find. Any present day life is likely to be sparse and occur in only a few niche habitats. Past life is likely to be degraded by cosmic radiation over geological time periods if exposed in the top few meters of the Mars surface. Also, only certain special deposits of salts or clays on Mars would have the capability to preserve organics for billions of years. So, they argue, there is a high risk that a Mars sample-return at our current stage of understanding would return samples that are no more conclusive about the origins of life on Mars or present day life than the Martian meteorite samples we already have.

Another consideration is the difficulty of keeping the sample completely free from Earth life contamination during the return journey and during handling procedures on Earth. This might make it hard to show conclusively that any biosignatures detected does not result from contamination of the samples.

Instead they advocate sending more sensitive instruments on Mars surface rovers. These could examine many different rocks and soil types, and search for biosignatures on the surface and so examine a wide range of materials which could not all be returned to Earth with current technology at reasonable cost.

A sample-return to Earth would then be considered at a later stage, once we have a reasonably thorough understanding of conditions on Mars, and possibly have already detected life there, either current or past life, through biosignatures and other in situ analyses.

Instruments under development for in situ analyses

• NASA Marshall Space Flight Center is leading a research effort to develop a Miniaturized Variable Pressure Scanning Electron Microscope (MVP-SEM) for future lunar and Martian missions.

• Several teams, including Jonathan Rothberg, and J. Craig Venter, are separately developing solutions for sequencing alien DNA directly on the Martian surface itself.
• Levin is working on updated versions of the Labeled release instrument flown on Viking. For instance versions that rely on detecting chirality. This is of special interest because it can enable detection of life even if it is not based on standard life chemistry.
• The Urey Mars Organic and Oxidant Detector instrument for detection of biosignatures has been descoped, but was due to be flown on ExoMars in 2018. It is designed with much higher levels of sensitivity for biosignatures than any previous instruments.

Study and analyses from orbit

During the “Exploration Telerobotics Symposium" in 2012 experts on telerobotics from industry, NASA and academics met to discuss telerobotics, and its applications to space exploration. Amongst other issues, particular attention was given to Mars missions and a Mars sample-return.

They came to the conclusion that telerobotic approaches could permit direct study of the samples on the Mars surface via telepresence from Mars orbit, permitting rapid exploration and use of human cognition to take advantage of chance discoveries and feedback from the results obtained so far.

They found that telepresence exploration of Mars has many advantages. The astronauts have near real-time control of the robots, and can respond immediately to discoveries. It also prevents contamination both ways and has mobility benefits as well.

Return of the sample to orbit has the advantage that it permits analysis of the sample without delay, to detect volatiles that may be lost during a voyage home. This was the conclusion of a meeting of researchers at the NASA Goddard Space Flight Center in 2012.

Telerobotics exploration of Mars

Similar methods could be used to directly explore other biologically sensitive moons such as Europa, Titan, or Enceladus, once the human presence in the vicinity becomes possible.

Panspermia

From Wikipedia, the free encyclopedia

 

Panspermia proposes that bodies such as comets transported life forms such as bacteria - complete with their DNA - through space to the Earth.

Panspermia (from Ancient Greek πᾶν (pan), meaning 'all', and σπέρμα (sperma), meaning 'seed') is the hypothesis that life exists throughout the Universe, distributed by space dust, meteoroids, asteroids, comets, planetoids, and also by spacecraft carrying unintended contamination by microorganisms.

Panspermia hypotheses propose (for example) that microscopic life-forms that can survive the effects of space (such as extremophiles) can become trapped in debris ejected into space after collisions between planets and small Solar System bodies that harbor life. Some organisms may travel dormant for an extended amount of time before colliding randomly with other planets or intermingling with protoplanetary disks. Under certain ideal impact circumstances (into a body of water, for example), and ideal conditions on a new planet's surfaces, it is possible that the surviving organisms could become active and begin to colonize their new environment. Panspermia studies concentrate not on how life began, but on the methods that may cause its distribution in the Universe.

Pseudo-panspermia (sometimes called "soft panspermia" or "molecular panspermia") argues that the pre-biotic organic building-blocks of life originated in space, became incorporated in the solar nebula from which planets condensed, and were further—and continuously—distributed to planetary surfaces where life then emerged (abiogenesis). From the early 1970s it started to become evident that interstellar dust included a large component of organic molecules. Interstellar molecules are formed by chemical reactions within very sparse interstellar or circumstellar clouds of dust and gas. The dust plays a critical role in shielding the molecules from the ionizing effect of ultraviolet radiation emitted by stars.

The chemistry leading to life may have begun shortly after the Big Bang, 13.8 billion years ago, during a habitable epoch when the Universe was only 10 to 17 million years old. Though the presence of life is confirmed only on the Earth, some scientists think that extraterrestrial life is not only plausible, but probable or inevitable. Probes and instruments have started examining other planets and moons in the Solar System and in other planetary systems for evidence of having once supported simple life, and projects such as SETI attempt to detect radio transmissions from possible extra-terrestrial civilizations.

