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Tianjin, Kuala Lumpur, Toronto, Milan, Shenyang, Dallas, Fort Worth, Boston, Belo Horizonte, Khartoum, Riyadh, Singapore, Washington, Detroit, Barcelona,, Houston, Athens, Berlin, Sydney, Atlanta, Guadalajara, San Francisco, Oakland, Montreal, Monterey, Melbourne, Ankara, Recife, Phoenix/Mesa, Durban, Porto Alegre, Dalian, Jeddah, Seattle, Cape Town, San Diego, Fortaleza, Curitiba, Rome, Naples, Minneapolis, St. Paul, Tel Aviv, Birmingham, Frankfurt, Lisbon, Manchester, San Juan, Katowice, Tashkent, Fukuoka, Baku, Sumqayit, St. Louis, Baltimore, Sapporo, Tampa, St. Petersburg, Taichung, Warsaw, Denver, Cologne, Bonn, Hamburg, Dubai, Pretoria, Vancouver, Beirut, Budapest, Cleveland, Pittsburgh, Campinas, Harare, Brasilia, Kuwait, Munich, Portland, Brussels, Vienna, San Jose, Damman , Copenhagen, Brisbane, Riverside, San Bernardino, Cincinnati and Accra Pioneer plaque The Pioneer plaques are a pair of gold-anodized aluminum plaques that were placed on board the 1972 Pioneer 10 and 1973 Pioneer 11 spacecraft, featuring a pictorial message, in case either Pioneer 10 or 11 is intercepted by intelligent extraterrestrial life. The plaques show the nude figures of a human male and female along with several symbols that are designed to provide information about the origin of the spacecraft.[1] The Pioneer 10 and 11 spacecraft were the first human-built objects to achieve escape velocity from the Solar System. The plaques were attached to the spacecraft's antenna support struts in a position that would shield them from erosion by interstellar dust. History The original idea, that the Pioneer spacecraft should carry a message from mankind, was first mentioned by Eric Burgess[2] when he visited the Jet Propulsion Laboratory in Pasadena, California, during the Mariner 9 mission. He approached Carl Sagan, who had lectured about communication with intelligent extraterrestrials at a conference in Crimea. Sagan was enthusiastic about the idea of sending a message with the Pioneer spacecraft. NASA agreed to the plan and gave him three weeks to prepare a message. Together with Frank Drake he designed the plaque, and the artwork was prepared by Linda Salzman Sagan, who was Sagan's wife at the time. Additional artistic contributions were made by Jon Lomberg.[3] Both plaques were manufactured at Precision Engravers, San Carlos, California.[4] The first plaque was launched with Pioneer 10 on March 2, 1972, and the second followed with Pioneer 11 on April 5, 1973. In May 2017, a limited edition of 200 replicas engraved from the original master design at Precision Engravers was made available in a Kickstarter Campaign, which also offered laser-engraved replicas.[5] Physical properties Material: 6061 T6 gold-anodized aluminium Width: 9 inches (228.6 mm) Height: 6 inches (152.4 mm) Thickness: 0.05 inches (1.27 mm) Mean depth of engraving: 0.015 inches (0.381 mm) Mass: approx. 4.2 ounces (120 g) Symbolism Hyperfine transition of neutral hydrogen Hyperfine transition of neutral hydrogen At the top left of the plaque is a schematic representation of the hyperfine transition of hydrogen, which is the most abundant element in the universe. The spin-flip transition of a hydrogen atom's electron has a frequency of about 1420.405 MHz, which corresponds to a period of 0.704 ns. Light at this frequency has a vacuum wavelength of 21.106 centimetres (8.309 in) (which is also the distance the light travels in that time period). Below the symbol, the small vertical line—representing the binary digit 1—specifies a unit of length (21 cm) as well as a unit of time (0.7 ns). Both units are used as measurements in the other symbols.[6] Figures of a man and a woman Figures of a man and a woman On the right side of the plaque, a nude man and woman are shown in front of the spacecraft.[7] Between the brackets that indicate the height of the woman, the binary representation of the number 8 can be seen (1000). In units of the wavelength of the hyperfine transition of hydrogen this means 8 × 21 cm = 168 centimetres (66 in). The small defect in the first zero is only in reproductions of the plaque (like here) and not on the original.[citation needed] It probably dates back to a printing error in the original article "A Message from Earth" which is the primary source for many of the copies of the engraving.[1] In the replicas from the 2017 Kickstarter campaign mentioned above the delineation is correct.[5] The right hand of the man is raised as a sign of good will. Although this gesture may not be understood, it offers a way to show the opposable thumb and how the limbs can be moved.[8] Originally Sagan intended the humans to be shown holding hands, but soon realized that an extraterrestrial might perceive them as a single creature rather than two organisms.[8] The original drawings of the figures were based on drawings by Leonardo da Vinci and Greek sculptures.[8] The woman's genitals are not depicted in detail; only the mons pubis is shown. It has been claimed that Sagan, having little time to complete the plaque, suspected that NASA would have rejected a more intricate drawing and therefore made a compromise just to be safe.[9] Sagan said that the decision to not include the vertical line on the woman's genitalia (pudendal cleft) which would be caused by the intersection of the labia majora was due to two reasons. First, Greek sculptures of women do not include that line. Second, Sagan believed that a design with such an explicit depiction of a woman's genitalia would be considered too obscene to be approved by NASA.[10] According to the memoirs of Robert S. Kraemer, however, the original design that was presented to NASA headquarters included a line which indicated the woman's vulva,[11] and this line was erased as a condition for approval of the design by John Naugle, former head of NASA's Office of Space Science and the agency's former chief scientist.[11] Sun and galactic landmarks Relative position of the Sun to the center of the Galaxy and 14 pulsars with their periods denoted The radial pattern on the left of the plaque shows 15 lines emanating from the same origin. Fourteen of the lines have corresponding long binary numbers, which stand for the periods of pulsars, using the hydrogen spin-flip transition frequency as the unit. Since these periods will change over time, the epoch of the launch can be calculated from these values. The lengths of the lines show the relative distances of the pulsars to the Sun. A tick mark at the end of each line gives the Z coordinate perpendicular to the galactic plane. If the plaque is found, only some of the pulsars may be visible from the location of its discovery. Showing the location with as many as 14 pulsars provides redundancy so that the location of the origin can be triangulated even if only some of the pulsars are recognized. The data for one of the pulsars is misleading. When the plaque was designed, the frequency of pulsar "1240" (now known as J1243-6423) was known to only three significant decimal digits: 0.388 second.[1] The map lists the period of this pulsar in binary to much greater precision: 100000110110010110001001111000. Rounding this off at about 10 significant bits (100000110100000000000000000000) would have provided a hint of this uncertainty. This pulsar is represented by the long line pointing down and to the right. The fifteenth line on the plaque extends to the far right, behind the human figures. This line indicates the Sun's relative distance to the center of the galaxy. The pulsar map and hydrogen atom diagram are shared in common with the Voyager Golden Record. Solar System The Solar System with the trajectory of the Pioneer spacecraft At the bottom of the plaque is a schematic diagram of the Solar System. A small picture of the spacecraft is shown, and the trajectory shows its way past Jupiter and out of the Solar System. Both Pioneers 10 and 11 have identical plaques; however, after launch, Pioneer 11 was redirected toward Saturn and from there it exited the Solar System. In this regard the Pioneer 11 plaque is inaccurate. The Saturn flyby of Pioneer 11 would also greatly influence its future direction and destination as compared to Pioneer 10, but this fact is not depicted in the plaques. Saturn's rings could give a further hint to identifying the Solar System. Rings around the planets Jupiter, Uranus, and Neptune were unknown when the plaque was designed; however, unlike Saturn, the ring systems on these planets are not so easily visible and apparent as Saturn's. Pluto was considered to be a planet when the plaque was designed; in 2006 the IAU reclassified Pluto as a dwarf planet. Other large bodies classed as dwarf planets, such as Eris, are not depicted, as they were unknown at the time the plaque was made. The binary numbers above and below the planets show the relative distance to the Sun. The unit is 1 / 10  of Mercury's orbit. Rather than the familiar "1" and "0", "I" and "–" are used. Silhouette of the spacecraft Silhouette of the Pioneer spacecraft relative to the size of the humans Behind the figures of the human beings, the silhouette of the Pioneer spacecraft is shown in the same scale so that the size of the human beings can be deduced by measuring the spacecraft. Criticism One of the parts of the diagram that is among the easiest for humans to understand may be among the hardest for potential extraterrestrial finders to understand: the arrow showing the trajectory of Pioneer. Ernst Gombrich criticized the use of an arrow because arrows are an artifact of hunter-gatherer societies like those on Earth; finders with a different cultural heritage may find the arrow symbol meaningless.[12] Art critic Craig Owens said that sexual bias is exhibited by the decision to have the man in the diagram perform the raised hand gesture to greet the extraterrestrials while the woman in the diagram has her hands at her sides.[13] Feminists also took issue with this choice for the same reason.[14] To appease these criticisms, a second illustration of a similar couple was provided, with the woman raising her hand instead.[3] Carl Sagan regretted that the figures in the finished engraving failed to look panracial. Although this was the intent, the final figures were criticised for looking too white. Both had broad and flattened noses, and the woman was given epicanthial folds to resemble East Asian people.[14] In the original drawing, the man was drawn with an "Afro" haircut, so an additional African physical trait would be included in the man to make the figures look more panracial, but that detail was changed to a "non-African Mediterranean-curly haircut" in the finished engraving.[15] Furthermore, Carl Sagan said that Linda Sagan intended to portray both the man and woman as having brown hair, but the hair being only outlined, rather than being both outlined and shaded made their hair appear blonde instead.[16] Other people had different interpretations of the race of people depicted by the figures. White people, black people and East Asian people each tended to think that the figures resembled their own racial group, so, although some people were proud that their race appeared to have been selected to represent all of humankind, others viewed the figures as "terribly racist" for "the apparently blatant exclusion" of other races.