Outer space facts for kids
Outer space, or simply space, is the huge area beyond Earth's atmosphere. It lies between planets, stars, and other objects in the sky. Space is almost completely empty, like a super-strong vacuum. It mainly contains tiny bits of hydrogen and helium gas. You can also find light, cosmic rays, magnetic fields, and dust floating around. The basic temperature of outer space, from the Big Bang's leftover glow, is about -270 degrees Celsius.
The gas between galaxies makes up about half of all the normal matter in the universe. This gas is incredibly thin, with less than one hydrogen atom in a cubic meter. Yet, it can be millions of degrees hot! Over time, matter has gathered to form stars and galaxies. Most of the universe is empty space, even within galaxies and star systems. Scientists believe that most of the universe's remaining mass and energy is made of mysterious dark matter and dark energy.
Outer space doesn't start at a single, clear height above Earth. However, the Kármán line, about 100 kilometers (62 miles) above sea level, is often used as the official start of space. This line is important for space law and for keeping aerospace records. Some parts of Earth's upper atmosphere are sometimes called "near space". The main rules for international space law come from the Outer Space Treaty. This treaty began on October 10, 1967. It says that no country can own space, and all countries can freely explore it. Even though there are rules for peaceful uses of space, some anti-satellite weapons have been tested in Earth orbit.
People first thought the space between Earth and the Moon was a vacuum in the 1600s. This was after scientists learned that air pressure drops as you go higher. In the 1900s, we finally understood how truly vast outer space is when the distance to the Andromeda Galaxy was measured. Humans began exploring space physically with high-altitude balloon flights. Later, rockets carried people into space. Yuri Gagarin from the Soviet Union was the first person to orbit Earth in 1961. Sending people and objects into space is very expensive. This limits human spaceflight to low Earth orbit and the Moon. However, uncrewed spacecraft have visited all the known planets in our Solar System. Outer space is a tough place for humans to explore because of the vacuum and radiation. Also, weightlessness can make astronauts' muscles and bones weaker.
Contents
Understanding Space Terms
The word space, meaning "the area beyond Earth's sky," was used before the full term "outer space." One of the first times it was used this way was in a poem called Paradise Lost by John Milton in 1667.
The term outer space became popular through the writings of H. G. Wells after 1901. Theodore von Kármán used the idea of free space for altitudes above Earth where spacecraft don't feel much air resistance. This helped define the boundary between airspace (where countries have control) and outer space. This boundary is now known as the Kármán line.
Words like "spaceborne" mean something exists in outer space, especially if it's carried by a spacecraft. "Space-based" means something is located in outer space or on another planet or moon.
How Space Formed and What It's Like
We don't know how big the entire universe is; it might even be endless. The Big Bang theory says that the universe started about 13.8 billion years ago as something extremely hot and dense. It then expanded very quickly. About 380,000 years later, the universe cooled enough for tiny particles to form hydrogen atoms. At this point, light could travel freely through space as it continued to expand. The matter left after this early expansion slowly came together due to gravity. This formed stars, galaxies, and other objects. What was left behind is the deep vacuum we now call outer space. Because light travels at a certain speed, we can only see a part of the universe. This is called the observable universe.
Scientists have studied the cosmic microwave background (CMB) to understand the universe's shape. These studies show that the shape of the observable universe is "flat." This means that parallel light rays stay parallel as they travel through space, except where local gravity bends them. This "flat" universe, along with its measured density and speeding-up expansion, suggests that space has a special kind of energy called dark energy.
Scientists estimate that the universe's average energy density is like 5.9 protons in every cubic meter. This includes dark energy, dark matter, and normal matter (atoms). Normal atoms make up only about 4.6% of this total, or about one proton in every four cubic meters. The universe's density isn't the same everywhere. It's very dense in galaxies, stars, and black holes. But in huge empty areas called cosmic voids, the density is much lower. Unlike matter and dark matter, dark energy doesn't seem to gather in galaxies. Even though dark energy might be most of the universe's mass-energy, its effect in our Milky Way galaxy is much smaller than gravity from matter and dark matter.
The Space Environment
.jpg)
Outer space is the closest thing we know to a perfect vacuum. There's almost no friction there. This allows stars, planets, and moons to move freely in their orbits. Even the deep vacuum between galaxies isn't completely empty. It has a few hydrogen atoms per cubic meter. To compare, the air we breathe has about 1025 molecules in a cubic meter! Because matter is so spread out in space, light can travel huge distances without being scattered. However, extinction, where dust and gas absorb and scatter light, is still important in studying galaxies.
