Astronomy & Space Vocabulary: 100 Key English Terms
English is the working language of astronomy and spaceflight: research papers, observing guides, telescope manuals and the live commentary of rocket launches all use it. These terms take you from the planets of our solar system to distant galaxies and the beginning of the universe, and from the backyard telescope to the launch pad and the space station.
100 terms
10 topics
35 core terms
200 example sentences
Read, understand, keep going
No account needed. Read the meaning, see where the term is used, then study the examples. Click any word in the lesson for help. You can also select a word and press Alt + Enter.
Solar System & Planets
planet
noun
Meaning
A large, round object that orbits a star, shines only by reflected light and is massive enough to have cleared other large bodies from its orbit.
Where you use it
Used everywhere from school books to research papers. The eight planets of our solar system are often divided into rocky inner planets and giant outer planets. Astronomers observe, discover and name planets.
Examples
Venus is the brightest planet in the evening sky this month, low in the west after sunset.
Mercury is the smallest planet and the closest one to the Sun.
Usage note
The word comes from the Greek for 'wanderer', because planets slowly change position against the background stars from night to night.
The star at the centre of our solar system. Its gravity holds the planets in their orbits, and its light and heat make life on Earth possible.
Where you use it
Written with a capital S in astronomy when it means our own star, usually with 'the'. Astronomers call it a yellow dwarf or a G-type main-sequence star. The adjective is 'solar', as in solar energy, solar flare and solar eclipse.
Examples
Light from the Sun takes about eight minutes to reach Earth.
Never look at the Sun through a telescope without a proper solar filter.
Usage note
In everyday writing 'the sun' in lower case is also correct; astronomy texts prefer the capital letter.
Common phrases
the Sun's surface · orbit the Sun · observe the Sun safely · solar activity
A natural object that orbits a planet or a dwarf planet. Written as 'the Moon', it means Earth's only natural satellite.
Where you use it
Astronomers talk about the moons of Jupiter or Saturn and about icy moons that may hide oceans. For Earth's companion, 'the Moon' is used; the adjective is 'lunar', as in lunar eclipse and lunar surface.
Examples
The full Moon will rise just after sunset on Friday.
Europa is one of Jupiter's moons, and scientists think it has a salty ocean under its ice.
Usage note
Turkish 'uydu' covers both natural moons and artificial satellites. In English, 'moon' is natural and 'satellite' usually means a human-made one.
Common phrases
the full Moon · icy moon · the far side of the Moon · moons of Jupiter · land on the Moon
The Sun and everything that orbits it: eight planets and their moons, dwarf planets, asteroids, comets, dust and gas.
Where you use it
Often written with capitals, 'the Solar System', when it means ours. In lower case, 'a solar system' is sometimes used informally for another star and its planets, although astronomers prefer 'planetary system' there.
Examples
Jupiter is the largest planet in the solar system.
Comets are leftovers from the early days of the solar system.
Common phrases
the outer solar system · the inner solar system · the early solar system · the edge of the solar system
A small icy body that orbits the Sun. When it comes close to the Sun, its ice turns to gas and forms a glowing cloud and often long tails.
Where you use it
News stories report that a comet is 'visible to the naked eye' or 'approaching the Sun'. Astronomers name its parts: nucleus, coma and tail. Halley's Comet, the most famous, returns about every 76 years.
Examples
A comet's tails point away from the Sun, pushed by sunlight and the solar wind.
Halley's Comet will next return to the inner solar system in 2061.
Usage note
Do not translate 'kuyruklu yıldız' as 'tailed star'. Comets are not stars; the English word is simply 'comet'.
A small rocky or metallic body that orbits the Sun. Most asteroids are found in the asteroid belt between Mars and Jupiter.
Where you use it
Common in science news and planetary defence: astronomers discover, track and name asteroids and calculate whether one might hit Earth. A 'near-Earth asteroid' has an orbit that comes relatively close to Earth's.
Examples
In 2022 NASA's DART spacecraft hit a small asteroid and changed its orbit.
The asteroid will pass safely, at several times the distance of the Moon.
Common phrases
near-Earth asteroid · asteroid impact · track an asteroid · asteroid sample · asteroid belt
The bright streak of light seen when a small piece of rock or dust from space burns up in Earth's atmosphere. The piece itself is called a meteoroid.
Where you use it
Used by observers counting meteors during a shower and in news about very bright 'fireballs'. In informal English people say 'shooting star' or 'falling star'.
Examples
We counted more than forty meteors in an hour during the Perseids.
A very bright meteor lit up the sky over the city last night.
Usage note
Meteoroid (the piece in space), meteor (the light in the sky), meteorite (the rock that reaches the ground): three different words.
Common phrases
bright meteor · meteor shower · see a meteor · meteor trail · count meteors
A planet that orbits a star other than the Sun. The first one around a Sun-like star was found in 1995, and thousands are known today.
Where you use it
Astronomers detect, confirm and characterise exoplanets, mostly with the transit and radial velocity methods. Popular science often focuses on 'Earth-like' or 'potentially habitable' exoplanets. Also called an 'extrasolar planet'.
Examples
More than five thousand exoplanets have been confirmed so far.
The space telescope studied the atmosphere of the exoplanet and found water vapour.
A round object that orbits the Sun but has not cleared other objects from its orbit. Pluto, Ceres and Eris are well-known examples.
Where you use it
An official category created by the International Astronomical Union in 2006. It comes up often in discussions about whether Pluto should be called a planet.
Examples
Pluto was reclassified as a dwarf planet in 2006.
Ceres is the only dwarf planet in the asteroid belt.
Common phrases
reclassified as a dwarf planet · icy dwarf planet · the dwarf planet Pluto
A very large planet made mostly of hydrogen and helium, with no solid surface to stand on. Jupiter and Saturn are the gas giants of our solar system.
Where you use it
Used in planetary science and exoplanet research. Uranus and Neptune are often called 'ice giants' because they contain more water, ammonia and methane ices. 'Hot Jupiters' are gas giants that orbit very close to their stars.
Examples
Many of the first exoplanets discovered were gas giants orbiting close to their stars.
A spacecraft cannot land on a gas giant because it has no solid surface.
Common phrases
gas giant planet · ice giant · hot Jupiter · the gas giants
A piece of rock or metal from space that survives its fall through Earth's atmosphere and lands on the ground.
Where you use it
Used by scientists, collectors and museums. Meteorites are found, recovered, classified and cut for study. The main types are stony, iron and stony-iron meteorites. A 'fall' is a meteorite seen coming down; a 'find' is one discovered later.
Examples
Most meteorites come from asteroids, but a few come from the Moon and Mars.
Villagers collected dozens of meteorite fragments after the fireball.
Common phrases
iron meteorite · meteorite fall · meteorite fragment · Martian meteorite · find a meteorite
The region between the orbits of Mars and Jupiter where most of the solar system's asteroids orbit the Sun.
Where you use it
Often called 'the main belt'. Science fiction shows it crowded with rocks, but the asteroids are actually far apart and spacecraft cross it safely. Its largest object is the dwarf planet Ceres.
Examples
The Dawn spacecraft visited Vesta and Ceres, the two largest bodies in the asteroid belt.
The total mass of the asteroid belt is only a few percent of the Moon's mass.
Common phrases
the main asteroid belt · in the asteroid belt · between Mars and Jupiter
A wide ring of icy bodies beyond the orbit of Neptune, about 30 to 50 AU from the Sun. Pluto and other dwarf planets orbit there, and many short-period comets come from it.
