Analytic philosophy

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Analytic_philosophy 

Analytic philosophy is a branch and tradition of philosophy using analysis which is popular in the Western World and particularly the Anglosphere, which began around the turn of the 20th century in the contemporary era and continues today. In the United Kingdom, United States, Canada, Australia, New Zealand and Scandinavia, the majority of university philosophy departments today identify themselves as "analytic" departments.

Central figures in this historical development of analytic philosophy are Gottlob Frege, Bertrand Russell, G. E. Moore, and Ludwig Wittgenstein. Other important figures in its history include the logical positivists (particularly Rudolf Carnap), W. V. O. Quine, Saul Kripke, and Karl Popper.

Analytic philosophy is characterized by an emphasis on language, known as the linguistic turn, and for its clarity and rigor in arguments, making use of formal logic and mathematics, and, to a lesser degree, the natural sciences. It also takes things piecemeal, in "an attempt to focus philosophical reflection on smaller problems that lead to answers to bigger questions."

Analytic philosophy is often understood in contrast to other philosophical traditions, most notably continental philosophies such as existentialism, phenomenology, and Hegelianism.

History

The history of analytic philosophy (taken in the narrower sense of "20th-/21st-century analytic philosophy") is usually thought to begin with the rejection of British idealism, a neo-Hegelian movement.

British idealism as taught by philosophers such as F. H. Bradley (1846–1924) and T. H. Green (1836–1882), dominated English philosophy in the late 19th century. Since its beginning, a basic goal of analytic philosophy has been conceptual clarity, in the name of which Moore and Russell rejected Hegelianism for being obscure—see for example Moore's "A Defence of Common Sense" and Russell's critique of the doctrine of internal relations. Inspired by developments in modern formal logic, the early Russell claimed that the problems of philosophy can be solved by showing the simple constituents of complex notions. An important aspect of British idealism was logical holism—the opinion that there are aspects of the world that can be known only by knowing the whole world. This is closely related to the opinion that relations between items are internal relations, that is, properties of the nature of those items. Russell, along with Wittgenstein, in response promulgated logical atomism and the doctrine of external relations—the belief that the world consists of independent facts.

Russell, during his early career, along with his collaborator Alfred North Whitehead, was much influenced by Gottlob Frege (1848–1925), who developed predicate logic, which allowed a much greater range of sentences to be parsed into logical form than was possible using the ancient Aristotelian logic. Frege was also influential as a philosopher of mathematics in Germany at the beginning of the 20th century. In contrast to Edmund Husserl's 1891 book Philosophie der Arithmetik, which argued that the concept of the cardinal number derived from psychical acts of grouping objects and counting them, Frege argued that mathematics and logic have their own validity, independent of the judgments or mental states of individual mathematicians and logicians (which were the basis of arithmetic according to the "psychologism" of Husserl's Philosophie). Frege further developed his philosophy of logic and mathematics in The Foundations of Arithmetic (1884) and The Basic Laws of Arithmetic (German: Grundgesetze der Arithmetik, 1893–1903), where he provided an alternative to psychologistic accounts of the concept of number.

Like Frege, Russell argued that mathematics is reducible to logical fundamentals in The Principles of Mathematics (1903). Later, his book written with Whitehead, Principia Mathematica (1910–1913), encouraged many philosophers to renew their interest in the development of symbolic logic. Additionally, Russell adopted Frege's predicate logic as his primary philosophical method, a method Russell thought could expose the underlying structure of philosophical problems. For example, the English word "is" has three distinct meanings which predicate logic can express as follows:

• For the sentence 'the cat is asleep', the is of predication means that "x is P" (denoted as P(x)).
• For the sentence 'there is a cat', the is of existence means that "there is an x" (∃x).
• For the sentence 'three is half of six', the is of identity means that "x is the same as y" (x=y).

Russell sought to resolve various philosophical problems by applying such logical distinctions, most famously in his analysis of definite descriptions in "On Denoting" (1905).

Ideal language

From about 1910 to 1930, analytic philosophers like Russell and Ludwig Wittgenstein emphasized creating an ideal language for philosophical analysis, which would be free from the ambiguities of ordinary language that, in their opinion, often made philosophy invalid. During this phase, Russell and Wittgenstein sought to understand language (and hence philosophical problems) by using logic to formalize how philosophical statements are made.

Logical atomism

Russell became an advocate of logical atomism. Wittgenstein developed a comprehensive system of logical atomism in his Tractatus Logico-Philosophicus (German: Logisch-Philosophische Abhandlung, 1921). He thereby argued that the universe is the totality of actual states of affairs and that these states of affairs can be expressed by the language of first-order predicate logic. Thus a picture of the universe can be construed by expressing facts in the form of atomic propositions and linking them using logical operators.

Logical positivism

Main article: Logical positivism

During the late 1920s to 1940s, a group of philosophers of the Vienna Circle and the Berlin Circle developed Russell and Wittgenstein's formalism into a doctrine known as "logical positivism" (or logical empiricism). Logical positivism used formal logical methods to develop an empiricist account of knowledge. Philosophers such as Rudolf Carnap and Hans Reichenbach, along with other members of the Vienna Circle, claimed that the truths of logic and mathematics were tautologies, and those of science were verifiable empirical claims. These two constituted the entire universe of meaningful judgments; anything else was nonsense. The claims of ethics, aesthetics, and theology were consequently reduced to pseudo-statements, neither empirically true nor false and therefore meaningless. In reaction to what he considered excesses of logical positivism, Karl Popper insisted on the role of falsification in the philosophy of science—although his general method was also part of the analytic tradition. With the coming to power of Adolf Hitler and Nazism in 1933, many members of the Vienna and Berlin Circles fled to Britain and the US, which helped to reinforce the dominance of logical positivism and analytic philosophy in anglophone countries.

Logical positivists typically considered philosophy as having a minimal function. For them, philosophy concerned the clarification of thoughts, rather than having a distinct subject matter of its own. The positivists adopted the verification principle, according to which every meaningful statement is either analytic or is capable of being verified by experience. This caused the logical positivists to reject many traditional problems of philosophy, especially those of metaphysics or ontology, as meaningless.

