Molecular evolution

From Wikipedia, the free encyclopedia

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

Molecular evolution describes how inherited DNA and/or RNA change over evolutionary time, and the consequences of this for proteins and other components of cells and organisms. Molecular evolution is the basis of phylogenetic approaches to describing the tree of life. Molecular evolution overlaps with population genetics, especially on shorter timescales. Topics in molecular evolution include the origins of new genes, the genetic nature of complex traits, the genetic basis of adaptation and speciation, the evolution of development, and patterns and processes underlying genomic changes during evolution.

History

Main article: History of molecular evolution

The history of molecular evolution starts in the early 20th century with comparative biochemistry, and the use of "fingerprinting" methods such as immune assays, gel electrophoresis, and paper chromatography in the 1950s to explore homologous proteins. The advent of protein sequencing allowed molecular biologists to create phylogenies based on sequence comparison, and to use the differences between homologous sequences as a molecular clock to estimate the time since the most recent common ancestor. The surprisingly large amount of molecular divergence within and between species inspired the neutral theory of molecular evolution in the late 1960s. Neutral theory also provided a theoretical basis for the molecular clock, although this is not needed for the clock's validity. After the 1970s, nucleic acid sequencing allowed molecular evolution to reach beyond proteins to highly conserved ribosomal RNA sequences, the foundation of a reconceptualization of the early history of life. The Society for Molecular Biology and Evolution was founded in 1982.

Molecular phylogenetics

Main articles: Molecular systematics and Phylogenetics

Multiple sequence alignment (in this case DNA sequences) and illustrations of the use of substitution models to make evolutionary inferences. The data in this alignment (in this case a toy example with 18 sites) is converted to a set of site patterns. The site patterns are shown along with the number of times they occur in alignment. These site patterns are used to calculate the likelihood given the substitution model and a phylogenetic tree (in this case an unrooted four-taxon tree). It is also necessary to assume a substitution model to estimate evolutionary distances for pairs of sequences (distances are the number of substitutions that have occurred since sequences had a common ancestor). The evolutionary distance equation (d₁₂) is based on the simple model proposed by Jukes and Cantor in 1969. The equation transforms the proportion of nucleotide differences between taxa 1 and 2 (p₁₂ = 4/18; the four site patterns that differ between taxa 1 and 2 are indicated with asterisks) into an evolutionary distance (in this case d₁₂=0.2635 substitutions per site).

Molecular phylogenetics uses DNA, RNA, or protein sequences to resolve questions in systematics, i.e. about their correct scientific classification from the point of view of evolutionary history. The result of a molecular phylogenetic analysis is expressed in a phylogenetic tree. Phylogenetic inference is conducted using data from DNA sequencing. This is aligned to identify which sites are homologous. A substitution model describes what patterns are expected to be common or rare. Sophisticated computational inference is then used to generate one or more plausible trees.

Some phylogenetic methods account for variation among sites and among tree branches. Different genes, e.g. hemoglobin vs. cytochrome c, generally evolve at different rates. These rates are relatively constant over time (e.g., hemoglobin does not evolve at the same rate as cytochrome c, but hemoglobins from humans, mice, etc. do have comparable rates of evolution), although rapid evolution along one branch can indicate increased directional selection on that branch. Purifying selection causes functionally important regions to evolve more slowly, and amino acid substitutions involving similar amino acids occurs more often than dissimilar substitutions.

Five Stages of Molecular Phylogenetic Analysis

Gene family evolution

Main article: Gene family

Gene phylogeny as lines within grey species phylogeny. Top: An ancestral gene duplication produces two paralogs (histone H1.1 and 1.2). A speciation event produces orthologs in the two daughter species (human and chimpanzee). Bottom: in a separate species (E. coli), a gene has a similar function (histone-like nucleoid-structuring protein) but has a separate evolutionary origin and so is an analog.

Gene duplication can produce multiple homologous proteins (paralogs) within the same species. Phylogenetic analysis of proteins has revealed how proteins evolve and change their structure and function over time.

For example, ribonucleotide reductase (RNR) has evolved a multitude of structural and functional variants. Class I RNRs use a ferritin subunit and differ by the metal they use as cofactors. In class II RNRs, the thiyl radical is generated using an adenosylcobalamin cofactor and these enzymes do not require additional subunits (as opposed to class I which do). In class III RNRs, the thiyl radical is generated using S-adenosylmethionine bound to a [4Fe-4S] cluster. That is, within a single family of proteins numerous structural and functional mechanisms can evolve.

In a proof-of-concept study, Bhattacharya and colleagues converted myoglobin, a non-enzymatic oxygen storage protein, into a highly efficient Kemp eliminase using only three mutations. This demonstrates that only few mutations are needed to radically change the function of a protein. Directed evolution is the attempt to engineer proteins using methods inspired by molecular evolution.

Molecular evolution at one site

Change at one locus begins with a new mutation, which might become fixed due to some combination of natural selection, genetic drift, and gene conversion.

Mutation

Main article: Mutation

This hedgehog has no pigmentation due to a mutation.

Mutations are permanent, transmissible changes to the genetic material (DNA or RNA) of a cell or virus. Mutations result from errors in DNA replication during cell division and by exposure to radiation, chemicals, other environmental stressors, viruses, or transposable elements. When point mutations to just one base-pair of the DNA fall within a region coding for a protein, they are characterized by whether they are synonymous (do not change the amino acid sequence) or non-synonymous. Other types of mutations modify larger segments of DNA and can cause duplications, insertions, deletions, inversions, and translocations.