History

The first known mention of the term was in the writings of the 5th-century BC Greek philosopher Anaxagoras. Panspermia began to assume a more scientific form through the proposals of Jöns Jacob Berzelius (1834), Hermann E. Richter (1865), Kelvin (1871), Hermann von Helmholtz (1879), and finally reaching the level of a detailed scientific hypothesis through the efforts of the Swedish chemist Svante Arrhenius (1903).

Fred Hoyle (1915–2001) and Chandra Wickramasinghe (born 1939) were influential proponents of panspermia. In 1974 they proposed the hypothesis that some dust in interstellar space was largely organic (containing carbon), which Wickramasinghe later proved to be correct. Hoyle and Wickramasinghe further contended that life forms continue to enter the Earth's atmosphere, and may be responsible for epidemic outbreaks, new diseases, and the genetic novelty necessary for macroevolution.

In an Origins Symposium presentation on April 7, 2009, physicist Stephen Hawking stated his opinion about what humans may find when venturing into space, such as the possibility of alien life through the theory of panspermia: "Life could spread from planet to planet or from stellar system to stellar system, carried on meteors."

Three series of astrobiology experiments have been conducted outside the International Space Station between 2008 and 2015 (EXPOSE) where a wide variety of biomolecules, microorganisms, and their spores were exposed to the solar flux and vacuum of space for about 1.5 years. Some organisms survived in an inactive state for considerable lengths of time, and those samples sheltered by simulated meteorite material provide experimental evidence for the likelihood of the hypothetical scenario of lithopanspermia.

Several simulations in laboratories and in low Earth orbit suggest that ejection, entry and impact is survivable for some simple organisms. In 2015, remains of biotic material were found in 4.1 billion-year-old rocks in Western Australia, when the young Earth was about 400 million years old. According to one researcher, "If life arose relatively quickly on Earth … then it could be common in the universe."

In April 2018 a Russian team published a paper which disclosed that they found DNA on the exterior of the ISS from land and marine bacteria similar to those previously observed in superficial micro layers at the Barents and Kara seas' coastal zones. They conclude "The presence of the wild land and marine bacteria DNA on the ISS suggests their possible transfer from the stratosphere into the ionosphere with the ascending branch of the global atmospheric electrical circuit. Alternatively, the wild land and marine bacteria as well as the ISS bacteria may all have an ultimate space origin."

Proposed mechanisms

Panspermia can be said to be either interstellar (between star systems) or interplanetary (between planets in the same star system); its transport mechanisms may include comets,  radiation pressure and lithopanspermia (microorganisms embedded in rocks). Interplanetary transfer of nonliving material is well documented, as evidenced by meteorites of Martian origin found on Earth. Space probes may also be a viable transport mechanism for interplanetary cross-pollination in the Solar System or even beyond. However, space agencies have implemented planetary protection procedures to reduce the risk of planetary contamination, although, as recently discovered, some microorganisms, such as Tersicoccus phoenicis, may be resistant to procedures used in spacecraft assembly clean room facilities. In 2012, mathematician Edward Belbruno and astronomers Amaya Moro-Martín and Renu Malhotra proposed that gravitational low-energy transfer of rocks among the young planets of stars in their birth cluster is commonplace, and not rare in the general galactic stellar population. Deliberate directed panspermia from space to seed Earth or sent from Earth to seed other planetary systems have also been proposed. One twist to the hypothesis by engineer Thomas Dehel (2006), proposes that plasmoid magnetic fields ejected from the magnetosphere may move the few spores lifted from the Earth's atmosphere with sufficient speed to cross interstellar space to other systems before the spores can be destroyed.

Radiopanspermia

In 1903, Svante Arrhenius published in his article The Distribution of Life in Space, the hypothesis now called radiopanspermia, that microscopic forms of life can be propagated in space, driven by the radiation pressure from stars. Arrhenius argued that particles at a critical size below 1.5 μm would be propagated at high speed by radiation pressure of the Sun. However, because its effectiveness decreases with increasing size of the particle, this mechanism holds for very tiny particles only, such as single bacterial spores. The main criticism of radiopanspermia hypothesis came from Iosif Shklovsky and Carl Sagan, who pointed out the proofs of the lethal action of space radiations (UV and X-rays) in the cosmos. Regardless of the evidence, Wallis and Wickramasinghe argued in 2004 that the transport of individual bacteria or clumps of bacteria, is overwhelmingly more important than lithopanspermia in terms of numbers of microbes transferred, even accounting for the death rate of unprotected bacteria in transit.

Then, data gathered by the orbital experiments ERA, BIOPAN, EXOSTACK and EXPOSE, determined that isolated spores, including those of B. subtilis, were killed by several orders of magnitude if exposed to the full space environment for a mere few seconds, but if shielded against solar UV, the spores were capable of surviving in space for up to six years while embedded in clay or meteorite powder (artificial meteorites). Though minimal protection is required to shelter a spore against UV radiation, exposure to solar UV and cosmic ionizing radiation of unprotected DNA, break it up into its bases. Also, exposing DNA to the ultrahigh vacuum of space alone is sufficient to cause DNA damage, so the transport of unprotected DNA or RNA during interplanetary flights powered solely by light pressure is extremely unlikely. The feasibility of other means of transport for the more massive shielded spores into the outer Solar System – for example, through gravitational capture by comets – is at this time unknown.