Ordinary language

After World War II, during the late 1940s and 1950s, analytic philosophy became involved with ordinary-language analysis. This resulted in two main trends. One continued Wittgenstein's later philosophy, which differed dramatically from his early work of the Tractatus. The other, known as "Oxford philosophy", involved J. L. Austin. In contrast to earlier analytic philosophers (including the early Wittgenstein) who thought philosophers should avoid the deceptive trappings of natural language by constructing ideal languages, ordinary-language philosophers claimed that ordinary language already represents many subtle distinctions not recognized in the formulation of traditional philosophical theories or problems. While schools such as logical positivism emphasize logical terms, supposed to be universal and separate from contingent factors (such as culture, language, historical conditions), ordinary-language philosophy emphasizes the use of language by ordinary people. The most prominent ordinary-language philosophers during the 1950s were the aforementioned Austin and Gilbert Ryle.

Ordinary-language philosophers often sought to dissolve philosophical problems by showing them to be the result of ordinary misunderstanding language. Examples include Ryle, who tried to dispose of "Descartes' myth", and Wittgenstein.

Contemporary analytic philosophy

Although contemporary philosophers who self-identify as "analytic" have widely divergent interests, assumptions, and methods—and have often rejected the fundamental premises that defined analytic philosophy before 1960—analytic philosophy today is usually considered to be determined by a particular style, characterized by precision and thoroughness about a specific topic, and resistance to "imprecise or cavalier discussions of broad topics".

During the 1950s, logical positivism was challenged influentially by Wittgenstein in the Philosophical Investigations, Quine in "Two Dogmas of Empiricism", and Sellars in Empiricism and the Philosophy of Mind. After 1960, anglophone philosophy began to incorporate a wider range of interests, opinions, and methods. Still, many philosophers in Britain and America still consider themselves "analytic philosophers". They have done so largely by expanding the notion of "analytic philosophy" from the specific programs that dominated anglophone philosophy before 1960 to a much more general notion of an "analytic" style.

Many philosophers and historians have attempted to define or describe analytic philosophy. Those definitions often include an emphasis on conceptual analysis: A.P. Martinich draws an analogy between analytic philosophy's interest in conceptual analysis and analytic chemistry, which aims to determine chemical compositions. Steven D. Hales described analytic philosophy as one of three types of philosophical method practiced in the West: "[i]n roughly reverse order by number of proponents, they are phenomenology, ideological philosophy, and analytic philosophy".

Scott Soames agrees that clarity is important: analytic philosophy, he says, has "an implicit commitment—albeit faltering and imperfect—to the ideals of clarity, rigor and argumentation" and it "aims at truth and knowledge, as opposed to moral or spiritual improvement [...] the goal in analytic philosophy is to discover what is true, not to provide a useful recipe for living one's life". Soames also states that analytic philosophy is characterized by "a more piecemeal approach. There is, I think, a widespread presumption within the tradition that it is often possible to make philosophical progress by intensively investigating a small, circumscribed range of philosophical issues while holding broader, systematic questions in abeyance".

A few of the most important and active topics and subtopics of analytic philosophy are summarized by the following sections.

Philosophy of mind and cognitive science

Motivated by the logical positivists' interest in verificationism, logical behaviorism was the most prominent theory of mind of analytic philosophy for the first half of the 20th century. Behaviorists tended to opine either that statements about the mind were equivalent to statements about behavior and dispositions to behave in particular ways or that mental states were directly equivalent to behavior and dispositions to behave. Behaviorism later became much less popular, in favor of type physicalism or functionalism, theories that identified mental states with brain states. During this period, topics of the philosophy of mind were often related strongly to topics of cognitive science such as modularity or innateness. Finally, analytic philosophy has featured a certain number of philosophers who were dualists, and recently forms of property dualism have had a resurgence; the most prominent representative is David Chalmers.

John Searle suggests that the obsession with the philosophy of language during the 20th century has been superseded by an emphasis on the philosophy of mind, in which functionalism is currently the dominant theory. In recent years, a central focus of research in the philosophy of mind has been consciousness. While there is a general consensus for the global neuronal workspace model of consciousness, there are many opinions as to the specifics. The best known theories are Daniel Dennett's heterophenomenology, Fred Dretske and Michael Tye's representationalism, and the higher-order theories of either David M. Rosenthal—who advocates a higher-order thought (HOT) model—or David Armstrong and William Lycan—who advocate a higher-order perception (HOP) model. An alternative higher-order theory, the higher-order global states (HOGS) model, is offered by Robert van Gulick.

Ethics in analytic philosophy

Due to the commitments to empiricism and symbolic logic in the early analytic period, early analytic philosophers often thought that inquiry in the ethical domain could not be made rigorous enough to merit any attention. It was only with the emergence of ordinary language philosophers that ethics started to become an acceptable area of inquiry for analytic philosophers. Philosophers working with the analytic tradition have gradually come to distinguish three major types of moral philosophy.

• Meta-ethics which investigates moral terms and concepts;
• Normative ethics which examines and produces normative ethical judgments;
• Applied ethics, which investigates how existing normative principles should be applied to difficult or borderline cases, often cases created by new technology or new scientific knowledge.

Meta-ethics

Twentieth-century meta-ethics has two origins. The first is G.E. Moore's investigation into the nature of ethical terms (e.g., good) in his Principia Ethica (1903), which identified the naturalistic fallacy. Along with Hume's famous is/ought distinction, the naturalistic fallacy was a major topic of investigation for analytical philosophers.

The second is in logical positivism and its attitude that unverifiable statements are meaningless. Although that attitude was adopted originally to promote scientific investigation by rejecting grand metaphysical systems, it had the side effect of making (ethical and aesthetic) value judgments (as well as religious statements and beliefs) meaningless. But because value judgments are of significant importance in human life, it became incumbent on logical positivism to develop an explanation of the nature and meaning of value judgments. As a result, analytic philosophers avoided normative ethics and instead began meta-ethical investigations into the nature of moral terms, statements, and judgments.

The logical positivists opined that statements about value—including all ethical and aesthetic judgments—are non-cognitive; that is, they cannot be objectively verified or falsified. Instead, the logical positivists adopted an emotivist theory, which was that value judgments expressed the attitude of the speaker. For example, in this view, saying, "Killing is wrong", is equivalent to saying, "Boo to murder", or saying the word "murder" with a particular tone of disapproval.

While analytic philosophers generally accepted non-cognitivism, emotivism had many deficiencies. It evolved into more sophisticated non-cognitivist theories such as the expressivism of Charles Stevenson, and the universal prescriptivism of R.M. Hare, which was based on J.L. Austin's philosophy of speech acts.

These theories were not without their critics. Philippa Foot contributed several essays attacking all these theories. J.O. Urmson's article "On Grading" called the is/ought distinction into question.