Stars, planets, and moons hold onto their atmospheres with gravity. Atmospheres don't have a sharp edge. The gas slowly gets thinner as you go higher until it blends into outer space. Earth's air pressure drops a lot at 100 kilometers (62 miles) up. Above this height, the gas pressure becomes very small compared to the push from the Sun's light and the solar wind. The thermosphere layer of Earth's atmosphere, in this range, changes a lot due to space weather.
The temperature of outer space is measured by how much the gas particles move. The light in outer space has a different temperature than the gas itself. This means the gas and light aren't in perfect balance. The entire observable universe is filled with light from the Big Bang. This is called the cosmic microwave background radiation (CMB). This background radiation has a temperature of about -270 degrees Celsius. Gas temperatures in space can vary a lot. For example, the Boomerang Nebula is about -272 degrees Celsius, while the solar corona (the Sun's outer atmosphere) can reach millions of degrees Celsius.
Scientists have found magnetic fields around many space objects. Star formation in spiral galaxies can create magnetic fields. These fields are weak but can be detected. The way dust grains line up with a galaxy's magnetic field helps us see these fields in nearby galaxies. Powerful processes in active elliptical galaxies create their famous jets and radio lobes. Even very distant objects show signs of magnetic fields.
Outside of a planet's protective atmosphere and magnetic field, there's little to stop energetic particles called cosmic rays. These particles have very high energies. The most common cosmic rays are protons (87%), helium nuclei (12%), and heavier nuclei (1%). Cosmic rays can harm electronic devices and pose a health risk to astronauts.
Humans in Space
How Space Affects Our Bodies
Even though space is a harsh place, some living things can survive its extreme conditions for a long time. For example, certain types of lichen survived ten days in space in 2007. Seeds from two plant species sprouted after being in space for 1.5 years. A type of bacteria called Bacillus subtilis survived 559 days in low Earth orbit or a simulated Martian environment.
The idea of lithopanspermia suggests that rocks blasted into space from planets with life might carry living things to another planet where life can grow. Some scientists think this might have happened early in our Solar System's history. Rocks carrying tiny living things could have traveled between Venus, Earth, and Mars. Since bacteria can survive for millions of years, it's possible for life to spread across galaxies.
The Vacuum of Space
The lack of pressure in space is the most immediate danger to humans. As you go higher above Earth, the pressure drops. At about 19.14 kilometers (11.9 miles) up, the pressure is so low that water boils at normal body temperature. This height is called the Armstrong line. Above this line, fluids in your throat and lungs would boil away. This means that at or above the Armstrong line, humans need a special pressure suit or a pressurized capsule to survive.
If an unprotected person is suddenly exposed to very low pressure in space, like during a quick loss of air from a spacecraft, their lungs could be damaged. Even if they breathe out, the air might not leave fast enough to prevent injury. A rapid pressure drop can also hurt eardrums and sinuses. Bruises and bleeding can happen in soft tissues. The body's need for oxygen can increase, leading to a lack of oxygen in the brain.
When pressure drops quickly, oxygen in the blood rushes into the lungs to try and balance the pressure. Once blood without enough oxygen reaches the brain, a person would lose consciousness in seconds. They would die from lack of oxygen within minutes. Blood and other body fluids boil when the pressure gets too low. This is called ebullism. The steam can make the body swell to twice its normal size and slow blood flow. However, tissues are stretchy and porous enough to prevent the body from bursting. Blood vessels help keep some blood liquid, slowing down ebullism.
Special pressure suits can reduce swelling and ebullism. The Crew Altitude Protection Suit (CAPS), designed for astronauts in the 1960s, prevents ebullism at very low pressures. Above 8 kilometers (5 miles) up, extra oxygen is needed for breathing and to prevent water loss. Above 20 kilometers (12 miles), pressure suits are essential to prevent ebullism. Most space suits use pure oxygen at a pressure similar to the oxygen pressure at Earth's surface. This pressure is enough to prevent ebullism. However, nitrogen dissolved in the blood could still cause decompression sickness if not managed.
Weightlessness and Radiation
Humans grew up with Earth's gravity. Being in weightlessness has harmful effects on health. At first, more than half of astronauts get space motion sickness. This can cause nausea, vomiting, dizziness, headaches, tiredness, and a general feeling of being unwell. Space sickness usually lasts 1 to 3 days, then the body adjusts. Longer time in weightlessness leads to muscle loss and weaker bones, called spaceflight osteopenia. Regular exercise can help reduce these effects. Other problems include fluids shifting in the body, a slower cardiovascular system, fewer red blood cells, balance issues, and a weaker immune system. Astronauts might also lose body mass, get stuffy noses, have trouble sleeping, and have puffy faces.