Based on experimental data on radiation effects and DNA stability, it has been concluded that for such long travel times, boulder-sized rocks which are greater than or equal to 1 meter in diameter are required to effectively shield resistant microorganisms, such as bacterial spores against galactic cosmic radiation. These results clearly negate the radiopanspermia hypothesis, which requires single spores accelerated by the radiation pressure of the Sun, requiring many years to travel between the planets, and support the likelihood of interplanetary transfer of microorganisms within asteroids or comets, the so-called lithopanspermia hypothesis.

Lithopanspermia

Lithopanspermia, the transfer of organisms in rocks from one planet to another either through interplanetary or interstellar space, remains speculative. Although there is no evidence that lithopanspermia has occurred in the Solar System, the various stages have become amenable to experimental testing.

• Planetary ejection — For lithopanspermia to occur, researchers have suggested that microorganisms must survive ejection from a planetary surface which involves extreme forces of acceleration and shock with associated temperature excursions. Hypothetical values of shock pressures experienced by ejected rocks are obtained with Martian meteorites, which suggest the shock pressures of approximately 5 to 55 GPa, acceleration of 3 Mm/s² and jerk of 6 Gm/s³ and post-shock temperature increases of about 1 K to 1000 K. To determine the effect of acceleration during ejection on microorganisms, rifle and ultracentrifuge methods were successfully used under simulated outer space conditions.
• Survival in transit — The survival of microorganisms has been studied extensively using both simulated facilities and in low Earth orbit. A large number of microorganisms have been selected for exposure experiments. It is possible to separate these microorganisms into two groups, the human-borne, and the extremophiles. Studying the human-borne microorganisms is significant for human welfare and future manned missions; whilst the extremophiles are vital for studying the physiological requirements of survival in space.
• Atmospheric entry — An important aspect of the lithopanspermia hypothesis to test is that microbes situated on or within rocks could survive hypervelocity entry from space through Earth's atmosphere (Cockell, 2008). As with planetary ejection, this is experimentally tractable, with sounding rockets and orbital vehicles being used for microbiological experiments. B. subtilis spores inoculated onto granite domes were subjected to hypervelocity atmospheric transit (twice) by launch to a ∼120 km altitude on an Orion two-stage rocket. The spores were shown to have survived on the sides of the rock, but they did not survive on the forward-facing surface that was subjected to a maximum temperature of 145 °C. In separate experiments, as part of the ESA STONE experiment, numerous organisms were embedded in different types or rocks and were mounted in the heat shield of six Foton re-entry capsules. During reentry, the rock samples were subjected to temperatures and pressure loads comparable to those experienced in meteorites. The exogenous arrival of photosynthetic microorganisms could have quite profound consequences for the course of biological evolution on the inoculated planet. As photosynthetic organisms must be close to the surface of a rock to obtain sufficient light energy, atmospheric transit might act as a filter against them by ablating the surface layers of the rock. Although cyanobacteria have been shown to survive the desiccating, freezing conditions of space in orbital experiments, this would be of no benefit as the STONE experiment showed that they cannot survive atmospheric entry. Thus, non-photosynthetic organisms deep within rocks have a chance to survive the exit and entry process. (See also: Impact survival.) Research presented at the European Planetary Science Congress in 2015 suggests that ejection, entry and impact is survivable for some simple organisms.

Accidental panspermia

Thomas Gold, a professor of astronomy, suggested in 1960 the hypothesis of "Cosmic Garbage", that life on Earth might have originated accidentally from a pile of waste products dumped on Earth long ago by extraterrestrial beings.

Directed panspermia

Directed panspermia concerns the deliberate transport of microorganisms in space, sent to Earth to start life here, or sent from Earth to seed new planetary systems with life by introduced species of microorganisms on lifeless planets. The Nobel prize winner Francis Crick, along with Leslie Orgel proposed that life may have been purposely spread by an advanced extraterrestrial civilization, but considering an early "RNA world" Crick noted later that life may have originated on Earth. It has been suggested that 'directed' panspermia was proposed in order to counteract various objections, including the argument that microbes would be inactivated by the space environment and cosmic radiation before they could make a chance encounter with Earth.

Conversely, active directed panspermia has been proposed to secure and expand life in space. This may be motivated by biotic ethics that values, and seeks to propagate, the basic patterns of our organic gene/protein life-form. The panbiotic program would seed new planetary systems nearby, and clusters of new stars in interstellar clouds. These young targets, where local life would not have formed yet, avoid any interference with local life.