As non-cognitivism, the is/ought distinction, and the naturalistic fallacy began to be called into question, analytic philosophers showed a renewed interest in the traditional questions of moral philosophy. Perhaps the most influential being Elizabeth Anscombe, whose monograph Intention was called by Donald Davidson "the most important treatment of action since Aristotle". A favorite student and friend of Ludwig Wittgenstein, her 1958 article "Modern Moral Philosophy" introduced the term "consequentialism" into the philosophical lexicon, declared the "is-ought" impasse to be unproductive, and resulted in a revival of virtue ethics.

Normative ethics

The first half of the 20th century was marked by skepticism toward and neglect of normative ethics. Related subjects, such as social and political philosophy, aesthetics, and philosophy of history, became only marginal topics of English-language philosophy during this period.

During this time, utilitarianism was the only non-skeptical type of ethics to remain popular. However, as the influence of logical positivism began to decrease mid-century, analytic philosophers had renewed interest in ethics. G.E.M. Anscombe's 1958 "Modern Moral Philosophy" sparked a revival of Aristotle's virtue ethical approach and John Rawls's 1971 A Theory of Justice restored interest in Kantian ethical philosophy. Today, contemporary normative ethics is dominated by three schools: consequentialism, virtue ethics, and deontology.

Applied ethics

A significant feature of analytic philosophy since approximately 1970 has been the emergence of applied ethics—an interest in the application of moral principles to specific practical issues. The philosophers following this orientation view ethics as involving humanistic values, which involve practical implications and applications in the way people interact and lead their lives socially.

Topics of special interest for applied ethics include environmental issues, animal rights, and the many challenges created by advancing medical science. In education, applied ethics addressed themes such as punishment in schools, equality of educational opportunity, and education for democracy.

Analytic philosophy of religion

In Analytic Philosophy of Religion, Harris noted that

analytic philosophy has been a very heterogeneous 'movement'.... some forms of analytic philosophy have proven very sympathetic to the philosophy of religion and have provided a philosophical mechanism for responding to other more radical and hostile forms of analytic philosophy.

As with the study of ethics, early analytic philosophy tended to avoid the study of philosophy of religion, largely dismissing (as per the logical positivists) the subject as part of metaphysics and therefore meaningless. The demise of logical positivism renewed interest in philosophy of religion, prompting philosophers like William Alston, John Mackie, Alvin Plantinga, Robert Merrihew Adams, Richard Swinburne, and Antony Flew not only to introduce new problems, but to re-study classical topics such as the nature of miracles, theistic arguments, the problem of evil, the rationality of belief in God, concepts of the nature of God, and many more.

Plantinga, Mackie and Flew debated the logical validity of the free will defense as a way to solve the problem of evil. Alston, grappling with the consequences of analytic philosophy of language, worked on the nature of religious language. Adams worked on the relationship of faith and morality. Analytic epistemology and metaphysics has formed the basis for some philosophically-sophisticated theistic arguments, like those of the reformed epistemologists like Plantinga.

Analytic philosophy of religion has also been preoccupied with Wittgenstein, as well as his interpretation of Søren Kierkegaard's philosophy of religion. Using first-hand remarks (which was later published in Philosophical Investigations, Culture and Value, and other works), philosophers such as Peter Winch and Norman Malcolm developed what has come to be known as contemplative philosophy, a Wittgensteinian school of thought rooted in the "Swansea tradition," and which includes Wittgensteinians such as Rush Rhees, Peter Winch, and D.Z. Phillips, among others. The name "contemplative philosophy" was first coined by D.Z. Phillips in Philosophy's Cool Place, which rests on an interpretation of a passage from Wittgenstein's "Culture and Value." This interpretation was first labeled, "Wittgensteinian Fideism," by Kai Nielsen but those who consider themselves Wittgensteinians in the Swansea tradition have relentlessly and repeatedly rejected this construal as a caricature of Wittgenstein's considered position; this is especially true of D.Z. Phillips. Responding to this interpretation, Kai Nielsen and D.Z. Phillips became two of the most prominent philosophers on Wittgenstein's philosophy of religion.

Political philosophy

Liberalism

Current analytic political philosophy owes much to John Rawls, who in a series of papers from the 1950s onward (most notably "Two Concepts of Rules" and "Justice as Fairness") and his 1971 book A Theory of Justice, produced a sophisticated defense of a generally liberal egalitarian account of distributive justice. This was followed soon by Rawls's colleague Robert Nozick's book Anarchy, State, and Utopia, a defence of free-market libertarianism. Isaiah Berlin also had a lasting influence on both analytic political philosophy and liberalism with his lecture "Two Concepts of Liberty".

During recent decades there have also been several critiques of liberalism, including the feminist critiques of Catharine MacKinnon and Andrea Dworkin, the communitarian critiques of Michael Sandel and Alasdair MacIntyre (although neither of them endorses the term), and the multiculturalist critiques of Amy Gutmann and Charles Taylor. Although not an analytic philosopher, Jürgen Habermas is another prominent—if controversial—author of contemporary analytic political philosophy, whose social theory is a blend of social science, Marxism, neo-Kantianism, and American pragmatism.

Consequentialist libertarianism also derives from the analytic tradition.

Analytical Marxism

Another development of political philosophy was the emergence of the school of analytical Marxism. Members of this school seek to apply techniques of analytic philosophy and modern social science such as rational choice theory to clarify the theories of Karl Marx and his successors. The best-known member of this school is G. A. Cohen, whose 1978 work, Karl Marx's Theory of History: A Defence, is generally considered to represent the genesis of this school. In that book, Cohen used logical and linguistic analysis to clarify and defend Marx's materialist conception of history. Other prominent analytical Marxists include the economist John Roemer, the social scientist Jon Elster, and the sociologist Erik Olin Wright. The work of these later philosophers have furthered Cohen's work by bringing to bear modern social science methods, such as rational choice theory, to supplement Cohen's use of analytic philosophical techniques in the interpretation of Marxian theory.

Cohen himself would later engage directly with Rawlsian political philosophy to advance a socialist theory of justice that contrasts with both traditional Marxism and the theories advanced by Rawls and Nozick. In particular, he indicates Marx's principle of from each according to his ability, to each according to his need.

Communitarianism

Communitarians such as Alasdair MacIntyre, Charles Taylor, Michael Walzer, and Michael Sandel advance a critique of liberalism that uses analytic techniques to isolate the main assumptions of liberal individualists, such as Rawls, and then challenges these assumptions. In particular, communitarians challenge the liberal assumption that the individual can be considered as fully autonomous from the community in which he lives and is brought up. Instead, they argue for a conception of the individual that emphasizes the role that the community plays in forming his or her values, thought processes and opinions.