The distribution of rates for diverse kinds of mutations is called the "mutation spectrum" (see App. B of ). Mutations of different types occur at widely varying rates. Point mutation rates for most organisms are very low, roughly 10⁻⁹ to 10⁻⁸ per site per generation, though some viruses have higher mutation rates on the order of 10⁻⁶ per site per generation. Transitions (A ↔ G or C ↔ T) are more common than transversions (purine (adenine or guanine)) ↔ pyrimidine (cytosine or thymine, or in RNA, uracil)). Perhaps the most common type of mutation in humans is a change in the length of a short tandem repeat (e.g., the CAG repeats underlying various disease-associated mutations). Such STR mutations may occur at rates on the order of 10⁻³ per generation.

Different frequencies of different types of mutations can play an important role in evolution via bias in the introduction of variation (arrival bias), contributing to parallelism, trends, and differences in the navigability of adaptive landscapes. Mutation bias makes systematic or predictable contributions to parallel evolution. Since the 1960s, genomic GC content has been thought to reflect mutational tendencies. Mutational biases also contribute to codon usage bias. Although such hypotheses are often associated with neutrality, recent theoretical and empirical results have established that mutational tendencies can influence both neutral and adaptive evolution via bias in the introduction of variation (arrival bias).

Selection

Main article: Natural selection

Selection can occur when an allele confers greater fitness, i.e. greater ability to survive or reproduce, on the average individual than carries it. A selectionist approach emphasizes e.g. that biases in codon usage are due at least in part to the ability of even weak selection to shape molecular evolution.

Selection can also operate at the gene level at the expense of organismal fitness, resulting in intragenomic conflict. This is because there can be a selective advantage for selfish genetic elements in spite of a host cost. Examples of such selfish elements include transposable elements, meiotic drivers, and selfish mitochondria.

Selection can be detected using the Ka/Ks ratio, the McDonald–Kreitman test. Rapid adaptive evolution is often found for genes involved in intragenomic conflict, sexual antagonistic coevolution, and the immune system.

Genetic drift

Main article: Genetic drift

Genetic drift is the change of allele frequencies from one generation to the next due to stochastic effects of random sampling in finite populations. These effects can accumulate until a mutation becomes fixed in a population. For neutral mutations, the rate of fixation per generation is equal to the mutation rate per replication. A relatively constant mutation rate thus produces a constant rate of change per generation (molecular clock).

Slightly deleterious mutations with a selection coefficient less than a threshold value of 1 / the effective population size can also fix. Many genomic features have been ascribed to accumulation of nearly neutral detrimental mutations as a result of small effective population sizes. With a smaller effective population size, a larger variety of mutations will behave as if they are neutral due to inefficiency of selection.

Gene conversion

Main article: Gene conversion

Gene conversion occurs during recombination, when nucleotide damage is repaired using an homologous genomic region as a template. It can be a biased process, i.e. one allele may have a higher probability of being the donor than the other in a gene conversion event. In particular, GC-biased gene conversion tends to increase the GC-content of genomes, particularly in regions with higher recombination rates. There is also evidence for GC bias in the mismatch repair process. It is thought that this may be an adaptation to the high rate of methyl-cytosine deamination which can lead to C→T transitions.

The dynamics of biased gene conversion resemble those of natural selection, in that a favored allele will tend to increase exponentially in frequency when rare.

Genome architecture

Main article: Genome evolution

Genome size

Main article: Genome size

Genome size is influenced by the amount of repetitive DNA as well as number of genes in an organism. Some organisms, such as most bacteria, Drosophila, and Arabidopsis have particularly compact genomes with little repetitive content or non-coding DNA. Other organisms, like mammals or maize, have large amounts of repetitive DNA, long introns, and substantial spacing between genes. The C-value paradox refers to the lack of correlation between organism 'complexity' and genome size. Explanations for the so-called paradox are two-fold. First, repetitive genetic elements can comprise large portions of the genome for many organisms, thereby inflating DNA content of the haploid genome. Repetitive genetic elements are often descended from transposable elements.

Secondly, the number of genes is not necessarily indicative of the number of developmental stages or tissue types in an organism. An organism with few developmental stages or tissue types may have large numbers of genes that influence non-developmental phenotypes, inflating gene content relative to developmental gene families.

Neutral explanations for genome size suggest that when population sizes are small, many mutations become nearly neutral. Hence, in small populations repetitive content and other 'junk' DNA can accumulate without placing the organism at a competitive disadvantage. There is little evidence to suggest that genome size is under strong widespread selection in multicellular eukaryotes. Genome size, independent of gene content, correlates poorly with most physiological traits and many eukaryotes, including mammals, harbor very large amounts of repetitive DNA.

However, birds likely have experienced strong selection for reduced genome size, in response to changing energetic needs for flight. Birds, unlike humans, produce nucleated red blood cells, and larger nuclei lead to lower levels of oxygen transport. Bird metabolism is far higher than that of mammals, due largely to flight, and oxygen needs are high. Hence, most birds have small, compact genomes with few repetitive elements. Indirect evidence suggests that non-avian theropod dinosaur ancestors of modern birds also had reduced genome sizes, consistent with endothermy and high energetic needs for running speed. Many bacteria have also experienced selection for small genome size, as time of replication and energy consumption are so tightly correlated with fitness.

Chromosome number and organization

The ant Myrmecia pilosula has only a single pair of chromosomes whereas the Adders-tongue fern Ophioglossum reticulatum has up to 1260 chromosomes. The number of chromosomes in an organism's genome does not necessarily correlate with the amount of DNA in its genome. The genome-wide amount of recombination is directly controlled by the number of chromosomes, with one crossover per chromosome or per chromosome arm, depending on the species.