[17] Linda Sagan decided to make the figures nude to address the problem of the type of clothes they should wear to represent all of humanity and to make the figures more anatomically educational for extraterrestrials, but some viewed their nudity as pornographic.[18] According to astronomer Frank Drake, there were many negative reactions to the plaque because the human beings were displayed naked.[19] When images of the final design were published in American newspapers, one newspaper published the image with the man's genitalia removed and another newspaper published the image with both the man's genitalia and the woman's nipples removed.[20] In one letter to a newspaper, a person angrily wrote that they felt that the nudity of the images made the images obscene.[21] In contrast, in another letter to the same newspaper, a person was critical of the prudishness of the people who found depictions of nudity to be obscene.[a] There have also been criticisms of the censorship of the female figure's genitals. Scientist and artist Joe Davis protested the depiction with his Poetica Vaginal project wherein he used an MIT radar dish to transmit the recordings of a vaginal detector.[22] See also Alien language Arecibo message Communication with extraterrestrial intelligence Lunar plaque Pioneer program Search for extraterrestrial intelligence (SETI) Voyager Golden Record References Notes  The writer is being sarcastic in second quotation on page 25. Using sarcasm, the writer is mocking the prudishness which led to the censorship of the woman's vulva rather than supporting the censorship.[21] Citations  Sagan, Sagan & Drake 1972, pp. 881–884.  Pournelle 1982, pp. 212–240.  Scott, Jonathan (21 March 2019). The Vinyl Frontier: The Story of NASA's Interstellar Mixtape. Bloomsbury Publishing. p. 92. ISBN 978-1-4729-5611-8.  "Plaque Engraving, Sign Engraving - Precision Engravers, San Carlos". www.precision-engravers.com. Retrieved 2020-06-29.  King, Duane. "Pioneer Plaque: a Message from Earth". Kickstarter.com. Retrieved 27 December 2017.  Rosenthal, Jake (20 January 2016). "The Pioneer Plaque: Science as a Universal Language". planetary.org. Retrieved 9 October 2019.  Barnett, David (10 September 2015). "Send aliens modern messages of Earth's equality and diversity, say scientists". the Guardian. Retrieved 9 October 2019.  Sagan 2000, p. 22.  Fletcher 2001.  Sagan 2000.  Wolverton 2004, p. 80.  Gombrich 1983, pp. 85–89.  Spariosu 1991, p. 176.  Achenbach 1999, p. 92.  Geppert 2012, p. 297.  Spangenburg, Moser & Moser 2004, p. 74.  Wolverton 2004, p. 82.  Wolverton 2004, p. 79.  Sagan 1978.  Sagan 2000, p. 23.  Sagan 2000, p. 25.  Hay 2020. Sources Achenbach, Joel (1999). Captured by Aliens: The Search for Life and Truth in a Very Large Universe. New York: Simon & Schuster. ISBN 978-0-684-84856-3. Fletcher, Alan (2001). The Art of Looking Sideways. Phaidon Press. ISBN 978-0-7148-3449-8. Geppert, Alexander C.T. (2012). Geppert, Alexander C. T. (ed.). Imagining Outer Space: European Astroculture in the Twentieth Century. Palgrave Macmillan. doi:10.1057/9780230361362. ISBN 978-0-230-23172-6. Gombrich, E. H. (1983). "The Image and the Eye: Further Studies in the Psychology of Pictorial Representation". The Journal of Aesthetics and Art Criticism. 42 (1): 85. doi:10.2307/429951. ISSN 0021-8529. JSTOR 429951. Hay, Mark (26 February 2020). "NASA's Fight to Protect Aliens From Naked Ladies". OZY. Archived from the original on 2021-06-02. Morgan, Edwin (1997). Collected Poems. Manchester: Carcanet Press. ISBN 978-1-857541-88-5. Pournelle, Jerry (April 1982). "The Osborne 1, Zeke's New Friends, and Spelling Revisited". BYTE. pp. 212–240. Sagan, C.; Sagan, L. S.; Drake, F. (1972). "A Message from Earth". Science. 175 (4024): 881–884. Bibcode:1972Sci...175..881S. doi:10.1126/science.175.4024.881. ISSN 0036-8075. PMID 17781060. Sagan, Carl (1978). Murmurs of Earth: the Voyager interstellar record. Random House. ISBN 978-0-394-41047-0. Sagan, Carl (2000). Carl Sagan's Cosmic Connection: An Extraterrestrial Perspective. Cambridge University Press. pp. 22–25. ISBN 978-0-521-78303-3. Spangenburg, Ray; Moser, Kit; Moser, Diane (2004). Carl Sagan: A Biography. Greenwood. ISBN 978-0-313-32265-5. Spariosu, Mihai (1991). Bogue, Ronald (ed.). Mimesis in contemporary theory : an interdisciplinary approach. Vol. 2, Mimesis, semiosis, and power. Philadelphia; Amsterdam: J. Benjamins. ISBN 9789027242259. OCLC 185840954. Wolverton, Mark (2004). The Depths of Space: The Story of the Pioneer Planetary Probes. National Academies Press. ISBN 978-0-309-09050-6. External links Wikimedia Commons has media related to Pioneer plaque. Hans Mark: Origin Story of Carl Sagan's Plaque on Pioneer 10 on YouTube – recording from 1994 interview Reading the Pioneer/Voyager Pulsar Map (Wm. Robert Johnston), updated 11 March 2003. 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verification. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. Find sources: "Uncrewed spacecraft" – news · newspapers · books · scholar · JSTOR (January 2023) (Learn how and when to remove this template message) The uncrewed resupply vessel Progress M-06M Galileo space probe, prior to departure from Earth orbit in 1989 Uncrewed spacecraft Buran launched, orbited Earth, and landed as an uncrewed spacecraft in 1988 (shown here at an airshow) Model of James Webb Space Telescope Top: The uncrewed resupply vessel Progress M-06M (left). Galileo space probe, prior to departure from Earth orbit in 1989 (right). Bottom: Spaceplane Buran was launched, orbited Earth, and landed as an uncrewed spacecraft in 1988 (left). Model of James Webb Space Telescope (right). Part of a series on Spaceflight History     History of spaceflight Space Race Timeline of spaceflight Space probes Lunar missions Mars missions Applications     Communications Earth observation Exploration Espionage Military Navigation Settlement Telescopes Tourism Spacecraft Robotic spacecraft     Satellite Space probe Cargo spacecraft Crewed spacecraft     Apollo LM Space capsules Space Shuttle Space stations Spaceplanes Vostok Space launch     Spaceport Launch pad Expendable and reusable launch vehicles Escape velocity Non-rocket spacelaunch Spaceflight types     Sub-orbital Orbital Interplanetary Interstellar Intergalactic List of space organizations     Space agencies Space forces Companies  Spaceflight portal     vte Uncrewed spacecraft or robotic spacecraft are spacecraft without people on board. Uncrewed spacecraft may have varying levels of autonomy from human input; they may be remote controlled, remote guided or autonomous: they have a pre-programmed list of operations, which they will execute unless otherwise instructed. A robotic spacecraft for scientific measurements is often called a space probe or space observatory. Many space missions are more suited to telerobotic rather than crewed operation, due to lower cost and risk factors. In addition, some planetary destinations such as Venus or the vicinity of Jupiter are too hostile for human survival, given current technology. Outer planets such as Saturn, Uranus, and Neptune are too distant to reach with current crewed spaceflight technology, so telerobotic probes are the only way to explore them. Telerobotics also allows exploration of regions that are vulnerable to contamination by Earth micro-organisms since spacecraft can be sterilized. Humans can not be sterilized in the same way as a spaceship, as they coexist with numerous micro-organisms, and these micro-organisms are also hard to contain within a spaceship or spacesuit. The first uncrewed space mission was Sputnik, launched October 4, 1957 to orbit the Earth. Nearly all satellites, landers and rovers are robotic spacecraft. Not every uncrewed spacecraft is a robotic spacecraft; for example, a reflector ball is a non-robotic uncrewed spacecraft. Space missions where other animals but no humans are on-board are called uncrewed missions. Many habitable spacecraft also have varying levels of robotic features. For example, the space stations Salyut 7 and Mir, and the International Space Station module Zarya, were capable of remote guided station-keeping and docking maneuvers with both resupply craft and new modules. Uncrewed resupply spacecraft are increasingly used for crewed space stations. History A replica of Sputnik 1 at the U.S. National Air and Space Museum A replica of Explorer 1 The first robotic spacecraft was launched by the Soviet Union (USSR) on 22 July 1951, a suborbital flight carrying two dogs Dezik and Tsygan.[1] Four other such flights were made through the fall of 1951. The first artificial satellite, Sputnik 1, was put into a 215-by-939-kilometer (116 by 507 nmi) Earth orbit by the USSR on 4 October 1957. On 3 November 1957, the USSR orbited Sputnik 2. Weighing 113 kilograms (249 lb), Sputnik 2 carried the first animal into orbit, the dog Laika.[2] Since the satellite was not designed to detach from its launch vehicle's upper stage, the total mass in orbit was 508.3 kilograms (1,121 lb).[3] In a close race with the Soviets, the United States launched its first artificial satellite, Explorer 1, into a 357-by-2,543-kilometre (193 by 1,373 nmi) orbit on 31 January 1958. Explorer I was an 205-centimetre (80.75 in) long by 15.2-centimetre (6.00 in) diameter cylinder weighing 14.0 kilograms (30.8 lb), compared to Sputnik 1, a 58-centimeter (23 in) sphere which weighed 83.6 kilograms (184 lb). Explorer 1 carried sensors which confirmed the existence of the Van Allen belts, a major scientific discovery at the time, while Sputnik 1 carried no scientific sensors. On 17 March 1958, the US orbited its second satellite, Vanguard 1, which was about the size of a grapefruit, and remains in a 670-by-3,850-kilometre (360 by 2,080 nmi) orbit as of 2016. The first attempted lunar probe was the Luna E-1 No.1, launched on 23 September 1958. The goal of a lunar probe repeatedly failed until 4 January 1959 when Luna 1 orbited around the Moon and then the Sun. The success of these early missions began a race between the US and the USSR to outdo each other with increasingly ambitious probes. Mariner 2 was the first probe to study another planet, revealing Venus' extremely hot temperature to scientists in 1962, while the Soviet Venera 4 was the first atmospheric probe to study Venus. Mariner 4's 1965 Mars flyby snapped the first images of its cratered surface, which the Soviets responded to a few months later with images from on its surface from Luna 9. In 1967, America's Surveyor 3 gathered information about the Moon's surface that would prove crucial to the Apollo 11 mission that landed humans on the Moon two years later.[4] The first interstellar probe was Voyager 1, launched 5 September 1977. It entered interstellar space on 25 August 2012,[5] followed by its twin Voyager 2 on 5 November 2018.[6] Nine other countries have successfully launched satellites using their own launch vehicles: France (1965),[7] Japan[8] and China (1970),[9] the United Kingdom (1971),[10] India (1980),[11] Israel (1988),[12] Iran (2009),[13] North Korea (2012),[14] and South Korea (2022).