For example, microbial payloads launched by solar sails at speeds up to 0.0001 c (30,000 m/s) would reach targets at 10 to 100 light-years in 0.1 million to 1 million years. Fleets of microbial capsules can be aimed at clusters of new stars in star-forming clouds, where they may land on planets or captured by asteroids and comets and later delivered to planets. Payloads may contain extremophiles for diverse environments and cyanobacteria similar to early microorganisms. Hardy multicellular organisms (rotifer cysts) may be included to induce higher evolution.

The probability of hitting the target zone can be calculated from

 ${\displaystyle P(target)={\frac {A(target)}{\pi (dy)^{2}}}={\frac {ar(target)^{2}v^{2}}{(tp)^{2}d^{4}}}}$

where A(target) is the cross-section of the target area, dy is the positional uncertainty at arrival; a – constant (depending on units), r(target) is the radius of the target area; v the velocity of the probe; (tp) the targeting precision (arcsec/yr); and d the distance to the target, guided by high-resolution astrometry of 1×10⁻⁵ arcsec/yr (all units in SIU). These calculations show that relatively near target stars(Alpha PsA, Beta Pictoris) can be seeded by milligrams of launched microbes; while seeding the Rho Ophiochus star-forming cloud requires hundreds of kilograms of dispersed capsules.

Directed panspermia to secure and expand life in space is becoming possible because of developments in solar sails, precise astrometry, extrasolar planets, extremophiles and microbial genetic engineering. After determining the composition of chosen meteorites, astroecologists performed laboratory experiments that suggest that many colonizing microorganisms and some plants could obtain many of their chemical nutrients from asteroid and cometary materials. However, the scientists noted that phosphate (PO₄) and nitrate (NO₃–N) critically limit nutrition to many terrestrial lifeforms. With such materials, and energy from long-lived stars, microscopic life planted by directed panspermia could find an immense future in the galaxy.

A number of publications since 1979 have proposed the idea that directed panspermia could be demonstrated to be the origin of all life on Earth if a distinctive 'signature' message were found, deliberately implanted into either the genome or the genetic code of the first microorganisms by our hypothetical progenitor.

In 2013 a team of physicists claimed that they had found mathematical and semiotic patterns in the genetic code which they think is evidence for such a signature. This claim has been refuted by biologist PZ Myers who said, writing in Pharyngula:

Unfortunately, what they’ve so honestly described is good old honest garbage ... Their methods failed to recognize a well-known functional association in the genetic code; they did not rule out the operation of natural law before rushing to falsely infer design ... We certainly don’t need to invoke panspermia. Nothing in the genetic code requires design. and the authors haven’t demonstrated otherwise.

In a later peer-reviewed article, the authors address the operation of natural law in an extensive statistical test, and draw the same conclusion as in the previous article. In special sections they also discuss methodological concerns raised by PZ Myers and some others.

Pseudo-panspermia

Pseudo-panspermia (sometimes called soft panspermia, molecular panspermia or quasi-panspermia) proposes that the organic molecules used for life originated in space and were incorporated in the solar nebula, from which the planets condensed and were further —and continuously— distributed to planetary surfaces where life then emerged (abiogenesis). From the early 1970s it was becoming evident that interstellar dust consisted of a large component of organic molecules. The first suggestion came from Chandra Wickramasinghe, who proposed a polymeric composition based on the molecule formaldehyde (CH₂O). Interstellar molecules are formed by chemical reactions within very sparse interstellar or circumstellar clouds of dust and gas. Usually this occurs when a molecule becomes ionized, often as the result of an interaction with cosmic rays. This positively charged molecule then draws in a nearby reactant by electrostatic attraction of the neutral molecule's electrons. Molecules can also be generated by reactions between neutral atoms and molecules, although this process is generally slower. The dust plays a critical role of shielding the molecules from the ionizing effect of ultraviolet radiation emitted by stars.

A 2008 analysis of ¹²C/¹³C isotopic ratios of organic compounds found in the Murchison meteorite indicates a non-terrestrial origin for these molecules rather than terrestrial contamination. Biologically relevant molecules identified so far include uracil, an RNA nucleobase, and xanthine. These results demonstrate that many organic compounds which are components of life on Earth were already present in the early Solar System and may have played a key role in life's origin.

In August 2009, NASA scientists identified one of the fundamental chemical building-blocks of life (the amino acid glycine) in a comet for the first time.

In August 2011, a report, based on NASA studies with meteorites found on Earth, was published suggesting building blocks of DNA (adenine, guanine and related organic molecules) may have been formed extraterrestrially in outer space. In October 2011, scientists reported that cosmic dust contains complex organic matter ("amorphous organic solids with a mixed aromatic-aliphatic structure") that could be created naturally, and rapidly, by stars. One of the scientists suggested that these complex organic compounds may have been related to the development of life on Earth and said that, "If this is the case, life on Earth may have had an easier time getting started as these organics can serve as basic ingredients for life."

In August 2012, and in a world first, astronomers at Copenhagen University reported the detection of a specific sugar molecule, glycolaldehyde, in a distant star system. The molecule was found around the protostellar binary IRAS 16293-2422, which is located 400 light years from Earth. Glycolaldehyde is needed to form ribonucleic acid, or RNA, which is similar in function to DNA. This finding suggests that complex organic molecules may form in stellar systems prior to the formation of planets, eventually arriving on young planets early in their formation.