Changes in chromosome number can play a key role in speciation, as differing chromosome numbers can serve as a barrier to reproduction in hybrids. Human chromosome 2 was created from a fusion of two chimpanzee chromosomes and still contains central telomeres as well as a vestigial second centromere. Polyploidy, especially allopolyploidy, which occurs often in plants, can also result in reproductive incompatibilities with parental species. Agrodiatus blue butterflies have diverse chromosome numbers ranging from n=10 to n=134 and additionally have one of the highest rates of speciation identified to date.

Cilliate genomes house each gene in individual chromosomes.

Organelles

Main article: Organelle

Animal cell showing organelles.

In addition to the nuclear genome, endosymbiont organelles contain their own genetic material. Mitochondrial and chloroplast DNA varies across taxa, but membrane-bound proteins, especially electron transport chain constituents are most often encoded in the organelle. Chloroplasts and mitochondria are maternally inherited in most species, as the organelles must pass through the egg. In a rare departure, some species of mussels are known to inherit mitochondria from father to son.

Origins of new genes

New genes arise from several different genetic mechanisms including gene duplication, de novo gene birth, retrotransposition, chimeric gene formation, recruitment of non-coding sequence into an existing gene, and gene truncation.

Gene duplication initially leads to redundancy. However, duplicated gene sequences can mutate to develop new functions or specialize so that the new gene performs a subset of the original ancestral functions. Retrotransposition duplicates genes by copying mRNA to DNA and inserting it into the genome. Retrogenes generally insert into new genomic locations, lack introns, and sometimes develop new expression patterns and functions.

Chimeric genes form when duplication, deletion, or incomplete retrotransposition combines portions of two different coding sequences to produce a novel gene sequence. Chimeras often cause regulatory changes and can shuffle protein domains to produce novel adaptive functions.

De novo gene birth can give rise to protein-coding genes and non-coding genes from previously non-functional DNA. For instance, Levine and colleagues reported the origin of five new genes in the D. melanogaster genome. Similar de novo origin of genes has also been shown in other organisms such as yeast, rice and humans. De novo genes may evolve from spurious transcripts that are already expressed at low levels.

Constructive neutral evolution

Main article: Constructive neutral evolution

Constructive neutral evolution (CNE) explains that complex systems can emerge and spread into a population through neutral transitions with the principles of excess capacity, presuppression, and ratcheting, and it has been applied in areas ranging from the origins of the spliceosome to the complex interdependence of microbial communities.

Journals and societies

The Society for Molecular Biology and Evolution publishes the journals "Molecular Biology and Evolution" and "Genome Biology and Evolution" and holds an annual international meeting. Other journals dedicated to molecular evolution include Journal of Molecular Evolution and Molecular Phylogenetics and Evolution. Research in molecular evolution is also published in journals of genetics, molecular biology, genomics, systematics, and evolutionary biology.

Space manufacturing

From Wikipedia, the free encyclopedia

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

A vision of a future Moon base that could be produced and maintained using 3D printing

Crystals grown by American scientists on the Russian Space Station Mir in 1995: (a) rhombohedral canavalin, (b) creatine kinase, (c) lysozyme, (d) beef catalase, (e) porcine alpha amylase, (f) fungal catalase, (g) myglobin, (h) concanavalin B, (i) thaumatin, (j) apoferritin, (k) satellite tobacco mosaic virus and (l) hexagonal canavalin.

Comparison of insulin crystals growth in outer space (left) and on Earth (right)

Space manufacturing or In-space manufacturing (ISM in short) is the fabrication, assembly or integration of tangible goods beyond Earth's atmosphere (or more generally, outside a planetary atmosphere), involving the transformation of raw or recycled materials into components, products, or infrastructure in space, where the manufacturing process is executed either by humans or automated systems by taking advantage of the unique characteristics of space. Synonyms of Space/In-space manufacturing are In-orbit manufacturing (since most production capabilities are limited to low Earth orbit), Off-Earth manufacturing, Space-based manufacturing, Orbital manufacturing, In-situ manufacturing, In-space fabrication, In-space production, etc. In-space manufacturing is a part of the broader activity of in-space servicing, assembly and manufacturing (ISAM) and is related to in situ resource utilization (ISRU).

Three major domains of In-space manufacturing are ISM for space (space-for-space) where products remain in space, ISM for Earth (space-for-Earth) where goods with improved properties produced in outer-space microgravity are transported back to Earth, and ISM for surface where goods are produced on or sent to surfaces of celestial bodies like the Moon, Mars, and asteroids.

In-space manufacturing uses processes such as additive manufacturing (printing a 3D object in successive layers), subtractive manufacturing (making 3D objects by successively removing material from a solid), hybrid manufacturing (usually combining additive manufacturing and subtractive manufacturing) and welding (joining pieces of material by melting or plasticizing along a joint line).

In-space manufacturing removes spacecraft design limitations due to launch parameters (mass, vibration, structural load, etc.) and volume limitations imposed by payload size. It allows for recycling of launched materials, utilization space-mined resources and on-demand spare parts production, which enables on-site repair of critical parts (increasing reliability and redundancy) and infrastructure development. It takes advantage of unique space features such as microgravity, ultra-vacuum and containerless processing, which are difficult to do on Earth.

Areas

In-space manufacturing (ISM) can be categorized into three different areas according to the end use of manufactured products. In-space manufacturing for space (space-for-space) involves activities focused on in-orbit construction intended for use in space. ISM for Earth (space-for-Earth) is the production of new materials and products that exhibit enhanced properties when manufactured in microgravity, subsequently transported back to Earth. Lastly, ISM for surface extends to surface operations on celestial bodies such as the Moon, Mars, and asteroids.