[15] Telepresence Telerobotics becomes telepresence when the time delay is short enough to permit control of the spacecraft in close to real time by humans. Even the two seconds light speed delay for the Moon is too far away for telepresence exploration from Earth. The L1 and L2 positions permit 400-millisecond round trip delays, which is just close enough for telepresence operation. Telepresence has also been suggested as a way to repair satellites in Earth orbit from Earth. The Exploration Telerobotics Symposium in 2012 explored this and other topics.[16] Design      This section may have too many subsection headers. Please help consolidate the section. (May 2023) (Learn how and when to remove this template message) In spacecraft design, the United States Air Force considers a vehicle to consist of the mission payload and the bus (or platform). The bus provides physical structure, thermal control, electrical power, attitude control and telemetry, tracking and commanding.[17] Subsystems JPL divides the "flight system" of a spacecraft into subsystems.[18] These include: Structure An illustration's of NASA's planned Orion spacecraft approaching a robotic asteroid capture vehicle The physical backbone structure, which     provides overall mechanical integrity of the spacecraft     ensures spacecraft components are supported and can withstand launch loads Data handling This is sometimes referred to as the command and data subsystem. It is often responsible for:     command sequence storage     maintaining the spacecraft clock     collecting and reporting spacecraft telemetry data (e.g. spacecraft health)     collecting and reporting mission data (e.g. photographic images) Attitude determination and control See also: Attitude control system This system is mainly responsible for the correct spacecraft's orientation in space (attitude) despite external disturbance-gravity gradient effects, magnetic-field torques, solar radiation and aerodynamic drag; in addition it may be required to reposition movable parts, such as antennas and solar arrays.[19] Entry, descent, and landing Integrated sensing incorporates an image transformation algorithm to interpret the immediate imagery land data, perform a real-time detection and avoidance of terrain hazards that may impede safe landing, and increase the accuracy of landing at a desired site of interest using landmark localization techniques. Integrated sensing completes these tasks by relying on pre-recorded information and cameras to understand its location and determine its position and whether it is correct or needs to make any corrections (localization). The cameras are also used to detect any possible hazards whether it is increased fuel consumption or it is a physical hazard such as a poor landing spot in a crater or cliff side that would make landing very not ideal (hazard assessment). Landing on hazardous terrain In planetary exploration missions involving robotic spacecraft, there are three key parts in the processes of landing on the surface of the planet to ensure a safe and successful landing.[20] This process includes an entry into the planetary gravity field and atmosphere, a descent through that atmosphere towards an intended/targeted region of scientific value, and a safe landing that guarantees the integrity of the instrumentation on the craft is preserved. While the robotic spacecraft is going through those parts, it must also be capable of estimating its position compared to the surface in order to ensure reliable control of itself and its ability to maneuver well. The robotic spacecraft must also efficiently perform hazard assessment and trajectory adjustments in real time to avoid hazards. To achieve this, the robotic spacecraft requires accurate knowledge of where the spacecraft is located relative to the surface (localization), what may pose as hazards from the terrain (hazard assessment), and where the spacecraft should presently be headed (hazard avoidance). Without the capability for operations for localization, hazard assessment, and avoidance, the robotic spacecraft becomes unsafe and can easily enter dangerous situations such as surface collisions, undesirable fuel consumption levels, and/or unsafe maneuvers. Telecommunications Components in the telecommunications subsystem include radio antennas, transmitters and receivers. These may be used to communicate with ground stations on Earth, or with other spacecraft.[21] Electrical power The supply of electric power on spacecraft generally come from photovoltaic (solar) cells or from a radioisotope thermoelectric generator. Other components of the subsystem include batteries for storing power and distribution circuitry that connects components to the power sources.[22] Temperature control and protection from the environment Main article: Spacecraft thermal control Spacecraft are often protected from temperature fluctuations with insulation. Some spacecraft use mirrors and sunshades for additional protection from solar heating. They also often need shielding from micrometeoroids and orbital debris.[23] Propulsion Main article: Spacecraft propulsion Spacecraft propulsion is a method that allows a spacecraft to travel through space by generating thrust to push it forward.[24] However, there is not one universally used propulsion system: monopropellant, bipropellant, ion propulsion, etc. Each propulsion system generates thrust in slightly different ways with each system having its own advantages and disadvantages. But, most spacecraft propulsion today is based on rocket engines. The general idea behind rocket engines is that when an oxidizer meets the fuel source, there is explosive release of energy and heat at high speeds, which propels the spacecraft forward. This happens due to one basic principle known as Newton's Third Law. According to Newton, "to every action there is an equal and opposite reaction." As the energy and heat is being released from the back of the spacecraft, gas particles are being pushed around to allow the spacecraft to propel forward. The main reason behind the usage of rocket engine today is because rockets are the most powerful form of propulsion there is. Monopropellant For a propulsion system to work, there is usually an oxidizer line and a fuel line. This way, the spacecraft propulsion is controlled. But in a monopropellant propulsion, there is no need for an oxidizer line and only requires the fuel line.[25] This works due to the oxidizer being chemically bonded into the fuel molecule itself. But for the propulsion system to be controlled, the combustion of the fuel can only occur due to a presence of a catalyst. This is quite advantageous due to making the rocket engine lighter and cheaper, easy to control, and more reliable. But, the downfall is that the chemical is very dangerous to manufacture, store, and transport. Bipropellant A bipropellant propulsion system is a rocket engine that uses a liquid propellent.[26] This means both the oxidizer and fuel line are in liquid states. This system is unique because it requires no ignition system, the two liquids would spontaneously combust as soon as they come into contact with each other and produces the propulsion to push the spacecraft forward. The main benefit for having this technology is because that these kinds of liquids have relatively high density, which allows the volume of the propellent tank to be small, therefore increasing space efficacy. The downside is the same as that of monopropellant propulsion system: very dangerous to manufacture, store, and transport. Ion An ion propulsion system is a type of engine that generates thrust by the means of electron bombardment or the acceleration of ions.[27] By shooting high-energy electrons to a propellant atom (neutrally charge), it removes electrons from the propellant atom and this results the propellant atom becoming a positively charged atom. The positively charged ions are guided to pass through positively charged grids that contains thousands of precise aligned holes are running at high voltages. Then, the aligned positively charged ions accelerates through a negative charged accelerator grid that further increases the speed of the ions up to 40 kilometres per second (90,000 mph). The momentum of these positively charged ions provides the thrust to propel the spacecraft forward. The advantage of having this kind of propulsion is that it is incredibly efficient in maintaining constant velocity, which is needed for deep-space travel. However, the amount of thrust produced is extremely low and that it needs a lot of electrical power to operate. Mechanical devices Mechanical components often need to be moved for deployment after launch or prior to landing. In addition to the use of motors, many one-time movements are controlled by pyrotechnic devices.[28] Robotic vs. uncrewed spacecraft Robotic spacecraft are specifically designed system for a specific hostile environment.[29] Due to their specification for a particular environment, it varies greatly in complexity and capabilities. While an uncrewed spacecraft is a spacecraft without personnel or crew and is operated by automatic (proceeds with an action without human intervention) or remote control (with human intervention). The term 'uncrewed spacecraft' does not imply that the spacecraft is robotic. Control Robotic spacecraft use telemetry to radio back to Earth acquired data and vehicle status information. Although generally referred to as "remotely controlled" or "telerobotic", the earliest orbital spacecraft – such as Sputnik 1 and Explorer 1 – did not receive control signals from Earth. Soon after these first spacecraft, command systems were developed to allow remote control from the ground. Increased autonomy is important for distant probes where the light travel time prevents rapid decision and control from Earth. Newer probes such as Cassini–Huygens and the Mars Exploration Rovers are highly autonomous and use on-board computers to operate independently for extended periods of time.[30][31] Space probes and observatories Further information: List of Solar System probes, List of space telescopes, and Lists of spacecraft A space probe is a robotic spacecraft that does not orbit Earth, but instead, explores further into outer space. Space probes have different sets of scientific instruments onboard. A space probe may approach the Moon; travel through interplanetary space; flyby, orbit, or land on other planetary bodies; or enter interstellar space. Space probes send collected data to Earth. Space probes can be orbiters, landers, and rovers. Space probes can also gather materials from its target and return it to Earth.[32][33] Once a probe has left the vicinity of Earth, its trajectory will likely take it along an orbit around the Sun similar to the Earth's orbit. To reach another planet, the simplest practical method is a Hohmann transfer orbit. More complex techniques, such as gravitational slingshots, can be more fuel-efficient, though they may require the probe to spend more time in transit. Some high Delta-V missions (such as those with high inclination changes) can only be performed, within the limits of modern propulsion, using gravitational slingshots. A technique using very little propulsion, but requiring a considerable amount of time, is to follow a trajectory on the Interplanetary Transport Network.