Analytic metaphysics

One striking difference with respect to early analytic philosophy was the revival of metaphysical theorizing during the second half of the 20th century. Philosophers such as David Kellogg Lewis and David Armstrong developed elaborate theories on a range of topics such as universals,causation, possibility and necessity, and abstract objects.

Among the developments that resulted in the revival of metaphysical theorizing were Quine's attack on the analytic–synthetic distinction, which was generally considered to weaken Carnap's distinction between existence questions internal to a framework and those external to it. Important also for the revival of metaphysics was the further development of modal logic, including the work of Saul Kripke, who argued in Naming and Necessity and elsewhere for the existence of essences and the possibility of necessary, a posteriori truths.

Metaphysics remains a fertile topic of research, having recovered from the attacks of A.J. Ayer and the logical positivists. Although many discussions are continuations of old ones from previous decades and centuries, the debate remains active. The philosophy of fiction, the problem of empty names, and the debate over existence's status as a property have all become major concerns, while perennial issues such as free will, possible worlds, and the philosophy of time have been revived.

Science has also had an increasingly significant role in metaphysics. The theory of special relativity has had a profound effect on the philosophy of time, and quantum physics is routinely discussed in the free will debate. The weight given to scientific evidence is largely due to widespread commitments among philosophers to scientific realism and naturalism.

Philosophy of language

Philosophy of language is a topic that has decreased in activity during the last four decades, as evidenced by the fact that few major philosophers today treat it as a primary research topic. Indeed, while the debate remains fierce, it is still strongly influenced by those authors from the first half of the century: Gottlob Frege, Bertrand Russell, Ludwig Wittgenstein, J.L. Austin, Alfred Tarski, and W.V.O. Quine.

In Saul Kripke's publication Naming and Necessity, he argued influentially that flaws in common theories of proper names are indicative of larger misunderstandings of the metaphysics of necessity and possibility. By wedding the techniques of modal logic to a causal theory of reference, Kripke was widely regarded as reviving theories of essence and identity as respectable topics of philosophical discussion.

Another influential philosopher, Pavel Tichý initiated Transparent Intensional Logic, an original theory of the logical analysis of natural languages—the theory is devoted to the problem of saying exactly what it is that we learn, know and can communicate when we come to understand what a sentence means.

Philosophy of science

Reacting against both the verificationism of the logical positivists as well as the critiques of the philosopher of science Karl Popper, who had suggested the falsifiability criterion on which to judge the demarcation between science and non-science, discussions of philosophy of science during the last 40 years were dominated by social constructivist and cognitive relativist theories of science. Thomas Samuel Kuhn with his formulation of paradigm shifts and Paul Feyerabend with his epistemological anarchism are significant for these discussions. The philosophy of biology has also undergone considerable growth, particularly due to the considerable debate in recent years over the nature of evolution, particularly natural selection. Daniel Dennett and his 1995 book Darwin's Dangerous Idea, which defends Neo-Darwinism, stand at the foreground of this debate.

Epistemology

Owing largely to Gettier's 1963 paper "Is Justified True Belief Knowledge?", epistemology resurged as a topic of analytic philosophy during the last 50 years. A large portion of current epistemological research is intended to resolve the problems that Gettier's examples presented to the traditional justified true belief model of knowledge, including developing theories of justification in order to deal with Gettier's examples, or giving alternatives to the justified true belief model. Other and related topics of contemporary research include debates between internalism and externalism, basic knowledge, the nature of evidence, the value of knowledge, epistemic luck, virtue epistemology, the role of intuitions in justification, and treating knowledge as a primitive concept.

Aesthetics

As a result of attacks on the traditional aesthetic notions of beauty and sublimity from post-modern thinkers, analytic philosophers were slow to consider art and aesthetic judgment. Susanne Langer and Nelson Goodman addressed these problems in an analytic style during the 1950s and 1960s. Since Goodman, aesthetics as a discipline for analytic philosophers has flourished. Rigorous efforts to pursue analyses of traditional aesthetic concepts were performed by Guy Sircello in the 1970s and 1980s, resulting in new analytic theories of love, sublimity, and beauty.

Planetary protection

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Planetary_protection 

A Viking lander being prepared for dry heat sterilization – this remains the "silver standard" of present-day planetary protection.

Planetary protection is a guiding principle in the design of an interplanetary mission, aiming to prevent biological contamination of both the target celestial body and the Earth in the case of sample-return missions. Planetary protection reflects both the unknown nature of the space environment and the desire of the scientific community to preserve the pristine nature of celestial bodies until they can be studied in detail.

There are two types of interplanetary contamination. Forward contamination is the transfer of viable organisms from Earth to another celestial body. Back contamination is the transfer of extraterrestrial organisms, if such exist, back to the Earth's biosphere.

History

The potential problem of lunar and planetary contamination was first raised at the International Astronautical Federation VIIth Congress in Rome in 1956.

In 1958 the U.S. National Academy of Sciences (NAS) passed a resolution stating, “The National Academy of Sciences of the United States of America urges that scientists plan lunar and planetary studies with great care and deep concern so that initial operations do not compromise and make impossible forever after critical scientific experiments.” This led to creation of the ad hoc Committee on Contamination by Extraterrestrial Exploration (CETEX), which met for a year and recommended that interplanetary spacecraft be sterilized, and stated, “The need for sterilization is only temporary. Mars and possibly Venus need to remain uncontaminated only until study by manned ships becomes possible”.

In 1959, planetary protection was transferred to the newly formed Committee on Space Research (COSPAR). COSPAR in 1964 issued Resolution 26 affirming that:

the search for extraterrestrial life is an important objective of space research, that the planet of Mars may offer the only feasible opportunity to conduct this search during the foreseeable future, that contamination of this planet would make such a search far more difficult and possibly even prevent for all time an unequivocal result, that all practical steps should be taken to ensure that Mars be not biologically contaminated until such time as this search has been satisfactorily carried out, and that cooperation in proper scheduling of experiments and use of adequate spacecraft sterilization techniques is required on the part of all deep space probe launching authorities to avoid such contamination.

Signatories of the Outer Space Treaty - includes all current and aspiring space faring nation states. By signing the treaty, these nation states have all committed themselves to planetary protection.

  Signed and ratified

  Signed only

  Not signed

In 1967, the US, USSR, and UK ratified the United Nations Outer Space Treaty. The legal basis for planetary protection lies in Article IX of this treaty:

"Article IX: ... States Parties to the Treaty shall pursue studies of outer space, including the Moon and other celestial bodies, and conduct exploration of them so as to avoid their harmful contamination and also adverse changes in the environment of the Earth resulting from the introduction of extraterrestrial matter and, where necessary, shall adopt appropriate measures for this purpose...