Rationale

There are several motivating factors behind in-space manufacturing. The space environment, in particular the effects of microgravity and vacuum, enable the research of and production of goods that could otherwise not be manufactured on Earth. Secondly, the extraction and processing of raw materials from other astronomical bodies, also called In-Situ Resource Utilisation (ISRU), could enable more sustainable space exploration missions at reduced cost compared to launching all required resources from Earth. Furthermore, raw materials could be transported to low Earth orbit where they could be processed into goods that are shipped to Earth. By replacing terrestrial production on Earth, this seeks to preserve the Earth. Moreover, raw materials of very high value, for example gold, silver, or platinum, could be transported to low Earth orbit for processing or transfer to Earth which is thought to have the potential to become economically viable. In-space manufacturing supports long-duration space missions and colonization by enabling on-site repair and infrastructure development beyond Earth. Additionally, in the area of spaceflight technology, space manufacturing enhances mission safety by decentralizing manufacturing activities and establishing redundancy in critical systems, allows for customized production tailored to specific mission requirements, fostering rapid iteration and adaptation of designs, drives technological innovation in materials science, robotics, and additive manufacturing, with applications extending beyond space exploration, and lays the foundation for space-based infrastructure development, supporting a wide range of commercial activities and scientific research.

History

During the Soyuz 6 mission of 1969, Russian cosmonauts performed the first welding experiments in space. Three different welding processes were tested using a hardware unit called Vulkan. The tests included welding aluminum, titanium, and stainless steel.

The Skylab mission, launched in May 1973, served as a laboratory to perform various space manufacturing experiments. The station was equipped with a materials processing facility that included a multi-purpose electric furnace, a crystal growth chamber, and an electron beam gun. Among the experiments to be performed was research on molten metal processing; photographing the behavior of ignited materials in zero-gravity; crystal growth; processing of immiscible alloys; brazing of stainless steel tubes, electron beam welding, and the formation of spheres from molten metal. The crew spent a total of 32 man-hours on materials science and space manufacturing investigation during the mission.

The Space Studies Institute began hosting a bi-annual Space Manufacturing Conference in 1977.

Microgravity research in materials processing continued in 1983 using the Spacelab facility. This module has been carried into orbit 26 times aboard the Space Shuttle, as of 2002. In this role the shuttle served as an interim, short-duration research platform before the completion of the International Space Station.

The Wake Shield Facility is deployed by the Space Shuttle's robotic arm. NASA image

In February 1994 and September 1995, the Wake Shield Facility was carried into orbit by the Space Shuttle. This demonstration platform used the vacuum created in the orbital wake to manufacture thin films of gallium arsenide and aluminum gallium arsenide.

On May 31, 2005, the recoverable, uncrewed Foton-M2 laboratory was launched into orbit. Among the experiments were crystal growth and the behavior of molten-metal in weightlessness.

The completion of the International Space Station has provided expanded and improved facilities for performing industrial research. These have and will continue to lead to improvements in our knowledge of materials sciences, new manufacturing techniques on Earth, and potentially some important discoveries in space manufacturing methods. NASA and Tethers Unlimited will test the Refabricator aboard the ISS, which is intended to recycle plastic for use in space additive manufacturing.

The Material Science Laboratory Electromagnetic Levitator (MSL-EML) on board the Columbus Laboratory is a science facility that can be used to study the melting and solidification properties of various materials. The Fluid Science Laboratory (FSL) is used to study the behavior of liquids in microgravity.

Material properties in the space environment

There are several unique differences between the properties of materials in space compared to the same materials on the Earth. These differences can be exploited to produce unique or improved manufacturing techniques.

• The microgravity environment allows control of convection in liquids or gasses, and the elimination of sedimentation. Diffusion becomes the primary means of material mixing, allowing otherwise immiscible materials to be intermixed.
• The environment allows enhanced growth of larger, higher-quality crystals in solution.
• The ultraclean vacuum of space allows the creation of very pure materials and objects. The use of vapor deposition can be used to build up materials layer by layer, free from defects.
• Surface tension causes liquids in microgravity to form perfectly round spheres. This can cause problems when trying to pump liquids through a conduit, but it is very useful when perfect spheres of consistent size are needed for an application.
• Space can provide readily available extremes of heat and cold. Sunlight can be focused to concentrate enough heat to melt the materials, while objects kept in perpetual shade are exposed to temperatures close to absolute zero. The temperature gradient can be exploited to produce strong, glassy materials.

Material processing

For most manufacturing applications, specific material requirements must be satisfied. Mineral ores need to be refined to extract specific metals, and volatile organic compounds will need to be purified. Ideally these raw materials are delivered to the processing site in an economical manner, where time to arrival, propulsion energy expenditure, and extraction costs are factored into the planning process. Minerals can be obtained from asteroids, the lunar surface, or a planetary body. Volatiles could potentially be obtained from a comet, carbonaceous chondrite or "C-Type" asteroids, or the moons of Mars or other planets. It may also prove possible to extract hydrogen in the form of water ice or hydrated minerals from cold traps on the poles of the Moon.

Unless the materials processing and the manufacturing sites are co-located with the resource extraction facilities, the raw materials would need to be moved about the Solar System. There are several proposed means of providing propulsion for this material, including solar sails, electric sails, magnetic sails, electric ion thrusters, microwave electrothermal thrusters, or mass drivers (this last method uses a sequence of electromagnets mounted in a line to accelerate a conducting material).

At the materials processing facility, the incoming materials will need to be captured by some means. Maneuvering rockets attached to the load can park the content in a matching orbit. Alternatively, if the load is moving at a low delta-v relative to the destination, then it can be captured by means of a mass catcher. This could consist of a large, flexible net or inflatable structure that would transfer the momentum of the mass to the larger facility. Once in place, the materials can be moved into place by mechanical means or by means of small thrusters.

