Space exploration

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

Buzz Aldrin taking a core sample of the Moon during the Apollo 11 mission

Self-portrait of Curiosity rover on Mars's surface

Space exploration is the physical investigation of outer space by uncrewed robotic space probes and through human spaceflight.

While the observation of objects in space, known as astronomy, predates reliable recorded history, it was the development of large and relatively efficient rockets during the mid-twentieth century that allowed physical space exploration to become a reality. Common rationales for exploring space include advancing scientific research, national prestige, uniting different nations, ensuring the future survival of humanity, and developing military and strategic advantages against other countries.

The early era of space exploration was driven by a "Space Race" in which the Soviet Union and the United States vied to demonstrate their technological superiority. Landmarks of this era include the launch of the first human-made object to orbit Earth, the Soviet Union's Sputnik 1, on 4 October 1957, and the first Moon landing by the American Apollo 11 mission on 20 July 1969. The Soviet space program achieved many of the first milestones, including the first living being in orbit in 1957, the first human spaceflight (Yuri Gagarin aboard Vostok 1) in 1961, the first spacewalk (by Alexei Leonov) on 18 March 1965, the first automatic landing on another celestial body in 1966, and the launch of the first space station (Salyut 1) in 1971.

In the 1970s, focus shifted from one-off flights to renewable hardware, such as the Space Shuttle program, and from competition to cooperation, the foremost example being the International Space Station (ISS), built between 1998 and 2011.

The 2000s brought advancements in the national space-exploration programs of China, the European Union, Japan, and India. The 2010s saw the rise of the private space industry in earnest with the development of private launch vehicles, space capsules, and satellite manufacturing. In the 2020s, the two primary global programs gaining traction are Moon-focused: the Chinese-led International Lunar Research Station and the U.S.-led Artemis Program, with its plan to build the Lunar Gateway and the Artemis Base Camp, each with a set of international partners.

History of exploration

See also: History of astronomy, Discovery and exploration of the Solar System, Timeline of space exploration, Timeline of first orbital launches by country, and Outer space § Discovery

A V-2 Rocket in the Peenemünde Museum in Germany

First telescopes

The first telescope is said to have been invented in 1608 in the Netherlands by an eyeglass maker named Hans Lippershey, but their first recorded use in astronomy was by Galileo Galilei in 1609. In 1668 Isaac Newton built his own reflecting telescope, the first fully functional telescope of this kind, and a landmark for future developments due to its superior features over the previous Galilean telescope.

A string of discoveries in the Solar System (and beyond) followed, then and in the next centuries: the mountains of the Moon, the phases of Venus, the main satellites of Jupiter and Saturn, the rings of Saturn, many comets, the asteroids, the new planets Uranus and Neptune, and many more satellites.

The Orbiting Astronomical Observatory 2 was the first space telescope launched 1968, but the launch of the Hubble Space Telescope in 1990 set a milestone. As of 1 December 2022, there were 5,284 confirmed exoplanets discovered. The Milky Way is estimated to contain 100–400 billion stars and more than 100 billion planets. There are at least 2 trillion galaxies in the observable universe. HD1 is the most distant known object from Earth, reported as 33.4 billion light-years away.

First outer space flights

Model of a Russian Vostok spacecraft

Apollo Command Service Module in lunar orbit

MW 18014 was a German V-2 rocket test launch that took place on 20 June 1944, at the Peenemünde Army Research Center in Peenemünde. It was the first human-made object to reach outer space, attaining an apogee of 176 kilometers, which is well above the Kármán line. It was a vertical test launch. Although the rocket reached space, it did not reach orbital velocity, and therefore returned to Earth in an impact, becoming the first sub-orbital spaceflight. In 1949, the Bumper-WAC reached an altitude of 393 kilometres (244 mi), becoming the first human-made object to enter space, according to NASA.

First object in orbit

The first successful orbital launch was of the Soviet uncrewed Sputnik 1 ("Satellite 1") mission on 4 October 1957. The satellite weighed about 83 kg (183 lb), and is believed to have orbited Earth at a height of about 250 km (160 mi). It had two radio transmitters (20 and 40 MHz), which emitted "beeps" that could be heard by radios around the globe. Analysis of the radio signals was used to gather information about the electron density of the ionosphere, while temperature and pressure data were encoded in the duration of radio beeps. The results indicated that the satellite was not punctured by a meteoroid. Sputnik 1 was launched by an R-7 rocket. It burned up upon re-entry on 3 January 1958.

First human outer space flight

The first successful human spaceflight was Vostok 1 ("East 1"), carrying the 27-year-old Russian cosmonaut, Yuri Gagarin, on 12 April 1961. The spacecraft completed one orbit around the globe, lasting about 1 hour and 48 minutes. Gagarin's flight resonated around the world; it was a demonstration of the advanced Soviet space program and it opened an entirely new era in space exploration: human spaceflight.

First astronomical body space explorations

The first artificial object to reach another celestial body was Luna 2 which reached the Moon in 1959. The first soft landing on another celestial body was performed by Luna 9 landing on the Moon on 3 February 1966. Luna 10 became the first artificial satellite of the Moon, entering in a lunar orbit on 3 April 1966.

The first crewed landing on another celestial body was performed by Apollo 11 on 20 July 1969, landing on the Moon. There have been a total of six spacecraft with humans landing on the Moon starting from 1969 to the last human landing in 1972.

The first interplanetary flyby was the 1961 Venera 1 flyby of Venus, though the 1962 Mariner 2 was the first flyby of Venus to return data (closest approach 34,773 kilometers). Pioneer 6 was the first satellite to orbit the Sun, launched on 16 December 1965. The other planets were first flown by in 1965 for Mars by Mariner 4, 1973 for Jupiter by Pioneer 10, 1974 for Mercury by Mariner 10, 1979 for Saturn by Pioneer 11, 1986 for Uranus by Voyager 2, 1989 for Neptune by Voyager 2. In 2015, the dwarf planets Ceres and Pluto were orbited by Dawn and passed by New Horizons, respectively. This accounts for flybys of each of the eight planets in the Solar System, the Sun, the Moon, and Ceres and Pluto (two of the five recognized dwarf planets).

The first interplanetary surface mission to return at least limited surface data from another planet was the 1970 landing of Venera 7, which returned data to Earth for 23 minutes from Venus. In 1975, Venera 9 was the first to return images from the surface of another planet, returning images from Venus. In 1971, the Mars 3 mission achieved the first soft landing on Mars returning data for almost 20 seconds. Later, much longer duration surface missions were achieved, including over six years of Mars surface operation by Viking 1 from 1975 to 1982 and over two hours of transmission from the surface of Venus by Venera 13 in 1982, the longest ever Soviet planetary surface mission. Venus and Mars are the two planets outside of Earth on which humans have conducted surface missions with uncrewed robotic spacecraft.

First space station

Salyut 1 was the first space station of any kind, launched into low Earth orbit by the Soviet Union on 19 April 1971. The International Space Station (ISS) is currently the largest and oldest of the 2 current fully functional space stations, inhabited continuously since the year 2000. The other, Tiangong space station built by China, is now fully crewed and operational.

First interstellar space flight

Voyager 1 became the first human-made object to leave the Solar System into interstellar space on 25 August 2012. The probe passed the heliopause at 121 AU to enter interstellar space.

Farthest from Earth

In 1970, Apollo 13 flight passed the far side of the Moon at an altitude of 254 kilometers (158 miles; 137 nautical miles) above the lunar surface, and 400,171 km (248,655 mi) from Earth, marking the record for the farthest humans had ever traveled from Earth until the Artemis II lunar flyby in April 2026, which was 406,773 km (252,757 mi) from Earth at its furthest point.

As of 9 February 2025, Voyager 1 was at a distance of 166.4 AU (24.89 billion km; 15.47 billion mi) from Earth. It is the most distant human-made object from Earth.