This treaty has since been signed and ratified by 104 nation states. Another 24 have signed but not ratified. All the current space-faring nation states have both signed and ratified it. Amongst nations with space faring aspirations, some have not yet ratified: the United Arab Emirates, Syria and North Korea have signed but not yet ratified.

The Outer Space Treaty has consistent and widespread international support, and as a result of this, together with the fact that it is based on the 1963 declaration which was adopted by consensus in the UN National Assembly, it has taken on the status of customary international law. The provisions of the Outer Space Treaty are therefore binding on all states, even those who have neither signed nor ratified it.

For forward contamination, the phrase to be interpreted is "harmful contamination". Two legal reviews came to differing interpretations of this clause (both reviews were unofficial). However the currently accepted interpretation is that “any contamination which would result in harm to a state’s experiments or programs is to be avoided”. NASA policy states explicitly that “the conduct of scientific investigations of possible extraterrestrial life forms, precursors, and remnants must not be jeopardized”.

COSPAR recommendations and categories

The Committee on Space Research (COSPAR) meets every two years, in a gathering of 2000 to 3000 scientists, and one of its tasks is to develop recommendations for avoiding interplanetary contamination. Its legal basis is Article IX of the Outer Space Treaty.

Its recommendations depend on the type of space mission and the celestial body explored. COSPAR categorizes the missions into 5 groups:

• Category I: Any mission to locations not of direct interest for chemical evolution or the origin of life, such as the Sun or Mercury. No planetary protection requirements.
• Category II: Any mission to locations of significant interest for chemical evolution and the origin of life, but only a remote chance that spacecraft-borne contamination could compromise investigations. Examples include the Moon, Venus, and comets. Requires simple documentation only, primarily to outline intended or potential impact targets, and an end of mission report of any inadvertent impact site if such occurred.
• Category III: Flyby and orbiter missions to locations of significant interest for chemical evolution or the origin of life, and with a significant chance that contamination could compromise investigations e.g., Mars, Europa, Enceladus. Requires more involved documentation than Category II. Other requirements, depending on the mission, may include trajectory biasing, clean room assembly, bioburden reduction, and if impact is a possibility, inventory of organics.
• Category IV: Lander or probe missions to the same locations as Category III. Measures to be applied depend on the target body and the planned operations. "Sterilization of the entire spacecraft may be required for landers and rovers with life-detection experiments, and for those landing in or moving to a region where terrestrial microorganisms may survive and grow, or where indigenous life may be present. For other landers and rovers, the requirements would be for decontamination and partial sterilization of the landed hardware."

Missions to Mars in category IV are subclassified further:

• Category IVa. Landers that do not search for Martian life - uses the Viking lander pre-sterilization requirements, a maximum of 300,000 spores per spacecraft and 300 spores per square meter.
• Category IVb. Landers that search for Martian life. Adds stringent extra requirements to prevent contamination of samples.
• Category IVc. Any component that accesses a Martian special region (see below) must be sterilized to at least to the Viking post-sterilization biological burden levels of 30 spores total per spacecraft.

• Category V: This is further divided into unrestricted and restricted sample return.

• Unrestricted Category V: samples from locations judged by scientific opinion to have no indigenous lifeforms. No special requirements.
• Restricted Category V: (where scientific opinion is unsure) the requirements include: absolute prohibition of destructive impact upon return, containment of all returned hardware which directly contacted the target body, and containment of any unsterilized sample returned to Earth.

For Category IV missions, a certain level of biological burden is allowed for the mission. In general this is expressed as a 'probability of contamination', required to be less than one chance in 10,000 of forward contamination per mission, but in the case of Mars Category IV missions (above) the requirement has been translated into a count of Bacillus spores per surface area, as an easy to use assay method.

More extensive documentation is also required for Category IV. Other procedures required, depending on the mission, may include trajectory biasing, the use of clean rooms during spacecraft assembly and testing, bioload reduction, partial sterilization of the hardware having direct contact with the target body, a bioshield for that hardware, and, in rare cases, complete sterilization of the entire spacecraft.

For restricted Category V missions, the current recommendation is that no uncontained samples should be returned unless sterilized. Since sterilization of the returned samples would destroy much of their science value, current proposals involve containment and quarantine procedures. For details, see Containment and quarantine below. Category V missions also have to fulfill the requirements of Category IV to protect the target body from forward contamination.

Mars special regions

A special region is a region classified by COSPAR where terrestrial organisms could readily propagate, or thought to have a high potential for existence of Martian life forms. This is understood to apply to any region on Mars where liquid water occurs, or can occasionally occur, based on the current understanding of requirements for life.

If a hard landing risks biological contamination of a special region, then the whole lander system must be sterilized to COSPAR category IVc.

Target categories

Some targets are easily categorized. Others are assigned provisional categories by COSPAR, pending future discoveries and research.

The 2009 COSPAR Workshop on Planetary Protection for Outer Planet Satellites and Small Solar System Bodies covered this in some detail. Most of these assessments are from that report, with some future refinements. This workshop also gave more precise definitions for some of the categories:

Category I

“not of direct interest for understanding the process of chemical evolution or the origin of life.” 

• Io, Sun, Mercury, undifferentiated metamorphosed asteroids

Category II

… where there is only a remote chance that contamination carried by a spacecraft could jeopardize future exploration”. In this case we define “remote chance” as “the absence of niches (places where terrestrial microorganisms could proliferate) and/or a very low likelihood of transfer to those places.”

• Callisto, comets, asteroids of category P, D, and C, Venus, Kuiper belt objects (KBO) < 1/2 size of Pluto.

Provisional Category II

• Ganymede, Titan, Triton, the Pluto–Charon system, and other large KBOs (> 1/2 size of Pluto), Ceres

Provisionally, they assigned these objects to Category II. However, they state that more research is needed, because there is a remote possibility that the tidal interactions of Pluto and Charon could maintain some water reservoir below the surface. Similar considerations apply to the other larger KBOs.

Triton is insufficiently well understood at present to say it is definitely devoid of liquid water. The only close up observations to date are those of Voyager 2.

In a detailed discussion of Titan, scientists concluded that there was no danger of contamination of its surface, except short term adding of negligible amounts of organics, but Titan could have a below surface water reservoir that communicates with the surface, and if so, this could be contaminated.