[34] A space telescope or space observatory is a telescope in outer space used to observe astronomical objects. Space telescopes avoid the filtering and distortion of electromagnetic radiation which they observe, and avoid light pollution which ground-based observatories encounter. They are divided into two types: satellites which map the entire sky (astronomical survey), and satellites which focus on selected astronomical objects or parts of the sky and beyond. Space telescopes are distinct from Earth imaging satellites, which point toward Earth for satellite imaging, applied for weather analysis, espionage, and other types of information gathering. Cargo spacecraft Further information: Comparison of space station cargo vehicles A collage of automated cargo spacecraft used in the past or present to resupply the International Space Station Cargo or resupply spacecraft are robotic spacecraft that are designed specifically to carry cargo, possibly to support space stations' operation by transporting food, propellant and other supplies. This is different from a space probe, whose missions are to conduct scientific investigations. Automated cargo spacecraft have been used since 1978 and have serviced Salyut 6, Salyut 7, Mir, the International Space Station and Tiangong space station. As of 2023, three different cargo spacecraft are used to supply the International Space Station: Russian Progress, American SpaceX Dragon 2 and Cygnus. Chinese Tianzhou is used to supply Tiangong space station. See also     Spaceflight portal     Beacon mode service     Geosynchronous satellite     Human spaceflight     List of passive satellites     Timeline of Solar System exploration References Asif Siddiqi, Sputnik and the Soviet Space Challenge, University Press of Florida, 2003, ISBN 081302627X, p. 96 Whitehouse, David (28 October 2002). "First dog in space died within hours". BBC News World Edition. Archived from the original on 17 July 2013. Retrieved 10 May 2013. "The animal, launched on a one-way trip on board Sputnik 2 in November 1957, was said to have died painlessly in orbit about a week after blast-off. Now, it has been revealed she died from overheating and panic just a few hours after the mission started." "Sputnik 2, Russian Space Web". 3 November 2012. Archived from the original on 2 February 2023. Retrieved 7 January 2023. "NASA - What Is a Space Probe?". www.nasa.gov. Archived from the original on 30 August 2021. Retrieved 9 January 2023. Barnes, Brooks (12 September 2013). "In a Breathtaking First, NASA's Voyager 1 Exits the Solar System". The New York Times. ISSN 0362-4331. Archived from the original on 7 April 2019. Retrieved 1 August 2022. Potter, Sean (9 December 2018). "NASA's Voyager 2 Probe Enters Interstellar Space". NASA. Archived from the original on 21 May 2022. Retrieved 1 August 2022. "France launches first satellite". UPI. 26 November 1965. Retrieved 4 March 2023. "11 February 1970. This Day in History: Japan launches its first satellite". History Channel. Archived from the original on 5 March 2023. Retrieved 4 March 2023. "Timeline: Major milestones in Chinese space exploration". Reuters. 22 November 2020. Archived from the original on 5 March 2023. Retrieved 4 March 2023. Ben Judge (28 October 2020). "28 October 1971: Britain's only independent satellite launch". Money Week. Archived from the original on 5 March 2023. Retrieved 4 March 2023. VP Sandlas (31 August 2018). "Blast from the past: An insider's account of India's first successful experimental satellite launch". Archived from the original on 8 November 2023. Retrieved 4 March 2023. Glenn Frankel (20 September 1988). "Israel Launches its First Satellite into Orbit". Washington Post. Archived from the original on 8 November 2023. Retrieved 4 March 2023. "Iran launches first satellite, draws concern". Los Angeles Times. 3 February 2009. Archived from the original on 5 March 2023. Retrieved 4 March 2023. "North Korea Launches First Satellite into Orbit". Space News. 14 December 2012. Archived from the original on 8 November 2023. Retrieved 4 March 2023. "South Korea launches first satellite with homegrown rocket". NBC News. 22 June 2022. Archived from the original on 8 November 2023. Retrieved 5 March 2023. Exploration Telerobotics Symposium Archived 2015-07-05 at the Wayback Machine May 2–3, 2012 at NASA Goddard Space Flight Center. "Air University Space Primer, Chapter 10 – Spacecraft Design, Structure And Operation" (PDF). USAF. Archived from the original (PDF) on 21 December 2016. Retrieved 13 October 2007. "Chapter 11. Typical Onboard Systems". JPL. Archived from the original on 28 April 2015. Retrieved 10 June 2008. Wiley J. Larson; James R. Wertz(1999). Space Mission Analysis and Design, 3rd ed. Microcosm. pp. 354. ISBN 978-1-881883-10-4, Howard, Ayanna (January 2011). "Rethinking public–private space travel". Space Policy. 29 (4): 266–271. Bibcode:2013SpPol..29..266A. doi:10.1016/j.spacepol.2013.08.002. LU. K. Khodarev (1979). "Space Communications". The Great Soviet Encyclopedia. Archived from the original on 10 May 2013. Retrieved 10 May 2013. "The transmission of information between the earth and spacecraft, between two or more points on the earth via spacecraft or using artificial means located in space (a belt of needles, a cloud of ionized particles, and so on), and between two or more spacecraft." Wiley J. Larson; James R. Wertz (1999). Space Mission Analysis and Design, 3rd ed.. Microcosm. pp. 409. ISBN 978-1-881883-10-4, "Micrometeoroid and Orbital Debris (MMOD) Protection" (PDF). NASA. Archived from the original (PDF) on 29 October 2009. Retrieved 10 May 2013. Hall, Nancy (5 May 2015). "Welcome to the Beginner's Guide to Propulsion". NASA. Archived from the original on 8 November 2023. Retrieved 7 January 2023. Zhang, Bin (October 2014). "A verification framework with application to a propulsion system". Expert Systems with Applications. 41 (13): 5669–5679. doi:10.1016/j.eswa.2014.03.017. Chen, Yang (April 2017). "Dynamic modeling and simulation of an integral bipropellant propulsion double-valve combined test system" (PDF). Acta Astronautica. 133: 346–374. Bibcode:2017AcAau.133..346C. doi:10.1016/j.actaastro.2016.10.010. Archived from the original on 8 November 2023. Retrieved 7 January 2023. Patterson, Michael (August 2017). "Ion Propulsion". NASA. Archived from the original on 31 December 2018. Retrieved 7 January 2023. Wiley J. Larson; James R. Wertz(1999). Space Mission Analysis and Design, 3rd ed. Microcosm. pp. 460. ISBN 978-1-881883-10-4, Davis, Phillips. "Basics of Space Flight". NASA. Archived from the original on 2 June 2019. Retrieved 7 January 2023. K. Schilling; W. Flury (11 April 1989). "AUTONOMY AND ON-BOARD MISSION MANAGEMENT ASPECTS FOR THE CASSINI-TITAN PROBE". ATHENA MARS EXPLORATION ROVERS. Archived from the original (PDF) on 5 May 2013. Retrieved 10 May 2013. "Current space missions exhibit a rapid growth in the requirements for on-board autonomy. This is the result of increases in mission complexity, intensity of mission activity and mission duration. In addition, for interplanetary spacecraft, the operations are characterized by complicated ground control access, due to the large distances and the relevant solar system environment[…] To handle these problemsn, the spacecraft design has to include some form of autonomous control capability." "Frequently Asked Questions (Athena for kids): Q) Is the rover controlled by itself or controlled by scientists on Earth?" (PDF). ATHENA MARS EXPLORATION ROVERS. 2005. Archived from the original (PDF) on 29 October 2009. Retrieved 10 May 2013. "Communication with Earth is only twice per sol (martian day) so the rover is on its own (autonomous) for much of its journey across the martian landscape. Scientists send commands to the rover in a morning "uplink" and gather data in an afternoon "downlink." During an uplink, the rover is told where to go, but not exactly how to get there. Instead, the command contains the coordinates of waypoints toward a desired destination. The rover must navigate from waypoint to waypoint without human help. The rover has to use its "brain" and its "eyes" for these instances. The "brain" of each rover is the onboard computer software that tells the rover how to navigate based on what the Hazcams (hazard avoidance cameras) see. It is programmed with a given set of responses to a given set of circumstances. This is called "autonomy and hazard avoidance."" "NASA - What Is a Space Probe?". www.nasa.gov. Archived from the original on 30 August 2021. Retrieved 26 February 2023. "Space Probes". education.nationalgeographic.org. Archived from the original on 26 February 2023. Retrieved 26 February 2023.     Ross, S. D. (2006). "The Interplanetary Transport Network" (PDF). American Scientist. 94 (3): 230–237. doi:10.1511/2006.59.994. Archived (PDF) from the original on 20 October 2013. 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Extraterrestrial life, life that may exist or may have existed in the universe outside of Earth. The search for extraterrestrial life encompasses many fundamental scientific questions. What are the basic requirements for life? Could life have arisen elsewhere in the solar system? Are there other planets like Earth? How likely is the evolution of intelligent life? (Read Britannica’s biography of Carl Sagan, co-author of this entry.) Universal criteria No one knows which aspects of living systems are necessary, in the sense that living systems everywhere must have them, and which are contingent, in the sense that they are the result of evolutionary accidents such that elsewhere a different sequence of events might have led to different properties of life. In this respect the discovery of even a single example of extraterrestrial life, no matter how elementary in form or substance, would represent a fundamental revolution in science. Do a vast array of biological themes and counterpoints exist in the universe, or are there places with living fugues, compared with which Earth’s one tune is a bit thin and reedy? Or is Earth’s the only tune around? Life on Earth, structurally based on carbon, hydrogen, nitrogen, and other elements, uses water as its interaction medium. Phosphorus, as phosphate bound to an organic residue, is required for energy storage and transport; sulfur is involved in the three-dimensional configuration of protein molecules; and other elements are present in smaller concentrations. Must these particular atoms be the atoms of life everywhere, or might there be a wide range of atomic possibilities in extraterrestrial organisms? What are the general physical constraints on extraterrestrial life? In approaching these questions, several criteria can be used. The major atoms should tend to have a high cosmic abundance. Structural molecules of organisms at the temperature of the planet in question should not be so extremely stable that chemical reactions are impossible, but neither should they be extremely unstable, or else the organism would fall to pieces. A medium for molecular interaction must be present. Solids are inappropriate because of their inertness. The medium, most likely a liquid but possibly a very dense gas, must be stable in a number of respects. It should have a large temperature range (for a liquid, the temperature difference between freezing point and boiling point should be large). The liquid should be difficult to vaporize and to freeze; in general, it should be difficult to change its