During long trips in space, radiation can be a serious danger. Exposure to high-energy cosmic rays can cause tiredness, nausea, vomiting. It can also damage the immune system and change white blood cell counts. Over longer periods, there's a higher risk of cancer. It can also damage the eyes, nervous system, lungs, and digestive system. On a three-year round trip to Mars, many cells in an astronaut's body could be hit and damaged by high-energy particles. Spacecraft walls can block some of these particles. Water containers and other barriers can help even more. However, cosmic rays hitting the shielding can create more radiation that affects the crew. More research is needed to understand radiation dangers and find better ways to protect astronauts.
The Edge of Space
There isn't a clear physical line where Earth's atmosphere ends and outer space begins. The air pressure just gets thinner and thinner until it blends with the solar wind. People have suggested different boundaries, from 30 kilometers (19 miles) to 1,600,000 kilometers (994,000 miles). In 2009, scientists used a sounding rocket to measure how ions (charged particles) move in the atmosphere. They found that 118 kilometers (73 miles) above Earth was the middle point where these particles changed from gentle atmospheric winds to the faster flows of outer space. These outer space flows can move much faster than 268 meters per second (880 feet per second).
High-altitude aircraft, like high-altitude balloons, have reached altitudes up to 50 kilometers (31 miles) above Earth. Until 2021, the United States called anyone who traveled above 50 miles (80 kilometers) an astronaut. Now, astronaut wings are only given to spacecraft crew members who did important things for public safety or human spaceflight safety during their flight.
The area between airspace and outer space is called "near space." There's no official legal definition for it, but it usually means the altitude range from 20 to 100 kilometers (12 to 62 miles). For safety, commercial aircraft usually fly below 12 kilometers (7.5 miles). Air traffic control services only go up to 18 to 20 kilometers (11 to 12 miles). The upper limit of "near space" is the Kármán line. Above this line, aerodynamics (how things fly in air) no longer works, and astrodynamics (how things fly in space) takes over. This range includes the stratosphere, mesosphere, and lower thermosphere layers of Earth's atmosphere.
Some people use larger ranges for near space, like 18 to 160 kilometers (11 to 99 miles). These ranges extend to where orbital flight in very low Earth orbits becomes practical. Spacecraft have flown in very stretched-out orbits with their lowest point as low as 80 to 90 kilometers (50 to 56 miles) and survived many orbits. At 120 kilometers (75 miles) up, spacecraft coming back to Earth start to feel atmospheric drag. For spaceplanes like NASA's Space Shuttle, this is where they switch from using thrusters to using aerodynamic wings to steer.
The Kármán line, set by the Fédération Aéronautique Internationale and used by the United Nations, is 100 kilometers (62 miles) high. It's a working definition for the boundary between flying in air and flying in space. This line is named after Theodore von Kármán. Around this height, a vehicle can't get enough aerodynamic lift from the atmosphere to stay up.
There's no international law setting a specific height limit for national airspace. However, the Kármán line is most often used. Some worry that setting this limit too high could make space activities difficult due to concerns about entering another country's airspace. Others argue that there shouldn't be one single limit. Instead, different limits might apply depending on the type of craft and its purpose. More commercial and military flights that go almost into space have raised questions about where airspace laws and outer space laws apply. Spacecraft, like the Space Shuttle, have flown over other countries as low as 30 kilometers (19 miles).
Space Laws and Rules
The Outer Space Treaty is the main set of rules for international space law. It covers how countries can legally use outer space, including the Moon and other celestial bodies. The treaty says that all countries are free to explore outer space. No country can claim to own any part of it. It calls outer space the "province of all mankind." This idea means that all nations, especially those without space programs, should have equal access to space. The treaty also forbids putting nuclear weapons in outer space. The United Nations General Assembly approved this treaty in 1963. The Soviet Union, the United States, and the United Kingdom signed it in 1967. As of 2017, 105 countries have agreed to follow the treaty. Another 25 countries have signed it but haven't fully approved it.
Since 1958, the United Nations has passed many resolutions about outer space. More than 50 of these have been about countries working together for peaceful uses of space and preventing an arms race there. The UN's Committee on the Peaceful Uses of Outer Space has also created four more space law treaties. Still, there are no laws against putting regular weapons in space. The USA, USSR, China, and India (in 2019) have all successfully tested anti-satellite weapons. These weapons create space junk. The 1979 Moon Treaty gave control of all heavenly bodies (and their orbits) to the international community. However, no country that currently sends humans into space has approved this treaty.
In 1976, eight countries near the equator (Ecuador, Colombia, Brazil, Congo, Zaire, Uganda, Kenya, and Indonesia) met in Bogotá, Colombia. In their "Bogotá Declaration," they claimed control over the part of the geosynchronous orbit that was above their countries. Other countries do not accept these claims.