Targets of exploration

Starting in the mid-20th century probes and then human missions were sent into Earth orbit, and then on to the Moon. Also, probes were sent throughout the known Solar System, and into Solar orbit. Uncrewed spacecraft have been sent into orbit around Saturn, Jupiter, Mars, Venus, and Mercury by the 21st century, and the most distance active spacecraft, Voyager 1 and 2 traveled beyond 100 times the Earth-Sun distance. The instruments were enough though that it is thought they have left the Sun's heliosphere, a sort of bubble of particles made in the Galaxy by the Sun's solar wind.

The Sun

The Sun is a major focus of space exploration. Being above the atmosphere in particular and Earth's magnetic field gives access to the solar wind and infrared and ultraviolet radiations that cannot reach Earth's surface. The Sun generates most space weather, which can affect power generation and transmission systems on Earth and interfere with, and even damage, satellites and space probes. Numerous spacecraft dedicated to observing the Sun, beginning with the Apollo Telescope Mount, have been launched and still others have had solar observation as a secondary objective. Parker Solar Probe, launched in 2018, will approach the Sun to within 1/9th the orbit of Mercury.

Mercury

Main article: Exploration of Mercury

A MESSENGER image from 18,000 km showing a region about 500 km across (2008)

Mercury remains the least explored of the Terrestrial planets. As of May 2013, the Mariner 10 and MESSENGER missions have been the only missions that have made close observations of Mercury. MESSENGER entered orbit around Mercury in March 2011, to further investigate the observations made by Mariner 10 in 1975 (Munsell, 2006b). A third mission to Mercury, scheduled to arrive in 2025, BepiColombo is to include two probes. BepiColombo is a joint mission between Japan and the European Space Agency. MESSENGER and BepiColombo are intended to gather complementary data to help scientists understand many of the mysteries discovered by Mariner 10's flybys.

Flights to other planets within the Solar System are accomplished at a cost in energy, which is described by the net change in velocity of the spacecraft, or delta-v. Due to the relatively high delta-v to reach Mercury and its proximity to the Sun, it is difficult to explore and orbits around it are rather unstable.

Venus

Main article: Observations and explorations of Venus

Venus was the first target of interplanetary flyby and lander missions and, despite one of the most hostile surface environments in the Solar System, has had more landers sent to it (nearly all from the Soviet Union) than any other planet in the Solar System. The first flyby was the 1961 Venera 1, though the 1962 Mariner 2 was the first flyby to successfully return data. Mariner 2 has been followed by several other flybys by multiple space agencies often as part of missions using a Venus flyby to provide a gravitational assist en route to other celestial bodies. In 1967, Venera 4 became the first probe to enter and directly examine the atmosphere of Venus. In 1970, Venera 7 became the first successful lander to reach the surface of Venus and by 1985 it had been followed by eight additional successful Soviet Venus landers which provided images and other direct surface data. Starting in 1975, with the Soviet orbiter Venera 9, some ten successful orbiter missions have been sent to Venus, including later missions which were able to map the surface of Venus using radar to pierce the obscuring atmosphere.

Earth

Main article: Earth observation satellite

First television image of Earth from space, taken by TIROS-1 (1960)

Space exploration has been used as a tool to understand Earth as a celestial object. Orbital missions can provide data for Earth that can be difficult or impossible to obtain from a purely ground-based point of reference.

For example, the existence of the Van Allen radiation belts was unknown until their discovery by the United States' first artificial satellite, Explorer 1. These belts contain radiation trapped by Earth's magnetic fields, which currently renders construction of habitable space stations above 1000 km impractical. Following this early unexpected discovery, a large number of Earth observation satellites have been deployed specifically to explore Earth from a space-based perspective. These satellites have significantly contributed to the understanding of a variety of Earth-based phenomena. For instance, the hole in the ozone layer was found by an artificial satellite that was exploring Earth's atmosphere, and satellites have allowed for the discovery of archeological sites or geological formations that were difficult or impossible to otherwise identify.

Moon

Main article: Exploration of the Moon

Apollo 16 LEM Orion, the Lunar Roving Vehicle and astronaut John Young (1972)

The Moon was the first celestial body to be the object of space exploration. It holds the distinctions of being the first remote celestial object to be flown by, orbited, and landed upon by spacecraft, and the only remote celestial object ever to be visited by humans.

In 1959, the Soviets obtained the first images of the far side of the Moon, never previously visible to humans. The U.S. exploration of the Moon began with the Ranger 4 impactor in 1962. Starting in 1966, the Soviets successfully deployed a number of landers to the Moon which were able to obtain data directly from the Moon's surface; just four months later, Surveyor 1 marked the debut of a successful series of U.S. landers. The Soviet uncrewed missions culminated in the Lunokhod program in the early 1970s, which included the first uncrewed rovers and also successfully brought lunar soil samples to Earth for study. This marked the first (and to date the only) automated return of extraterrestrial soil samples to Earth. Uncrewed exploration of the Moon continues with various nations periodically deploying lunar orbiters. China's Chang'e 4 in 2019 and Chang'e 6 in 2024 achieved the world's first landing and sample return on the far side of the Moon. India's Chandrayaan-3 in 2023 achieved the world's first landing on the lunar south pole region.

Crewed exploration of the Moon began in 1968 with the Apollo 8 mission that successfully orbited the Moon, the first time any extraterrestrial object was orbited by humans. In 1969, the Apollo 11 mission marked the first time humans set foot upon another world. Crewed exploration of the Moon did not continue for long. The Apollo 17 mission in 1972 marked the sixth landing and the most recent human visit. Artemis II will complete a crew flyby of the Moon in 2026, and Artemis IV will perform the first lunar landing since Apollo 17 with launch planned in 2028. Robotic missions are still pursued vigorously.

Mars

Main article: Exploration of Mars

Surface of Mars by the Spirit rover (2004)

The exploration of Mars has been an important part of the space exploration programs of the Soviet Union (later Russia), the United States, Europe, Japan, and India. Dozens of robotic spacecraft, including orbiters, landers, and rovers, have been launched toward Mars since the 1960s. These missions were aimed at gathering data about current conditions and answering questions about the history of Mars. The questions raised by the scientific community are expected to not only give a better appreciation of the Red Planet but also yield further insight into the past, and possible future, of Earth.

The exploration of Mars has come at a considerable financial cost with roughly two-thirds of all spacecraft destined for Mars failing before completing their missions, with some failing before they even began. Such a high failure rate can be attributed to the complexity and large number of variables involved in an interplanetary journey, and has led researchers to jokingly speak of The Great Galactic Ghoul which subsists on a diet of Mars probes. This phenomenon is also informally known as the "Mars Curse". In contrast to overall high failure rates in the exploration of Mars, India has become the first country to achieve success of its maiden attempt. India's Mars Orbiter Mission (MOM) is one of the least expensive interplanetary missions ever undertaken with an approximate total cost of ₹ 450 Crore (US$73 million). The first mission to Mars by any Arab country has been taken up by the United Arab Emirates. Called the Emirates Mars Mission, it was launched on 19 July 2020 and went into orbit around Mars on 9 February 2021. The uncrewed exploratory probe was named "Hope Probe" and was sent to Mars to study its atmosphere in detail.

Phobos

Main article: Exploration of Phobos

The Russian space mission Fobos-Grunt, which launched on 9 November 2011, experienced a failure leaving it stranded in low Earth orbit. It was to begin exploration of the Phobos and Martian circumterrestrial orbit, and study whether the moons of Mars, or at least Phobos, could be a "trans-shipment point" for spaceships traveling to Mars.