temperature. The interaction medium needs to be an excellent solvent. A fluid phase must be present on the planet in question, for material must cycle to the organism as food and away from the organism as waste. The planet should therefore have an atmosphere and some liquid near the surface, although not necessarily a water ocean. If the intensity of ultraviolet light or charged particles from its sun is intense at the planetary surface, then some area, perhaps below the surface, should be shielded from this radiation (although some forms or intensity of radiation might permit useful chemical reactions to occur). Finally, it is imperative that conditions allow the existence of autotrophy (the ability of an organism to synthesize at least some of its own nutrients) or other means of net production of necessary compounds. Special offer for students! Check out our special academic rate and excel this spring semester! Thermodynamically, photosynthesis based on stellar radiation may be the optimal source of energy for extraterrestrial life. Photosynthetic organisms and the radiation they receive are not in thermodynamic equilibrium. On Earth, for example, a green plant may have a temperature of about 300 K (23 °C, or 73 °F); the Sun’s temperature is about 6,000 K. (K = kelvin. On the Kelvin temperature scale, in which 0 K [−273 °C, or −460 °F] is absolute zero, 273 K [0 °C, or 32 °F] is the freezing point of water, and 373 K [100 °C, or 212 °F] is the boiling point of water at one atmosphere pressure.) Photosynthetic processes are possible because energy is transported from a hotter object (the Sun) to a cooler object (Earth). Were the source of radiation at the same or at a colder temperature than the photosynthesizer, no photosynthetic activity would be possible. For this reason, the idea that a subterranean green plant will photosynthesize by use of thermal infrared radiation emitted by its surroundings is untenable. Equally unfeasible is the idea that a cold star, with a surface temperature similar to that of Earth, could sustain photosynthetic organisms. One can use these conditions to establish the limits for the chemical requirements of life. When atoms chemically combine, the energy necessary to separate them is called the bond energy, and the measure of this energy determines how tightly the two atoms are bound to each other. Bond energies generally vary from about 10 electron volts (eV) to about 0.03 eV. Covalent bonds, where electrons are shared between atoms, tend to be more energetic than hydrogen bonds, where a hydrogen atom is shared between atoms, and hydrogen bonds in turn are more energetic than van der Waals forces, which arise from the attraction of the electrons of one atom for the nucleus of another. Atoms, free or bound, move with an average kinetic energy corresponding to about 0.02 eV. The higher the temperature, the more atoms move with energy sufficient to break a given bond spontaneously. Specific atoms have circumscribed functions in modern biology, but, aside from structure and the need for the liquid interaction medium, they may not be fundamental. The energy-rich phosphate bonds in adenosine triphosphate (ATP), about as energetic as the hydrogen bonds, are in fact of relatively low energy. Cells store large numbers of these bonds to drive a molecular degradation or synthesis. One expects the energy currency on high-temperature worlds to be much more energetic per bond and on low-temperature worlds to be much less energetic per bond. In The Fitness of the Environment (1913), American biochemist Lawrence Joseph Henderson first stressed the advantages of carbon and water for life in terms of comparative chemistry. Henderson was struck by the fact that the very atoms needed are exactly those that are around. It remains a remarkable fact that the atoms most useful for life have very high cosmic abundances. The search for extraterrestrial life Mars Mars Global map of Mars in epithermal (intermediate-energy) neutrons created from data collected by the 2001 Mars Odyssey spacecraft. Odyssey mapped the location and concentrations of epithermal neutrons knocked off the Martian surface by incoming cosmic rays. Deep blue areas at the high latitudes mark the lowest levels of neutrons, which scientists have interpreted to indicate the presence of high levels of hydrogen. The hydrogen enrichment, in turn, is suggestive of large reservoirs of water ice below the surface. Astrobiology, a term coined for the study of all life anywhere in the universe (including Earth), has replaced exobiology, the study of extraterrestrial life exclusively and therefore criticizable as “a science that lacks a subject matter.” Unlike exobiology, astrobiology respects the scientific possibility that life beyond Earth may never be found. Indeed, no evidence for life beyond Earth has been adduced. However, the design of astrobiological experiments forces critical examination of the generality of assumptions derived from Earth life. There is an entire spectrum of possibilities for life on another planet. A planet may be lifeless and lack any vestiges of organic matter or fossils. Alternatively, it may be lifeless but contain organic matter or fossils. There may be life having simple or quite complex biochemistry, physiology, and behaviour. Even intelligent life with a technical civilization may be found. Confirmation of any of these possibilities would be of great scientific importance. The search for extraterrestrial life is most clearly grasped by imagining the reverse situation. For example, if humans were on Mars, examination of Earth for life with the full armoury of contemporary scientific instrumentation and knowledge would be illuminating. Both remote and in situ testing might be attempted. In remote testing, light of any wavelength reflected from or emitted by the target planet can be examined. Remote-sensing methods seek thermodynamic disequilibrium, especially in the fluid phases (atmosphere and hydrosphere) of the planet. With in situ studies, samples of a planet must be acquired by instrumentation that lands there and performs experiments. Chemical, mechanical, or spectral disequilibria may also be sought. Earth’s atmosphere contains large amounts of molecular oxygen and about 1.7–2 parts per million (106) of methane, but in thermodynamic equilibrium the abundance of methane should be less than one part in 1035. This huge discrepancy implies that some process continuously and rapidly generates methane on Earth such that methane increases to a very large steady-state abundance before it can be oxidized. Although the methane disequilibrium mechanism need not be biological (e.g., relatively stable aromatic hydrocarbons could have been produced nonbiologically early in Earth’s history, and slow degradation may then have led to a continuous loss of methane from the planetary subsurface), a biological explanation seems more likely. As seen from Mars, the methane discrepancy could be considered as a preliminary positive test for life on Earth. Indeed, the methane abundance on Earth is due to bacteria. Some methanogenic bacteria live in wetlands (hence the term marsh gas for methane), and others live in the intestinal tracts of cows and other ruminants. Similarly, the large amount of free oxygen gas might be considered a sign of life. The possibility that the photodissociation of water and the subsequent escape to space of hydrogen are the source of oxygen would need to be excluded. Also, spectroscopic detection of such relatively complex reduced organic molecules as terpenes (hydrocarbons given off by plants and found over forests) could be used as a test for life. Earth at night Earth at night Earth at night as seen in a composite of images taken by satellites of the Defense Meteorological Satellite Program of the U.S. Air Force. By contrast, photographic observations of the daytime Earth from Mars would not necessarily detect life. Even at a resolution of 100 metres (330 feet)—that is, an ability to discriminate fine detail at high contrast only if its components are more than 100 metres apart—cities, canals, bridges, the Great Wall of China (long erroneously believed to be visible from the Moon), highways, and other large-scale accoutrements of Earth’s technical civilization would be extremely difficult to discern. As resolution progressively improves, it becomes increasingly easy to distinguish the regular geometric patterns of cultivated fields, highways, and airports. However, these are products of recent civilization; thus, only 100,000 years ago no clear sign of life would have been visible with remote-sensing techniques. Today lights of the largest cities are detectable from Mars, as are seasonal changes in the colour of plants. telecommunication antennas telecommunication antennas Short-wave, microwave, cellular telephone, and other types of telecommunication antennas receive and send messages from high ground near Phoenix, Arizona. Scanning of the electromagnetic spectrum offers another technique for detecting life. Domestic television transmissions, the high-frequency end of the AM broadcast band, and radar defense networks make up some of the enormous amount of energy put out by Earth into space at certain radio frequencies. According to an estimate made by the Russian astrophysicist Iosif S. Shklovskii, if this radiation were to be interpreted as ordinary thermal emission, the implied temperature of Earth would be hundreds of millions of degrees. This radio “brightness temperature” of Earth would have steadily increased over the last several decades. The frequency and the time variation of these signals are not purely random noise. In situ studies by vehicles that enter Earth’s atmosphere and land on the surface could detect life at many places on Earth. However, there are many other places where large organisms are infrequent such that life-detection attempts based solely on television searches for large animals would be inconclusive. Of course, if such an experiment were successful—for example, if the camera recorded a cavorting dolphin, a strolling camel, or a flying peacock—it would provide quite convincing evidence of life. Although the open ocean, the Gobi Desert, and Antarctica are relatively devoid of large life-forms, they are—like other, less-barren ecosystems—replete with microorganisms. A television camera coupled to an optical or electron microscope might be an optimal life detector. The 17th-century Dutch microscopist Antonie van Leeuwenhoek had no difficulty in identifying as alive the little “animalcules” he found in a drop of water, even though nothing similar had been seen before in human history. Metabolic and chemical criteria might be used for detecting life with in situ studies. The fixation of gas (such as carbon dioxide) when illuminated might be due to photosynthesis or chemosynthesis. Direct tests of soil or water for optical activity might be made. Organic molecules could certainly be sought with gas chromatography, mass spectrometry, or remote analytic chemistry. The detection of organic matter would then lead to experiments that would determine if it was biological in origin. In general, many tests for life are intrinsically ambiguous. There remains the omnipresent problem of contamination. Any spacecraft might carry living organisms from