Materials can be used for manufacturing either in their raw form, or by processing them to extract the constituent elements. Processing techniques include various chemical, thermal, electrolytic, and magnetic methods for separation. In the near term, relatively straightforward methods can be used to extract aluminum, iron, oxygen, and silicon from lunar and asteroidal sources. Less concentrated elements will likely require more advanced processing facilities, which may have to wait until a space manufacturing infrastructure is fully developed.

Some of the chemical processes will require a source of hydrogen for the production of water and acid mixtures. Hydrogen gas can also be used to extract oxygen from the lunar regolith, although the process is not very efficient. So a readily available source of useful volatiles is a positive factor in the development of space manufacturing. Alternatively, oxygen can be liberated from the lunar regolith without reusing any imported materials by heating the regolith to 4,530 °F (2,500 °C) in a vacuum. This was tested on Earth with lunar simulant in a vacuum chamber. As much as 20% of the sample was released as free oxygen. Eric Cardiff calls the remainder slag. This process is highly efficient in terms of imported materials used up per batch, but is not the most efficient process in energy per kilogram of oxygen.

One proposed method of purifying asteroid materials is through the use of carbon monoxide (CO). Heating the material to 500 °F (260 °C) and exposing it to CO causes the metals to form gaseous carbonyls. This vapor can then be distilled to separate out the metal components, and the CO can then be recovered by another heating cycle. Thus an automated ship can scrape up loose surface materials from, say, the relatively nearby 4660 Nereus (in delta-v terms), process the ore using solar heating and CO, and eventually return with a load of almost pure metal. The economics of this process can potentially allow the material to be extracted at one-twentieth the cost of launching from Earth, but it would require a two-year round trip to return any mined ore.

Manufacturing

Due to speed of light constraints on communication, manufacturing in space at a distant point of resource acquisition will either require completely autonomous robotics to perform the labor, or a human crew with all the accompanying habitat and safety requirements. If the plant is built in orbit around the Earth, or near a crewed space habitat, however, telerobotic devices can be used for certain tasks that require human intelligence and flexibility.

Solar power provides a readily available power source for thermal processing. Even with heat alone, simple thermally-fused materials can be used for basic construction of stable structures. Bulk soil from the Moon or asteroids has a very low water content, and when melted to form glassy materials is very durable. These simple, glassy solids can be used for the assembly of habitats on the surface of the Moon or elsewhere. The solar energy can be concentrated in the manufacturing area using an array of steerable mirrors.

The availability and favorable physical properties of metals will make them a major component of space manufacturing. Most of the metal handling techniques used on Earth can also be adopted for space manufacturing. A few of these techniques will need significant modifications due to the microgravity environment.

The production of hardened steel in space will introduce some new factors. Carbon only appears in small proportions in lunar surface materials and will need to be delivered from elsewhere. Waste materials carried by humans from the Earth is one possible source, as are comets. The water normally used to quench steel will also be in short supply, and require strong agitation.

Casting steel can be a difficult process in microgravity, requiring special heating and injection processes, or spin forming. Heating can be performed using sunlight combined with electrical heaters. The casting process would also need to be managed to avoid the formation of voids as the steel cools and shrinks.

Various metal-working techniques can be used to shape the metal into the desired form. The standard methods are casting, drawing, forging, machining, rolling, and welding. Both rolling and drawing metals require heating and subsequent cooling. Forging and extrusion can require powered presses, as gravity is not available. Electron beam welding has already been demonstrated on board the Skylab, and will probably be the method of choice in space. Machining operations can require precision tools which will need to be imported from the Earth for some duration.

New space manufacturing technologies are being studied at places such as Marshall's National Center for Advanced Manufacturing. The methods being investigated include coatings that can be sprayed on surfaces in space using a combination of heat and kinetic energy, and electron beam free form fabrication of parts. Approaches such as these, as well as examination of material properties that can be investigated in an orbiting laboratory, will be studied on the International Space Station by NASA and Made In Space, Inc.

3D-printing in space

Additive manufacturing in space has progressed from initial research (2014–2016) to early development (2016–2017), with patent activity since early 2020 indicating accelerated advancements.

The option of 3D printing items in space holds many advantages over manufacturing situated on Earth. With 3D printing technologies, rather than exporting tools and equipment from Earth into space, astronauts have the option to manufacture needed items directly. On-demand patterns of manufacturing make long-distance space travel more feasible and self-sufficient as space excursions require less cargo. Mission safety is also improved.

The Made In Space, Inc. 3D printers, which launched in 2014 to the International Space Station, are designed specifically for a zero-gravity or micro-gravity environment. The effort was awarded the Phase III Small Business Innovation and Research Contract. The Additive Manufacturing Facility will be used by NASA to carry out repairs (including during emergency situations), upgrades, and installation. Made In Space lists the advantages of 3D printing as easy customization, minimal raw material waste, optimized parts, faster production time, integrated electronics, limited human interaction, and option to modify the printing process.

The Refabricator experiment, under development by Firmamentum, a division of Tethers Unlimited, Inc. under a NASA Phase III Small Business Innovation Research contract, combines a recycling system and a 3D printer to perform demonstration of closed-cycle in-space manufacturing on the International Space Station (ISS). The Refabricator experiment, which was delivered to the ISS aboard Cygnus NG-10 on November 19, 2018, processes plastic feedstock through multiple printing and recycling cycles to evaluate how many times the plastic materials can be re-used in the microgravity environment before their polymers degrade to unacceptable levels.

Additionally, 3D printing in space can also account for the printing of meals. NASA's Advanced Food Technology program is currently investigating the possibility of printing food items in order to improve food quality, nutrient content, and variety.