In the case of Ganymede, the question is, given that its surface shows pervasive signs of resurfacing, is there any communication with its subsurface ocean? They found no known mechanism by which this could happen, and the Galileo spacecraft found no evidence of cryovolcanism. Initially, they assigned it as Priority B minus, meaning that precursor missions are needed to assess its category before any surface missions. However, after further discussion they provisionally assigned it to Category II, so no precursor missions are required, depending on future research.

If there is cryovolcanism on Ganymede or Titan, the undersurface reservoir is thought to be 50 – 150 km below the surface. They were unable to find a process that could transfer the surface melted water back down through 50 km of ice to the under surface sea. This is why both Ganymede and Titan were assigned a reasonably firm provisional Category II, but pending results of future research.

Icy bodies that show signs of recent resurfacing need further discussion and might need to be assigned to a new category depending on future research. This approach has been applied, for instance, to missions to Ceres. The planetary protection Category is subject for review during the mission of the Ceres orbiter (Dawn) depending on the results found.

Category III / IV

“…where there is a significant chance that contamination carried by a spacecraft could jeopardize future exploration.” We define “significant chance” as “the presence of niches (places where terrestrial microorganisms could proliferate) and the likelihood of transfer to those places.” 

• Mars because of possible surface habitats.
• Europa because of its subsurface ocean.
• Enceladus because of evidence of water plumes.

Category V

Unrestricted Category V: “Earth-return missions from bodies deemed by scientific opinion to have no indigenous life forms.”

Restricted Category V: "Earth-return missions from bodies deemed by scientific opinion to be of significant interest to the process of chemical evolution or the origin of life."

In the category V for sample return the conclusions so far are:

• Unrestricted Category V: Venus, the Moon.
• Restricted Category V: Mars, Europa, Enceladus.

The Coleman–Sagan equation

The aim of the current regulations is to keep the number of microorganisms low enough so that the probability of contamination of Mars (and other targets) is acceptable. It is not an objective to make the probability of contamination zero.

The aim is to keep the probability of contamination of 1 chance in 10,000 of contamination per mission flown. This figure is obtained typically by multiplying together the number of microorganisms on the spacecraft, the probability of growth on the target body, and a series of bioload reduction factors.

In detail the method used is the Coleman–Sagan equation.

${\displaystyle P_{c}=N_{0}RP_{S}P_{t}P_{R}P_{g}}$.

where

${\displaystyle N_{0}}$ = the number of microorganisms on the spacecraft initially

${\displaystyle R}$ = Reduction due to conditions on spacecraft before and after launch

${\displaystyle P_{S}}$ = Probability that microorganisms on the spacecraft reach the surface of the planet

${\displaystyle P_{t}}$ = Probability that spacecraft will hit the planet - this is 1 for a lander

${\displaystyle P_{R}}$ = Probability of microorganism to be released in the environment when on the ground, usually set to 1 for crashlanding.

${\displaystyle P_{g}}$ = Probability of growth. For targets with liquid water this is set to 1 for sake of the calculation.

Then the requirement is ${\displaystyle P_{c}<10^{-4}}$

The ${\displaystyle 10^{-4}}$ is a number chosen by Sagan et al., somewhat arbitrarily. Sagan and Coleman assumed that about 60 missions to the Mars surface would occur before the exobiology of Mars is thoroughly understood, 54 of those successful, and 30 flybys or orbiters, and the number was chosen to endure a probability to keep the planet free from contamination of at least 99.9% over the duration of the exploration period.

Critiques

The Coleman–Sagan equation has been criticised because the individual parameters are often not known to better than a magnitude or so. For example, the thickness of the surface ice of Europa is unknown, and may be thin in places, which can give rise to a high level of uncertainty in the equation. It has also been criticised because of the inherent assumption made of an end to the protection period and future human exploration. In the case of Europa, this would only protect it with reasonable probability for the duration of the period of exploration.

Greenberg has suggested an alternative, to use the natural contamination standard — that our missions to Europa should not have a higher chance of contaminating it than the chance of contamination by meteorites from Earth.

As long as the probability of people infecting other planets with terrestrial microbes is substantially smaller than the probability that such contamination happens naturally, exploration activities would, in our view, be doing no harm. We call this concept the natural contamination standard.

Another approach for Europa is the use of binary decision trees which is favoured by the Committee on Planetary Protection Standards for Icy Bodies in the Outer Solar System under the auspices of the Space Studies Board. This goes through a series of seven steps, leading to a final decision on whether to go ahead with the mission or not.

Recommendation: Approaches to achieving planetary protection should not rely on the multiplication of bioload estimates and probabilities to calculate the likelihood of contaminating Solar System bodies with terrestrial organisms unless scientific data unequivocally define the values, statistical variation, and mutual independence of every factor used in the equation.

Recommendation: Approaches to achieving planetary protection for missions to icy Solar System bodies should employ a series of binary decisions that consider one factor at a time to determine the appropriate level of planetary protection procedures to use.

Containment and quarantine for restricted Category V sample return

In the case of restricted Category V missions, Earth would be protected through quarantine of sample and astronauts in a yet to be built Biosafety level 4 facility. In the case of a Mars sample return, missions would be designed so that no part of the capsule that encounters the Mars surface is exposed to the Earth environment. One way to do that is to enclose the sample container within a larger outer container from Earth, in the vacuum of space. The integrity of any seals is essential and the system must also be monitored to check for the possibility of micro-meteorite damage during return to Earth.

The recommendation of the ESF report is that

“No uncontained Mars materials, including space craft surfaces that have been exposed to the Mars environment should be returned to Earth unless sterilised"

..."For unsterilised samples returned to Earth, a programme of life detection and biohazard testing, or a proven sterilisation process, shall be undertaken as an absolute precondition for the controlled distribution of any portion of the sample.”

No restricted category V returns have been carried out. During the Apollo program, the sample-returns were regulated through the Extra-Terrestrial Exposure Law. This was rescinded in 1991, so new regulations would need to be enacted. The Apollo era quarantine procedures are of interest as the only attempt to date of a return to Earth of a sample that, at the time, was thought to have a remote possibility of including extraterrestrial life.

Samples and astronauts were quarantined in the Lunar Receiving Laboratory. The methods used would be considered inadequate for containment by modern standards. Also the lunar receiving laboratory would be judged a failure by its own design criteria as the sample return didn't contain the lunar material, with two failure points during the Apollo 11 return mission, at the splashdown and at the facility itself.

However the Lunar Receiving Laboratory was built quickly with only two years from start to finish, a time period now considered inadequate. Lessons learned from it can help with design of any Mars sample return receiving facility.