the home planet and report them as detected on the target planet. Great care must be taken to ensure that the spacecraft is rigorously sterilized and travels without life from home. Even the detection of significant quantities of extraterrestrial organic matter can be misleading. Carbonaceous chondrite meteorites fall on Earth from the asteroid belt. They contain up to 4 percent organic matter by mass. This matter has been ascertained to be of nonbiological origin. Microscopic inclusions also have been detected. The most abundant of these inclusions are mineralogical in origin. Highly structured inclusions, such as filaments or microspheres with central dots, are rare and sometimes the result of obvious contamination (one inclusion contained ragweed pollen). Claims of the extraction of viable microorganisms from the interiors of carbonaceous chondrites were not supported by subsequent evidence. These meteorites are porous and “breathe” air in and out during their entry into the atmosphere and during their storage prior to study. Significant opportunities exist for contamination after their arrival on Earth because of the ubiquity of microorganisms. Some bacteria extracted from a meteorite were facultative aerobes. As no planet in the solar system except Earth harbours significant quantities of oxygen gas, it is unlikely that the electron-transfer multienzyme pathways required for oxygen respiration evolved in the asteroid belt. Nevertheless, the large amounts of organic matter found in carbonaceous chondrites suggest that organic molecule production occurs with great efficiency in certain extraterrestrial locations. This production may serve as a natural precursor to life elsewhere. No single unambiguous “life detector” exists. Instruments of great generality that make few ambiguous assumptions about the nature of extraterrestrial life require luck (e.g., an animal or protist must walk or swim by during the operating lifetime of the camera) or the solution of difficult instrumental problems (e.g., the acquisition and preparation of samples for remote electron microscopic examination). Highly sensitive instruments, such as metabolism detectors, are directed at organisms presumably vastly more abundant than animals. These instruments critically depend on assumptions that are basically informed guesses (e.g., that extraterrestrial organisms eat sugars). Therefore, an array of both very general and very specific instruments is recommended to establish, or preclude, the existence of extraterrestrial life in the solar system. Life in the solar system Saturn: surface of Titan Saturn: surface of Titan Image of the surface of Titan from the Huygens probe's High Resolution Imager. A brief survey of life’s prospects on the moons and planets of the solar system follows. In the solar system there are many different environments that could contain significant clues to the origin of life and perhaps even life itself. However, there is not yet definitive evidence for or against extraterrestrial life on these planets. The Moon and Mercury Moon Moon The familiar near side of Earth's Moon, photographed on December 7, 1992, by the Galileo spacecraft on its way to Jupiter. Two primary kinds of terrain are visible—the lighter areas, which constitute the heavily cratered and very old highlands, and the darker, roughly circular plains, traditionally called maria, which are relatively young lava-filled impact basins. Among the maria are (from left) the crescent-shaped Oceanus Procellarum near the left limb, the large, almost perfectly circular Mare Imbrium, or Imbrium Basin (with the crater Copernicus a bright dot at its lower margin), Mare Serenitatis immediately to the right of Imbrium, Mare Tranquillitatis to the lower right of Serenitatis, and Mare Crisium, isolated near the right limb. Another bright crater, Tycho, stands out at the bottom left of the image. The Moon’s surface is inhospitable to life of any sort. Diurnal temperatures range from about 100 K (−173 °C, or −279 °F) to about 400 K (127 °C, or 261 °F). In the absence of either an atmosphere or a magnetic field, ultraviolet light and charged particles from the Sun penetrate unimpeded to the lunar surface. In less than an hour, they deliver a dose lethal to the most radiation-resistant bacteria known. The subsurface environment of the Moon is not nearly so inclement. Ultraviolet light and solar protons do not penetrate more than 1 metre (3.3 feet) below the surface, and the temperature is maintained at a relatively constant value of about 230 K (−43 °C, or −45 °F). Nevertheless, the absence of any surface fluid, atmosphere, or liquid to cycle matter and energy makes prospects for life dim. Mercury Mercury Photo mosaic of Mercury, taken by the Mariner 10 spacecraft, 1974. The environment of Mercury is rather like that of the Moon. Its surface temperatures range from about 100 K to about 620 K (347 °C, or 657 °F), but, about 1 metre below the surface, the temperature is constant at roughly room temperature. However, the absence of any significant atmosphere, the unlikelihood of bodies of liquid, and the intense solar radiation make the prospect for life on Mercury remote. Martian “vegetation” and “canals” the possibility of life on Mars the possibility of life on Mars A discussion of the possibility of microbial life on Mars.See all videos for this article seasons of Mars seasons of Mars The seasons of Mars, a result of the planet's inclination of 24.9° to its orbital plane. At present, southern summer occurs when Mars's elongated orbit brings it nearest the Sun. As the seasons change, the polar caps alternately grow and shrink. At its maximum size the southern cap extends about 5° more equatorward than the northern cap. Evidence for life on Mars has been claimed for more than a century. The first such argument was posed by a French astronomer, Étienne L. Trouvelot, in 1884: Judging from the changes that I have seen to occur from year to year in these spots, one could believe that these changing grayish areas are due to Martian vegetation undergoing seasonal changes. The seasonal changes on Mars have been reliably observed, not only visually but also photometrically. There is a conspicuous springtime increase in the contrast between the bright and dark areas of Mars. Colour changes with season have also been reported. Space probes have found no vegetation on Mars, but seasonally variable dust storms provide a convincing explanation of the colour changes. Mars as seen by Hubble Space Telescope compared with an 1894 map of Mars Mars as seen by Hubble Space Telescope compared with an 1894 map of Mars Mars as seen by the Hubble Space Telescope (left) compared with a map of Mars based on French astronomer Eugène Antoniadi's observations in 1894. Antoniadi later concluded that what he thought were Martian canals are an optical illusion. Historically, life on Mars was argued for on the basis of the “canals.” This apparent set of thin straight lines across the Martian bright areas extends for hundreds, even thousands, of kilometres and changes seasonally. First systematically observed in 1887 by Italian astronomer Giovanni V. Schiaparelli, the lines were further catalogued and popularized about the turn of the 20th century by American astronomer Percival Lowell. From the unerring straightness of the lines, Lowell argued they could not be natural in origin. Instead he interpreted them as artificial constructs built by intelligent Martians. Lowell suggested they might be channels that carry water from the melting polar caps to the parched equatorial cities. However, many other astronomers were not able to see the canals, and the canals are now believed to be an optical illusion. Approximately rectilinear features do exist on the Martian surface, but these are natural features such as crater chains, terrain contour boundaries, faults, mountain chains, and ridges analogous to the suboceanic ridge features of Earth. surface of Mars from Viking 1 lander surface of Mars from Viking 1 lander Martian surface of rocks and fine-grained material, photographed in 1976 by the Viking 1 spacecraft. In July and August 1976 two U.S. probes, Viking 1 and 2, successfully landed on Mars with equipment designed to detect the presence or remains of organic material. Analyses of atmospheric and soil samples yielded conclusive results; the data were interpreted as negative. At least in the vicinity of these probes, no evidence for life exists. In 1996 analysis of the Allan Hills Martian meteorite (ALH84001) yielded structures and sedimentary magnetite that some have interpreted as direct evidence for extremely small microbial life on Mars. However, most scientists are very skeptical that the Allan Hills meteorite actually contains traces of past Martian life. The culprits are more likely to be tiny carbonate crystals and abiogenic magnetite. The search for past and current life on Mars continues. Venus Venus Venus Venus photographed in ultraviolet light by the Pioneer Venus Orbiter (Pioneer 12) spacecraft, February 26, 1979. Although Venus's cloud cover is nearly featureless in visible light, ultraviolet imaging reveals distinctive structure and pattern, including global-scale V-shaped bands that open toward the west (left). Added colour in the image emulates Venus's yellow-white appearance to the eye. profile of Venus's atmosphere profile of Venus's atmosphere Profile of Venus's middle and lower atmospheres as derived from measurements made by the Pioneer Venus mission's atmospheric probes and other spacecraft. Below 100 km (60 miles) the temperature rises slowly at first and then more rapidly with decreasing altitude, well surpassing the melting point of lead at the surface. By contrast, the wind, which near the top of the middle atmosphere is comparable in speed to the more powerful tropical cyclones on Earth, slows dramatically to a light breeze at the surface. The average surface temperature of Venus is approximately 750 K (477 °C, or 891 °F). Even at the poles or on the tops of the highest Venusian mountains, surface temperatures do not fall below 400 K (127 °C, or 261 °F). The temperatures on Venus are too hot for Earth-style life. However, carbon dioxide, sunlight, and water (according to the results of the Venera space vehicles) are found in the clouds of Venus. These three are the prerequisites for photosynthesis. Molecular nitrogen also is expected at the cloud level, and some minerals are likely convectively raised to the cloud level from surface dust. The cloud pressures are about the same as those on the surface of Earth, and the temperatures in the lower clouds also are quite Earth-like. Although highly acidic by virtue of their sulfur, the lower clouds of Venus are the most Earth-like extraterrestrial environment known. No organisms on Earth lead a completely airborne existence, so most scientists dispute the possibility that organisms exist buoyed in the clouds of Venus. Jovian planets Jupiter's Great Red Spot Jupiter's Great Red Spot Jupiter's Great Red Spot and its surroundings, photographed by Voyager 1, February 25, 1979. Included are the white ovals, observed since the 1930s, and immense areas of turbulence to the left of the Great Red Spot. The atmosphere of Jupiter is composed of hydrogen, helium, methane, ammonia, some neon, and water