Where you use it
Objects there are called Kuiper Belt objects (KBOs) or trans-Neptunian objects. The New Horizons probe flew past one of them, Arrokoth, in 2019. It should not be confused with the much more distant Oort Cloud.
Examples
Pluto is the best-known object in the Kuiper Belt.
Many short-period comets began their journey in the Kuiper Belt.
Usage note
Kuiper is the surname of a Dutch-American astronomer and is pronounced KY-per, rhyming with 'piper'.
Common phrases
Kuiper Belt object · in the Kuiper Belt · beyond the Kuiper Belt
A constant stream of charged particles flowing out from the Sun into space at hundreds of kilometres per second.
Where you use it
Used in space weather forecasts and planetary science. When gusts of solar wind hit Earth's magnetic field, they can cause auroras and disturb satellites and radio links. The solar wind also shapes the tails of comets.
Examples
Earth's magnetic field protects us from most of the solar wind.
A fast stream of solar wind could bring auroras to northern Europe tonight.
Common phrases
fast solar wind · solar wind particles · solar wind speed · space weather
A huge ball of hot gas, mainly hydrogen and helium, that produces light and heat through nuclear reactions in its core. The Sun is the nearest star to Earth.
Where you use it
A basic word in every part of astronomy. Astronomers describe a star by its brightness, colour, temperature, mass and age, and talk about how stars are born, live and die. In everyday English, 'stars' can mean any points of light in the night sky.
Examples
Betelgeuse is a cool red star, while Rigel in the same constellation is hot and blue-white.
On a clear night far from the city, you can see a few thousand stars with the naked eye.
Usage note
A 'shooting star' is not a star at all; it is a meteor burning up in Earth's atmosphere.
Common phrases
bright star · star formation · massive star · Sun-like star · the life cycle of a star
A region of space where matter is packed so tightly that gravity stops anything, even light, from escaping. Many form when very massive stars collapse.
Where you use it
Astronomers detect black holes through their effect on nearby stars and gas, through X-rays from hot material around them and through gravitational waves. Main types are stellar-mass and supermassive black holes. In everyday English, 'a black hole' is also something that swallows money or time.
Examples
In 2019 astronomers released the first image of the shadow of a black hole.
When the two black holes merged, they sent gravitational waves across the universe.
Common phrases
stellar-mass black hole · supermassive black hole · fall into a black hole · black hole merger · the shadow of a black hole
A huge cloud of gas and dust in space. Some nebulae are places where new stars are born; others are material thrown off by dying stars.
Where you use it
Popular targets for telescopes and astrophotography. Types include emission nebulae, dark nebulae, planetary nebulae and supernova remnants. The Orion Nebula can be seen with the naked eye as a fuzzy patch.
Examples
The Orion Nebula is a stellar nursery where thousands of young stars are forming.
With a narrowband filter, the nebula shows much more detail in photographs.
Usage note
The plural is 'nebulae' (NEB-yuh-lee) in scientific writing and 'nebulas' in everyday English. A planetary nebula has nothing to do with planets; the name comes from how round it looked in old telescopes.
Common phrases
planetary nebula · emission nebula · the Orion Nebula · dark nebula · star-forming nebula
A huge stellar explosion that can briefly shine as brightly as a whole galaxy. It happens when a massive star collapses or when a white dwarf gains too much mass.
Where you use it
A star 'goes supernova' or 'explodes as a supernova'. Astronomers study supernova remnants and use Type Ia supernovae to measure distances to faraway galaxies. Supernovae spread heavy elements such as iron into space.
Examples
The Crab Nebula is what remains of a supernova seen from Earth in 1054.
Betelgeuse is expected to go supernova within the next 100,000 years.
Usage note
The plural is 'supernovae' in scientific writing and 'supernovas' in general texts.
Common phrases
go supernova · supernova remnant · Type Ia supernova · supernova explosion
A reaction in which light atomic nuclei, such as hydrogen, join to form heavier ones, such as helium, releasing a huge amount of energy. It powers the Sun and other stars.
Where you use it
In astronomy it explains how stars shine and how chemical elements are made. Stars 'fuse hydrogen into helium' in their cores. Outside astronomy the term is common in energy research and news about fusion reactors.
Examples
Nuclear fusion in the Sun's core turns hydrogen into helium.
When a star runs out of fuel for nuclear fusion, its core begins to collapse.
Usage note
Do not confuse fusion (joining nuclei) with fission (splitting nuclei), which is what today's nuclear power plants use.
Common phrases
hydrogen fusion · fusion reaction · nuclear fusion in the core · fuse hydrogen into helium
A large, cool and very bright star in a late stage of its life. It forms when a star like the Sun uses up the hydrogen in its core and its outer layers swell.
Where you use it
Used when describing the life cycle of stars. Popular science often explains that the Sun will become a red giant in about five billion years and may swallow Mercury and Venus. Arcturus and Aldebaran are bright examples.
Examples
In about five billion years the Sun will swell into a red giant.
Aldebaran, the orange eye of Taurus, is a red giant.
Common phrases
become a red giant · red giant phase · red giant star
The small, very dense and hot core left behind when a Sun-like star dies. It is about the size of Earth but has a mass comparable to the Sun's.
Where you use it
Common in explanations of stellar evolution. A white dwarf no longer produces energy by fusion; it slowly cools over billions of years. Sirius B, the faint companion of Sirius, is a famous example.
Examples
Our Sun will end its life as a white dwarf surrounded by a planetary nebula.
A teaspoon of white dwarf material would weigh several tonnes on Earth.
Common phrases
white dwarf star · cooling white dwarf · become a white dwarf
The extremely dense core left after a massive star explodes as a supernova. It packs more mass than the Sun into a ball only about 20 kilometres across.
Where you use it
Used in astrophysics and in news about gravitational waves. Rapidly spinning neutron stars that we see as regular pulses are called pulsars. When two neutron stars collide, they can create heavy elements such as gold.
Examples
In 2017 detectors recorded gravitational waves from two merging neutron stars.
This neutron star spins hundreds of times per second.
Common phrases
neutron star merger · spinning neutron star · neutron star collision
The long, stable stage of a star's life when it fuses hydrogen into helium in its core. On a chart of star brightness against temperature, these stars form a diagonal band.
Where you use it
Used in astronomy courses together with the Hertzsprung–Russell (H–R) diagram. A star is 'on the main sequence' or 'leaves the main sequence'. About 90% of stars, including the Sun, are main-sequence stars.
Examples
The Sun has been on the main sequence for about 4.6 billion years.
Massive stars leave the main sequence much sooner than small ones.
Common phrases
main-sequence star · on the main sequence · leave the main sequence · main-sequence lifetime
A rapidly spinning neutron star that sends out beams of radio waves or other radiation, which we see from Earth as very regular pulses, like the beam of a lighthouse.
Where you use it
Used in radio astronomy and astrophysics. The first pulsar was discovered in 1967 by Jocelyn Bell Burnell. Because their pulses are so regular, pulsars are used as natural clocks, for example in the search for gravitational waves.
Examples
The pulsar in the Crab Nebula spins about 30 times per second.
At first, the regular signal from the pulsar was jokingly labelled 'LGM-1', for 'little green men'.
Common phrases
millisecond pulsar · radio pulsar · pulsar timing · discover a pulsar
The boundary around a black hole beyond which nothing, not even light, can escape. Anything that crosses it can never come back out.
Where you use it
Used in astrophysics and in popular science about black holes. Its size depends on the black hole's mass. The Event Horizon Telescope, a worldwide network of radio dishes, is named after it.