Asteroids

Main article: Exploration of the asteroids

Asteroid 4 Vesta, imaged by the Dawn spacecraft (2011)

Until the advent of space travel, objects in the asteroid belt were merely pinpricks of light in even the largest telescopes, their shapes and terrain remaining a mystery. Several asteroids have now been visited by probes, the first of which was Galileo, which flew past two: 951 Gaspra in 1991, followed by 243 Ida in 1993. Both of these lay near enough to Galileo's planned trajectory to Jupiter that they could be visited at acceptable cost. The first landing on an asteroid was performed by the NEAR Shoemaker probe in 2000, following an orbital survey of the object, 433 Eros. The dwarf planet Ceres and the asteroid 4 Vesta, two of the three largest asteroids, were visited by NASA's Dawn spacecraft, launched in 2007.

Hayabusa was a robotic spacecraft developed by the Japan Aerospace Exploration Agency to return a sample of material from the small near-Earth asteroid 25143 Itokawa to Earth for further analysis. Hayabusa was launched on 9 May 2003 and rendezvoused with Itokawa in mid-September 2005. After arriving at Itokawa, Hayabusa studied the asteroid's shape, spin, topography, color, composition, density, and history. In November 2005, it landed on the asteroid twice to collect samples. The spacecraft returned to Earth on 13 June 2010.

Jupiter

Main article: Exploration of Jupiter

Tupan Patera on Jupiter's moon Io

The exploration of Jupiter has consisted solely of a number of automated NASA spacecraft visiting the planet since 1973. A large majority of the missions have been "flybys", in which detailed observations are taken without the probe landing or entering orbit; such as in Pioneer and Voyager programs. The Galileo and Juno spacecraft are the only spacecraft to have entered the planet's orbit. As Jupiter is believed to have only a relatively small rocky core and no real solid surface, a landing mission is precluded.

Reaching Jupiter from Earth requires a delta-v of 9.2 km/s, which is comparable to the 9.7 km/s delta-v needed to reach low Earth orbit. Fortunately, gravity assists through planetary flybys can be used to reduce the energy required at launch to reach Jupiter, albeit at the cost of a significantly longer flight duration.

Jupiter has 95 known moons, many of which have relatively little known information about them.

Saturn

Main article: Exploration of Saturn

Saturn has been explored only through uncrewed spacecraft launched by NASA, including one mission (Cassini–Huygens) planned and executed in cooperation with other space agencies. These missions consist of flybys in 1979 by Pioneer 11, in 1980 by Voyager 1, in 1982 by Voyager 2 and an orbital mission by the Cassini spacecraft, which lasted from 2004 until 2017.

Saturn has at least 62 known moons, although the exact number is debatable since Saturn's rings are made up of vast numbers of independently orbiting objects of varying sizes. The largest of the moons is Titan, which holds the distinction of being the only moon in the Solar System with an atmosphere denser and thicker than that of Earth. Titan holds the distinction of being the only object in the Outer Solar System that has been explored with a lander, the Huygens probe deployed by the Cassini spacecraft.

Uranus

Main article: Exploration of Uranus

The exploration of Uranus has been entirely through the Voyager 2 spacecraft. However, there are plans for the Chinese Tianwen 4 mission to visit it or Callisto. Given its axial tilt of 97.77°, with its polar regions exposed to sunlight or darkness for long periods, scientists were not sure what to expect at Uranus. The closest approach to Uranus occurred on 24 January 1986. Voyager 2 studied the planet's unique atmosphere and magnetosphere. Voyager 2 also examined its ring system and the moons of Uranus including all five of the previously known moons, while discovering an additional ten previously unknown moons.

Images of Uranus proved to have a uniform appearance, with no evidence of the dramatic storms or atmospheric banding evident on Jupiter and Saturn. Great effort was required to even identify a few clouds in the images of the planet. The magnetosphere of Uranus, however, proved to be unique, being profoundly affected by the planet's unusual axial tilt. In contrast to the bland appearance of Uranus itself, striking images were obtained of the Moons of Uranus, including evidence that Miranda had been unusually geologically active.

Neptune

Main article: Exploration of Neptune

The exploration of Neptune began with the 25 August 1989 Voyager 2 flyby, the sole visit to the system. The possibility of a Neptune Orbiter has been discussed, but no other missions have been given serious thought.

Although the extremely uniform appearance of Uranus during Voyager 2's visit in 1986 had led to expectations that Neptune would also have few visible atmospheric phenomena, the spacecraft found that Neptune had obvious banding, visible clouds, auroras, and even a conspicuous anticyclone storm system rivaled in size only by Jupiter's Great Red Spot. Neptune also proved to have the fastest winds of any planet in the Solar System, measured as high as 2,100 km/h. Voyager 2 also examined Neptune's ring and moon system. It discovered 900 complete rings and additional partial ring "arcs" around Neptune. In addition to examining Neptune's three previously known moons, Voyager 2 also discovered five previously unknown moons, one of which, Proteus, proved to be the last largest moon in the system. Data from Voyager 2 supported the view that Neptune's largest moon, Triton, is a captured Kuiper belt object.

Pluto

Main article: Pluto § Exploration

The dwarf planet Pluto presents significant challenges for spacecraft because of its great distance from Earth (requiring high velocity for reasonable trip times) and small mass (making capture into orbit difficult at present). Voyager 1 could have visited Pluto, but controllers opted instead for a close flyby of Saturn's moon Titan, resulting in a trajectory incompatible with a Pluto flyby. Voyager 2 never had a plausible trajectory for reaching Pluto.

After an intense political battle, a mission to Pluto dubbed New Horizons was granted funding from the United States government in 2003. New Horizons was launched successfully on 19 January 2006. In early 2007 the craft made use of a gravity assist from Jupiter. Its closest approach to Pluto was on 14 July 2015; scientific observations of Pluto began five months prior to closest approach and continued for 16 days after the encounter.

Kuiper Belt Objects

The New Horizons mission also performed a flyby of the small planetesimal Arrokoth, in the Kuiper belt, in 2019. This was its first extended mission.

Comets

Main article: List of missions to comets

Comet 103P/Hartley (2010)

Although many comets have been studied from Earth sometimes with centuries-worth of observations, only a few comets have been closely visited. In 1985, the International Cometary Explorer conducted the first comet fly-by (21P/Giacobini-Zinner) before joining the Halley Armada studying the famous comet. The Deep Impact probe smashed into 9P/Tempel to learn more about its structure and composition and the Stardust mission returned samples of another comet's tail. The Philae lander successfully landed on Comet Churyumov–Gerasimenko in 2014 as part of the broader Rosetta mission.

Deep space exploration

Main article: Deep space exploration

This high-resolution image of the Hubble Ultra Deep Field includes galaxies of various ages, sizes, shapes, and colors. The smallest, reddest galaxies, are some of the most distant galaxies to have been imaged by an optical telescope.

Deep space exploration is the branch of astronomy, astronautics and space technology that is involved with the exploration of distant regions of outer space. Physical exploration of space is conducted both by human spaceflights (deep-space astronautics) and by robotic spacecraft.

Some of the best candidates for future deep space engine technologies include anti-matter, nuclear power and beamed propulsion. Beamed propulsion, appears to be the best candidate for deep space exploration presently available, since it uses known physics and known technology that is being developed for other purposes.

Future of space exploration

Main article: Future of space exploration

Concept art for a NASA Vision mission

Artistic image of a rocket lifting from a Saturn moon

Breakthrough Starshot

Main article: Breakthrough Starshot

Breakthrough Starshot is a research and engineering project by the Breakthrough Initiatives to develop a proof-of-concept fleet of light sail spacecraft named StarChip, to be capable of making the journey to the Alpha Centauri star system 4.37 light-years away. It was founded in 2016 by Yuri Milner, Stephen Hawking, and Mark Zuckerberg.

