Space Glossary

The process where a heat shield slowly burns away during reentry, carrying heat away from the spacecraft and protecting the crew inside.

The gradual gathering of matter by gravity, like when dust and gas slowly clump together to form planets or stars over millions of years.

A measure of how much light a surface reflects. A perfectly white surface has an albedo of 1, while a perfectly black one has an albedo of 0. The Moon's albedo is only about 0.12.

The point in a planet's orbit where it is farthest from the Sun. Earth reaches aphelion in early July, about 152.1 million km from the Sun.

The point in an orbit around Earth where a spacecraft or satellite is farthest from our planet. The opposite of perigee.

A rocky object orbiting the Sun that is smaller than a planet but larger than a meteoroid. Most asteroids live in the asteroid belt between Mars and Jupiter.

The study of life in the universe — where it might exist, how it began, and what forms it could take. Scientists in this field search for signs of life on Mars, Europa, and beyond.

A person trained to travel and work in space. The word comes from Greek and means star sailor. Different countries use different titles: cosmonaut (Russia), taikonaut (China), and vyomanaut (India).

The layer of gases surrounding a planet or moon, held in place by gravity. Earth's atmosphere is mostly nitrogen and oxygen and protects us from harmful radiation.

Beautiful curtains of colored light in the sky near the poles, caused by charged particles from the Sun hitting gases in Earth's atmosphere. The northern version is called Aurora Borealis.

An imaginary line that a planet or moon spins around. Earth's axis is tilted about 23.5 degrees, which is why we have seasons.

The compass direction of a star or object in the sky, measured in degrees from north (0°) clockwise around the horizon. Used to point telescopes and antennas.

The common center of mass around which two or more bodies orbit. The Earth-Moon barycenter is actually inside Earth, about 4,670 km from its center.

A system of two stars that orbit around each other, held together by their shared gravity. More than half of all star systems are binaries.

A region in space where gravity is so incredibly strong that nothing — not even light — can escape. They form when massive stars collapse at the end of their lives.

An extremely bright meteor — basically a fireball — that explodes in the atmosphere. Bolides are brighter than magnitude -4, rivaling Venus in brightness.

A powerful rocket (or set of rockets) that provides extra thrust during launch to help a spacecraft break free of Earth's gravity. Some modern boosters can land and be reused.

A small pressurized spacecraft designed to carry crew or cargo, typically cone or bell-shaped. Examples include SpaceX Dragon, Boeing Starliner, and the classic Apollo command module.

The apparent outward force felt by an object moving in a circle. Space station designs use spinning sections to create artificial gravity through centrifugal force.

The maximum mass a white dwarf star can have — about 1.4 times the mass of our Sun. Above this limit, the star collapses further into a neutron star or black hole.

Surrounding or orbiting a star. A circumstellar disk of dust and gas around a young star is where planets form.

A ball of ice, dust, and rock that orbits the Sun. When a comet gets close to the Sun, it heats up and releases gas and dust, forming a glowing tail that can stretch millions of kilometers.

A group of stars that forms a pattern in the night sky, like Orion the Hunter or the Big Dipper. There are 88 officially recognized constellations.

The deflection of moving objects caused by a rotating reference frame. On Earth, it curves weather patterns. On rotating space stations, it would make walking feel oddly different.

The outermost layer of the Sun's atmosphere — a halo of superheated plasma that is mysteriously much hotter than the Sun's surface. It is visible during a total solar eclipse.

The faint glow of radiation left over from the Big Bang, filling the entire universe. It is the oldest light we can detect, dating back about 13.8 billion years.

The Russian word for a space traveler. Russian and some other former Soviet-bloc astronauts are called cosmonauts.

A bowl-shaped hole on the surface of a planet, moon, or asteroid, usually formed by the impact of a meteorite or by a volcanic explosion.

The science of extremely cold temperatures, used in space to keep rocket fuel (like liquid hydrogen and oxygen) cold enough to stay liquid. Also used to cool sensitive instruments.