Examples
Once matter crosses the event horizon, it is lost from our view forever.
The event horizon of a black hole with the Sun's mass would have a radius of only about three kilometres.
Common phrases
cross the event horizon · beyond the event horizon · size of the event horizon
An enormous system of stars, gas, dust and dark matter held together by gravity. A large galaxy can contain hundreds of billions of stars.
Where you use it
Astronomers classify galaxies by shape: spiral, elliptical and irregular. They talk about galaxies forming, merging and colliding. With a capital G, 'the Galaxy' sometimes means our own Milky Way.
Examples
The telescope captured light from a galaxy that formed less than a billion years after the Big Bang.
Our galaxy and Andromeda are moving towards each other.
The barred spiral galaxy that contains our solar system and a few hundred billion stars. It also names the pale band of light it forms across a dark night sky.
Where you use it
Always used with 'the'. Photographers and stargazers travel to dark sites to see or shoot the Milky Way, especially its bright core in summer. Astronomers also say 'our galaxy' or 'the Galaxy'.
Examples
The Sun takes about 230 million years to go once around the centre of the Milky Way.
Away from the city lights, the Milky Way stretched across the whole sky.
Usage note
The English name means 'milky road' and the Turkish one 'straw road'; both come from old images of a pale path across the sky.
Common phrases
the Milky Way galaxy · the core of the Milky Way · photograph the Milky Way · see the Milky Way
A flat, rotating galaxy with a bright central bulge and arms that curve outwards like a pinwheel. The arms are rich in young stars, gas and dust.
Where you use it
One of the main classes in the Hubble classification. Many spiral galaxies, including the Milky Way, have a bar of stars across the centre and are called barred spirals. Astronomers describe their arms as tightly or loosely wound.
Examples
Andromeda is a large spiral galaxy, similar in size to our own.
Seen edge-on, a spiral galaxy looks like a thin line with a bulge in the middle.
A galaxy with a smooth, oval or round shape and no spiral arms. It contains mostly old, reddish stars and very little gas for making new ones.
Where you use it
Used when classifying galaxies. The largest elliptical galaxies sit at the centres of galaxy clusters and often grew through mergers of smaller galaxies. M87, home of the first black hole ever imaged, is a giant elliptical.
Examples
Elliptical galaxies form few new stars because they have lost most of their gas.
If the Milky Way and Andromeda merge one day, they will probably form a large elliptical galaxy.
A group of stars born from the same cloud and held together by gravity. Open clusters are loose and young; globular clusters are dense balls of very old stars.
Where you use it
Favourite targets for binoculars and small telescopes. The Pleiades, or Seven Sisters, is a famous open cluster visible to the naked eye. Astronomers use clusters to study how stars of the same age evolve.
Examples
The Pleiades is a young star cluster about 440 light-years away.
Through binoculars the star cluster looked like a handful of diamonds.
Usage note
Do not confuse a star cluster with a galaxy cluster, which is a group of hundreds or thousands of galaxies.
Common phrases
open star cluster · globular star cluster · young star cluster · the Pleiades star cluster
A black hole with the mass of millions or billions of Suns. Almost every large galaxy, including the Milky Way, has one at its centre.
Where you use it
Used in galaxy research and science news. The one at the centre of our galaxy is called Sagittarius A* (said 'A-star') and has about four million times the Sun's mass. When it pulls in gas, a supermassive black hole can power a bright quasar.
Examples
The supermassive black hole at the centre of the Milky Way was imaged in 2022.
Astronomers are surprised by how quickly the first supermassive black holes grew.
Common phrases
central supermassive black hole · the supermassive black hole at the centre · growth of supermassive black holes
The nearest large spiral galaxy to the Milky Way, about 2.5 million light-years away. Under dark skies it can be seen with the naked eye as a faint smudge.
Where you use it
Also known as M31. It is a classic first deep-sky target for stargazers, and science stories often mention that it is moving towards our galaxy and may collide with it in a few billion years.
Examples
On autumn nights in the Northern Hemisphere, the Andromeda Galaxy is high in the sky.
The light we see from the Andromeda Galaxy left it about 2.5 million years ago.
Common phrases
the Andromeda Galaxy (M31) · observe the Andromeda Galaxy · Andromeda and the Milky Way
The extremely bright centre of a distant galaxy, powered by a supermassive black hole pulling in gas. A single quasar can outshine its entire galaxy.
Where you use it
The name is short for 'quasi-stellar object', because quasars first looked like stars in telescopes. Astronomers use them to study the early universe and the gas between galaxies. They belong to the wider class of active galactic nuclei (AGN).
Examples
The light from this quasar left it more than 12 billion years ago.
Quasars were most common when the universe was a few billion years old.
All of space and time and everything in it: matter, energy, planets, stars and galaxies. Its observable part is about 93 billion light-years across.
Where you use it
Usually 'the universe', in lower or upper case. Cosmologists talk about the early universe, the age and expansion of the universe and the observable universe. 'Cosmos' is a more literary synonym.
Examples
The universe is about 13.8 billion years old.
Ordinary matter makes up only about five percent of the universe.
Usage note
Do not confuse 'universe' (evren) with 'space' (uzay). 'Space' is the almost empty region beyond Earth's atmosphere; 'the universe' is everything that exists.
Common phrases
the early universe · the observable universe · the expanding universe · the age of the universe
The scientific model in which the universe began about 13.8 billion years ago in an extremely hot, dense state and has been expanding and cooling ever since.
Where you use it
Usually 'the Big Bang' or 'the Big Bang theory'. Scientists talk about the first seconds after the Big Bang and the evidence for it, such as the cosmic microwave background and the redshift of distant galaxies.
Examples
Hydrogen and helium nuclei formed in the first few minutes after the Big Bang.
The Big Bang theory explains why distant galaxies are moving away from us.
Usage note
It was not an explosion inside space; space itself has been expanding everywhere. The name was first used on a radio programme in 1949 by Fred Hoyle, an astronomer who did not accept the theory.
Common phrases
the Big Bang theory · after the Big Bang · evidence for the Big Bang · the hot Big Bang
An invisible form of matter that neither gives off nor absorbs light but whose gravity holds galaxies and galaxy clusters together. Its nature is still unknown.
Where you use it
A central idea in modern cosmology. Scientists infer dark matter from how fast galaxies rotate and how light bends around galaxy clusters, and they search for its particles with underground detectors. It makes up about 85% of all matter.
Examples
Without dark matter, the outer stars of the galaxy would fly off into space.
No experiment has directly detected a dark matter particle yet.
Usage note
Do not confuse it with dark energy: dark matter holds things together through gravity, while dark energy speeds up the expansion of the universe.
Common phrases
dark matter halo · detect dark matter · dark matter particle · evidence for dark matter
The unknown cause of the accelerating expansion of the universe. It makes up about two thirds of the total energy content of the universe.
Where you use it
A term from cosmology research and science journalism. The accelerating expansion was discovered in 1998 through studies of distant supernovae, work that won the 2011 Nobel Prize in Physics. Large sky surveys now measure how dark energy behaves over time.
Examples
Dark energy is pushing galaxies apart faster and faster.
Nobody yet knows what dark energy really is.
Common phrases
the nature of dark energy · dark energy survey · dark energy and dark matter
The stretching of light from an object that is moving away from us or whose light crosses expanding space, so that its spectral lines shift to longer, redder wavelengths.
Where you use it
A key measurement in cosmology, written as z. Astronomers measure a galaxy's redshift to find how far away it is; 'high-redshift galaxies' are very distant and seen as they were long ago. The opposite is 'blueshift'.