Asteroids

Main article: Exploration of the asteroids

An article in the science magazine Nature suggested the use of asteroids as a gateway for space exploration, with the ultimate destination being Mars. In order to make such an approach viable, three requirements need to be fulfilled: first, "a thorough asteroid survey to find thousands of nearby bodies suitable for astronauts to visit"; second, "extending flight duration and distance capability to ever-increasing ranges out to Mars"; and finally, "developing better robotic vehicles and tools to enable astronauts to explore an asteroid regardless of its size, shape or spin". Furthermore, using asteroids would provide astronauts with protection from galactic cosmic rays, with mission crews being able to land on them without great risk to radiation exposure.

Artemis program

Main article: Artemis program

The Artemis program is an ongoing crewed spaceflight program carried out by NASA, U.S. commercial spaceflight companies, and international partners such as ESA, with the goal of landing "the first woman and the next man" on the Moon, specifically at the lunar south pole region. Artemis would be the next step towards the long-term goal of establishing a sustainable presence on the Moon, laying the foundation for private companies to build a lunar economy, and eventually sending humans to Mars.

In 2017, the lunar campaign was authorized by Space Policy Directive 1, using various ongoing spacecraft programs such as Orion, the Lunar Gateway, Commercial Lunar Payload Services, and adding an undeveloped crewed lander. The Space Launch System will serve as the primary launch vehicle for Orion, while commercial launch vehicles are planned for use to launch other elements of the campaign. NASA requested $1.6 billion in additional funding for Artemis for fiscal year 2020, while the U.S. Senate Appropriations Committee requested from NASA a five-year budget profile which is needed for evaluation and approval by the U.S. Congress. As of 2026, the first Artemis mission was launched in 2022 with the second mission, a crewed lunar flyby launched in April 2026. Construction on the Lunar Gateway is underway with initial capabilities set for the 2025–2027 timeframe. The first CLPS lander landed in 2024, marking the first US spacecraft to land since Apollo 17.

Rationales

Main article: Space advocacy

Astronaut Buzz Aldrin had a personal Communion service when he first arrived on the surface of the Moon.

The research that is conducted by national space exploration agencies, such as NASA and Roscosmos, is one of the reasons supporters cite to justify government expenses. Economic analyses of the NASA programs often showed ongoing economic benefits (such as NASA spin-offs), generating many times the revenue of the cost of the program. It is also argued that space exploration would lead to the extraction of resources on other planets and especially asteroids, which contain billions of dollars' worth of minerals and metals. Such expeditions could generate substantial revenue. In addition, it has been argued that space exploration programs help inspire youth to study in science and engineering. Space exploration also gives scientists the ability to perform experiments in other settings and expand humanity's knowledge.

Another claim is that space exploration is a necessity to humankind and that staying on Earth will eventually lead to extinction. Some of the reasons are lack of natural resources, comets, nuclear war, and worldwide epidemic. Stephen Hawking, renowned British theoretical physicist, said, "I don't think the human race will survive the next thousand years, unless we spread into space. There are too many accidents that can befall life on a single planet. But I'm an optimist. We will reach out to the stars." Author Arthur C. Clarke (1950) presented a summary of motivations for the human exploration of space in his non-fiction semi-technical monograph Interplanetary Flight. He argued that humanity's choice is essentially between expansion off Earth into space, versus cultural (and eventually biological) stagnation and death.

These motivations could be attributed to one of the first rocket scientists in NASA, Wernher von Braun, and his vision of humans moving beyond Earth. The basis of this plan was to:

Develop multi-stage rockets capable of placing satellites, animals, and humans in space.

Development of large, winged reusable spacecraft capable of carrying humans and equipment into Earth orbit in a way that made space access routine and cost-effective.

Construction of a large, permanently occupied space station to be used as a platform both to observe Earth and from which to launch deep space expeditions.

Launching the first human flights around the Moon, leading to the first landings of humans on the Moon, with the intent of exploring that body and establishing permanent lunar bases.

Assembly and fueling of spaceships in Earth orbit for the purpose of sending humans to Mars with the intent of eventually colonizing that planet.

Known as the Von Braun Paradigm, the plan was formulated to lead humans in the exploration of space. Von Braun's vision of human space exploration served as the model for efforts in space exploration well into the twenty-first century, with NASA incorporating this approach into the majority of their projects. The steps were followed out of order, as seen by the Apollo program reaching the moon before the space shuttle program was started, which in turn was used to complete the International Space Station. Von Braun's Paradigm formed NASA's drive for human exploration, in the hopes that humans discover the far reaches of the universe.

NASA has produced a series of public service announcement videos supporting the concept of space exploration.

Overall, the U.S. public remains largely supportive of both crewed and uncrewed space exploration. According to an Associated Press Poll conducted in July 2003, 71% of U.S. citizens agreed with the statement that the space program is "a good investment", compared to 21% who did not.

Human nature

Space advocacy and space policy regularly invokes exploration as a human nature.

Topics

Main articles: Space science and Human presence in space

Spaceflight

Main articles: Spaceflight and Astronautics

Delta-v's in km/s for various orbital maneuvers

Spaceflight is the use of space technology to achieve the flight of spacecraft into and through outer space.

Spaceflight is used in space exploration, and also in commercial activities like space tourism and satellite telecommunications. Additional non-commercial uses of spaceflight include space observatories, reconnaissance satellites and other Earth observation satellites.

A spaceflight typically begins with a rocket launch, which provides the initial thrust to overcome the force of gravity and propels the spacecraft from the surface of Earth. Once in space, the motion of a spacecraft—both when unpropelled and when under propulsion—is covered by the area of study called astrodynamics. Some spacecraft remain in space indefinitely, some disintegrate during atmospheric reentry, and others reach a planetary or lunar surface for landing or impact.

Satellites

Main article: Satellite

Satellites are used for a large number of purposes. Common types include military (spy) and civilian Earth observation satellites, communication satellites, navigation satellites, weather satellites, and research satellites. Space stations and human spacecraft in orbit are also satellites.

Commercialization of space

Main article: Commercialization of space

The commercialization of space first started out with the launching of private satellites by NASA or other space agencies. Current examples of the commercial satellite use of space include satellite navigation systems, satellite television, satellite communications (such as internet services) and satellite radio. The next step of commercialization of space was seen as human spaceflight. Flying humans safely to and from space had become routine to NASA and Russia. Reusable spacecraft were an entirely new engineering challenge, something only seen in novels and films like Star Trek and War of the Worlds. Astronaut Buzz Aldrin supported the use of making a reusable vehicle like the space shuttle. Aldrin held that reusable spacecraft were the key in making space travel affordable, stating that the use of "passenger space travel is a huge potential market big enough to justify the creation of reusable launch vehicles". Space tourism is a next step in the use of reusable vehicles in the commercialization of space. The purpose of this form of space travel is personal pleasure.

Private spaceflight companies such as SpaceX and Blue Origin, and commercial space stations such as the Axiom Space and the Bigelow Commercial Space Station have changed the cost and overall landscape of space exploration, and are expected to continue to do so in the near future.

Alien life

Main articles: Astrobiology and Extraterrestrial life

Astrobiology is the interdisciplinary study of life in the universe, combining aspects of astronomy, biology and geology. It is focused primarily on the study of the origin, distribution and evolution of life. It is also known as exobiology (from Greek: έξω, exo, "outside"). The term "Xenobiology" has been used as well, but this is technically incorrect because its terminology means "biology of the foreigners". Astrobiologists must also consider the possibility of life that is chemically entirely distinct from any life found on Earth. In the Solar System, some of the prime locations for current or past astrobiology are on Enceladus, Europa, Mars, and Titan.