In September 2012, NASA scientists reported that polycyclic aromatic hydrocarbons (PAHs), subjected to interstellar medium (ISM) conditions, are transformed, through hydrogenation, oxygenation and hydroxylation, to more complex organics – "a step along the path toward amino acids and nucleotides, the raw materials of proteins and DNA, respectively".^[100][101] Further, as a result of these transformations, the PAHs lose their spectroscopic signature which could be one of the reasons "for the lack of PAH detection in interstellar ice grains, particularly the outer regions of cold, dense clouds or the upper molecular layers of protoplanetary disks."

In 2013, the Atacama Large Millimeter Array (ALMA Project) confirmed that researchers have discovered an important pair of prebiotic molecules in the icy particles in interstellar space (ISM). The chemicals, found in a giant cloud of gas about 25,000 light-years from Earth in ISM, may be a precursor to a key component of DNA and the other may have a role in the formation of an important amino acid. Researchers found a molecule called cyanomethanimine, which produces adenine, one of the four nucleobases that form the "rungs" in the ladder-like structure of DNA. The other molecule, called ethanamine, is thought to play a role in forming alanine, one of the twenty amino acids in the genetic code. Previously, scientists thought such processes took place in the very tenuous gas between the stars. The new discoveries, however, suggest that the chemical formation sequences for these molecules occurred not in gas, but on the surfaces of ice grains in interstellar space. NASA ALMA scientist Anthony Remijan stated that finding these molecules in an interstellar gas cloud means that important building blocks for DNA and amino acids can 'seed' newly formed planets with the chemical precursors for life.

In March 2013, a simulation experiment indicate that dipeptides (pairs of amino acids) that can be building blocks of proteins, can be created in interstellar dust.

In February 2014, NASA announced a greatly upgraded database for tracking polycyclic aromatic hydrocarbons (PAHs) in the universe. According to scientists, more than 20% of the carbon in the universe may be associated with PAHs, possible starting materials for the formation of life. PAHs seem to have been formed shortly after the Big Bang, are widespread throughout the universe, and are associated with new stars and exoplanets.

In March 2015, NASA scientists reported that, for the first time, complex DNA and RNA organic compounds of life, including uracil, cytosine and thymine, have been formed in the laboratory under outer space conditions, using starting chemicals, such as pyrimidine, found in meteorites. Pyrimidine, like polycyclic aromatic hydrocarbons (PAHs), the most carbon-rich chemical found in the Universe, may have been formed in red giants or in interstellar dust and gas clouds, according to the scientists.

In May 2016, the Rosetta Mission team reported the presence of glycine, methylamine and ethylamine in the coma of 67P/Churyumov-Gerasimenko. This, plus the detection of phosphorus, is consistent with the hypothesis that comets played a crucial role in the emergence of life on Earth.

Extraterrestrial life

The chemistry of life may have begun shortly after the Big Bang, 13.8 billion years ago, during a habitable epoch when the Universe was only 10–17 million years old. According to the panspermia hypothesis, microscopic life—distributed by meteoroids, asteroids and other small Solar System bodies—may exist throughout the universe. Nonetheless, Earth is the only place in the universe known by humans to harbor life. The sheer number of planets in the Milky Way galaxy, however, may make it probable that life has arisen somewhere else in the galaxy and the universe. It is generally agreed that the conditions required for the evolution of intelligent life as we know it are probably exceedingly rare in the universe, while simultaneously noting that simple single-celled microorganisms may be more likely.

The extrasolar planet results from the Kepler mission estimate 100–400 billion exoplanets, with over 3,500 as candidates or confirmed exoplanets. On 4 November 2013, astronomers reported, based on Kepler space mission data, that there could be as many as 40 billion Earth-sized planets orbiting in the habitable zones of sun-like stars and red dwarf stars within the Milky Way Galaxy. 11 billion of these estimated planets may be orbiting sun-like stars. The nearest such planet may be 12 light-years away, according to the scientists.

It is estimated that space travel over cosmic distances would take an incredibly long time to an outside observer, and with vast amounts of energy required. However, there are reasons to hypothesize that faster-than-light interstellar space travel might be feasible. This has been explored by NASA scientists since at least 1995.

Hypotheses on extraterrestrial sources of illnesses

Hoyle and Wickramasinghe have speculated that several outbreaks of illnesses on Earth are of extraterrestrial origins, including the 1918 flu pandemic, and certain outbreaks of polio and mad cow disease. For the 1918 flu pandemic they hypothesized that cometary dust brought the virus to Earth simultaneously at multiple locations—a view almost universally dismissed by experts on this pandemic. Hoyle also speculated that HIV came from outer space. After Hoyle's death, The Lancet published a letter to the editor from Wickramasinghe and two of his colleagues, in which they hypothesized that the virus that causes severe acute respiratory syndrome (SARS) could be extraterrestrial in origin and not originated from chickens. The Lancet subsequently published three responses to this letter, showing that the hypothesis was not evidence-based, and casting doubts on the quality of the experiments referenced by Wickramasinghe in his letter. A 2008 encyclopedia notes that "Like other claims linking terrestrial disease to extraterrestrial pathogens, this proposal was rejected by the greater research community."