An even more mysterious force that makes up about 68% of the universe and is causing the expansion of the universe to speed up. Scientists still do not fully understand what it is.

A mysterious invisible substance that makes up about 27% of the universe. We cannot see it, but we know it exists because its gravity affects how galaxies move and hold together.

Pieces of old satellites, spent rocket stages, and other junk orbiting Earth. Space debris is a growing problem because even a tiny piece can damage a spacecraft at orbital speeds.

Generally refers to regions of space far beyond Earth and the Moon — interplanetary space and beyond. Deep Space Network (DSN) is NASA's system for communicating with far-flung spacecraft.

The change in velocity needed to perform a space maneuver, like entering orbit or landing on the Moon. It is the most important number in mission planning.

A controlled maneuver that lowers a spacecraft's orbit, causing it to reenter Earth's atmosphere. Old satellites are deorbited to prevent them from becoming space debris.

The process of connecting two spacecraft together in orbit. It requires incredibly precise control because both vehicles are moving at thousands of kilometers per hour.

The change in frequency of light or sound as a source moves toward or away from you. Astronomers use it to measure how fast stars and galaxies are moving.

A round object orbiting the Sun that is too small to have cleared its orbital neighborhood of other debris. Pluto, Ceres, and Eris are all dwarf planets.

A hypothetical megastructure that completely surrounds a star to capture most or all of its energy output. Proposed by physicist Freeman Dyson as a sign of an advanced civilization.

When one object in space blocks the light from another. A solar eclipse happens when the Moon passes between the Sun and Earth; a lunar eclipse when Earth's shadow falls on the Moon.

The imaginary plane of Earth's orbit around the Sun. Most planets orbit close to this plane, which is why they appear to travel along a similar path across the sky.

The full range of energy waves, from radio waves to gamma rays, with visible light in the middle. Astronomers study all types to learn about the universe.

An orbit shaped like an oval rather than a perfect circle. Most objects in space, including planets, follow elliptical orbits.

The minimum speed an object needs to break free of a planet's or moon's gravity without any further thrust. For Earth, it is about 11.2 km/s (40,320 km/h).

Extra-Vehicular Activity — the technical name for a spacewalk. Any time an astronaut leaves the safety of the spacecraft and works in open space, it counts as an EVA.

The boundary around a black hole beyond which nothing can return. Once you cross it, even light cannot escape — it is the point of no return.

A planet that orbits a star other than our Sun. Thousands have been discovered, and some sit in the habitable zone where liquid water could exist.

A living organism that thrives in extreme conditions — like boiling water, intense radiation, or crushing pressure — giving scientists clues about where alien life might survive.

The protective nose cone at the top of a rocket that shields the payload during launch and ascent through the atmosphere. It is jettisoned once in space.

A spacecraft maneuver where the probe passes close to a planet or moon without entering orbit or landing, collecting data and photos as it zooms past.

The condition of falling under gravity with no other forces acting on you. Astronauts on the ISS are in free fall around Earth, which is why they float — they are not actually weightless.

The process of smashing lightweight atoms (like hydrogen) together to form heavier ones (like helium), releasing enormous amounts of energy. This is what powers the Sun and all stars.

A massive collection of stars, gas, dust, and dark matter held together by gravity. Our Milky Way contains 100–400 billion stars, and there are trillions of galaxies in the observable universe.

The highest-energy form of light in the electromagnetic spectrum. Gamma-ray bursts from distant galaxies are the most powerful explosions in the universe.

A major disturbance of Earth's magnetosphere caused by a burst of solar wind or a coronal mass ejection hitting Earth's magnetic field. Can disrupt GPS, power grids, and radio communications.

An orbit about 35,786 km above the equator where a satellite moves at exactly the same speed as Earth's rotation, so it appears to hover over one spot. Used for weather and communications satellites.