Human spaceflight and habitation

Main articles: Human spaceflight, Bioastronautics, Effect of spaceflight on the human body, Space medicine, Space architecture, Space station, Space habitat (facility), and Space habitat (settlement)

Crew quarters on Zvezda, the base ISS crew module

To date, the longest human occupation of space is the International Space Station which has been in continuous use for 25 years, 160 days. Valeri Polyakov's record single spaceflight of almost 438 days aboard the Mir space station has not been surpassed. The health effects of space have been well documented through years of research conducted in the field of aerospace medicine. Analog environments similar to those experienced in space travel (like deep sea submarines), have been used in this research to further explore the relationship between isolation and extreme environments. It is imperative that the health of the crew be maintained as any deviation from baseline may compromise the integrity of the mission as well as the safety of the crew, hence the astronauts must endure rigorous medical screenings and tests prior to embarking on any missions. However, it does not take long for the environmental dynamics of spaceflight to commence its toll on the human body; for example, space motion sickness (SMS) – a condition which affects the neurovestibular system and culminates in mild to severe signs and symptoms such as vertigo, dizziness, fatigue, nausea, and disorientation – plagues almost all space travelers within their first few days in orbit. Space travel can also have an impact on the psyche of the crew members as delineated in anecdotal writings composed after their retirement. Space travel can adversely affect the body's natural biological clock (circadian rhythm); sleep patterns causing sleep deprivation and fatigue; and social interaction; consequently, residing in a Low Earth Orbit (LEO) environment for a prolonged amount of time can result in both mental and physical exhaustion. Long-term stays in space reveal issues with bone and muscle loss in low gravity, immune system suppression, problems with eyesight, and radiation exposure. The lack of gravity causes fluid to rise upward which can cause pressure to build up in the eye, resulting in vision problems; the loss of bone minerals and densities; cardiovascular deconditioning; and decreased endurance and muscle mass.

Radiation is an insidious health hazard to space travelers as it is invisible and can cause cancer. When above the Earth's magnetic field, spacecraft are no longer protected from the sun's radiation; the danger of radiation is even more potent in deep space. The hazards of radiation can be ameliorated through protective shielding on the spacecraft, alerts, and dosimetry.

Fortunately, with new and rapidly evolving technological advancements, those in Mission Control are able to monitor the health of their astronauts more closely using telemedicine. One may not be able to completely evade the physiological effects of space flight, but those effects can be mitigated. For example, medical systems aboard space vessels such as the International Space Station (ISS) are well equipped and designed to counteract the effects of lack of gravity and weightlessness; on-board treadmills can help prevent muscle loss and reduce the risk of developing premature osteoporosis. Additionally, a crew medical officer is appointed for each ISS mission and a flight surgeon is available 24/7 via the ISS Mission Control Center located in Houston, Texas. Although the interactions are intended to take place in real time, communications between the space and terrestrial crew may become delayed – sometimes by as much as 20 minutes – as their distance from each other increases when the spacecraft moves further out of low Earth orbit; because of this the crew are trained and need to be prepared to respond to any medical emergencies that may arise on the vessel as the ground crew are hundreds of miles away.

Many past and current concepts for the continued exploration and colonization of space focus on a return to the Moon as a "steppingstone" to the other planets, especially Mars. At the end of 2006, NASA announced they were planning to build a permanent Moon base with continual presence by 2024.

Beyond the technical factors that could make living in space more widespread, it has been suggested that the lack of private property, the inability or difficulty in establishing property rights in space, has been an impediment to the development of space for human habitation. Since the advent of space technology in the latter half of the twentieth century, the ownership of property in space has been murky, with strong arguments both for and against. In particular, the making of national territorial claims in outer space and on celestial bodies has been specifically proscribed by the Outer Space Treaty, which had been, as of 2012, ratified by all spacefaring nations. Space colonization, also called space settlement and space humanization, would be the permanent autonomous (self-sufficient) human habitation of locations outside Earth, especially of natural satellites or planets such as the Moon or Mars, using significant amounts of in-situ resource utilization.

Human representation and participation

See also: Space law

Participation and representation of humanity in space is an issue ever since the first phase of space exploration. Some rights of non-spacefaring countries have been mostly secured through international space law, declaring space the "province of all mankind", understanding spaceflight as its resource, though sharing of space for all humanity is still criticized as imperialist and lacking. Additionally to international inclusion, the inclusion of women and people of colour has also been lacking. To reach a more inclusive spaceflight, some organizations like the Justspace Alliance and IAU featured Inclusive Astronomy have been formed in recent years.

Women

Main article: Women in space

The first woman to go to space was Valentina Tereshkova. She flew in 1963 but it was not until the 1980s that another woman entered space again. All astronauts were required to be military test pilots at the time and women were not able to join this career. This is one reason for the delay in allowing women to join space crews. After the rule changed, Svetlana Savitskaya became the second woman to go to space, she was also from the Soviet Union. Sally Ride became the next woman in space and the first woman to fly to space through the United States program.

Since then, eleven other countries have allowed women astronauts. The first all-female space walk occurred in 2018, including Christina Koch and Jessica Meir. They had both previously participated in space walks with NASA. The first woman to go to the Moon is planned for 2026.

Despite these developments, women are underrepresented among astronauts and especially cosmonauts. Issues that block potential applicants from the programs, and limit the space missions they are able to go on, include:

• agencies limiting women to half as much time in space than men, arguing that there may be unresearched additional risks for cancer.
• a lack of space suits sized appropriately for female astronauts.

Art

See also: Space art § Art in space

Artistry in and from space ranges from signals, capturing and arranging material like Yuri Gagarin's selfie in space or the image The Blue Marble, over drawings like the first one in space by cosmonaut and artist Alexei Leonov, music videos like Chris Hadfield's cover of Space Oddity on board the ISS, to permanent installations on celestial bodies like on the Moon.

Astronomy

From Wikipedia, the free encyclopedia

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

The Paranal Observatory of European Southern Observatory creating a laser guide star in the atmosphere to observe the Galactic Center

Astronomy is a natural science that studies celestial objects and the phenomena that occur in the cosmos. It uses mathematics, physics, and chemistry to explain their origin and their overall evolution. Objects of interest include planets, moons, stars, nebulae, galaxies, meteoroids, asteroids, and comets. Relevant phenomena include supernova explosions, gamma ray bursts, quasars, blazars, pulsars, and cosmic microwave background radiation. More generally, astronomy studies everything that originates beyond Earth's atmosphere. Cosmology is the branch of astronomy that studies the universe as a whole.

Astronomy is one of the oldest natural sciences. The early civilizations in recorded history made methodical observations of the night sky. These include the Egyptians, Babylonians, Greeks, Indians, Chinese, Maya, and many ancient indigenous peoples of the Americas. In the past, astronomy included disciplines as diverse as astrometry, celestial navigation, observational astronomy, and the making of calendars.

Astronomy is one of the few sciences in which amateurs play an active role. This is especially true for the discovery and observation of transient events. Amateur astronomers have helped with many important discoveries, such as finding new comets.

Etymology

Astronomy (from the Greek ἀστρονομία from ἄστρον astron, "star" and -νομία -nomia from νόμος nomos, "law" or "rule") means study of celestial objects. Astronomy should not be confused with astrology, the belief system which claims that human affairs are correlated with the positions of celestial objects. The two fields share a common origin but became distinct, astronomy being supported by physics while astrology is not.

Use of terms "astronomy" and "astrophysics"

"Astronomy" and "astrophysics" are broadly synonymous in modern usage. In dictionary definitions, "astronomy" is "the study of objects and matter outside the Earth's atmosphere and of their physical and chemical properties", while "astrophysics" is the branch of astronomy dealing with "the behavior, physical properties, and dynamic processes of celestial objects and phenomena". Sometimes, as in the introduction of the introductory textbook The Physical Universe by Frank Shu, "astronomy" means the qualitative study of the subject, whereas "astrophysics" is the physics-oriented version of the subject. Some fields, such as astrometry, are in this sense purely astronomy rather than also astrophysics. Research departments may use "astronomy" and "astrophysics" according to whether the department is historically affiliated with a physics department, and many professional astronomers have physics rather than astronomy degrees. Thus, in modern use, the two terms are often used interchangeably.