vapour. These are exactly the gases used in experiments that simulate the early Earth. Laboratory and computer experiments have been performed on the application of energy to simulated Jovian atmospheres. Immediate gas-phase products include significant quantities of hydrogen cyanide and acetylene. More-complex organic molecules, including aromatic hydrocarbons, are formed in lower yields. The clouds of Jupiter are vividly coloured, and their hue may be attributable to organic compounds. An apparent absorption feature near 260 nanometres in Jupiter’s ultraviolet spectrum may be due to aromatic hydrocarbons or even due to nucleotide bases. Jupiter may be a vast planetary brew that has operated for 4.5 billion years as a laboratory of organic chemistry. Saturn Saturn Saturn and its spectacular rings, in a natural-colour composite of 126 images taken by the Cassini spacecraft on October 6, 2004. The view is directed toward Saturn's southern hemisphere, which is tipped toward the Sun. Shadows cast by the rings are visible against the bluish northern hemisphere, while the planet's shadow is projected on the rings to the left. The other Jovian planets, Saturn, Uranus, and Neptune, resemble Jupiter, although less is known about them. Their cloud-top temperatures progressively decrease with distance from the Sun. Microwave studies of Saturn indicate that the atmospheric temperature increases with depth below the clouds. A similar situation is expected to exist on Jupiter, Uranus, and Neptune. These planets of the solar system are associated with many natural satellites. Some, such as Titan, a satellite of Saturn, and Io, a satellite of Jupiter, have atmospheres. Despite the relative suitability for life’s preconditions, no evidence is known for life on the outer planets or their satellites. Europa, other Jovian moons, comets, and asteroids Know about the icy surface of Jupiter's moon Europa and the possibility of life beneath it Know about the icy surface of Jupiter's moon Europa and the possibility of life beneath it An overview of Jupiter's moon Europa, including a discussion about the possibility of extraterrestrial life beneath its icy surface. See all videos for this article Europa Europa Crescent view of Europa, one of Jupiter's four large, Galilean moons, in a composite of images made by the Galileo spacecraft in 1995 and 1998. Colours have been exaggerated in processing to reveal subtle differences in surface materials. The reddish lines in the moon's icy crust are cracks and ridges, some of them thousands of kilometres long, while the reddish mottling indicates areas of disrupted ice, where large ice blocks have shifted. The red material may be salt minerals deposited by liquid water that emerged from below the surface. The relatively few craters indicate that the icy crust has been relatively warm and mobile for at least a good part of Europa's early history. Europa, the fourth largest satellite of Jupiter, may be the best candidate for extraterrestrial life in the solar system. The Galileo orbiter revealed a crust of water ice and a complex surface on this moon. Optical imaging, thermographic temperature probes, and magnetic field measurements support the strong inference that a liquid saltwater ocean surges beneath the frozen crust. A wisp of an oxygen atmosphere has also been detected by spectrographic techniques. Furthermore, since organic molecules including methane and nitrogen-rich gases such as ammonia abound on Jupiter and some of its other moons, such “prebiotic chemicals” are highly likely to be present on Europa. The Galileo flyby also detected abundant sulfuric acid, a potential chemical power source, on the surface of Europa. (Such discoveries in the Jovian planets inspire further investigation of the limits to diversity of life on Earth. Lakes such as Vostok in Antarctica reside under more than 3 km [2 miles] of ice. Studies of bacteria in these lakes and of water seeps within cavities in granitic and carbonate rocks provide models for the viability of possible Earth-like life-forms on Europa and other Jovian moons.) Ufo, alien, space over trees. More From Britannica Unidentified Flying Objects: What We Know Io is the most volcanically active place in the solar system, and Ganymede and Callisto may also have water ice under their surfaces. The immense tidal influence of Jupiter regularly pumps energy into these planetary systems. Now that it has become clear that chemoautotrophic life-forms do not require sunlight as sources of energy, some scientists argue that a shift of focus from Mars and the other inner planets is in order. The outer planets’ satellites, especially Europa and Saturn’s Titan, promise new insights into the search for extraterrestrial life in the solar system. In 2008, for example, the Cassini spacecraft reported several hundred lakes and seas of organic materials on Titan, dozens of which contain more liquid hydrocarbon (such as methane and ethane) than all of Earth’s oil and gas reserves combined. Tens of thousands of comets, as well as some thousands of asteroids and asteroidal fragments revolving about the Sun between the orbits of Mars and Jupiter, contain organic molecules. The asteroids are the presumed sources of the carbonaceous chondrites’ organic matter. Pluto has a predominantly nitrogen atmosphere covering a surface of frozen nitrogen, carbon dioxide, and methane. The intense cold and paucity of solar radiation on Pluto and the lack of atmosphere and liquid waters on the asteroids argue against the likelihood of finding life on these bodies. Life beyond the solar system Eagle Nebula Eagle Nebula Newly formed stars emerging from the Eagle Nebula, as seen by the Hubble Space Telescope. For thousands of years humans have wondered whether they were alone in the universe or whether other worlds populated by more or less humanlike creatures might exist. In ancient times and throughout the Middle Ages, the common view was that Earth was the only “world” in the universe. Many mythologies populated the sky with divine beings, certainly a kind of extraterrestrial life. Some philosophers held that life was not unique to Earth. Metrodorus, an Epicurean in the 3rd and 4th centuries BCE, argued that To consider the Earth the only populated world in infinite space is as absurd as to assert that in an entire field sown with millet, only one grain will grow. Since the Renaissance, fashionable belief has fluctuated. Practically all informed opinion in the late 18th century held that each planet was populated by intelligent beings. However, except for those who followed Percival Lowell, the prevailing informed opinion in the early 20th century held that chances for extraterrestrial intelligent life were insignificant. The subject of extraterrestrial intelligent life is for many people a touchstone of their beliefs and desires. Some urgently desire evidence for extraterrestrial intelligence, and others equally fervently deny the possibility of its existence. The subject should be approached in as unbiased a frame of mind as possible. The probability of advanced technical civilizations in the Milky Way Galaxy depends on many controversial issues. The Drake equation and extrasolar life three views of the Milky Way Galaxy three views of the Milky Way Galaxy American astrophysicist Frank D. Drake devised a simple approach that illuminates the uncertainties involved in determining whether extraterrestrial intelligence is possible. The number of extant technical civilizations in the Milky Way Galaxy is estimated by the following equation (the so-called Drake equation, or Green Bank formula): N = R*fpneflfifcL where R* is the average rate of star formation over the lifetime of the Milky Way Galaxy, fp is the fraction of stars with planetary systems, ne is the mean number of planets per star that are ecologically suitable for the origin and evolution of life, fl is the fraction of such planets on which life arises, fi is the fraction of such planets on which intelligent life evolves, fc is the fraction of such planets on which a technical civilization develops, and L is the mean lifetime of a technical civilization. A consideration of the factors involved in the choice of numerical values for each parameter follows. These estimates are little better than informed guesses; no great reliability should be pretended for them. HD 209458b HD 209458b Artist's conception of the extrasolar planet HD 209458b, some 150 light-years from Earth. There are about 200 billion stars in the Milky Way Galaxy. The age of the Milky Way Galaxy is about 10 billion years. A value of R* = 10 stars per year is probably fairly reliable. While most contemporary theories of star formation imply that the origin of planets accompanies the origin of stars, such theories are not developed well enough to merit much confidence. More than 250 extrasolar planets have been confirmed. They have been observed via several different means: by “wobble,” which detects the changing wavelength of a star’s light as the star gets closer and then farther away from Earth as a massive planet tugs it from the centre of the system; by transit, which detects the dimming of a star as a planet crosses between it and Earth, much like a solar eclipse; and by infrared observation, which observes a planet directly. Owing to limitations of current detection methods, most of the planets discovered so far have masses at least as great as the solar system’s largest planets, Jupiter and Saturn. Most of these planets are also very close to their stars, much closer than Earth is to the Sun, so life systems similar to Earth’s could not exist on them. This difference has led scientists to consider new models for planetary formation. For example, Gliese 876, a red dwarf star one-third the mass of the Sun and 15 light-years away in Aquarius, has three planets: a gas giant half the mass of Jupiter that orbits Gliese 876 every 30 Earth days, another twice the mass of Jupiter that orbits exactly once for every two orbits of its neighbour, and a third six times the mass of Earth that orbits every 2 Earth days. HD 168443, lying 123 light-years away in Serpens, has one planet 8 times Jupiter’s mass and another 18 times the mass of Jupiter, which is beyond the scale that had been considered possible for a planet; this giant may be a brown dwarf. Several planets even smaller than Saturn have been found. Gliese 581, at 20 light-years away, has three planets, one of which is only five times the mass of Earth. NASA’s Kepler mission, launched in 2009, used space-based telescopes to observe Sun-like stars that host Earth-sized planets and discovered thousands of extrasolar planets. Because the wobble method can detect only a planet that has been observed for a significant portion of its orbit, finding a planet like Jupiter with an orbital period of 12 Earth years requires several years of observations. Nevertheless, some planetary systems similar to the solar system have been found. HD 190360A, at 52 light-years away, is very similar to the Sun and has a detectable planet similar in size and orbital distance to Jupiter. Systems like HD 190360A could also have smaller planets arranged like those in the solar system. The existence of large planets around so many nearby stars demonstrates that a significant fraction of stars do indeed