A growing problem in international space law is the danger from the increasing amount of space junk.
Since 2020, the Artemis Accords have aimed to guide lunar exploration. They provide a legal framework for the United States-led Artemis program, which plans to send humans back to the Moon.
Earth Orbit
When a rocket launches to reach orbit, it needs to push against gravity and speed up to orbital speed. After the rocket stops firing its engines, it starts to fall back towards Earth because of gravity. If it reaches the right speed and height, this fall turns into an oval-shaped loop around the planet. This means a spacecraft successfully enters Earth orbit when gravity pulls it down just enough to stop it from flying off into outer space.
For a low Earth orbit, the speed needed is about 7.8 kilometers per second (17,400 mph). To compare, the fastest piloted airplane ever flew at 2.2 kilometers per second (4,900 mph) in 1967. The highest orbital speed, 11.2 kilometers per second (25,000 mph), is the escape velocity. This is the speed needed to break free from Earth's gravity completely and go into an orbit around the Sun. The energy to reach Earth orbital speed at 600 kilometers (370 miles) up is about 36 million joules per kilogram. This is six times the energy just to climb to that height.
Very low Earth orbit (VLEO) is defined as orbits below 450 kilometers (280 miles). These are good for Earth observation with small satellites. Low Earth orbits generally range from 180 to 2000 kilometers (110 to 1240 miles) high and are used for scientific satellites. Medium Earth orbits are from 2000 to 35780 kilometers (1240 to 22230 miles). These are good for navigation and special satellites. Above 35780 kilometers (22230 miles) are high Earth orbits, used for weather and some communication satellites.
Spacecraft in orbit with their lowest point below about 2000 kilometers (1240 miles) (low Earth orbit) are affected by drag from Earth's atmosphere. This drag slowly lowers their orbit. How fast an orbit decays depends on the satellite's size and mass. It also depends on changes in the upper atmosphere's air density, which is affected by space weather. At altitudes above 800 kilometers (500 miles), orbits can last for centuries. Below about 300 kilometers (190 miles), orbits decay much faster, lasting only days. Once a satellite drops to 180 kilometers (110 miles), it has only hours before it burns up in the atmosphere.
Radiation around Earth is trapped in Van Allen radiation belts. These belts hold solar and galactic radiation. Radiation is a danger to astronauts and space systems. It's hard to shield against, and space weather makes the radiation levels change. The radiation belts are donut-shaped regions around the equator, bending towards Earth's poles. The South Atlantic Anomaly is where charged particles get closest to Earth. The inner Van Allen belt is strongest at about 3000 kilometers (1860 miles) above the equator. It overlaps with the upper edge of low Earth orbit.
Regions of Space
.png)
.png)
.png)
.png)
.png)
Space Near Earth
The very top layer of Earth's atmosphere is called the exosphere. It starts from the thermopause, which is between 250 and 500 kilometers (155 to 310 miles) high, depending on how much sunlight hits it. Above this height, molecules rarely bump into each other, and the atmosphere blends into interplanetary space. The area close to Earth has many satellites orbiting it and has been studied a lot. This area is divided into different regions.
Near-Earth space is the region from low Earth orbits out to geostationary orbits. This is where most artificial satellites are and where most human space activity happens. This area has a lot of space junk, which can threaten satellites. Some of this debris falls back into Earth's atmosphere over time. Even though it's outer space, the air in low-Earth orbit (the first few hundred kilometers above the Kármán line) is still thick enough to cause significant drag on satellites.
Geospace is a part of outer space that includes Earth's upper atmosphere and magnetosphere. The Van Allen radiation belts are inside geospace. The outer edge of geospace is the magnetopause, which is where Earth's magnetosphere meets the solar wind. The inner edge is the ionosphere.
The changing space weather in geospace is affected by the Sun and the solar wind. Studying geospace is linked to heliophysics, which is the study of the Sun and its effects on the Solar System's planets. The magnetopause on the day side of Earth is squished by the solar wind. On the night side, the solar wind stretches the magnetosphere into a long magnetotail that can reach far beyond Earth. For about four days each month, the Moon is protected from the solar wind as it passes through this magnetotail.
Geospace contains electrically charged particles that are very spread out. Earth's magnetic field controls how these particles move. These charged gases can create storm-like disturbances powered by the solar wind. These disturbances can send electric currents into Earth's upper atmosphere. Geomagnetic storms can affect the radiation belts and the ionosphere. These storms increase the flow of energetic electrons, which can permanently damage satellite electronics. They can also interfere with shortwave radio communication and GPS location and timing. Magnetic storms can be dangerous for astronauts, even in low Earth orbit. They also create the beautiful auroras seen near Earth's poles.