History

Main article: History of astronomy

For a chronological guide, see Timeline of astronomy.

Pre-historic

Megaliths from Nabta Playa, constructed by Neolithic populations, located in Aswan, Upper Egypt.

The Nebra sky disc found on Mittenberg hill in Germany and dated to c. 1800–1600 BCE.

The initial development of astronomy was driven by practical needs like agricultural calendars. Before recorded history archeological sites such as Stonehenge provide evidence of ancient interest in astronomical observations. Evidence also comes from artefacts such as the Nebra sky disc inlaid with symbols interpreted as a sun, moon, and stars including a cluster of seven stars. Megalithic structures located in Nabta Playa, Upper Egypt featured astronomical calendar arrangements in alignment with the heliacal rising of Sirius and supported calibration the yearly calendar for the annual Nile flood.

Classical

A Babylonian planisphere (7th century BCE) was an early astronomical instrument. Its use of sexagesimals (e.g. 12, 24, 60, 360) is still being used today through having been broadly adopted for timekeeping and astrometry.

Civilizations such as Egypt, Mesopotamia, Greece, India, China independently but with cross-cultural influences created astronomical observatories and developed ideas on the nature of the Universe, along with calendars and astronomical instruments. A key early development was the beginning of mathematical and scientific astronomy among the Babylonians, laying the foundations for astronomical traditions in other civilizations. The Babylonians discovered that lunar eclipses recurred in the saros cycle of 223 synodic months.

Following the Babylonians, significant advances were made in ancient Greece and the Hellenistic world. Greek astronomy sought a rational, physical explanation for celestial phenomena. In the 4th century BC, Heracleides Ponticus was the first to proposed that the Earth rotates on its own axis. In the 3rd century BC, Aristarchus of Samos estimated the size and distance of the Moon and Sun, and he proposed a model of the Solar System where the Earth and planets rotated around the Sun, now called the heliocentric model. In the 2nd century BC, Hipparchus calculated the size and distance of the Moon and invented the earliest known astronomical devices such as the astrolabe. He also observed the small drift in the positions of the equinoxes and solstices with respect to the fixed stars that we now know is caused by precession. Hipparchus also created a catalog of 1020 stars, and most of the constellations of the northern hemisphere derive from Greek astronomy. The Antikythera mechanism (c. 150–80 BC) was an early analog computer designed to calculate the location of the Sun, Moon, and planets for a given date. Technological artifacts of similar complexity did not reappear until the 14th century, when mechanical astronomical clocks appeared in Europe.

Post-classical

After the classical Greek era, astronomy was dominated by the geocentric model of the Universe, or the Ptolemaic system, named after Claudius Ptolemy. His 13-volume astronomy work, named the Almagest in its Arabic translation, became the primary reference for over a thousand years. In this system, the Earth was believed to be the center of the Universe with the Sun, the Moon and the stars rotating around it. While the system would eventually be discredited, it gave the most accurate predictions for the positions of astronomical bodies available at that time.

With the arrival of Hellenistic astronomy in India through trade and cultural contacts, Indian astronomy entered a new phase during the early centuries CE. Earlier indigenous traditions, such as those recorded in the Vedāṅga Jyotiṣa, provided calendrical foundations, while Greek astronomical models were later integrated by scholars including Āryabhaṭa, Varāhamihira, and Brahmagupta. Āryabhaṭa notably improved methods for calculating planetary motions and eclipses. In the later medieval period, the Kerala school contributed to astronomy through refined observational practices and more accurate planetary and eclipse calculations.

Portrait of Alfraganus in the Compilatio astronomica, 1493. Islamic astronomers collected and translated Indian, Persian and Greek texts, adding their own work.

Astronomy flourished in the medieval Islamic world. Astronomical observatories were established there by the early 9th century. In 964, the Andromeda Galaxy, the largest galaxy in the Local Group, was described by the Persian Muslim astronomer Abd al-Rahman al-Sufi in his Book of Fixed Stars. The SN 1006 supernova, the brightest apparent magnitude stellar event in the last 1000 years, was observed by the Egyptian Arabic astronomer Ali ibn Ridwan and Chinese astronomers in 1006. Iranian scholar Al-Biruni observed that, contrary to Ptolemy, the Sun's apogee (highest point in the heavens) was mobile, not fixed. Arabic astronomers introduced many Arabic names now used for individual stars.

The ruins at Great Zimbabwe and Timbuktu may have housed astronomical observatories. In Post-classical West Africa, astronomers studied the movement of stars and relation to seasons, crafting charts of the heavens and diagrams of orbits of the other planets based on complex mathematical calculations. Songhai historian Mahmud Kati documented a meteor shower in 1583.

In medieval Europe, Richard of Wallingford (1292–1336) invented the first astronomical clock, the Rectangulus which allowed for the measurement of angles between planets and other astronomical bodies, as well as an equatorium called the Albion which could be used for astronomical calculations such as lunar, solar and planetary longitudes. Nicole Oresme (1320–1382) discussed evidence for the rotation of the Earth. Jean Buridan (1300–1361) developed the theory of impetus, describing motions including of the celestial bodies. For over six centuries (from the recovery of ancient learning during the late Middle Ages into the Enlightenment), the Roman Catholic Church gave more financial and social support to the study of astronomy than probably all other institutions. Among the Church's motives was finding the date for Easter.

Copernicus

During the Renaissance, Nicolaus Copernicus proposed a heliocentric model of the solar system. While his model maintained circular orbits, it was sufficient to calculate the size of planetary orbits and their period. The appealing simplicity of Copernican astronomy led to its adoption among astronomers even before it was confirmed by Galileo's telescopic observations in the 1600s.

Early telescopic

The first sketches of the Moon's topography, from Galileo's ground-breaking Sidereus Nuncius (1610)

Sometime around 1608 the telescope was invented and by 1610, Galileo Galilei observed phases on the planet Venus similar to those of the Moon, supporting the heliocentric model. Around the same time the heliocentric model was organized quantitatively by Johannes Kepler. Analyzing two decades of careful observations by Tycho Brahe, Kepler devised a system that described the details of the motion of the planets around the Sun. While Kepler discarded the uniform circular motion of Copernicus in favor of elliptical motion, he did not succeed in formulating a theory behind the laws he wrote down. It was Isaac Newton, with his invention of celestial dynamics and his law of gravitation, who finally explained the motions of the planets. Newton also developed the reflecting telescope. Newton, in collaboration with Richard Bentley proposed that stars are like the Sun only much further away.

The new telescopes also altered ideas about stars. By 1610 Galileo discovered that the band of light crossing the sky at night that we call the Milky Way was composed of numerous stars. In 1668 James Gregory compared the luminosity of Jupiter to Sirius to estimate its distance at over 83,000 AU. The English astronomer John Flamsteed, Britain's first Astronomer Royal, catalogued over 3000 stars but the data were published against his wishes in 1712. The astronomer William Herschel made a detailed catalog of nebulosity and clusters, and in 1781 discovered the planet Uranus, the first new planet found. Friedrich Bessel developed the technique of stellar parallax in 1838 but it was so difficult to apply that only about 100 stars were measured by 1900.

During the 18–19th centuries, the study of the three-body problem by Leonhard Euler, Alexis Claude Clairaut, and Jean le Rond d'Alembert led to more accurate predictions about the motions of the Moon and planets. This work was further refined by Joseph-Louis Lagrange and Pierre Simon Laplace, allowing the masses of the planets and moons to be estimated from their perturbations.

Significant advances in astronomy came about with the introduction of new technology, including the spectroscope and astrophotography. In 1814–15, Joseph von Fraunhofer discovered some 574 dark lines in the spectrum of the sun and of other stars. In 1859, Gustav Kirchhoff ascribed these lines to the presence of different elements.