Airbus is developing and planning with the European Space Agency to send and test the first 3D-printer printing metals in space at the ISS in a year from 2022, and establishing space manufacturing in three to four years from 2022.

Mitsubishi Electric Research Laboratories (MERL) has developed a UV-curable resin that is stable in the LEO environment, as well as an extruder and positioner to fabricate structures in orbit.

Products

There are thought to be a number of useful products that can potentially be manufactured in space and result in an economic benefit. Research and development is required to determine the best commodities to be produced, and to find efficient production methods. The following products are considered prospective early candidates:

• Growth of protein crystals
• Improved semiconductor wafers
• Micro-encapsulation

As the infrastructure is developed and the cost of assembly drops, some of the manufacturing capacity can be directed toward the development of expanded facilities in space, including larger scale manufacturing plants. These will likely require the use of lunar and asteroid materials, and so follow the development of mining bases.

Rock is the simplest product, and at minimum is useful for radiation shielding. It can also be subsequently processed to extract elements for various uses.

Water from lunar sources, Near Earth Asteroids or Martian moons is thought to be relatively cheap and simple to extract, and gives adequate performance for many manufacturing and material shipping purposes. Separation of water into hydrogen and oxygen can be easily performed in small scale, but some scientists believe that this will not be performed on any large scale initially due to the large quantity of equipment and electrical energy needed to split water and liquify the resultant gases. Water used in steam rockets gives a specific impulse of about 190 seconds; less than half that of hydrogen/oxygen, but this is adequate for delta-v's that are found between Mars and Earth. Water is useful as a radiation shield and in many chemical processes.

Ceramics made from lunar or asteroid soil can be employed for a variety of manufacturing purposes. These uses include various thermal and electrical insulators, such as heat shields for payloads being delivered to the Earth's surface.

Metals can be used to assemble a variety of useful products, including sealed containers (such as tanks and pipes), mirrors for focusing sunlight, and thermal radiators. The use of metals for electrical devices would require insulators for the wires, so a flexible insulating material such as plastic or fiberglass will be needed.

A notable output of space manufacturing is expected to be solar panels. Expansive solar energy arrays can be constructed and assembled in space. As the structure does not need to support the loads that would be experienced on Earth, huge arrays can be assembled out of proportionately smaller amounts of material. The generated energy can then be used to power manufacturing facilities, habitats, spacecraft, lunar bases, and even beamed down to collectors on the Earth with microwaves.

Other possibilities for space manufacturing include propellants for spacecraft, some repair parts for spacecraft and space habitats, and, of course, larger factories. Ultimately, space manufacturing facilities can hypothetically become nearly self-sustaining, requiring only minimal imports from the Earth. The microgravity environment allows for new possibilities in construction on a massive scale, including megascale engineering. These future projects might potentially assemble space elevators, massive solar array farms, very high capacity spacecraft, and rotating habitats capable of sustaining populations of tens of thousands of people in Earth-like conditions.

Challenges

The space environment is expected to be beneficial for production of a variety of products assuming the obstacles to it can be overcome. The most significant cost is overcoming the energy hurdle for boosting materials into orbit. Once this barrier is significantly reduced in cost per kilogram, the entry price for space manufacturing can make it much more attractive to entrepreneurs. After the heavy capitalization costs of assembling the mining and manufacturing facilities are paid, the production will need to be economically profitable in order to become self-sustaining and beneficial to society.

The economic requirements of space manufacturing imply a need to collect the requisite raw materials at a minimum energy cost. The cost of space transport is directly related to the delta-v, or change in velocity required to move from the mining sites to the manufacturing plants. Bringing material to Earth orbit from bodies such as Near-Earth asteroids, Phobos, Deimos or the lunar surface requires far less delta-v than launching from Earth itself, despite the greater distances involved. This makes these places economically attractive as sources of raw materials.

Habitable zone for complex life

From Wikipedia, the free encyclopedia

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

Natural shielding against space weather and solar wind, such as the magnetosphere depicted in this artistic rendition, is required for planets to sustain life for prolonged periods.

A Habitable Zone for Complex Life (HZCL) is a range of distances from a star suitable for complex aerobic life. Different types of limitations preventing complex life give rise to different zones. Conventional habitable zones are based on compatibility with water. Most zones start at a distance from the host star and then end at a distance farther from the star. A planet would need to orbit inside the boundaries of this zone. With multiple zonal constraints, the zones would need to overlap for the planet to support complex life. The requirements for bacterial life produce much larger zones than those for complex life, which requires a very narrow zone.

Exoplanets

Main articles: Exoplanets and Earth analog

The first confirmed exoplanets was discovered in 1992, several planets orbiting the pulsar PSR B1257+12. Since then the list of exoplanets has grown to the thousands. Most exoplanets are hot Jupiter planets, that orbit very close the star. Many exoplanets are super-Earths, that could be a gas dwarf or large rocky planet, like Kepler-442b at a mass 2.36 times Earths.

Star

Main articles: Star and Solar twin

Unstable stars are young and old stars, or very large or small stars. Unstable stars have changing solar luminosity that changes the size of the life habitable zones. Unstable stars also produce extreme solar flares and coronal mass ejections. Solar flares and coronal mass ejections can strip away a planet's atmosphere that is not replaceable. Thus life habitable zones require and very stable star like the Sun, at ±0.1% solar luminosity change. Finding a stable star, like the Sun, is the search for a solar twin, with solar analogs that have been found. Star metallicity, mass, age, color, and temperature all effect luminosity variations. The Sun, a G2V star, has a mid-range metallicity optimal for the formation of rocky planets. Dwarf stars (red dwarf/orange dwarf/brown dwarf/subdwarf) are not only unstable, but also emit low energy, so the habitable zone is very close to the star and planets become tidally locked on the timescales needed for the development of life. Giant stars (subgiant/giant star/red giant/red supergiant) are unstable and emit high energy, so the habitable zone is very far from the star. Multiple-star systems are also very common and are not suitable for complex life, as the planet orbit would be unstable due to multiple gravitational forces and solar radiation. Liquid water is possible in Multiple-star systems.