have planets in orbit around them. Another indication that planetary formation is a general process throughout the universe is the satellite systems of the major planets of the solar system. Jupiter with 79 satellites, Saturn with 82, and Uranus with 27 resemble miniature solar systems. Considering the wide range of temperatures that seem to be compatible with life, it can be tentatively concluded that fpne is about 1. However, since liquid water is considered to be crucial to life’s origin and evolution, fpne probably has a significantly smaller value. Because of the short time it took for life to arise on Earth, as implied by the fossil record, and because of the ease with which relevant organic molecules are produced in experiments that simulate the early Earth, the likelihood of life’s arising during a period of billions of years may be high. Some scientists believe that the appropriate value of fl, the fraction of planets with life, is about 1. For the quantities of fi, the fraction of planets with intelligent life, the parameters are even more uncertain. The evolutionary path leading to mammals depends on a great many specific circumstances and historical accidents; it is therefore highly unlikely that such a path will ever repeat. However, intelligence clearly has a great selective advantage and is not necessarily restricted to the single evolutionary path that occurred on Earth. Similar arguments are made for fc, the fraction of technical civilizations. Intelligence and technical civilization are clearly not equivalent. For example, dolphins appear to be intelligent, but their lack of manipulative organs limits their technology. Both intelligence and technical civilization evolved about halfway through the lifetime of Earth and the Sun. Some, but by no means all, evolutionary biologists would conclude that 1/100 is a conservative estimate for the product fifc. Still more uncertain is the value of the final parameter, L, the lifetime of a technical civilization. A technical civilization here is defined as one capable of interstellar radio communication. Thus, human technical civilization is only a few decades old. Technical civilizations may tend, through the use of weapons of mass destruction, to destroy themselves shortly after they come into being. If L is then taken to be 10 years, multiplication of all the factors assumed above leads to the conclusion that only one technical civilization exists in the Milky Way Galaxy—our own. But if technical civilizations do not produce massively destructive weapons or use them to annihilate themselves, then the lifetimes of technical civilizations may be very long. In that case, the number of technical civilizations in the Milky Way Galaxy may be immense. If even 1 percent of developing civilizations make peace with themselves, then about 1,000,000 technical civilizations may be extant in the Milky Way Galaxy. If such civilizations were randomly distributed in space, the nearest would be several hundred light-years from Earth. These conclusions are very uncertain. Searching for technical civilizations How would technical civilizations enter into communication with one another? Independent of the value of L, the Drake formula cited above implies that about one technical civilization arises every 10,000 years in the Milky Way Galaxy. Accordingly, it would be extraordinarily unlikely for humans to find a technical civilization as backward as Earth’s. The rate of technical advance on Earth in the past few hundred years makes it clear that no serious and reliable projection of future scientific and technical advances can be made. Advanced civilizations are expected to have techniques and sciences unknown to 21st-century people. Nevertheless, humanity is already capable of communication by radio over interstellar distances. If Earth’s largest radio telescope, the 500-metre- (1,600-foot-) diameter dish at the Five-hundred-metre Aperture Spherical Telescope in China and its receivers, is employed and if identical equipment is employed on some transmitting planet, how far apart could the transmitting and receiving planets be for intelligible signals to be passed? The rather astonishing answer is about 2,500 light-years. Within a volume centred on Earth with a radius of 2,500 light-years, there are more than 150,000,000 stars. Problems would definitely surface in the establishment of such radio communication. The frequency, target star, longevity, and character of the message would all have to be selected by the transmitting planet so that the receiving planet would be able to deduce them without too much effort. None of these problems seems insuperable. One choice might be to listen to stars of approximately solar spectral type. Certain natural radio frequencies, such as the 1,420-megahertz (21-cm) line of neutral hydrogen, might also be used. In the absence of any symbols or language in common, messages that use the neutral hydrogen line might be the most appropriate for discerning intelligent origin and intellectual content from life-forms that do not share human evolutionary history. Very few anthropocentric assumptions would be needed. Because Earth’s technologies are relatively new, it makes little sense to transmit messages to hypothetical planets of other stars. But it does make sense to listen for radio transmissions from planets of other stars. Other communication techniques include laser transmission and interstellar spaceflight, but these may not be feasible. American engineers Christopher Rose and Gregory Wright have argued that sending a physical artifact is a preferable communication technique because radio waves tend to disperse, whereas physical artifacts retain their information in compact form and are more likely to be readable when they arrive at their destination. However, such “messages in a bottle” would travel 1,000 times slower than light. If the measure of effectiveness is the amount of information communicated across a broad area per unit cost, then radio transmission is the method of choice. A scientific search for intelligent extraterrestrial life that could communicate beyond its own celestial home was first called for in 1959 by Italian physicist Giuseppe Cocconi and American physicist Philip Morrison. Using the radio telescope at Green Bank, West Virginia, in 1960, Drake mounted the first (very brief) search, Project Ozma, which was oriented to two nearby stars, Epsilon Eridani and Tau Ceti. On the basis of the Drake equation, it would be very unlikely that success would greet an effort aimed at two stars only 12 light-years away. Not surprisingly, Project Ozma was unsuccessful. Related programs organized on a larger scale were mounted with great enthusiasm in the 1960s in the U.S.S.R. After Project Ozma ended, various government and private projects continued the search for extraterrestrial intelligence (SETI). The Planetary Society, founded in 1980 by American astronomer Carl Sagan, planetary scientist Bruce Murray, and aerospace engineer Louis Friedman, has as one of its aims the bringing together of professionals and amateurs in support of SETI. Funding by American movie director Steven Spielberg permitted the society to start the first privately funded SETI project, the Megachannel Extraterrestrial Assay, in 1982. SETI@home SETI@home The SETI@home screen saver. Several searches for extraterrestrial signals that might indicate attempts of extraterrestrial life to communicate with itself are under way. Both radio and optical light transmissions are sought, with instruments receiving cosmic signals in both the Northern and Southern hemispheres. The involvement of amateurs is encouraged. Even the pooling of resources of home computers to analyze the prodigious amounts of data received from outer space helps in the effort. With the communication advantages of the World Wide Web, astronomers from all parts of the globe may aid in the effort. An array of radio dishes near Buenos Aires searches millions of channels for radio transmissions in the southern sky. Professionals and their amateur colleagues at Harvard University search for signals from the visible regions of the electromagnetic spectrum in the Optical SETI project at the Oak Ridge Observatory in Harvard, Massachusetts. SETI is an extraordinary pursuit, in part because of the potential significance of success. SETI brings unity to a wide range of scientific disciplines as well. Astrobiology, which includes SETI, as the study of the origin and evolution of stars, planets, and life and of the evolution of intelligence and of technical civilizations, is arguably the most important science for understanding the human condition. Astrobiology includes the political problem of recognizing ourselves less as members of tribes and more as citizens of the universe. To pursue these studies, a number of modern methods—molecular evolution via computational proteomics and genomics, geochronological analyses, chemical element detections coupled with scanning electron microscopy, immunocytochemistry for study of protein dynamics, to name only a few—promise to refine definitions of life as well as detect life under extreme conditions on Earth and beyond. Science fiction routinely depicts extraterrestrial beings as thinly disguised men and women. The unique circuitous one-way path of evolution on Earth makes it extremely unlikely that any mammal or flowering plant, to say nothing of a child, would have evolved on a moon of Jupiter or an extrasolar planet. In the words of Loren Eiseley (from The Immense Journey [1957]), Lights come and go in the night sky. Men, troubled at last by the things they build, may toss in their sleep and dream bad dreams, or lie awake while the meteors whisper greenly overhead. But nowhere in all space or on a thousand worlds will there be men to share our loneliness. There may be wisdom; there may be power; somewhere across space great instruments, handled by strange, manipulative organs, may stare vainly at our floating cloud wrack, their owners yearning as we yearn. Nevertheless, in the nature of life and in principles of evolution we have had our answer. Of men [as are known on Earth] elsewhere, and beyond, there will be none forever. Although there is only an infinitesimal possibility that humanlike beings will be discovered in outer space (to serve as a cosmic example of convergent evolution), the discovery of any other living matter anywhere else in the cosmos would be of the utmost scientific significance. Moreover, if no evidence at all for life beyond Earth is found after a significant search, this too would be of great scientific moment. The absence of the evolving matter-energy flow systems that are life would reinforce the awesome responsibility of protecting its diversity in this biosphere, which includes that precious, cosmically fragile, and recent growth form, human civilization.

• Condition: In Excellent Condition
• Number of Pieces: 1
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• Time Period: 2000s
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• Denomination: Commerative
• Year of Issue: 2024
• Collection: Pioneer
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• Country/Region of Manufacture: United Kingdom
• Country of Origin: Great Britain

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