Ripples in the fabric of space-time caused by massive accelerating objects, like merging black holes. First detected in 2015 by LIGO, confirming Einstein's prediction.

A technique where a spacecraft flies close to a planet and uses the planet's gravity like a slingshot to speed up or change direction — saving fuel on long missions.

The region around a star where conditions are just right for liquid water to exist on a planet's surface — not too hot and not too cold. Sometimes called the Goldilocks zone.

A protective layer on a spacecraft that absorbs or deflects the extreme heat generated during reentry into a planet's atmosphere, keeping the crew and cargo safe.

The boundary where the solar wind is stopped by the interstellar medium — the outer edge of the heliosphere. Voyager 1 crossed the heliopause in August 2012.

The giant bubble of charged particles blown outward by the solar wind, stretching far beyond Pluto. It shields our solar system from some of the harsh radiation of interstellar space.

An efficient orbital maneuver using two engine burns to move a spacecraft between two circular orbits. It is the most fuel-efficient way to travel between planets.

A number that describes how fast the universe is expanding. It tells us that for every megaparsec (about 3.26 million light-years) of distance, galaxies are moving apart about 70 km/s faster.

A type of rocket fuel that ignites instantly when its two components (fuel and oxidizer) touch, without needing a spark. Very reliable, which is why it is used for critical maneuvers.

The tilt of an orbit relative to the equator. An inclination of 0° means the orbit follows the equator; 90° means it passes over the poles.

Electromagnetic radiation with wavelengths longer than visible light but shorter than microwaves. Infrared telescopes like the James Webb Space Telescope can peer through cosmic dust to see hidden stars.

The space between stars. Interstellar space is not truly empty — it contains thin gas, dust, and cosmic rays. Voyager 1 became the first human-made object to enter interstellar space in 2012.

A type of engine that creates thrust by electrically accelerating ions (charged atoms) to very high speeds. It produces gentle but incredibly efficient thrust, perfect for long deep-space missions.

The International Space Station — a football-field-sized laboratory orbiting Earth at about 400 km altitude. Built by 15 nations, it has been continuously occupied since November 2000.

To deliberately release or throw away a part of a spacecraft that is no longer needed, like an empty fuel tank or a protective fairing, to reduce weight.

Relating to Jupiter or planets similar to Jupiter — large gas giants with thick atmospheres, no solid surface, and many moons. Saturn, Uranus, and Neptune are also Jovian worlds.

A way to measure a civilization's advancement based on how much energy it can harness — from all the energy on its planet (Type I) to its star (Type II) to its entire galaxy (Type III).

A vast ring of icy objects beyond Neptune's orbit, stretching from about 30 to 55 AU from the Sun. Home to Pluto, Eris, and countless smaller icy worlds.

One of five special spots in space where the gravitational pull of two large bodies (like Earth and the Sun) balances perfectly with orbital motion. The James Webb Space Telescope sits at L2.

A spacecraft designed to touch down on the surface of a planet, moon, or asteroid. Unlike rovers, landers stay in one place after landing.

The specific period of time during which a rocket must launch to reach its intended orbit or destination. Miss it, and you may wait days, weeks, or even years for the next one.

The distance light travels in one year — about 9.46 trillion kilometers (5.88 trillion miles). It is a unit of distance, not time, used to measure the vast distances between stars.

The region of space from about 160 to 2,000 km above Earth's surface. The ISS, most satellites, and crewed missions operate here. Objects in LEO circle Earth roughly every 90 minutes.

Anything related to the Moon. Lunar missions include the Apollo landings, the Artemis program, and robotic explorers like India's Chandrayaan series.

A type of neutron star with an extraordinarily powerful magnetic field — a trillion times stronger than Earth's. Magnetars can produce intense bursts of X-rays and gamma rays.

The region around a planet controlled by its magnetic field, which deflects charged particles from the solar wind. Earth's magnetosphere is essential for protecting life.