Named habitable zones

Main article: Habitable zone

A conventional habitable zone is defined by liquid water.

• Habitable zone (HZ) (also called the circumstellar habitable zone), the orbit around a star that would allow liquid water to remain for a short period of time (a given period of time) on at least a small part of the planet's surface. Thus within the HZ, water, (H₂O) is between 0 °C (32 °F; 273 K) and 100 °C (212 °F; 373 K) temperature. This zone is a temperature zone, set by the star's radiation and distance from the star. In the Solar System the planet Mars is just at the outer boundary of the habitable zone. The planet Venus is at the inner edge of the habitable zone, but due to its thick atmosphere it has no water. The HZ includes planets with elliptic orbits; such planets might orbit into and out of the HZ. When a planet moves out of the HZ, all its water would freeze to ice on the outside of the HZ, and/or all water would become steam on the inner side. The HZ could be defined as the region where bacteria, a form of life, could possibly survive for a short period of time. The HZ is also sometimes called the "Goldilocks" zone.
• Optimistic habitable zone (OHZ): a zone where liquid surface water could have been on a planet at some time in its past history. This zone would be larger than the HZ. Mars is an example of a planet in the OHZ.: it is just beyond the HZ today, but had liquid water for a short time span before the Mars carbonate catastrophe, some 4 billion years ago.
• Continuously habitable zone (CHZ): a zone where liquid water persists on the surface of a planet for years. This requires a near-circular planetary orbit and a stable star. The zone may be much smaller than the habitable zone.
• Conservative habitable zone: a zone where liquid surface water remains on a planet over a long time span, as on Earth. This might also need a greenhouse effect provided by gases such as CO₂ and water vapor to maintain the correct temperature. Rayleigh scattering would also be needed.

Named habitable zones for complex life

Over time and with more research, astronomers, cosmologists and astrobiologist have discovered more parameters needed for life. Each parameter could have a corresponding zone. Some of the named zones include:

• Ultraviolet habitable zone: a zone where the ultraviolet (UV) radiation from a star is neither too weak nor too strong for life to exist. Life needs the correct amount of ultraviolet for synthesis of biochemicals. The extent of the zone depends on the amount of ultraviolet radiation from the star, the range of UV wavelengths, the age of the star, and the atmosphere of the planet. In humans UV is used to produce vitamin D. Extreme ultraviolet (EUV) can cause atmospheric loss.
• Photosynthetic habitable zone: a zone where both long-term liquid water and oxygenic photosynthesis can occur.
• Tropospheric habitable zone, or ozone habitable zone: a zone where the planet would have the correct amount of ozone needed for life. Inhaling too much ozone causes inflammation and irritation, whereas too little troposphere ozone would produce biochemical smog. On Earth, the troposphere ozone is part of the ground-level ozone protection. Tropospheric ozone is formed by the interaction of ultraviolet light with hydrocarbons and nitrogen oxides.
• Planet rotation rate habitable zone: the zone where a planet's rotation rate is best for life. If the rotation is too slow, the day/night temperature difference is too great. The rotation rate also changes the planet's reflectivity and thus temperature, rotation as altered cloud distributions, cloud altitudes, and cloud opacities. A fast rotation rate increases wind speed on the planet. The rotation rate affects the planet's clouds and their reflectivity. Slowing the rotation rate changes cloud distributions, cloud altitudes, and cloud opacities. These changes in the clouds changes the temperature of the planet. A high rotation rate also can cause continuous, very fast winds on the surface. High rotation rates, coupled with strong Coriolis effects, generates continuous, high-speed winds by enabling the formation of persistent, fast-moving jet streams and atmospheric belts.
• Planet rotation axis tilt habitable zone, or obliquity habitable zone: the region where a stable axial tilt for a planet's rotation is maintained. Earth's axis is tilted 23.5°; this gives seasons, providing snow and ice that can melt to provide water run off in the summer. Obliquity has a major impact on a planet's temperature, thus its habitable zone.
• Tidal habitable zone. Planets too close to the star become tidally locked. The mass of the star and the distance from the star set the tidal habitable zone. A planet tidally locked has one side of the planet facing the star, this side would be very hot. The face away from the star would be well below freezing. A planet too close to the star will also have tidal heating from the star. Tidal heating can vary the planet's orbital eccentricity. Too far from the star and the planet will not receive enough solar heat.
• Astrosphere habitable zone: the zone in which a planet's astrosphere will be strong enough to protect the planet from the solar wind and cosmic rays. The astrosphere must be long lasting to protect the planet. Mars lost its water and most of its atmosphere after the losing its magnetic field and Mars carbonate catastrophe event. Star-Sun's solar wind is made of charged particles, including plasma, electrons, protons and alpha particles. The solar wind is different for each star. Earth's magnetic field is very large and has protected Earth since its formation.
• Atmosphere electric field habitable zone: the place in which the ambipolar electric field is correct for the planet's electric field to help ions overcome gravity. The planet's ionosphere must be correct to protect against the loss of the atmosphere. This is addition to a strong magnetic field to protect against the solar wind stripping away the atmosphere and water into outer space.
• Orbital eccentricity habitable zone: the zone in which planets maintain a nearly circular orbit. As orbits with eccentricity have the planets move in and out of the habitable zones. In the Solar System, the grand tack hypothesis proposes the theory of the unique placement of the gas giants, the Solar System belts and the planets near circular orbits.^[
• Coupled planet-moon - Magnetosphere habitable zone: the zone that planet's moon and the planet's core produce a strong magnetosphere, magnetic field to protect against the solar wind stripping away the planet's atmosphere and water into outer space. Just as Mars had a magnetic field for a short time. Earth's Moon had a large magnetosphere for several hundred million years after its formation, as proposed in a 2020 study by Saied Mighani. The Moon's magnetosphere would have given added protection of Earth's atmosphere as the early Sun was not as stable as it today. In 2020, James Green modeled the coupled planet-moon-magnetosphere habitable zone. The modeling showed a coupled planet–moon magnetosphere that would give planet the protection from stellar wind in the early Solar System. In the case of Earth, the Moon was closer to Earth in the early formation of the Solar System, giving added protection. This protection was needed then as the Sun was less stable.
• Pressure-dependent habitable zone: the zone in which planets may have the correct atmospheric pressure to have liquid surface water. With a low atmospheric pressure, the temperature at which water boils is much lower, and at pressures below that of the triple point, liquid water cannot exist. The average surface pressure on Mars today is close to that of the triple point of water; thus, liquid water cannot exist there. Planets with high-pressure atmospheres may have liquid surface water, but life forms may experience difficulty with respiratory systems in high-pressure atmospheres. As pressure increases, gas becomes denser, making it more difficult to move in and out of the lungs, also airway resistance is increased as the viscosity and density of the air mixture is greater thus the higher resistance within the airways.This greatly increases the effort required for breathing.
• Galactic habitable zone (GHZ): The GHZ, also called the Galactic Goldilocks zone, is the place in a galaxy in which heavy elements needed for a rocky planet and life are present, but also a place where strong cosmic rays will not kill life and strip the atmosphere off the planet. The term Goldilocks zone is used, as it is a fine balance between the two sites (heavy elements and strong cosmic rays). Not all galaxies are able to support life. In many galaxies, life-killing events such as gamma-ray bursts can occur. About 90% of galaxies have long and frequent gamma ray bursts, thus no life. Cosmic rays pose a threat to life. Galaxies with many stars too close together or without any dust protection also are not hospitable for life. Irregular galaxies and other small galaxies do not have enough heavy elements. Elliptical galaxies are full of lethal radiation and lack heavy elements. Large spiral galaxies, like the Milky Way, have the heavy element needed for life at the center and out to about half distance from the center bar. Not all large spiral galaxies are the same, as spiral galaxies with too much active star formation can be deadly to life. Too little star formation and the spiral arms will collapse. Not all spiral galaxies have the correct galactic ram pressure stripping parameters; too much ram pressure can deplete the galaxy of gas and thus end star formation. The Milky Way is a barred spiral galaxy, the bar is important to star formation and metallicity of the galaxy's stars and planets. A barred spiral galaxy must have stable arms with the just right star formation. Barred spiral galaxies make up about 65% of spiral galaxies, but most have too much star formation. Peculiar galaxies lack stable spiral arms, while irregular galaxies contain too many new stars and lack heavy elements. Unbarred spiral galaxies do not correct star formation and metallicity for a galactic Goldilocks zone. For long term life on a planet, the spiral arms must be stable for a long period of time, as in the Milky Way. The spiral arms must not be too close to each other, or there will be too much ultraviolet radiation. If the planet moves into or across a spiral arm, the orbits of the planets could change from gravitational disturbances. Movement across a spiral arms also would cause deadly asteroid impacts and high radiation. The planet must be in the correct place in the spiral galaxy: near the galactic center, radiation and gravitational forces are too great for life, whereas the outskirts of a spiral galaxy are metal-poor. The Sun in 28,000 light years from the center bar, in the galactic Goldilocks zone. At this distance, the Sun revolves in the galaxy at the same rate as the spiral-arm rotation, thus minimizing arm crossings.
• Supergalactic habitable zone: a place in a supercluster of galaxies that can provide for habitability of planets. The supergalactic habitable zone takes into account events in galaxies that can end habitability not only in a galaxy, but all galaxies nearby, such as galaxies merging, active galactic nucleus, starburst galaxy, supermassive black holes and merging black holes, all which output intense radiation. The supergalactic habitable zone also takes into account the abundance of various chemical elements in the galaxy, as not all galaxies or regions within have all the needed elements for life.
• Habitable zone for complex life (HZCL): the place that all the life habitable zones overlap for a long period of time, as in the Solar System. The list of habitable zones for complex life has grown longer with increasing understanding of the Universe, galaxies, and the Solar System. Complex life is normally defined as eukaryote life forms, including all animals, plants, fungi, and most unicellular organisms. Simple life forms are normally defined as prokaryotes.

Other orbital-distance related factors

Some factors that depend on planetary distance and may limit complex aerobic life have not been given zone names. These include:

• Milankovitch cycle The Milankovitch cycle and ice age have been key is shaping Earth. Life on Earth today is using water melting from the last ice age. The ice ages cannot be too long or too cold for life to survive. Milankovitch cycle has an impact on the planet's obliquity also.

Life

Main articles: Life and Carbon-based life

Life on Earth is carbon-based. However, some theories suggest that life could be based on other elements in the periodic table. Other elements proposed have been silicon, boron, arsenic, ammonia, methane and others. As more research has been done on life on Earth, it has been found that only carbon's organic molecules have the complexity and stability to form life. Carbon's properties allows for complex chemical bonding that produces covalent bonds needed for organic chemistry. Carbon molecules are lightweight and relatively small in size. Carbon's ability to bond to oxygen, hydrogen, nitrogen, phosphorus, and sulfur (called CHNOPS) is key to life.