Examples
In the 1920s astronomers found that a galaxy's redshift grows with its distance.
The galaxy has a redshift above 10, so we see it as it was a few hundred million years after the Big Bang.
Common phrases
high redshift · measure the redshift · cosmological redshift · redshift survey
The faint microwave radiation that fills the whole sky, left over from about 380,000 years after the Big Bang. It is often shortened to CMB.
Where you use it
One of the strongest pieces of evidence for the Big Bang. Two radio astronomers discovered it by accident in 1965. Its temperature is about 2.7 kelvin, and tiny ripples in it show the seeds of later galaxies.
Examples
The cosmic microwave background is the oldest light we can observe.
Satellites have mapped tiny temperature differences in the cosmic microwave background.
Usage note
A small part of the 'snow' on old analogue TV screens came from this radiation.
Common phrases
map the cosmic microwave background · CMB radiation · CMB temperature · fluctuations in the CMB
A ripple in space-time caused by massive objects accelerating, such as two black holes spiralling into each other. Einstein predicted them in 1916.
Where you use it
Usually plural. The first direct detection was made in 2015 by the LIGO observatory and won the 2017 Nobel Prize in Physics. Scientists now speak of 'gravitational-wave astronomy' and of 'detections' or 'events'.
Examples
The gravitational waves came from two black holes that merged 1.3 billion years ago.
To catch a gravitational wave, detectors must measure changes far smaller than an atom.
Usage note
Do not confuse them with 'gravity waves', which are waves in air or water studied in meteorology and oceanography.
An instrument that collects light or other radiation from distant objects with lenses, mirrors or antennas, so that faint and far-away things can be seen and studied.
Where you use it
Used by amateurs and professionals alike. You set up, point, align and focus a telescope; astronomers apply for 'telescope time' at observatories. Types include refractors, reflectors, radio telescopes and space telescopes.
Examples
Through a small telescope you can already see Saturn's rings.
We set up the telescope in the garden and waited for the clouds to clear.
Usage note
The stress is on the first syllable: TEL-e-scope. In the adjective 'telescopic' it moves to the third syllable: tel-e-SCOP-ic.
Common phrases
point the telescope at · set up a telescope · telescope time · amateur telescope · large telescope
A building or site with telescopes and instruments for observing the sky. Many large observatories are built on high, dry mountains far from city lights.
Where you use it
Used for both professional research sites and public facilities. Space telescopes are also called observatories, as in 'space observatory'. People visit an observatory or join a public observing night there.
Examples
The observatory sits at 2,500 metres, above most of the clouds and dust.
The local observatory holds public stargazing nights every Friday in summer.
Common phrases
national observatory · space observatory · visit an observatory · observatory dome · mountaintop observatory
A scientist who studies stars, planets, galaxies and other objects in space, usually by observing them and analysing the data.
Where you use it
Used for professionals and, with 'amateur', for hobbyists. Related job titles are 'astrophysicist' and 'planetary scientist'. Science news often begins with 'Astronomers have discovered...'.
Examples
Astronomers have found a new moon orbiting Saturn.
An amateur astronomer in Australia was the first to spot the new comet.
Usage note
Do not confuse it with 'astrologer'. Astronomy is a science; astrology claims that the positions of planets affect people's lives and is not a science.
Common phrases
amateur astronomer · professional astronomer · team of astronomers · radio astronomer
Too much artificial light at night from streetlights, buildings and signs, which brightens the sky and hides faint stars and galaxies.
Where you use it
A big topic for stargazers, astrophotographers and environmental groups. People escape, measure and reduce light pollution and look for 'dark sky' sites. The Bortle scale rates sky darkness from 1 (darkest) to 9 (inner city).
Examples
Because of light pollution, many city children have never seen the Milky Way.
We drove two hours away from the city to escape the light pollution.
Common phrases
reduce light pollution · light pollution map · heavy light pollution · escape light pollution · dark sky site
A telescope placed in orbit or deep space, above Earth's atmosphere, so it gets sharper images and can detect light the atmosphere blocks, such as infrared, ultraviolet and X-rays.
Where you use it
The best-known examples are the Hubble Space Telescope, launched in 1990, and the James Webb Space Telescope, launched in 2021. Astronomers compete for observing time on them, and agencies share the images with the public.
Examples
The James Webb Space Telescope observes mainly in infrared light.
Free from air turbulence, a space telescope produces much sharper images.
Common phrases
Hubble Space Telescope · James Webb Space Telescope · infrared space telescope · space telescope images
A telescope that collects radio waves from space with a large dish or a set of antennas instead of visible light. It can observe through clouds and during the day.
Where you use it
Used in radio astronomy to study cold gas, pulsars, the cosmic microwave background and black holes. Several radio telescopes can be linked into an array that works like one giant instrument.
Examples
A network of radio telescopes around the world produced the first black hole image.
Mobile phones near the site can interfere with the radio telescope.
Common phrases
radio telescope dish · radio telescope array · large radio telescope
The diameter of a telescope's main lens or mirror. A larger aperture collects more light, so fainter objects and finer details can be seen.
Where you use it
The most important specification when choosing a telescope, given in millimetres or inches. Experienced observers like to say 'aperture is king'. Photographers use the same word for the opening of a camera lens.
Examples
For galaxies and nebulae, aperture matters much more than magnification.
On a dark night, a telescope with a 200 mm aperture can show the spiral arms of bright galaxies.
Common phrases
large aperture · an aperture of 200 mm · aperture size · small-aperture telescope
The small lens at the end of a telescope that you look through. Changing the eyepiece changes the magnification and the field of view.
Where you use it
Amateur astronomers own sets of eyepieces with different focal lengths, such as 25 mm and 10 mm. Magnification equals the telescope's focal length divided by the eyepiece's focal length.
Examples
Start with a low-power eyepiece to find the object, then switch to a stronger one.
A 10 mm eyepiece on a telescope with a 1,000 mm focal length gives 100 times magnification.
The study of light by splitting it into its colours, or spectrum. The dark and bright lines in a spectrum reveal an object's chemical make-up, temperature and motion.
Where you use it
One of the most important tools in astronomy. Researchers 'take a spectrum' of a star or galaxy with a spectrograph and identify elements from its spectral lines. Exoplanet scientists use it to study planetary atmospheres.
Examples
Spectroscopy showed that the star is made mostly of hydrogen and helium.
Helium was first discovered through spectroscopy of the Sun, before it was found on Earth.
Common phrases
infrared spectroscopy · spectroscopic observations · spectral lines · take a spectrum
A group of stars that forms a recognisable pattern in the sky, and the area of sky around it. Astronomers officially recognise 88 constellations.
Where you use it
Stargazers use constellations to find their way around the sky: an object is 'in the constellation of' Orion or Leo. Most constellation names are Latin. In the space industry, a 'constellation' is also a group of satellites working together.
Examples
Orion is the easiest constellation to recognise on winter evenings.
The comet is now passing through the constellation of Ursa Major.
Usage note
The Big Dipper (UK: the Plough) is not a constellation but an asterism: a well-known pattern inside the larger constellation Ursa Major.
Common phrases
the constellation of Orion · zodiac constellation · satellite constellation · find a constellation
The distance light travels in one year, about 9.46 trillion kilometres. It is a unit of distance, not of time.
Where you use it
Used in popular science, documentaries and news to give distances to stars and galaxies. Professional astronomers more often use the parsec, about 3.26 light-years. Spelled 'light-year' or 'light year'.