The stage of a star's life when it is steadily fusing hydrogen into helium in its core. Our Sun has been on the main sequence for about 4.6 billion years and will stay there for another 5 billion.

A scientific instrument that identifies what chemicals are present in a sample by measuring the mass of individual molecules. Rovers and orbiters carry them to analyze alien soils and atmospheres.

The streak of light you see when a small piece of space rock burns up in Earth's atmosphere — commonly called a shooting star.

A piece of space rock that survives its fiery trip through the atmosphere and lands on Earth's surface. Some meteorites are older than Earth itself.

A small rocky or metallic body traveling through space, ranging from a grain of sand to about a meter across. When it enters an atmosphere, it becomes a meteor.

The condition of apparent near-weightlessness experienced in orbit. It is not zero gravity — gravity is still present — but everything is falling together at the same rate.

The ground-based team and facility that monitors, commands, and supports spacecraft during missions. NASA's Mission Control Center is in Houston, Texas.

A rocket built in sections (stages) that are discarded when their fuel runs out, making the remaining rocket lighter and more efficient. Nearly all launch vehicles use this design.

The point directly below an observer, opposite the zenith. In satellite imaging, nadir refers to the point on Earth's surface directly beneath the satellite.

A giant cloud of gas and dust in space. Some nebulae are the birthplaces of new stars, while others are the remains of stars that have died in spectacular explosions.

The incredibly dense collapsed core left behind after a massive star explodes as a supernova. A teaspoon of neutron-star material would weigh about a billion tons.

A sudden brightening of a star caused by a thermonuclear explosion on its surface, usually when it pulls material from a companion star. Unlike a supernova, the star survives.

The bell-shaped end of a rocket engine that directs and accelerates exhaust gases, converting heat energy into thrust. Its shape is precisely engineered for maximum efficiency.

A rocket engine concept that uses a nuclear reactor to heat hydrogen gas, producing thrust. It could cut Mars travel time roughly in half compared to chemical rockets.

When one celestial object passes in front of and hides another — like the Moon blocking a distant star. Scientists use occultations to study atmospheres and measure sizes of distant objects.

A hypothetical sphere of icy objects believed to surround our solar system at a distance of up to 100,000 AU. It is thought to be the source of long-period comets.

The curved path an object takes around another object due to gravity. The Moon orbits Earth, Earth orbits the Sun, and the Sun orbits the center of the Milky Way.

The science of how objects move under gravity in space. Understanding it is key to planning every launch, orbit change, and landing in spaceflight.

A chemical (like liquid oxygen) that provides the oxygen needed for rocket fuel to burn in the vacuum of space, where there is no atmospheric oxygen.

The apparent shift in position of a nearby star when viewed from different points in Earth's orbit. Astronomers use parallax to measure distances to nearby stars.

A unit of distance equal to about 3.26 light-years (31 trillion km). The name comes from parallax-second, a method of measuring star distances by observing their apparent shift.

Everything a rocket carries into space that is not part of the rocket itself — satellites, crew capsules, scientific instruments, or supplies.

The lighter, outer part of a shadow where the light source is only partially blocked. During a solar eclipse, observers in the penumbra see a partial eclipse.

The point in an orbit around Earth where a spacecraft or satellite is closest to our planet. The opposite of apogee.

The point in an orbit around the Sun where a planet or object is closest to the Sun. Earth reaches perihelion in early January each year.

A small gravitational disturbance to an orbit caused by nearby objects. Tracking perturbations is how astronomers have discovered unseen planets and measured asteroid masses.

The visible surface of the Sun — the layer that emits the light we see. Its temperature is about 5,500°C, and it features dark sunspots and bright faculae.

The fourth state of matter — an extremely hot gas where atoms are stripped of their electrons. Plasma makes up about 99% of the visible universe, including stars and the solar wind.