Galaxies

Diagram of the stars, from William Herschel's On the construction of the heavens.

In the late 1700s William Herschel mapped the distribution of stars in different directions from Earth, concluding that the universe consisted of the Sun near the center of disk of stars, the Milky Way. After John Michell demonstrated that stars differ in intrinsic luminosity and after Herschel's own observations with more powerful telescopes that additional stars appeared in all directions, astronomers began to consider that some of the fuzzy spiral nebulae were distant island Universes.

Photograph of the Great Andromeda "Nebula" by Isaac Roberts in 1888.

The existence of galaxies, including the Earth's galaxy, the Milky Way, as a group of stars was only demonstrated in the 20th century. In 1912, Henrietta Leavitt discovered Cepheid variable stars with well-defined, periodic luminosity changes which can be used to fix the star's true luminosity which then becomes an accurate tool for distance estimates. Using Cepheid variable stars, Harlow Shapley constructed the first accurate map of the Milky Way. Using the Hooker Telescope, Edwin Hubble identified Cepheid variables in several spiral nebulae and in 1922–1923 proved conclusively that Andromeda Nebula and Triangulum among others, were entire galaxies outside our own, thus proving that the universe consists of a multitude of galaxies.

Cosmology

Main article: History of physical cosmology

Albert Einstein's 1917 publication of general relativity began the modern era of theoretical models of the universe as a whole. In 1922, Alexander Friedman published simplified models for the universe showing static, expanding and contracting solutions. In 1929 Hubble published observations that the galaxies are all moving away from Earth with a velocity proportional to distance, a relation now known as Hubble's law. This relation is expected if the universe is expanding. The consequence that the universe was once very dense and hot, a Big Bang concept expounded by Georges Lemaître in 1927, was discussed but no experimental evidence was available to support it. From the 1940s on, nuclear reaction rates under high density conditions were studied leading to the development of a successful model of big bang nucleosynthesis in the late 1940s and early 1950s. Then in 1965 cosmic microwave background radiation was discovered, cementing the evidence for the Big Bang.

Astrophysics predicted the existence of objects such as black holes and neutron stars. These have been used to explain phenomena such as quasars and pulsars.

Space telescopes have enabled measurements in parts of the electromagnetic spectrum normally blocked or blurred by the atmosphere. The LIGO project detected evidence of gravitational waves in 2015.

Observational astronomy

Main article: Observational astronomy

Overview of types of observational astronomy, relating wavelengths and their observability

Observational astronomy relies on many different wavelengths of electromagnetic radiation and the forms of astronomy are categorized according to the corresponding region of the electromagnetic spectrum on which the observations are made. Specific information on these subfields is given below.

Radio

The Very Large Array in New Mexico, a radio telescope

Main article: Radio astronomy

Radio astronomy uses radiation with long wavelengths, mainly between 1 millimeter and 15 meters (frequencies from 20 MHz to 300 GHz), far outside the visible range. Hydrogen, otherwise an invisible gas, produces a spectral line at 21 cm (1420 MHz) which is observable at radio wavelengths. Objects observable at radio wavelengths include interstellar gas, pulsars, fast radio bursts, supernovae, and active galactic nuclei.

Infrared

Main article: Infrared astronomy

The Subaru Telescope (left) and Keck Observatory (center) on Mauna Kea, both observatories that operate at near-infrared and visible wavelengths. The NASA Infrared Telescope Facility (right) is an example of a telescope that operates only at near-infrared wavelengths.

Infrared astronomy detects infrared radiation with wavelengths longer than red visible light, outside the range of our vision. The infrared spectrum is useful for studying objects that are too cold to radiate visible light, such as planets, circumstellar disks or nebulae whose light is blocked by dust. The longer wavelengths of infrared can penetrate clouds of dust that block visible light, allowing the observation of young stars embedded in molecular clouds and the cores of galaxies. Observations from the Wide-field Infrared Survey Explorer (WISE) have been particularly effective at unveiling numerous galactic protostars and their host star clusters.

With the exception of infrared wavelengths close to visible light, such radiation is heavily absorbed by the atmosphere, or masked, as the atmosphere itself produces significant infrared emission. Consequently, infrared observatories have to be located in high, dry places on Earth or in space. Some molecules radiate strongly in the infrared. This allows the study of the chemistry of space.

The James Webb Space Telescope senses infrared radiation to detect very distant galaxies. Visible light from these galaxies was emitted billions of years ago and the expansion of the universe shifted the light in to the infrared range. By studying these distant galaxies astronomers hope to learn about the formation of the first galaxies.

Optical

Main article: Optical astronomy

Historically, optical astronomy, which has been also called visible light astronomy, is the oldest form of astronomy. Images of observations were originally drawn by hand. In the late 19th century and most of the 20th century, images were made using photographic equipment. Modern images are made using digital detectors, particularly using charge-coupled devices (CCDs) and recorded on modern medium. Although visible light itself extends from approximately 380 to 700 nm that same equipment can be used to observe some near-ultraviolet and near-infrared radiation.

Ultraviolet

Main article: Ultraviolet astronomy

Ultraviolet astronomy employs ultraviolet wavelengths which are absorbed by the Earth's atmosphere, requiring observations from the upper atmosphere or from space. Ultraviolet astronomy is best suited to the study of thermal radiation and spectral emission lines from hot blue OB stars that are very bright at these wavelengths.

X-ray

Main article: X-ray astronomy

X-ray jet made from a supermassive black hole found by NASA's Chandra X-ray Observatory, made visible by light from the early Universe

X-ray astronomy uses X-radiation, produced by extremely hot and high-energy processes. Since X-rays are absorbed by the Earth's atmosphere, observations must be performed at high altitude, such as from balloons, rockets, or specialized satellites. X-ray sources include X-ray binaries, supernova remnants, clusters of galaxies, and active galactic nuclei.^[94] Since the Sun's surface is relatively cool, X-ray images of the Sun and other stars give valuable information on the hot solar corona.

Gamma-ray

Main article: Gamma ray astronomy

Gamma ray astronomy observes astronomical objects at the shortest wavelengths (highest energy) of the electromagnetic spectrum. Gamma rays may be observed directly by satellites such as the Compton Gamma Ray Observatory, or by specialized telescopes called atmospheric Cherenkov telescopes. Cherenkov telescopes do not detect the gamma rays directly but instead detect the flashes of visible light produced when gamma rays are absorbed by the Earth's atmosphere. Gamma-ray astronomy provides information on the origin of cosmic rays, possible annihilation events for dark matter, relativistic particles outflows from active galactic nuclei (AGN), and, using AGN as distant sources, properties of intergalactic space. Gamma-ray bursts, which radiate transiently, are extremely energetic events, and are the brightest (most luminous) phenomena in the universe.

Non-electromagnetic observation

The underground ANTARES neutrino telescope

Some events originating from great distances may be observed from the Earth using systems that do not rely on electromagnetic radiation.

In neutrino astronomy, astronomers use heavily shielded underground facilities such as SAGE, GALLEX, and Kamioka II/III for the detection of neutrinos. The vast majority of the neutrinos streaming through the Earth originate from the Sun, but 24 neutrinos were also detected from supernova 1987A. Cosmic rays, which consist of very high energy particles (atomic nuclei) that can decay or be absorbed when they enter the Earth's atmosphere, result in a cascade of secondary particles which can be detected by current observatories.

Gravitational-wave astronomy employs gravitational-wave detectors to collect observational data about distant massive objects. A few observatories have been constructed, such as the Laser Interferometer Gravitational Observatory LIGO. LIGO made its first detection on 14 September 2015, observing gravitational waves from a binary black hole. A second gravitational wave was detected on 26 December 2015 and additional observations should continue but gravitational waves require extremely sensitive instruments.