Examples
Proxima Centauri, the nearest star to the Sun, is about 4.2 light-years away.
The Milky Way is roughly 100,000 light-years across.
Usage note
A common mistake is to use it for time. In everyday English, 'light-years ahead' is an idiom meaning 'far more advanced'.
Common phrases
light-years away · billions of light-years · light-years across · measured in light-years
An event in which one body in space blocks the light of another, for example when the Moon passes between the Sun and Earth, or Earth's shadow falls on the Moon.
Where you use it
A solar eclipse happens when the Moon covers the Sun; a lunar eclipse when the Moon moves into Earth's shadow. Eclipses can be total, partial or annular. People watch, chase and photograph eclipses.
Examples
Never look directly at a partial solar eclipse without eclipse glasses.
During the total lunar eclipse, the Moon turned a deep red colour.
Usage note
The stress is on the second syllable: ih-KLIPS. As a verb, 'eclipse' also means to make something seem less important: 'Her success eclipsed his'.
Common phrases
total solar eclipse · lunar eclipse · partial eclipse · annular eclipse · eclipse glasses
A unit of distance equal to the average distance between Earth and the Sun, about 149.6 million kilometres. It is usually written AU or au.
Where you use it
Used for distances inside the solar system and in other planetary systems, where kilometres give huge numbers and light-years tiny ones. Mission updates say, for example, that a probe is 160 AU from the Sun.
Examples
Jupiter orbits the Sun at a distance of about 5.2 astronomical units.
Voyager 1 is now more than 160 astronomical units from the Sun.
Common phrases
one astronomical unit · measured in astronomical units · at a distance of 5 AU
One of the changing shapes of the Moon's lit part as seen from Earth during a cycle of about 29.5 days, from new Moon to full Moon and back.
Where you use it
Usually plural: 'the phases of the Moon'. The main phases are new Moon, first quarter, full Moon and last quarter, with crescent and gibbous shapes in between. 'Waxing' means growing and 'waning' means shrinking.
Examples
Lunar phases are caused by the changing angle between the Sun, Earth and the Moon, not by Earth's shadow.
Plan your deep-sky observing around the lunar phase and choose nights close to the new Moon.
Common phrases
the phases of the Moon · lunar phase calendar · waxing crescent · waning gibbous · first quarter
A period when many meteors seem to come from the same point in the sky, as Earth passes through dust left by a comet or an asteroid. Showers return on the same dates every year.
Where you use it
Showers are named after the constellation they seem to come from: the Perseids in August and the Geminids in December are the best known. News stories give the 'peak' night and the expected number of meteors per hour.
Examples
The Perseid meteor shower peaks around 12 August.
To enjoy the meteor shower, lie back on a blanket and let your eyes adapt to the dark.
Usage note
Turkish news often says 'göktaşı yağmuru', but these meteors are tiny grains that burn up high in the atmosphere; almost none reach the ground.
Common phrases
annual meteor shower · the peak of the meteor shower · watch a meteor shower · Perseid meteor shower
A number that measures how bright a star or other object looks. The scale runs backwards: the lower the number, the brighter the object, and very bright objects have negative values.
Where you use it
Used in star charts, observing guides and news about bright planets or comets. 'Apparent magnitude' is brightness as seen from Earth; 'absolute magnitude' is true brightness. The faintest stars visible to the naked eye are around magnitude 6.
Examples
Sirius, the brightest star in the night sky, has a magnitude of -1.46.
The comet may reach magnitude 4 and become visible without binoculars.
Usage note
A difference of five magnitudes means exactly 100 times more or less brightness. In general English 'magnitude' means size or importance, as in 'an earthquake of magnitude 6'.
One of the two moments each year, around 21 June and 21 December, when the Sun reaches its highest or lowest midday point in the sky, giving the longest or shortest day.
Where you use it
People say 'the summer solstice' and 'the winter solstice'; in the Southern Hemisphere the dates are reversed. To avoid confusion, astronomers prefer 'June solstice' and 'December solstice'.
Examples
The summer solstice brings the longest day of the year in Turkey.
After the winter solstice, the days slowly start to get longer.
Common phrases
summer solstice · winter solstice · June solstice · December solstice
One of the two moments each year, around 20 March and 22 September, when the Sun is directly above the equator and day and night are roughly equal everywhere.
Where you use it
Called the spring (vernal) equinox and the autumn equinox, or the March and September equinoxes. It marks the start of astronomical spring or autumn. The plural is 'equinoxes'.
Examples
At the equinox, the Sun rises due east and sets due west.
The March equinox marks the beginning of spring in the Northern Hemisphere.
Usage note
British English says 'autumn equinox'; American English often says 'fall equinox'.
Common phrases
spring equinox · autumn equinox · vernal equinox · March equinox
The passage of a planet or moon across the face of a larger body, such as Mercury crossing the Sun's disc or an exoplanet passing in front of its star.
Where you use it
Exoplanet hunters use the 'transit method': they watch for small, regular dips in a star's brightness. Transits of Mercury and Venus across the Sun are rare events that observers plan for years in advance.
Examples
The next transit of Venus will not happen until 2117.
The planet was found with the transit method, from a 1 percent dip in the star's light.
Usage note
In everyday English 'transit' is about transport: 'public transit', 'lost in transit'. In astronomy it has this special meaning.
Common phrases
transit method · transit of Mercury · transit of Venus · planetary transit
The curved path that an object such as a planet, moon or satellite follows around another object because of gravity. As a verb, it means to move along this path.
Where you use it
Used as both noun and verb: a satellite is 'in orbit', a rocket 'puts a payload into orbit', and the Moon 'orbits Earth'. Engineers talk about reaching, raising and lowering an orbit.
Examples
The satellite reached orbit about nine minutes after liftoff.
Earth orbits the Sun once every 365.25 days.
Usage note
With the verb, no preposition is needed: say 'orbit Earth', not 'orbit around Earth'. With the noun, 'in orbit around Earth' is correct.
Common phrases
in orbit · reach orbit · orbit the Sun · elliptical orbit · raise the orbit
The force that pulls any two objects with mass towards each other. It keeps the planets in orbit around the Sun and keeps our feet on the ground.
Where you use it
A core word in physics and astronomy. Scientists talk about the gravity of a planet, surface gravity and gravitational pull. The adjective is 'gravitational': gravitational field, gravitational waves.
Examples
On the Moon, gravity is about one sixth of what it is on Earth.
The probe used Jupiter's gravity to speed up on its way to Saturn.
Common phrases
surface gravity · the force of gravity · gravity assist · low gravity · gravitational pull
The time an object takes to complete one full orbit around another object. Earth's orbital period around the Sun is about 365.25 days.
Where you use it
Used for planets, moons, satellites and exoplanets. Astronomers work out an exoplanet's orbital period from regular dips in its star's light. Satellite operators give orbital periods in minutes.
Examples
Mars has an orbital period of about 687 Earth days.
Satellites in low orbit have an orbital period of around 90 minutes.
Common phrases
an orbital period of · short orbital period · calculate the orbital period
The minimum speed an object needs to break free from the gravity of a planet or another body without any further push. For Earth it is about 11.2 km per second.
Where you use it
Used in physics lessons, mission planning and explanations of black holes: a black hole is a place where the escape velocity is greater than the speed of light. Reaching orbit needs less speed than escape velocity.
Examples
To leave Earth for Mars, the probe had to exceed escape velocity.
The Moon's escape velocity is much lower than Earth's, so launching from its surface is easier.