Design criteria for a proposed Mars Sample Return Facility, and for the return mission, have been developed by the American National Research Council, and the European Space Foundation. They concluded that it could be based on biohazard 4 containment but with more stringent requirements to contain unknown microorganisms possibly as small as or smaller than the smallest Earth microorganisms known, the ultramicrobacteria. The ESF study also recommended that it should be designed to contain the smaller gene transfer agents if possible, as these could potentially transfer DNA from martian microorganisms to terrestrial microorganisms if they have a shared evolutionary ancestry. It also needs to double as a clean room facility to protect the samples from terrestrial contamination that could confuse the sensitive life detection tests that would be used on the samples.

Before a sample return, new quarantine laws would be required. Environmental assessment would also be required, and various other domestic and international laws not present during the Apollo era would need to be negotiated.

Decontamination procedures

For all spacecraft missions requiring decontamination, the starting point is clean room assembly in US federal standard class 100 cleanrooms. These are rooms with fewer than 100 particles of size 0.5 µm or larger per cubic foot. Engineers wear cleanroom suits with only their eyes exposed. Components are sterilized individually before assembly, as far as possible, and they clean surfaces frequently with alcohol wipes during assembly. Spores of Bacillus subtilis was chosen for not only its ability to readily generate spores, but its well-established use as a model species. It is a useful tracker of UV irradiation effects because of its high resilience to a variety of extreme conditions. As such it is an important indicator species for forward contamination in the context of planetary protection.

For Category IVa missions (Mars landers that do not search for Martian life), the aim is to reduce the bioburden to 300,000 bacterial spores on any surface from which the spores could get into the Martian environment. Any heat tolerant components are heat sterilized to 114 °C. Sensitive electronics such as the core box of the rover including the computer, are sealed and vented through high-efficiency filters to keep any microbes inside.

For more sensitive missions such as Category IVc (to Mars special regions), a far higher level of sterilization is required. These need to be similar to levels implemented on the Viking landers, which were sterilized for a surface which, at the time, was thought to be potentially hospitable to life similar to special regions on Mars today.

In microbiology, it is usually impossible to prove that there are no microorganisms left viable, since many microorganisms are either not yet studied, or not cultivable. Instead, sterilization is done using a series of tenfold reductions of the numbers of microorganisms present. After a sufficient number of tenfold reductions, the chance that there any microorganisms left will be extremely low.

The two Viking Mars landers were sterilized using dry heat sterilization. After preliminary cleaning to reduce the bioburden to levels similar to present day Category IVa spacecraft, the Viking spacecraft were heat-treated for 30 hours at 112 °C, nominal 125 °C (five hours at 112 °C was considered enough to reduce the population tenfold even for enclosed parts of the spacecraft, so this was enough for a million-fold reduction of the originally low population).

Modern materials however are often not designed to handle such temperatures, especially since modern spacecraft often use "commercial off the shelf" components. Problems encountered include nanoscale features only a few atoms thick, plastic packaging, and conductive epoxy attachment methods. Also many instrument sensors cannot be exposed to high temperature, and high temperature can interfere with critical alignments of instruments.

As a result, new methods are needed to sterilize a modern spacecraft to the higher categories such as Category IVc for Mars, similar to Viking. Methods under evaluation, or already approved, include:

• Vapour phase hydrogen peroxide - effective, but can affect finishes, lubricants and materials that use aromatic rings and sulfur bonds. This has been established, reviewed, and a NASA/ESA specification for use of VHP has been approved by the Planetary Protection Officer, but it has not yet been formally published.
• Ethylene oxide - this is widely used in the medical industry, and can be used for materials not compatible with hydrogen peroxide. It is under consideration for missions such as ExoMars.
• Gamma radiation and electron beams have been suggested as a method of sterilization, as they are used extensively in the medical industry. They need to be tested for compatibility with spacecraft materials and hardware geometries, and are not yet ready for review.

Some other methods are of interest as they can sterilize the spacecraft after arrival on the planet.

• Supercritical carbon dioxide snow (Mars) - is most effective against traces of organic compounds rather than whole microorganisms. Has the advantage though that it eliminates the organic traces - while other methods kill the microorganisms, they leave organic traces that can confuse life detection instruments. Is under study by JPL and ESA.
• Passive sterilization through UV radiation (Mars). Highly effective against many microorganisms, but not all, as a Bacillus strain found in spacecraft assembly facilities is particularly resistant to UV radiation. Is also complicated by possible shadowing by dust and spacecraft hardware.
• Passive sterilization through particle fluxes (Europa). Plans for missions to Europa take credit for reductions due to this.

Bioburden detection and assessment

The spore count is used as an indirect measure of the number of microorganisms present. Typically 99% of microorganisms by species will be non-spore forming and able to survive in dormant states, and so the actual number of viable dormant microorganisms remaining on the sterilized spacecraft is expected to be many times the number of spore-forming microorganisms.

One new spore method approved is the "Rapid Spore Assay". This is based on commercial rapid assay systems, detects spores directly and not just viable microorganisms and gives results in 5 hours instead of 72 hours.

Challenges

It is also long been recognized that spacecraft cleaning rooms harbour polyextremophiles as the only microbes able to survive in them. For example, in a recent study, microbes from swabs of the Curiosity rover were subjected to desiccation, UV exposure, cold and pH extremes. Nearly 11% of the 377 strains survived more than one of these severe conditions. The genomes of resistant spore producing Bacillus sp. have been studied and genome level traits potentially linked to the resistance have been reported.

This does not mean that these microbes have contaminated Mars. This is just the first stage of the process of bioburden reduction. To contaminate Mars they also have to survive the low temperature, vacuum, UV and ionizing radiation during the months long journey to Mars, and then have to encounter a habitat on Mars and start reproducing there. Whether this has happened or not is a matter of probability. The aim of planetary protection is to make this probability as low as possible. The currently accepted target probability of contamination per mission is to reduce it to less than 0.01%, though in the special case of Mars, scientists also rely on the hostile conditions on Mars to take the place of the final stage of heat treatment decimal reduction used for Viking. But with current technology scientists cannot reduce probabilities to zero.

New methods

Two recent molecular methods have been approved for assessment of microbial contamination on spacecraft surfaces.

• Adenosine triphosphate (ATP) detection - this is a key element in cellular metabolism. This method is able to detect non cultivable organisms. It can also be triggered by non viable biological material so can give a "false positive".
• Limulus Amebocyte Lysate assay - detects lipopolysaccharides (LPS). This compound is only present in Gram-negative bacteria. The standard assay analyses spores from microbes that are primarily Gram-positive, making it difficult to relate the two methods.