The combination of observations made using electromagnetic radiation, neutrinos or gravitational waves and other complementary information, is known as multi-messenger astronomy.

Astrometry and celestial mechanics

Main articles: Astrometry and Celestial mechanics

Use of optical interferometry to determine precise positions of stars

One of the oldest fields in astronomy, and in all of science, is the measurement of the positions of celestial objects known as astrometry. Historically, accurate knowledge of the positions of the Sun, Moon, planets and stars has been essential in celestial navigation (the use of celestial objects to guide navigation) and in the making of calendars. Careful measurement of the positions of the planets has led to a solid understanding of gravitational perturbations, and an ability to determine past and future positions of the planets with great accuracy, a field known as celestial mechanics. The measurement of stellar parallax of nearby stars provides a fundamental baseline in the cosmic distance ladder that is used to measure the scale of the Universe. Parallax measurements of nearby stars provide an absolute baseline for the properties of more distant stars, as their properties can be compared. Measurements of the radial velocity and proper motion of stars allow astronomers to plot the movement of these systems through the Milky Way galaxy.

Subfields by scale

Physical cosmology

Main article: Physical cosmology

Hubble Extreme Deep Field

Physical cosmology, the study of large-scale structure of the Universe, seeks to understand the formation and evolution of the cosmos. Fundamental to modern cosmology is the well-accepted theory of the Big Bang, the concept that the universe begin extremely dense and hot, then expanded over the course of 13.8 billion years to its present condition. The concept of the Big Bang became widely accepted after the discovery of the microwave background radiation in 1965. Dark matter and dark energy are now thought form 96% of the mass of the Universe. For this reason, much effort is expended in trying to understand the physics of these components.

Extragalactic

The blue, loop-shaped objects are multiple images of the same galaxy, duplicated by gravitational lensing. The cluster's gravitational field bends light, magnifying and distorting the image of a more distant object.

Main article: Extragalactic astronomy

The study of objects outside our galaxy is concerned with the formation and evolution of galaxies, their morphology (description) and classification, the observation of active galaxies, and at a larger scale, the groups and clusters of galaxies. These assist the understanding of the large-scale structure of the cosmos.

Galactic

Main article: Galactic astronomy

Galactic astronomy studies galaxies including the Milky Way, a barred spiral galaxy that is a prominent member of the Local Group of galaxies and contains the Solar System. It is a rotating mass of gas, dust, stars and other objects, held together by mutual gravitational attraction. As the Earth is within the dusty outer arms, large portions of the Milky Way are obscured from view.

Kinematic studies of matter in the Milky Way and other galaxies show there is more mass than can be accounted for by visible matter. A dark matter halo appears to dominate the mass, although the nature of this dark matter remains undetermined.

Stellar

Further information: Star

The study of stars and stellar evolution is fundamental to our understanding of the Universe. The astrophysics of stars has been determined through observation and theoretical understanding; and from computer simulations of the interior. Aspects studied include star formation in giant molecular clouds; the formation of protostars; and the transition to nuclear fusion and main-sequence stars, carrying out nucleosynthesis. Further processes studied include stellar evolution, ending either with supernovae or white dwarfs. The ejection of the outer layers forms a planetary nebula. The remnant of a supernova is a dense neutron star, or, if the stellar mass was at least three times that of the Sun, a black hole.

Solar

An ultraviolet image of the Sun's active photosphere as viewed by the NASA's TRACE space telescope.

Main article: Solar astronomy

Solar astronomy is the study of the Sun, a typical main-sequence dwarf star of stellar class G2 V, and about 4.6 billion years (Gyr) old. Processes studied by the science include the sunspot cycle, the sun's changes in luminosity, both steady and periodic, and the behavior of the sun's various layers, namely its core with its nuclear fusion, the radiation zone, the convection zone, the photosphere, the chromosphere, and the corona.

Planetary science

The black spot at the top is a dust devil climbing a crater wall on Mars. This moving, swirling column of Martian atmosphere (comparable to a terrestrial tornado) created the long, dark streak.

Main article: Planetary science

Planetary science is the study of the assemblage of planets, moons, dwarf planets, comets, asteroids, and other bodies orbiting the Sun, as well as exoplanets orbiting distant stars. The Solar System has been relatively well-studied, initially through telescopes and then later by spacecraft.

Processes studied include planetary differentiation; the generation of, and effects created by, a planetary magnetic field; and the creation of heat within a planet, such as by collisions, radioactive decay, and tidal heating. In turn, that heat can drive geologic processes such as volcanism, tectonics, and surface erosion, studied by branches of geology.

Interdisciplinary subfields

Astrochemistry

Main article: Astrochemistry

Astrochemistry is an overlap of astronomy and chemistry. It studies the abundance and reactions of molecules in the Universe, and their interaction with radiation. The word "astrochemistry" may be applied to both the Solar System and the interstellar medium. Studies in this field contribute for example to the understanding of the formation of the Solar System.

Astrobiology

Main article: Astrobiology

Astrobiology (or exobiology) studies the origin of life and its development other than on earth. It considers whether extraterrestrial life exists, and how humans can detect it if it does. It makes use of astronomy, biochemistry, geology, microbiology, physics, and planetary science to investigate the possibility of life on other worlds and help recognize biospheres that might be different from that on Earth. The origin and early evolution of life is an inseparable part of the discipline of astrobiology. That encompasses research on the origin of planetary systems, origins of organic compounds in space, rock-water-carbon interactions, abiogenesis on Earth, planetary habitability, research on biosignatures for life detection, and studies on the potential for life to adapt to challenges on Earth and in outer space.

Other

Astronomy and astrophysics have developed interdisciplinary links with other major scientific fields. Archaeoastronomy is the study of ancient or traditional astronomies in their cultural context, using archaeological and anthropological evidence. Astrostatistics is the application of statistics to the analysis of large quantities of observational astrophysical data. As "forensic astronomy", finally, methods from astronomy have been used to solve problems of art history and occasionally of law.

Amateur

Amateur astronomers can build their own equipment, and hold star parties and gatherings, such as Stellafane.

Main article: Amateur astronomy

Astronomy is one of the sciences to which amateurs can contribute the most. Collectively, amateur astronomers observe celestial objects and phenomena, sometimes with consumer-level equipment or equipment that they build themselves. Common targets include the Sun, the Moon, planets, stars, comets, meteor showers, and deep-sky objects such as star clusters, galaxies, and nebulae. Astronomy clubs throughout the world have programs to help their members set up and run observational programs such as to observe all the objects in the Messier (110 objects) or Herschel 400 catalogues.  Most amateurs work at visible wavelengths, but some have experimented with wavelengths outside the visible spectrum. The pioneer of amateur radio astronomy, Karl Jansky, discovered a radio source at the centre of the Milky Way. Some amateur astronomers use homemade telescopes or radio telescopes originally built for astronomy research (e.g. the One-Mile Telescope).

Amateurs can make occultation measurements to refine the orbits of minor planets. They can discover comets, and perform regular observations of variable stars. Improvements in digital technology have allowed amateurs to make advances in astrophotography.

Unsolved problems

Main article: List of unsolved problems in astronomy

In the 21st century, there remain important unanswered questions in astronomy. Some are cosmic in scope: for example, what are the dark matter and dark energy that dominate the evolution and fate of the cosmos? What will be the ultimate fate of the universe? Why is the abundance of lithium in the cosmos four times lower than predicted by the standard Big Bang model? Others pertain to more specific classes of phenomena. For example, is the Solar System normal or atypical? What is the origin of the stellar mass spectrum, i.e. why do astronomers observe the same distribution of stellar masses—the initial mass function—regardless of initial conditions? Likewise, questions remain about the formation of the first galaxies, the origin of supermassive black holes, the source of ultra-high-energy cosmic rays, and whether there is other life in the Universe, especially other intelligent life.