XGEO space is a term used by the USA for high Earth orbits. The 'X' means a multiple of geosynchronous orbit (GEO), which is about 35,786 kilometers (22,236 miles) high. For example, the L2 Earth-Moon Lagrange point is about 448,900 kilometers (278,900 miles) away, which is about 10.67 times the GEO distance. Translunar space is the area of lunar transfer orbits, between the Moon and Earth.
Cislunar space is the region outside Earth that includes lunar orbits, the Moon's path around Earth, and the Earth-Moon Lagrange points. A body's sphere of influence is the region where its gravity is stronger than other bodies' gravity. For Earth, this includes all space out to about 1% of the distance from Earth to the Sun, or 1.5 million kilometers (930,000 miles). Beyond Earth's sphere of influence, its orbital path also has co-orbital space. This space is shared by groups of Near-Earth Objects (NEOs), like horseshoe librators and Earth trojans. Sometimes, some NEOs become temporary satellites or quasi-moons of Earth for a while.
Deep space is defined by the United States government as all of outer space that is farther from Earth than a typical low-Earth orbit. This means the Moon is considered part of deep space. Other definitions vary. Some say it's beyond the Moon's orbit, while others say it's beyond the farthest reaches of the Solar System. The International Telecommunication Union, which handles radio communication with satellites, defines deep space as "distances from the Earth equal to, or greater than, 2 million kilometers (1.2 million miles)." This is about five times the Moon's orbital distance, but still much less than the distance between Earth and any other planet.
Space Between Planets
Interplanetary space is inside a giant bubble created by the Sun's magnetic field. This space reaches out to the heliopause, which is 110 to 160 times the distance from Earth to the Sun. This is far beyond the orbit of Neptune, our outermost planet. The Sun constantly sends out a stream of charged particles called the solar wind. This creates a very thin atmosphere, called the heliosphere, that stretches billions of kilometers into space. This wind has a density of 5–10 protons per cubic centimeter and moves at 350–400 kilometers per second (780,000–890,000 mph). Throughout interplanetary space, the Sun's magnetic field, along with fields from the solar winds, pushes away low-energy galactic cosmic rays. How far and how strongly these rays are pushed away depends on how active the solar wind is.
Interplanetary space is almost a complete vacuum. The average distance a particle travels before hitting another is about the distance from Earth to the Sun. This space isn't totally empty. It has scattered cosmic rays, which include charged atomic nuclei and other tiny particles. There's also gas, plasma, dust, small meteors, and many types of organic molecules. All this matter together is called the interplanetary medium. A cloud of interplanetary dust can be seen at night as a faint band of light called the zodiacal light.
Interplanetary space contains the magnetic field created by the Sun. Planets like Jupiter, Saturn, Mercury, and Earth also have their own magnetic fields. The solar wind shapes these fields into a teardrop shape, with a long tail stretching out behind the planet. These magnetic fields can trap particles from the solar wind and other sources. This creates belts of charged particles, like the Van Allen radiation belts. Planets without magnetic fields, like Mars, slowly lose their atmospheres because of the solar wind.
Space Between Stars
Interstellar space is the physical space outside the bubbles of plasma around individual stars. These bubbles are called astrospheres and are created by stellar winds from the stars. Interstellar space is the area between stars or stellar systems within a nebula or galaxy. It contains a thin mix of matter and radiation called the interstellar medium. The boundary between an astrosphere and interstellar space is called an astropause. For our Sun, the astrosphere is the heliosphere, and its boundary is the heliopause.
About 70% of the interstellar medium is made of single hydrogen atoms. Most of the rest is helium atoms. This medium also has small amounts of heavier atoms. These heavier atoms are formed inside stars and then shot into the interstellar medium by stellar winds. They also come from old stars shedding their outer layers, like when a planetary nebula forms. When a supernova explodes, it sends out shock waves of stellar material. This spreads heavy elements, which were made in the star's core, throughout the interstellar medium. The density of matter in the interstellar medium can change a lot. The average is about 1 million particles per cubic meter. But cold molecular clouds can have 100 million to 10 trillion particles per cubic meter.
Many molecules exist in interstellar space. These can form dust particles as tiny as 0.1 micrometers. Scientists are constantly finding new molecules using radio astronomy, about four new types each year. Large, dense areas called molecular clouds allow chemical reactions to happen. This includes the formation of complex organic molecules. Much of this chemistry happens through collisions. Energetic cosmic rays go through these cold, dense clouds and ionize hydrogen and helium. This leads to the creation of molecules like the trihydrogen cation. An ionized helium atom can then break apart common carbon monoxide to make ionized carbon, which can then lead to other organic chemical reactions.