Impact prevention

This particularly applies to orbital missions, Category III, as they are sterilized to a lower standard than missions to the surface. It is also relevant to landers, as an impact gives more opportunity for forward contamination, and impact could be on an unplanned target, such as a special region on Mars.

The requirement for an orbital mission is that it needs to remain in orbit for at least 20 years after arrival at Mars with probability of at least 99% and for 50 years with probability at least 95%. This requirement can be dropped if the mission is sterilized to Viking sterilization standard.

In the Viking era (1970s), the requirement was given as a single figure, that any orbital mission should have a probability of less than 0.003% probability of impact during the current exploratory phase of exploration of Mars.

For both landers and orbiters, the technique of trajectory biasing is used during approach to the target. The spacecraft trajectory is designed so that if communications are lost, it will miss the target.

Issues with impact prevention

Despite these measures there has been one notable failure of impact prevention. The Mars Climate Orbiter which was sterilized only to Category III, crashed on Mars in 1999 due to a mix-up of imperial and metric units. The office of planetary protection stated that it is likely that it burnt up in the atmosphere, but if it survived to the ground, then it could cause forward contamination.

Mars Observer is another Category III mission with potential planetary contamination. Communications were lost three days before its orbital insertion maneuver in 1993. It seems most likely it did not succeed in entering into orbit around Mars and simply continued past on a heliocentric orbit. If it did succeed in following its automatic programming, and attempted the manoeuvre, however, there is a chance it crashed on Mars.

Three landers have had hard landings on Mars. These are Schiaparelli EDM lander, the Mars Polar Lander, and Deep Space 2. These were all sterilized for surface missions but not for special regions (Viking pre-sterilization only). Mars Polar Lander, and Deep Space 2 crashed into the polar regions which are now treated as special regions because of the possibility of forming liquid brines.

Controversies

Meteorite argument

Alberto G. Fairén and Dirk Schulze-Makuch published an article in Nature recommending that planetary protection measures need to be scaled down. They gave as their main reason for this, that exchange of meteorites between Earth and Mars means that any life on Earth that could survive on Mars has already got there and vice versa.

Robert Zubrin used similar arguments in favour of his view that the back contamination risk has no scientific validity.

Rebuttal by NRC

The meteorite argument was examined by the NRC in the context of back contamination. It is thought that all the Martian meteorites originate in relatively few impacts every few million years on Mars. The impactors would be kilometers in diameter and the craters they form on Mars tens of kilometers in diameter. Models of impacts on Mars are consistent with these findings.

Earth receives a steady stream of meteorites from Mars, but they come from relatively few original impactors, and transfer was more likely in the early Solar System. Also some life forms viable on both Mars and on Earth might be unable to survive transfer on a meteorite, and there is so far no direct evidence of any transfer of life from Mars to Earth in this way.

The NRC concluded that though transfer is possible, the evidence from meteorite exchange does not eliminate the need for back contamination protection methods.

Impacts on Earth able to send microorganisms to Mars are also infrequent. Impactors of 10 km across or larger can send debris to Mars through the Earth's atmosphere but these occur rarely, and were more common in the early Solar System.

Proposal to end planetary protection for Mars

In their 2013 paper "The Over Protection of Mars", Alberto Fairén and Dirk Schulze-Makuch suggested that we no longer need to protect Mars, essentially using Zubrin's meteorite transfer argument. This was rebutted in a follow up article "Appropriate Protection of Mars", in Nature by the current and previous planetary protection officers Catharine Conley and John Rummel.

Critique of Category V containment measures

The scientific consensus is that the potential for large-scale effects, either through pathogenesis or ecological disruption, is extremely small. Nevertheless, returned samples from Mars will be treated as potentially biohazardous until scientists can determine that the returned samples are safe. The goal is to reduce the probability of release of a Mars particle to less than one in a million.

Policy proposals

Non-biological contamination

A COSPAR workshop in 2010, looked at issues to do with protecting areas from non biological contamination. They recommended that COSPAR expand its remit to include such issues. Recommendations of the workshop include:

Recommendation 3 COSPAR should add a separate and parallel policy to provide guidance on requirements/best practices for protection of non-living/nonlife-related aspects of Outer Space and celestial bodies

Some ideas proposed include protected special regions, or "Planetary Parks" to keep regions of the Solar System pristine for future scientific investigation, and also for ethical reasons.

Proposed extensions

Astrobiologist Christopher McKay has argued that until we have better understanding of Mars, our explorations should be biologically reversible. For instance if all the microorganisms introduced to Mars so far remain dormant within the spacecraft, they could in principle be removed in the future, leaving Mars completely free of contamination from modern Earth lifeforms.

In the 2010 workshop one of the recommendations for future consideration was to extend the period for contamination prevention to the maximum viable lifetime of dormant microorganisms introduced to the planet.

"'Recommendation 4.' COSPAR should consider that the appropriate protection of potential indigenous extraterrestrial life shall include avoiding the harmful contamination of any habitable environment —whether extant or foreseeable— within the maximum potential time of viability of any terrestrial organisms (including microbial spores) that may be introduced into that environment by human or robotic activity."

In the case of Europa, a similar idea has been suggested, that it is not enough to keep it free from contamination during our current exploration period. It might be that Europa is of sufficient scientific interest that the human race has a duty to keep it pristine for future generations to study as well. This was the majority view of the 2000 task force examining Europa, though there was a minority view of the same task force that such strong protection measures are not required.

"One consequence of this view is that Europa must be protected from contamination for an open-ended period, until it can be demonstrated that no ocean exists or that no organisms are present. Thus, we need to be concerned that over a time scale on the order of 10 million to 100 million years (an approximate age for the surface of Europa), any contaminating material is likely to be carried into the deep ice crust or into the underlying ocean."

In July 2018, the National Academies of Sciences, Engineering, and Medicine issued a Review and Assessment of Planetary Protection Policy Development Processes. In part, the report urges NASA to create a broad strategic plan that covers both forward and back contamination. The report also expresses concern about private industry missions, for which there is no governmental regulatory authority.

Protecting objects beyond the Solar System

The proposal by the German physicist Claudius Gros, that the technology of the Breakthrough Starshot project may be utilized to establish a biosphere of unicellular organisms on otherwise only transiently habitable exoplanets, has sparked a discussion, to what extent planetary protection should be extended to exoplanets. Gros argues that the extended timescales of interstellar missions imply that planetary and exoplanetary protection have different ethical groundings.