An orbit relatively close to Earth, usually below about 2,000 km. The space station and most Earth observation satellites fly in low Earth orbit.
Where you use it
Usually shortened to LEO, said 'lee-oh' or letter by letter. Engineers compare it with medium Earth orbit (MEO) and geostationary orbit (GEO). Large internet satellite constellations also operate in LEO.
Examples
Satellites in low Earth orbit circle the planet in about an hour and a half.
The rocket can carry about 20 tonnes to low Earth orbit.
Common phrases
in low Earth orbit · payload to low Earth orbit · LEO satellite · LEO constellation
The path that a rocket, spacecraft or other moving object follows through space, especially when it is not a closed orbit.
Where you use it
Used by mission planners and flight dynamics teams. They design, calculate and correct a trajectory with small engine burns called trajectory correction manoeuvres. A probe can be 'on a trajectory to Mars'.
Examples
A short engine burn put the spacecraft back on the right trajectory.
The asteroid's trajectory shows it will miss Earth by a wide margin.
Usage note
Turkish 'yörünge' is used for both 'orbit' and 'trajectory'. In English an orbit goes around a body, while a trajectory is any path.
Common phrases
trajectory correction · flight trajectory · change the trajectory · on a trajectory to
A circular orbit about 35,786 km above the equator in which a satellite takes one day to circle Earth, so it seems to stay fixed over the same spot.
Where you use it
Used for communications, TV broadcasting and weather satellites. Engineers shorten it to GEO and talk about the 'orbital slots' assigned to countries or operators. Satellite dishes on houses point at satellites in this orbit.
Examples
Türksat satellites work in geostationary orbit, so home dishes never need to move.
Reaching geostationary orbit can take days or even months after launch.
Usage note
'Geosynchronous' is a wider term for any orbit with a one-day period; geostationary is the special case directly above the equator.
Common phrases
geostationary orbit (GEO) · geostationary satellite · orbital slot · geostationary transfer orbit
Dead satellites, spent rocket stages and fragments from collisions or explosions that stay in orbit around Earth and can damage working spacecraft.
Where you use it
Also called 'orbital debris' or, informally, 'space junk'. Agencies track debris, warn operators about possible collisions and ask that satellites be removed at the end of their life. The space station sometimes moves to avoid a piece of debris.
Examples
The station fired its engines to dodge a piece of space debris.
Because space debris travels at several kilometres per second, even a fleck of paint can cause damage.
Common phrases
track space debris · debris avoidance manoeuvre · orbital debris · space junk · debris field
Describes a moon or planet that always turns the same face towards the body it orbits, because it spins exactly once for every orbit it completes.
Where you use it
Used about the Moon, many other moons and exoplanets that orbit close to their stars. The noun is 'tidal locking'. Scientists discuss whether a tidally locked planet, with one side always in daylight and the other always dark, could support life.
Examples
The Moon is tidally locked, so we always see the same side from Earth.
Many planets around red dwarf stars are probably tidally locked.
Usage note
The far side of the Moon is not dark all the time; it gets as much sunlight as the near side. 'The dark side of the Moon' is a popular but inaccurate phrase.
Three rules published by Johannes Kepler in the early 1600s: planets move in ellipses, they move faster when closer to the Sun, and orbit size is linked to orbital period.
Where you use it
Taught in physics and astronomy courses and used in everyday orbit calculations. People refer to Kepler's first, second or third law. The third law lets astronomers work out the mass of a star from the orbits of its planets.
Examples
According to Kepler's second law, a comet moves fastest when it is closest to the Sun.
Using Kepler's third law, we can calculate the planet's distance from its orbital period.
Common phrases
Kepler's first law · Kepler's third law · obey Kepler's laws
The point in an orbit around Earth where a satellite or the Moon is farthest from Earth. The closest point is called the perigee.
Where you use it
Used in orbital mechanics and satellite operations; engines are often fired at apogee to raise or circularise an orbit. For orbits around the Sun the matching words are aphelion and perihelion. A full Moon near perigee is popularly called a 'supermoon'.
Examples
The satellite will fire its engine at apogee to circularise its orbit.
At apogee the Moon looks slightly smaller than at perigee.
Usage note
The g is soft, like the j in 'jeep', and the stress is on the first syllable: AP-uh-jee.
Common phrases
apogee and perigee · apogee burn · apogee altitude · raise the apogee
A vehicle or engine that moves forward by pushing hot gas out of the back at high speed. Rockets are used to send satellites, probes and crews into space.
Where you use it
Common verbs: build, launch, fly, fire, land and reuse a rocket. Engineers often prefer 'launch vehicle' in formal documents and say 'rocket engine' or 'rocket motor' for the part that produces thrust.
Examples
The rocket lifted off on time and placed twenty satellites into orbit.
The first stage of the rocket returned and landed on a ship at sea.
Usage note
As a verb in general English, 'rocket' means to rise very quickly: 'Prices rocketed last year'.
An object that orbits a planet or another body. Most often it means a machine sent into orbit for communication, navigation, weather forecasting or observation.
Where you use it
Satellites are launched, deployed, operated and finally deorbited. Common types are communications, weather, navigation (GPS) and Earth observation satellites. Astronomers may call moons 'natural satellites'.
Examples
The new navigation satellite was launched into orbit on Tuesday.
The weather satellite sends new images of the clouds every ten minutes.
Usage note
Turkish 'satelit' is not the usual word for a satellite in space; say 'uydu'. In English, 'satellite' also describes a smaller place linked to a bigger one: 'a satellite office'.
Any vehicle designed to travel or operate in space, with or without a crew, such as a satellite, a probe or a crew capsule.
Where you use it
The standard general term in agency and engineering English. People launch, operate and control a spacecraft and report on 'spacecraft health'. Like 'aircraft', the plural is the same: two spacecraft.
Examples
Engineers lost contact with the spacecraft for a few hours during the solar storm.
Both spacecraft are healthy and on course for Mars.
Usage note
Never add -s: 'three spacecraft', not 'three spacecrafts'.
Common phrases
crewed spacecraft · robotic spacecraft · control the spacecraft · spacecraft health · spacecraft design
The cargo a rocket carries into space for a customer or a mission, such as a satellite, a probe, supplies or a crew capsule, as opposed to the rocket itself.
Where you use it
A key word in the launch business. Engineers integrate, encapsulate and deploy payloads and talk about payload capacity, payload mass and the payload fairing that protects it during launch. On a satellite, 'payload' can also mean its main instruments.
Examples
The payload separated from the upper stage exactly as planned.
The rocket's payload capacity to geostationary transfer orbit is about eight tonnes.
Common phrases
payload capacity · payload fairing · deploy the payload · payload mass · rideshare payload
A rocket designed to carry a payload from Earth's surface into space. The term covers all its stages, engines and boosters together.
Where you use it
The formal term used by agencies, launch companies and engineers; journalists often just say 'rocket'. People speak of heavy-lift, medium-lift and reusable launch vehicles.
Examples
The new launch vehicle failed on its first test flight, but the second one was a success.
Choosing a launch vehicle depends on the mass of the satellite and the target orbit.
The return of a spacecraft from space into Earth's atmosphere, when the air squeezed in front of it heats its outer surface to very high temperatures.
Where you use it
Used for crew capsules, returning rocket stages and old satellites. Controllers mention a short radio blackout during re-entry. American English often writes 'reentry'. Unprotected satellites and debris usually burn up on re-entry.
Examples
During re-entry the capsule's heat shield reaches well over 1,000 degrees Celsius.
The old satellite is expected to burn up completely on re-entry next week.