The local interstellar medium is the region of space within 100 parsecs (about 326 light-years) of the Sun. This area is interesting because it's close to us and interacts with our Solar System. This region is mostly within an area called the Local Bubble. This bubble is known for not having many dense, cold clouds. It's a hollow space in the Orion Arm of the Milky Way Galaxy. Dense molecular clouds are found along its edges, like those in the constellations of Ophiuchus and Taurus. The actual distance to the edge of this bubble varies from 60 to 250 parsecs or more. This area contains about 10,000 to 100,000 stars. The local interstellar gas balances the astrospheres around these stars. The size of each astrosphere depends on how dense the local interstellar medium is. The Local Bubble has dozens of warm interstellar clouds. These clouds can be up to 7,000 Kelvin (6,727 degrees Celsius) hot and 0.5–5 parsecs wide.
When stars move very fast, their astrospheres can create bow shocks as they hit the interstellar medium. For many years, scientists thought the Sun had a bow shock. But in 2012, data from the IBEX and Voyager probes showed that the Sun does not have a bow shock. Instead, these scientists say a slower "bow wave" marks the change from the solar wind to the interstellar medium. A bow shock is a third boundary of an astrosphere, found outside the termination shock and the astropause.
Space Between Galaxies
Intergalactic space is the physical space between galaxies. Studies of how galaxies are spread out show that the universe looks like a foam. Groups and clusters of galaxies lie along long, thin structures called filaments. These filaments take up about a tenth of all space. The rest are cosmic voids, which are mostly empty of galaxies. A typical void is about 7–30 megaparsecs (23–98 million light-years) across.
The space around and between galaxies is filled with the intergalactic medium (IGM). This very thin gas is organized in a filamentary structure. The spread-out, ionized gas has denser filaments, with about one atom per cubic meter. This is 5–200 times denser than the average density of the universe. Scientists believe the IGM is mostly made of the original elements from the Big Bang, with 76% hydrogen by mass. It also has heavier elements that have been shot out from galaxies.
As gas falls into the intergalactic medium from the voids, it heats up to temperatures of 100,000 to 10 million Kelvin (99,727 to 9,999,727 degrees Celsius). At these temperatures, it's called the warm–hot intergalactic medium (WHIM). Even though it's very hot by Earth standards, 100,000 Kelvin is often called "warm" in space science. Computer models and observations suggest that up to half of all the atomic matter in the universe might exist in this warm, thin state. When gas falls from the WHIM's filamentary structures into galaxy clusters (where cosmic filaments cross), it can get even hotter. It can reach temperatures of 100 million Kelvin (99,999,727 degrees Celsius) and above in what's called the intracluster medium (ICM).
Discovering Space's Secrets
Around 350 BCE, the Greek philosopher Aristotle thought that nature hates a vacuum. This idea was called the horror vacui. It built on an earlier idea from the 5th century BCE by Parmenides, who said that empty space couldn't exist. Because of this, for many centuries in the West, people widely believed that space could not be empty. Even in the 1600s, the French philosopher René Descartes argued that all of space must be filled with something.
In ancient China, the astronomer Zhang Heng in the 2nd century believed that space was endless. He thought it stretched far beyond what held the Sun and stars. Old books from the Hsüan Yeh school said the heavens were boundless, "empty and void of substance." They also said the "sun, moon, and the company of stars float in the empty space, moving or standing still."
The Italian scientist Galileo Galilei knew that air has weight and is affected by gravity. In 1640, he showed that a force resisted the creation of a vacuum. His student, Evangelista Torricelli, later created a device that made a partial vacuum in 1643. This experiment led to the first mercury barometer and caused a scientific stir in Europe. Torricelli suggested that since air has weight, air pressure should decrease with height. The French mathematician Blaise Pascal proposed an experiment to test this. In 1648, his brother-in-law, Florin Périer, did the experiment on the Puy de Dôme mountain in France. He found that the mercury column was shorter by three inches. This drop in pressure was also shown by taking a half-full balloon up a mountain and watching it expand, then shrink on the way down.
In 1650, German scientist Otto von Guericke built the first vacuum pump. This device further proved that horror vacui was wrong. He correctly observed that Earth's atmosphere surrounds the planet like a shell, getting thinner with height. He concluded that there must be a vacuum between Earth and the Moon.