An uncrewed spacecraft sent to explore planets, moons, asteroids, or comets. Famous probes include Voyager, Cassini, and New Horizons.

The combination of fuel and oxidizer that a rocket burns to produce thrust. Common propellants include liquid hydrogen/oxygen, kerosene/oxygen, and solid fuels.

A rotating disk of dense gas and dust surrounding a newly formed star, from which planets may eventually form through accretion.

A rapidly spinning neutron star that emits beams of radiation from its poles, like a cosmic lighthouse. Some pulsars spin hundreds of times per second.

In space science, the practice of isolating astronauts or samples returning from other worlds to prevent possible contamination. Apollo crews were quarantined after returning from the Moon.

An extremely bright and distant object powered by a supermassive black hole actively devouring matter. Quasars can outshine entire galaxies and are among the most luminous objects in the universe.

Energy traveling through space as waves or particles. In space, radiation from the Sun and cosmic rays is a major hazard to astronauts and electronics.

Zones of energetic charged particles trapped by a planet's magnetic field. Earth's radiation belts (the Van Allen belts) can be hazardous to astronauts and electronics.

A telescope that detects radio waves from space instead of visible light. The largest, FAST in China, has a dish 500 meters across and can detect incredibly faint signals.

The stretching of light to longer (redder) wavelengths as a source moves away from an observer. The farther away a galaxy is, the greater its redshift, indicating the universe is expanding.

The process of a spacecraft returning to Earth's atmosphere from space. Friction with the air generates extreme heat — up to 1,650°C — requiring a heat shield for protection.

The layer of loose, broken rock and dust covering the surface of the Moon, Mars, and other bodies. The Moon's regolith is several meters deep and was formed by billions of years of impacts.

The precise meeting of two spacecraft in orbit. It requires careful matching of speed, altitude, and timing before docking can occur.

Moving in the opposite direction to the normal flow. A retrograde orbit goes opposite to a planet's rotation. A retrograde burn slows a spacecraft down.

The Tsiolkovsky rocket equation — the fundamental formula showing that a rocket's speed depends on its exhaust velocity and how much of its mass is fuel. It is the tyranny of rocketry.

A robotic vehicle designed to move across the surface of another world, collecting data and images. Famous rovers include Curiosity and Perseverance on Mars and Yutu on the Moon.

Any object that orbits a larger one. The Moon is a natural satellite of Earth. Artificial satellites are human-made objects placed in orbit for communication, weather, GPS, and science.

Another name for a gravity assist maneuver, where a spacecraft gains speed by swinging close to a planet and using its gravitational pull to accelerate.

A sudden, intense burst of energy and light from the Sun's surface, caused by magnetic field lines tangling and snapping. Strong flares can disrupt communications and power grids on Earth.

A continuous stream of charged particles (mostly protons and electrons) flowing outward from the Sun at 400–800 km/s. It shapes comet tails, causes auroras, and fills the heliosphere.

A large spacecraft in orbit where crew members live and work for weeks or months. The ISS is the current primary station; China operates Tiangong, and several commercial stations are in development.

Conditions in space driven by the Sun — solar wind speed, radiation levels, and geomagnetic storms. Monitoring space weather is critical for protecting satellites, astronauts, and power grids.

When an astronaut goes outside the spacecraft in a spacesuit to perform repairs, install equipment, or conduct experiments. Spacewalks typically last 6–7 hours.

An instrument that splits light into its component colors (spectrum), revealing the chemical composition, temperature, and motion of distant objects.

The technique of splitting light into its spectrum to identify the chemical composition, temperature, density, and motion of objects. It is one of astronomy's most powerful tools.

In rocketry, the point at which a used rocket stage separates and falls away. Staging makes rockets more efficient because they shed dead weight as fuel is consumed.

A flight that reaches space (above 100 km altitude) but does not achieve the speed needed to stay in orbit. The spacecraft goes up and comes right back down.