In April 2016, Jiangwen Qu of the Department of Infectious Disease Control in China presented a statistical study suggesting that "extremes of sunspot activity to within plus or minus 1  year may precipitate influenza pandemics." He discussed possible mechanisms of epidemic initiation and early spread, including speculation on primary causation by externally derived viral variants from space via cometary dust.

Case studies

• A meteorite originating from Mars known as ALH84001 was shown in 1996 to contain microscopic structures resembling small terrestrial nanobacteria. When the discovery was announced, many immediately conjectured that these were fossils and were the first evidence of extraterrestrial life — making headlines around the world. Public interest soon started to dwindle as most experts started to agree that these structures were not indicative of life, but could instead be formed abiotically from organic molecules. However, in November 2009, a team of scientists at Johnson Space Center, including David McKay, reasserted that there was "strong evidence that life may have existed on ancient Mars", after having reexamined the meteorite and finding magnetite crystals.
• On May 11, 2001, two researchers from the University of Naples claimed to have found viable extraterrestrial bacteria inside a meteorite. Geologist Bruno D'Argenio and molecular biologist Giuseppe Geraci claim the bacteria were wedged inside the crystal structure of minerals, but were resurrected when a sample of the rock was placed in a culture medium.
• An Indian and British team of researchers led by Chandra Wickramasinghe reported on 2001 that air samples over Hyderabad, India, gathered from the stratosphere by the Indian Space Research Organisation (ISRO) on Jan 21, 2001, contained clumps of living cells. Wickramasinghe calls this "unambiguous evidence for the presence of clumps of living cells in air samples from as high as 41 km, above which no air from lower down would normally be transported". Two bacterial and one fungal species were later independently isolated from these filters which were identified as Bacillus simplex, Staphylococcus pasteuri and Engyodontium album respectively. Pushkar Ganesh Vaidya from the Indian Astrobiology Research Centre reported in 2009 that "the three microorganisms captured during the balloon experiment do not exhibit any distinct adaptations expected to be seen in microorganisms occupying a cometary niche".
• In 2005 an improved experiment was conducted by ISRO. On April 20, 2005, air samples were collected from the upper atmosphere at altitudes ranging from 20 km to more than 40 km. The samples were tested at two labs in India. The labs found 12 bacterial and 6 different fungal species in these samples. The fungi were Penicillium decumbens, Cladosporium cladosporioides, Alternaria sp. and Tilletiopsis albescens. Out of the 12 bacterial samples, three were identified as new species and named Janibacter hoylei (after Fred Hoyle), Bacillus isronensis (named after ISRO) and Bacillus aryabhattai (named after the ancient Indian mathematician, Aryabhata). These three new species showed that they were more resistant to UV radiation than similar bacteria.

Some other researchers have retrieved bacteria from the stratosphere since the 1970s. Atmospheric sampling by NASA in 2010 before and after hurricanes, collected 314 different types of bacteria; the study suggests that large-scale convection during tropical storms and hurricanes can then carry this material from the surface higher up into the atmosphere.

• Another proposed mechanism of spores in the stratosphere is lifting by weather and Earth magnetism up to the ionosphere into low Earth orbit, where Russian astronauts retrieved DNA from a known sterile exterior surface of the International Space Station. The Russian scientists then also speculated the possibility "that common terrestrial bacteria are constantly being resupplied from space."

• On January 10, 2013, Chandra Wickramasinghe found fossil diatom frustules in what he thinks is a new kind of carbonaceous meteorite called Polonnaruwa that landed in the North Central Province of Sri Lanka on 29 December 2012. Early on, there was criticism that Wickramasinghe's report was not an examination of an actual meteorite but of some terrestrial rock passed off as a meteorite.

Wickramasinghe's team remark that they are aware that a large number of unrelated stones have been submitted for analysis, and have no knowledge regarding the nature, source or origin of the stones their critics have examined, so Wickramasinghe clarifies that he is using the stones submitted by the Medical Research Institute in Sri Lanka. In response to the criticism from other scientists, Wickramasinghe performed X-ray diffraction and isotope analyses to verify its meteoritic origin. His analysis revealed a 95% silica and 3% quartz content, and interpreted this result as a "carbonaceous meteorite of unknown type". In addition, Wickramasinghe's team remarked that the temperature at which sand must be heated by lightning to melt and form a fulgurite (1770 °C) would have vaporized and burned all carbon-rich organisms and melted and thus destroyed the delicately marked silica frustules of the diatoms, and that the oxygen isotope data confirms its meteoric origin. Wickramasinghe's team also argues that since living diatoms require nitrogen fixation to synthetize amino acids, proteins, DNA, RNA and other life-critical biomolecules, a population of extraterrestrial cyanobacteria must have been a required component of the comet (Polonnaruwa meteorite) "ecosystem".