Common phrases
atmospheric re-entry · survive re-entry · burn up on re-entry · re-entry blackout
A protective layer on a spacecraft that takes the extreme heat of entering an atmosphere, so that the crew or instruments inside stay safe.
Where you use it
Engineers describe ablative heat shields, which slowly burn away and carry heat with them, and reusable tiles. After a mission they inspect the heat shield for erosion and cracks. Probes entering other planets' atmospheres need one too.
Examples
The heat shield protected the capsule as it plunged into the atmosphere at about 28,000 km/h.
The rover's heat shield was released a few minutes before landing on Mars.
A rocket or rocket stage that gives extra thrust in the first minutes of a launch and then separates. Side boosters are attached to the outside of the main rocket.
Where you use it
Launch teams talk about solid rocket boosters, side boosters and booster separation. In the age of reusable rockets, 'booster' often means a first stage that flies back and lands so that it can be used again.
Examples
The two side boosters landed back at the launch site at almost the same moment.
This booster has now flown more than twenty missions.
One section of a rocket with its own engines and propellant. When a stage runs out of fuel, it separates and drops away, making the rest of the rocket lighter.
Where you use it
Rockets are described as two-stage or three-stage. Launch commentary includes 'stage separation', 'second-stage ignition' and 'upper stage'. In general English, 'stage' also means a step or phase.
Examples
We have confirmation of stage separation, and the second-stage engine has ignited.
The upper stage will place the satellite into its final orbit.
The pushing force a rocket or jet engine produces by throwing gas out at high speed. It is measured in newtons or kilonewtons.
Where you use it
Engineers compare engines by thrust and talk about throttling it up or down. A rocket can only lift off if its thrust is greater than its weight. 'Thrust vector control' means steering by tilting the engine.
Examples
At liftoff the nine engines produce more than 7,000 kilonewtons of thrust.
The engines reduced thrust for a few seconds to lower the stress on the rocket.
Common phrases
generate thrust · full thrust · thrust-to-weight ratio · thrust vector control
The fuel and the oxidiser that a rocket burns to produce thrust. Common examples are liquid hydrogen, kerosene, methane and liquid oxygen.
Where you use it
Engineers prefer 'propellant' to 'fuel' because a rocket also carries an oxidiser. Teams load, top off and drain propellant and talk about propellant tanks and margins. Satellites carry propellant for their small thrusters.
Examples
Propellant loading begins about 35 minutes before launch.
The satellite still has enough propellant to stay in service for ten more years.
Usage note
Do not confuse 'propellant' (itici yakıt) with 'propeller' (pervane).
The act of sending a rocket, spacecraft or satellite up from the ground into space. As a verb, to send it up in this way.
Where you use it
Very frequent in news and mission updates: a launch is scheduled, delayed, scrubbed or successful. Rockets are launched 'from' a site and satellites 'into' orbit. The 'launch window' is the period when a launch is possible.
Examples
The launch was scrubbed because of strong winds at the pad.
The agency plans to launch the new weather satellite next spring.
Usage note
Outside the space world, 'launch' also means to start or introduce something new, such as a product or a campaign.
Common phrases
launch window · launch date · successful launch · scrub a launch · launch site
A planned flight or project in space with a specific goal, such as sending a probe to Mars or taking a crew to a space station.
Where you use it
Space agencies plan, launch, extend and end missions. Common phrases include crewed mission, robotic mission, mission patch and mission objectives. A mission can last minutes or decades.
Examples
The mission was extended for another three years because the rover was still working well.
The main goal of the mission is to bring samples back from the asteroid.
A wheeled vehicle that drives across the surface of a planet or moon, either robotic and remote-controlled or driven by astronauts.
Where you use it
Most often heard about Mars rovers such as Curiosity and Perseverance. Teams drive and command a rover, plan its route and report how far it has travelled. The Apollo astronauts drove the Lunar Roving Vehicle on the Moon.
Examples
The rover drilled into the rock and stored the sample in a sealed tube.
Because of the signal delay, engineers cannot drive the Mars rover in real time.
Common phrases
Mars rover · lunar rover · drive a rover · rover team · rover's wheels
A robotic spacecraft without a crew that is sent to study planets, moons, comets or deep space and sends its data back to Earth.
Where you use it
Often shortened to 'probe'. News stories say a probe is launched, arrives at, flies past, orbits or lands on its target. Voyager 1, launched in 1977, is the most distant human-made object.
Examples
The space probe will reach Jupiter's moons after an eight-year journey.
Signals from the space probe now take more than twenty hours to reach Earth.
Common phrases
send a space probe · deep-space probe · the probe arrived · probe data
The team and the room on the ground that monitor a spaceflight, talk to the crew or spacecraft and make decisions during the mission.
Where you use it
Used with or without 'the'. Flight controllers sit at consoles in roles such as flight director. Mission Control in Houston is the best-known example; crews often call it simply 'Houston'.
Examples
Mission control told the crew to close the hatch and wait for new data.
The team in mission control cheered when the rover's first images arrived.
Common phrases
Mission Control Center · contact mission control · flight controllers in mission control
The counting of time backwards to the moment of launch, during which teams check that the rocket, the weather and the launch range are ready.
Where you use it
Times are given as 'T-minus', as in 'T-minus 10 minutes'. A countdown can be held (paused), recycled (taken back) or resumed. Launch commentators read out the final seconds.
Examples
The countdown was held at T-minus two minutes because of a sensor reading.
The weather is 90 percent favourable and the countdown is going smoothly.
Common phrases
final countdown · hold the countdown · resume the countdown · launch countdown
The exact moment a rocket leaves the launch pad and starts to rise. The verb form is 'lift off'.
Where you use it
Heard in every launch broadcast: '...and liftoff!'. Reports say a rocket 'lifted off at 14:32' and 'cleared the tower'. Also spelled 'lift-off', especially in British English.
Examples
Liftoff came exactly on time, at the opening of the launch window.
About eight minutes after liftoff, the second stage shut down its engine.
Usage note
The noun is one word, 'liftoff'; the verb is two words, 'lift off': 'The rocket lifted off'.
Common phrases
minutes after liftoff · liftoff time · clean liftoff · and liftoff!
The platform and surrounding structures from which a rocket is launched, including the tower, fuel lines and flame trench.
Where you use it
Launch teams usually just say 'the pad'. Rockets are rolled out to the pad and stand on the pad before launch. Also written 'launchpad'; a 'launch complex' is the wider site around it.
Examples
The rocket was rolled out to the launch pad two days before the flight.
Engineers inspected the launch pad for damage after the engine test.
Usage note
In general English, 'launch pad' also means a starting point for a career or project: 'The job was a launch pad for her career'.
Common phrases
roll out to the launch pad · on the launch pad · launch pad abort · launch complex
To join two spacecraft together in space, for example when a crew capsule connects to a space station. The noun is 'docking'.
Where you use it
Spacecraft dock with or dock to a station at a docking port. Modern capsules can dock automatically while the crew monitors. The opposite is 'undock'. 'Berthing' is when a robotic arm grabs a vehicle and attaches it.
Examples
The capsule docked with the space station about 28 hours after launch.
Docking was delayed while engineers checked a faulty thruster.
Common phrases
dock with the station · docking port · undock from · automated docking
A manoeuvre in which a spacecraft passes close to a planet, moon or other body to study it or use its gravity, without going into orbit or landing.
Where you use it
Mission teams talk about the closest approach during a flyby and use gravity-assist flybys to change speed and direction. The verb is 'fly by' (two words). British English also writes 'fly-by'.