In the 1400s, German theologian Nicolaus Cusanus thought the universe had no center or edge. He believed the universe wasn't infinite, but it also couldn't be finite because it had no boundaries. These ideas led to the Italian philosopher Giordano Bruno in the 1500s thinking about the infinite size of space. He expanded the heliocentric (Sun-centered) idea of Copernicus to an infinite universe filled with a substance he called aether. He thought this aether didn't stop heavenly bodies from moving. The English philosopher William Gilbert came to a similar conclusion. He argued that stars are only visible because they are surrounded by a thin aether or empty space. This idea of aether came from ancient Greek philosophers, like Aristotle, who thought it was the medium through which heavenly bodies moved.
The idea of a universe filled with a luminiferous aether was supported by some scientists until the early 1900s. This aether was thought to be the medium that light traveled through. In 1887, the Michelson–Morley experiment tried to find Earth's movement through this medium. They looked for changes in the speed of light depending on the planet's direction of motion. The experiment found no such changes, showing something was wrong with the idea. The concept of the luminiferous aether was then abandoned. It was replaced by Albert Einstein's theory of special relativity. This theory says that the speed of light in a vacuum is always the same, no matter how the observer is moving.
The first professional astronomer to support the idea of an infinite universe was the Englishman Thomas Digges in 1576. But the true size of the universe was unknown until 1838. That's when the German astronomer Friedrich Bessel successfully measured the distance to a nearby star. He showed that the star system 61 Cygni had a tiny parallax of just 0.31 arcseconds. This meant it was over 10 light years away. In 1917, Heber Curtis noticed that novae (exploding stars) in spiral nebulae were much fainter than those in our galaxy. This suggested they were 100 times farther away. In 1923, American astronomer Edwin Hubble determined the distance to the Andromeda Galaxy. He did this by measuring the brightness of cepheid variables in that galaxy, using a new method discovered by Henrietta Leavitt. This proved that the Andromeda Galaxy, and all other galaxies, were far outside the Milky Way. With this, Hubble created the Hubble constant. This allowed the first calculations of the universe's age and the size of the Observable Universe. These calculations started at 2 billion years and 280 million light-years. They became more precise with better measurements. By 2006, data from the Hubble Space Telescope allowed a very accurate calculation of the age and size of the Observable Universe.
The modern idea of outer space is based on the "Big Bang" cosmology. This theory was first suggested in 1931 by the Belgian physicist Georges Lemaître. It says that the universe began from a state of extreme energy and has been continuously expanding ever since.
The earliest known guess for the temperature of outer space was by the Swiss physicist Charles É. Guillaume in 1896. He used the estimated radiation from background stars and concluded that space must be heated to 5–6 Kelvin (-268 to -267 degrees Celsius). British physicist Arthur Eddington made a similar calculation in 1926, getting 3.18 Kelvin (-270 degrees Celsius). German physicist Erich Regener used the total energy of cosmic rays to estimate an intergalactic temperature of 2.8 Kelvin (-270 degrees Celsius) in 1933. American physicists Ralph Alpher and Robert Herman predicted 5 Kelvin (-268 degrees Celsius) for the temperature of space in 1948. This was based on the gradual decrease in background energy after the new Big Bang theory.
Exploring Outer Space
For most of history, people explored space by watching it from Earth, first with their eyes, then with telescopes. Before rockets, the closest humans got to space was with balloon flights. In 1935, the American Explorer II crewed balloon reached 22 kilometers (14 miles) high. This was far surpassed in 1942 when the German A-4 rocket climbed to about 80 kilometers (50 miles). In 1957, the uncrewed satellite Sputnik 1 was launched by a Russian R-7 rocket. It orbited Earth at 215–939 kilometers (134–583 miles) high. This was followed by the first human spaceflight in 1961, when Yuri Gagarin orbited Earth on Vostok 1. The first humans to leave low Earth orbit were Frank Borman, Jim Lovell, and William Anders in 1968. They flew on the American Apollo 8 mission, which orbited the Moon and reached 377,349 kilometers (234,474 miles) from Earth.
The Soviet Luna 1 was the first spacecraft to reach escape velocity. It flew past the Moon in 1959. In 1961, Venera 1 became the first probe to another planet. It found the solar wind and flew past Venus, though contact was lost before it reached Venus. The first successful mission to a planet was the 1962 fly-by of Venus by Mariner 2. Mariner 4 made the first fly-by of Mars in 1964. Since then, uncrewed spacecraft have successfully explored every planet in the Solar System, along with their moons, many minor planets, and comets. These probes are still key tools for exploring outer space and observing Earth. In August 2012, Voyager 1 became the first human-made object to leave the Solar System and enter interstellar space.
How We Use Space
Outer space has become a very important part of our global society. It offers many uses that help our economy and scientific research.