• In 2013, Dale Warren Griffin, a microbiologist working at the United States Geological Survey noted that viruses are the most numerous entities on Earth. Griffin speculates that viruses evolved in comets and on other planets and moons may be pathogenic to humans, so he proposed to also look for viruses on moons and planets of the Solar System.

Hoaxes

A separate fragment of the Orgueil meteorite (kept in a sealed glass jar since its discovery) was found in 1965 to have a seed capsule embedded in it, whilst the original glassy layer on the outside remained undisturbed. Despite great initial excitement, the seed was found to be that of a European Juncaceae or Rush plant that had been glued into the fragment and camouflaged using coal dust. The outer "fusion layer" was in fact glue. Whilst the perpetrator of this hoax is unknown, it is thought that they sought to influence the 19th century debate on spontaneous generation — rather than panspermia — by demonstrating the transformation of inorganic to biological matter.

Extremophiles

Hydrothermal vents are able to support extremophile bacteria on Earth and may also support life in other parts of the cosmos.

Until the 1970s, life was thought to depend on its access to sunlight. Even life in the ocean depths, where sunlight cannot reach, was believed to obtain its nourishment either from consuming organic detritus rained down from the surface waters or from eating animals that did. However, in 1977, during an exploratory dive to the Galapagos Rift in the deep-sea exploration submersible Alvin, scientists discovered colonies of assorted creatures clustered around undersea volcanic features known as black smokers. It was soon determined that the basis for this food chain is a form of bacterium that derives its energy from oxidation of reactive chemicals, such as hydrogen or hydrogen sulfide, that bubble up from the Earth's interior. This chemosynthesis revolutionized the study of biology by revealing that terrestrial life need not be Sun-dependent; it only requires water and an energy gradient in order to exist.

It is now known that extremophiles, microorganisms with extraordinary capability to thrive in the harshest environments on Earth, can specialize to thrive in the deep-sea, ice, boiling water, acid, the water core of nuclear reactors, salt crystals, toxic waste and in a range of other extreme habitats that were previously thought to be inhospitable for life. Living bacteria found in ice core samples retrieved from 3,700 metres (12,100 ft) deep at Lake Vostok in Antarctica, have provided data for extrapolations to the likelihood of microorganisms surviving frozen in extraterrestrial habitats or during interplanetary transport. Also, bacteria have been discovered living within warm rock deep in the Earth's crust.

In order to test some these organisms' potential resilience in outer space, plant seeds and spores of bacteria, fungi and ferns have been exposed to the harsh space environment. Spores are produced as part of the normal life cycle of many plants, algae, fungi and some protozoans, and some bacteria produce endospores or cysts during times of stress. These structures may be highly resilient to ultraviolet and gamma radiation, desiccation, lysozyme, temperature, starvation and chemical disinfectants, while metabolically inactive. Spores germinate when favourable conditions are restored after exposure to conditions fatal to the parent organism.

Although computer models suggest that a captured meteoroid would typically take some tens of millions of years before collision with a planet, there are documented viable Earthly bacterial spores that are 40 million years old that are very resistant to radiation, and others able to resume life after being dormant for 25 million years, suggesting that lithopanspermia life-transfers are possible via meteorites exceeding 1 m in size.

The discovery of deep-sea ecosystems, along with advancements in the fields of astrobiology, observational astronomy and discovery of large varieties of extremophiles, opened up a new avenue in astrobiology by massively expanding the number of possible extraterrestrial habitats and possible transport of hardy microbial life through vast distances.

Research in outer space

The question of whether certain microorganisms can survive in the harsh environment of outer space has intrigued biologists since the beginning of spaceflight, and opportunities were provided to expose samples to space. The first American tests were made in 1966, during the Gemini IX and XII missions, when samples of bacteriophage T1 and spores of Penicillium roqueforti were exposed to outer space for 16.8 h and 6.5 h, respectively. Other basic life sciences research in low Earth orbit started in 1966 with the Soviet biosatellite program Bion and the U.S. Biosatellite program. Thus, the plausibility of panspermia can be evaluated by examining life forms on Earth for their capacity to survive in space. The following experiments carried on low Earth orbit specifically tested some aspects of panspermia or lithopanspermia:

ERA

EURECA facility deployment in 1992

The Exobiology Radiation Assembly (ERA) was a 1992 experiment on board the European Retrievable Carrier (EURECA) on the biological effects of space radiation. EURECA was an unmanned 4.5 tonne satellite with a payload of 15 experiments. It was an astrobiology mission developed by the European Space Agency (ESA). Spores of different strains of Bacillus subtilis and the Escherichia coli plasmid pUC19 were exposed to selected conditions of space (space vacuum and/or defined wavebands and intensities of solar ultraviolet radiation). After the approximately 11-month mission, their responses were studied in terms of survival, mutagenesis in the his (B. subtilis) or lac locus (pUC19), induction of DNA strand breaks, efficiency of DNA repair systems, and the role of external protective agents. The data were compared with those of a simultaneously running ground control experiment:

• The survival of spores treated with the vacuum of space, however shielded against solar radiation, is substantially increased, if they are exposed in multilayers and/or in the presence of glucose as protective.
• All spores in "artificial meteorites", i.e. embedded in clays or simulated Martian soil, are killed.
• Vacuum treatment leads to an increase of mutation frequency in spores, but not in plasmid DNA.
• Extraterrestrial solar ultraviolet radiation is mutagenic, induces strand breaks in the DNA and reduces survival substantially.
• Action spectroscopy confirms results of previous space experiments of a synergistic action of space vacuum and solar UV radiation with DNA being the critical target.
• The decrease in viability of the microorganisms could be correlated with the increase in DNA damage.
• The purple membranes, amino acids and urea were not measurably affected by the dehydrating condition of open space, if sheltered from solar radiation. Plasmid DNA, however, suffered a significant amount of strand breaks under these conditions.

BIOPAN

BIOPAN is a multi-user experimental facility installed on the external surface of the Russian Foton descent capsule. Experiments developed for BIOPAN are designed to investigate the effect of the space environment on biological material after exposure between 13 and 17 days. The experiments in BIOPAN are exposed to solar and cosmic radiation, the space vacuum and weightlessness, or a selection thereof. Of the 6 missions flown so far on BIOPAN between 1992 and 2007, dozens of experiments were conducted, and some analyzed the likelihood of panspermia. Some bacteria, lichens (Xanthoria elegans, Rhizocarpon geographicum and their mycobiont cultures, the black Antarctic microfungi Cryomyces minteri and Cryomyces antarcticus), spores, and even one animal (tardigrades) were found to have survived the harsh outer space environment and cosmic radiation.

EXOSTACK

EXOSTACK on the Long Duration Exposure Facility satellite.

The German EXOSTACK experiment was deployed on 7 April 1984 on board the Long Duration Exposure Facility statellite. 30% of Bacillus subtilis spores survived the nearly 6 years exposure when embedded in salt crystals, whereas 80% survived in the presence of glucose, which stabilize the structure of the cellular macromolecules, especially during vacuum-induced dehydration.

If shielded against solar UV, spores of B. subtilis were capable of surviving in space for up to 6 years, especially if embedded in clay or meteorite powder (artificial meteorites). The data support the likelihood of interplanetary transfer of microorganisms within meteorites, the so-called lithopanspermia hypothesis.

EXPOSE

Location of the astrobiology EXPOSE-E and EXPOSE-R facilities on the International Space Station

EXPOSE is a multi-user facility mounted outside the International Space Station dedicated to astrobiology experiments. There have been three EXPOSE experiments flown between 2008 and 2015: EXPOSE-E, EXPOSE-R and EXPOSE-R2.

Results from the orbital missions, especially the experiments SEEDS and LiFE, concluded that after an 18-month exposure, some seeds and lichens (Stichococcus sp. and Acarospora sp., a lichenized fungal genus) may be capable to survive interplanetary travel if sheltered inside comets or rocks from cosmic radiation and UV radiation. The LIFE, SPORES, and SEEDS parts of the experiments provided information about the likelihood of lithopanspermia. These studies will provide experimental data to the lithopanspermia hypothesis, and they will provide basic data to planetary protection issues.

Tanpopo

Dust collector with aerogel blocks

The Tanpopo mission is an orbital astrobiology experiment by Japan that is currently investigating the possible interplanetary transfer of life, organic compounds, and possible terrestrial particles in low Earth orbit. The Tanpopo experiment is taking place at the Exposed Facility located on the exterior of Kibo module of the International Space Station. The mission will collect cosmic dusts and other particles for three years by using an ultra-low density silica gel called aerogel. The purpose is to assess the panspermia hypothesis and the possibility of natural interplanetary transport of life and its precursors. Some of these aerogels will be replaced every one or two years through 2018. Sample collection began in May 2015, and the first samples were be returned to Earth in mid-2016.

Criticism

Panspermia is often criticized because it does not answer the question of the origin of life but merely places it on another celestial body. It was also criticized because it was thought it could not be tested experimentally.

Wallis and Wickramasinghe argued in 2004 that the transport of individual bacteria or clumps of bacteria, is overwhelmingly more important than lithopanspermia in terms of numbers of microbes transferred, even accounting for the death rate of unprotected bacteria in transit. Then it was found that isolated spores of B. subtilis were killed by several orders of magnitude if exposed to the full space environment for a mere few seconds. These results clearly negate the original panspermia hypothesis, which requires single spores as space travelers accelerated by the radiation pressure of the Sun, requiring many years to travel between the planets. However, if shielded against solar UV, spores of Bacillus subtilis were capable of surviving in space for up to 6 years, especially if embedded in clay or meteorite powder (artificial meteorites). The data support the likelihood of interplanetary transfer of microorganisms within meteorites, the so-called lithopanspermia hypothesis.