A dark, cooler area on the Sun's surface caused by intense magnetic activity. Sunspot numbers rise and fall in an 11-year cycle that affects space weather.

The gigantic explosion of a massive star at the end of its life, briefly outshining an entire galaxy. It scatters heavy elements into space and can leave behind a neutron star or black hole.

The system of transmitting measurements and data from a spacecraft back to mission control on Earth. It tells engineers everything from engine temperature to an astronaut's heart rate.

The hypothetical process of transforming another planet's environment to make it more like Earth — breathable air, liquid water, and comfortable temperatures. Mars is the most-discussed candidate.

The layer of Earth's atmosphere from about 80 to 700 km altitude. Despite extremely thin air, temperatures can reach 2,500°C. The ISS orbits within the thermosphere.

The force that pushes a rocket forward, generated by expelling exhaust gases at high speed in the opposite direction. It follows Newton's third law of motion.

A small rocket engine used for fine adjustments in a spacecraft's position and orientation, rather than main propulsion. Thrusters enable precise attitude control and docking maneuvers.

When a moon or planet rotates at the same rate it orbits, so the same face always points toward its partner. The Moon is tidally locked to Earth — we always see the same side.

The path a spacecraft follows through space, shaped by gravity, thrust, and atmospheric drag. Mission planners calculate trajectories to minimize fuel use and travel time.

A temporary orbit used to move a spacecraft from one orbit to another. A geostationary transfer orbit, for example, carries satellites from LEO up to geostationary altitude.

When a smaller body passes in front of a larger one as seen from the observer. Astronomers detect exoplanets by measuring the tiny dip in starlight when a planet transits its star.

A type of electromagnetic radiation with wavelengths shorter than visible light but longer than X-rays. The Sun emits UV light, which Earth's ozone layer mostly blocks.

The darkest part of a shadow, where the light source is completely blocked. During a solar eclipse, you can only see the total eclipse if you stand within the Moon's umbra on Earth.

Everything that exists — all matter, energy, space, and time. The observable universe is about 93 billion light-years across and contains roughly 2 trillion galaxies.

A region of space with extremely low pressure, containing very few or no particles. Outer space is a near-perfect vacuum, which is why sound cannot travel there.

Two donut-shaped zones of intense radiation surrounding Earth, trapped by our magnetic field. Spacecraft must be shielded to pass safely through them.

A star whose brightness changes over time, either because of internal processes (pulsation) or external factors like an orbiting companion blocking its light.

The speed of an object in a specific direction. In space, knowing both how fast something is going and which way it is heading is critical for navigation.

A rare event where Venus passes directly between Earth and the Sun, appearing as a small dark dot crossing the solar disk. The last one occurred in 2012; the next will be in 2117.

A substance that easily evaporates at low temperatures. In planetary science, volatiles like water ice, carbon dioxide, and methane are key to understanding a world's history and potential for life.

A theoretical faster-than-light propulsion concept from physics (and science fiction) that would compress space in front of a ship and expand it behind, moving the ship without exceeding light speed locally.

The sensation of having no weight, experienced during free fall in orbit. Astronauts are not truly weightless — they and their spacecraft are both falling together around Earth.

The hot, dense remnant of a low-to-medium mass star after it has exhausted its nuclear fuel. About the size of Earth but with a mass similar to the Sun. Our Sun will become one in about 5 billion years.

The study of celestial objects that emit X-rays, like black holes, neutron stars, and hot gas in galaxy clusters. X-ray telescopes must orbit above Earth's atmosphere, which blocks X-rays.

Rotation of a spacecraft around its vertical axis — turning left or right, like a car steering. Along with pitch and roll, yaw is one of the three axes of rotation used for attitude control.

The point in the sky directly above an observer — straight up. The opposite of nadir, which is directly below your feet.

A popular (but slightly inaccurate) term for the weightless condition in space. In reality, gravity is still present — objects in orbit are in continuous free fall, creating the sensation of weightlessness.