Putting artificial satellites into Earth orbit has brought many benefits. This has become the biggest part of the space economy. Satellites help relay long-distance communications like television. They provide accurate navigation (like GPS). They also allow us to directly monitor weather and observe Earth from afar through remote sensing. This Earth observation helps with many things. It tracks soil moisture for farming, predicts water flow from melting snow, finds plant diseases, and helps with military surveillance. Satellites also help discover and track the effects of climate change. Satellites use the much lower drag in space to stay in stable orbits. This allows them to cover the whole globe efficiently, unlike stratospheric balloons or high-altitude platform stations, which have other benefits.
Because there's no air, outer space is a perfect place for astronomy. We can observe all wavelengths of the electromagnetic spectrum there. The pictures from the Hubble Space Telescope prove this. They let us see light from over 13 billion years ago, almost back to the time of the Big Bang. Not every spot in space is ideal for a telescope. The interplanetary zodiacal dust gives off a faint infrared light. This can hide the light from dim sources like planets outside our Solar System. Moving an infrared telescope past this dust makes it more effective. Also, a place like the Daedalus crater on the far side of the Moon could shield a radio telescope from radio interference that bothers Earth-based observations.
The deep vacuum of space could be useful for certain industrial processes that need super-clean surfaces. Like asteroid mining, space manufacturing would need a huge financial investment with little chance of quick profits. A big part of the cost is sending things into Earth orbit. In 2006, it cost $6,000–$20,000 per kilogram (after adjusting for inflation). The cost of getting to space has dropped since 2013. Partially reusable rockets like the Falcon 9 have lowered the cost to below $3,500 per kilogram. Even with these new rockets, the cost to send materials into space is still too high for many industries. Ideas to fix this include fully reusable launch systems, non-rocket spacelaunch, momentum exchange tethers, and space elevators.
Interstellar travel for humans is currently just a theoretical idea. The distances to the nearest stars mean we would need new technologies. We would also need to safely support crews for journeys lasting many decades. For example, the Daedalus Project studied a spacecraft powered by nuclear fusion. It would take 36 years to reach the "nearby" Alpha Centauri system. Other proposed ways to travel between stars include light sails, ramjets, and beam-powered propulsion. More advanced systems might use antimatter as fuel, possibly reaching speeds close to the speed of light.
From Earth's surface, the super-cold temperature of outer space can be used as a renewable cooling method. This is called passive daytime radiative cooling. It helps heat escape through the atmosphere's infrared window into outer space, lowering local temperatures. Special materials called Photonic metamaterials can be used to block heating from the Sun.
Space in Our Society
Outer space, or the heavens, has been a place of imagination for people throughout history and across the world. The space age has made it a real place. The study of how cultures view space is called cultural astronomy. This includes archaeoastronomy, the history of astrology, ethnoastronomy, and the history of astronomy. The presence of humans in space is studied by astrosociology and space archaeology.
Ideas about spaceflight and the start of the space age have been linked to thoughts of utopianism (perfect societies) and colonialism (settling new lands). This led to the idea of space colonization. These ideas influenced space exploration and thoughts about extraterrestrial life. The idea of "the other" was sometimes seen as "extraterrestrial" or from another world. One of the first times colonialism was linked to outer space was in 1638. John Wilkins suggested in his book A Discourse Concerning a New Planet that future adventurers, like Francis Drake and Christopher Columbus, might reach the Moon and people could live there.
Spaceflight has also changed how we see the world, especially through Earth observation. This has led to the overview effect, a feeling of awe and wonder when seeing Earth from space. It's also created "Moon joy" when leaving Earth. Many agree that the environmental movement has been especially inspired by Earth observation and images like the Blue Marble.
Related pages
Images for kids
-
This is an artist's concept of the expansion of space, where a volume of the Universe is shown at different times. On the left, you see the rapid expansion from the beginning, followed by steadier expansion to the present day, shown on the right.
-
Part of the Hubble Ultra-Deep Field image showing a typical section of space with galaxies spread out in the deep vacuum. Because light takes time to travel, this view shows us the past 13 billion years of outer space's history.
-
Because of the dangers of a vacuum, astronauts must wear a special pressurized space suit when they are outside their spacecraft in space.
-
Aurora australis (Southern Lights) observed from the Space Shuttle Discovery, in May 1991.
-
Cislunar space seen from Mars.
-
SpaceShipOne made the first human private spaceflight in 2004, reaching an altitude of 100.12 km (62.21 mi).
-
A 2008 launch of the SM-3 missile used to destroy an American reconnaissance satellite.
-
The original Magdeburg hemispheres (lower left) used to show how a vacuum pump works.
-
The first image taken by a human of the whole Earth, likely photographed by William Anders of Apollo 8. South is up; South America is in the middle.
See also
In Spanish: Espacio exterior para niños