Examples
During its 2015 flyby, New Horizons took the first close-up images of Pluto.
The probe will use a flyby of Venus to change its speed.
Common phrases
close flyby · gravity-assist flyby · flyby of Jupiter · during the flyby
A spacecraft designed to land on the surface of a planet, moon, asteroid or comet and usually stay in one place to make measurements.
Where you use it
Unlike a rover, a lander does not drive around, although it may carry a rover. Teams talk about the descent, touchdown and landing site, and describe a successful 'soft landing' or a failed 'hard landing'.
Examples
In 2023 India's Chandrayaan-3 lander touched down near the Moon's south pole.
The lander's solar panels must stay clean to keep its instruments running.
Common phrases
lunar lander · Mars lander · lander and rover · soft landing
A person who is trained to travel and work in space on board a spacecraft or a space station.
Where you use it
Space agencies select, train and fly astronauts. Common crew roles include commander, pilot and mission specialist. Russia uses the word 'cosmonaut', and English-language media often call Chinese astronauts 'taikonauts'.
Examples
The astronauts spent six months on the space station running experiments.
She trained as an astronaut for four years before her first flight.
Common phrases
astronaut training · astronaut candidate · become an astronaut · astronaut corps · fellow astronaut
A large spacecraft that stays in orbit for years, where crews live and work for long periods and carry out experiments in microgravity.
Where you use it
The best-known example is the International Space Station (ISS), continuously crewed since 2000; China operates the Tiangong station. People live 'on' or 'aboard' a space station, and visiting vehicles dock with it.
Examples
The International Space Station circles Earth more than fifteen times a day.
The new crew will spend six months aboard the space station.
Common phrases
International Space Station · aboard the space station · dock with the space station · space station crew
A period when an astronaut leaves the spacecraft and works outside in space, wearing a spacesuit. The technical name is extravehicular activity (EVA).
Where you use it
Astronauts go on, conduct or perform a spacewalk, usually to repair or install equipment. Mission updates give its length, for example 'a six-and-a-half-hour spacewalk'. Agencies use 'EVA' in technical communication.
Examples
Alexei Leonov made the first spacewalk in 1965.
Two astronauts will go on a spacewalk tomorrow to replace a failed battery.
Common phrases
go on a spacewalk · conduct a spacewalk · first spacewalk · spacewalk preparations
Travel into or through space by a spacecraft, with or without people on board. 'Human spaceflight' means missions that carry crews.
Where you use it
Common in agency names, books and policy texts: human spaceflight, commercial spaceflight, private spaceflight. Engineers also separate orbital from suborbital spaceflight.
Examples
Yuri Gagarin's flight in 1961 opened the era of human spaceflight.
Long spaceflight affects the bones, muscles and eyesight of crew members.
The condition of near weightlessness that people and objects experience in orbit, where everything falls around Earth together and seems to float.
Where you use it
The term space agencies prefer to 'zero gravity'. Scientists run microgravity experiments in biology, materials and fluids, and doctors study its effects on the body. Short periods of microgravity can also be created on parabolic flights.
Examples
In microgravity, water forms floating spheres instead of pouring.
Months in microgravity weaken astronauts' bones and muscles.
Usage note
At the height of the space station, gravity is about 90% as strong as on the ground. Astronauts float because they are in free fall, not because gravity has disappeared.
Common phrases
microgravity experiment · microgravity environment · in microgravity · microgravity research
A sealed, pressurised suit that protects an astronaut from vacuum, extreme temperatures and radiation and supplies oxygen to breathe.
Where you use it
Also written 'space suit'. Engineers separate launch and entry suits, worn inside the capsule, from spacewalk suits, which are small spacecraft in themselves. Astronauts put on, check and pressurise their suits.
Examples
The spacesuit's helmet has a gold-coated visor to block strong sunlight.
Each new spacesuit is tested in a vacuum chamber before it flies.
Common phrases
pressurised spacesuit · spacesuit helmet · put on a spacesuit · spacewalk suit
The small spacecraft that carries astronauts to and from space. It sits on top of a rocket for launch and returns to Earth under parachutes.
Where you use it
Examples include Soyuz, Crew Dragon, Shenzhou and Orion. Capsules land on the ground or splash down in the sea. Crews board the capsule, and its launch escape system can pull it away from a failing rocket.
Examples
The crew capsule splashed down off the coast of Florida after a 20-day mission.
The three astronauts climbed into the crew capsule two hours before launch.
Common phrases
crewed capsule · board the capsule · capsule splashdown · launch escape system
The systems in a spacecraft or spacesuit that keep people alive by supplying oxygen, removing carbon dioxide and controlling temperature, pressure and water.
Where you use it
Usually 'life support system', or ECLSS (Environmental Control and Life Support System) in technical texts. On the space station it recycles most of the water, including sweat and urine. In medicine, the same phrase means machines that keep a patient alive.
Examples
The life support system on the station recycles about 98 percent of the water.
A long mission to Mars will need life support that works for years without resupply.
Common phrases
life support system · closed-loop life support · life support failure · portable life support
Ten questions from this glossary. Pick the right meaning for each term and see your score.
How to learn astronomy & space vocabulary
Learn the distance units together: the astronomical unit for distances inside the solar system, the light-year for stars and galaxies. Knowing which unit fits which scale helps you read news and papers correctly.
Read real texts from the field: monthly sky guides, mission press kits, space agency blog updates and the abstracts of astronomy papers. They show how the terms work in context.
Watch live launch broadcasts with English captions. After a few launches you will recognise countdown, liftoff, stage separation and nominal without thinking.
Pay attention to capital letters: the Sun, the Moon and Earth are capitalised when they mean our own star, moon and planet, and so are names such as the Milky Way and the Kuiper Belt.
Watch for Latin and Greek plurals and pronunciations: nebula becomes nebulae, supernova becomes supernovae, and words such as eclipse, apogee and Kuiper have stress or sounds that Turkish speakers often get wrong.
Frequently asked questions
What is the difference between astronomy and astrology?
Astronomy is the science that studies objects in space through observation, physics and mathematics. Astrology claims that the positions of the Sun, the Moon and the planets shape people's character and lives; this claim is not supported by scientific evidence. In English the people are called astronomers and astrologers, so check which word you need.
Is a light-year a unit of time?
No. A light-year is a distance: how far light travels in one year, about 9.46 trillion kilometres. When you read that a star is 100 light-years away, it also means you see the star as it was 100 years ago, because its light needed that long to reach us.
What is the difference between a meteoroid, a meteor and a meteorite?
A meteoroid is a small piece of rock or dust travelling through space. When it enters Earth's atmosphere and burns up, the streak of light is a meteor, often called a shooting star. If part of it survives and lands on the ground, that piece is a meteorite. Turkish often uses 'göktaşı' for more than one of these, but English keeps them apart.
Why is Pluto no longer called a planet?
In 2006 the International Astronomical Union defined a planet as a body that orbits the Sun, is round because of its own gravity and has cleared its orbit of other objects. Pluto meets the first two conditions but not the third, because it shares its region with many Kuiper Belt objects. It is now classed as a dwarf planet.
Should I write 'Earth' or 'the Earth', 'the moon' or 'the Moon'?
Both 'Earth' and 'the Earth' are correct; scientific and space agency texts usually write 'Earth' without 'the', like other planet names. For our own star and natural satellite, astronomy texts write 'the Sun' and 'the Moon' with capital letters. In everyday writing, 'the sun' and 'the moon' in lower case are also common.
