Space Science Glossary

#A

• Astrometry: The measurement of the positions and motions of stars and other celestial objects.
• Aurora: A natural light display in the Earth’s upper atmosphere caused by the interaction of the solar wind with the Earth’s magnetic field.
• Asteroid: A small, rocky body that orbits the Sun.
• Astrophysics: The branch of astronomy deals with the physical properties and behavior of objects in space, such as stars, galaxies, and black holes.
• Absolute Magnitude: A measure of the intrinsic brightness of a celestial object, independent of its distance from Earth.
• Accretion: The process by which small objects, such as dust and gas, come together to form larger objects, such as planets and stars.
• Adaptive Optics: A technique used to improve the resolution of telescopes by compensating for the blurring effects of the Earth’s atmosphere.
• Aerobraking: A technique used by spacecraft to slow down by using the drag of a planet’s atmosphere to slow down and change its orbit.
• Altitude: The distance of an object above a reference point, such as the Earth’s surface.
• Anti-satellite weapon: A weapon system designed to intercept and destroy satellites in orbit.
• Artificial Intelligence: The simulation of human intelligence in machines that are programmed to think and learn like humans
• Astrodynamics: The study of the motion of spacecraft and other objects in space, including the effects of gravity, propulsion, and guidance.
• Astronaut: A person who has been trained to operate and perform tasks in space.
• Astronomical Unit (AU): A unit of length equal to the average distance between the Earth and the Sun, about 93 million miles (150 million kilometers).
• Astrometric Binaries: A binary system in which the components are too close together to be resolved with telescopes, but one component’s gravitational pull is noticeable on the other.
• Astrophotography: The practice of taking photographs of celestial objects and phenomena.
• Attitude Determination: The process of determining the orientation of a spacecraft in space.
• Atmosphere: The gaseous envelope that surrounds a planet or other celestial object.
• Atomic clock: A clock that uses the vibrations of atoms as a time-keeping mechanism.
• Atmospheric Entry: The process of a spacecraft or other object entering a planet’s atmosphere.
• Astrometric Data: Data collected by astrometry, including the positions and motions of stars and other celestial objects.
• Astrometric Surveys: Surveys that measure the positions and motions of stars and other celestial objects.
• Astrometric Parallax: The change in a star’s apparent position as seen from two different locations.
• Astrometric Binary: A binary system in which the components are too close together to be resolved with telescopes, but one component’s gravitational pull is noticeable on the other.
• Astrobiology: The study of life in the universe, including the search for extraterrestrial life and the study of the conditions that might support life.
• Astrolabe: An ancient instrument used for measuring the positions of stars and other celestial objects.
• Astrology: The study of the supposed influence of the stars and planets on human affairs and natural phenomena.
• Astrophysical Jets: Jets of material emitted from the poles of celestial objects, such as black holes and young stars.
• Astrophysical Plasma: A gas that is so hot that the electrons have been stripped from the atoms, creating a plasma.
• Astrophysical Fluid Dynamics: The study of the behavior of fluids in space, such as the behavior of gases in stars and galaxies.
• Astrophysical Spectroscopy: The study of the light emitted by celestial objects, used to determine their composition, temperature, and other properties.
• Astrophysical Object: Any object in space that can be studied using astrophysics, such as stars, galaxies, and black holes.
• Astrophysical Simulations: Computer simulations of the behavior of celestial objects and phenomena.
• Astroparticle Physics: The study of subatomic particles and radiation in space, such as cosmic rays and gamma rays.
• Astro technology: The technology used for space exploration and research, such as satellites, telescopes, and spacecraft.
• Astronavigation: The process of determining the position and motion of a spacecraft in space.
• Astrogeology: The study of the geology of other celestial bodies, such as the Moon and Mars.
• Astromaterials: Materials found in space, such as meteorites and samples collected by spacecraft.
• Astrometry: The measurement of the positions and motions of stars and other celestial objects.
• Astrophysical Cosmology: The study of the universe’s origin, structure, and evolution.
• Astrophysical Data Analysis: The process of analyzing data collected by telescopes and other instruments to study celestial objects and phenomena.
• Astrophysical Numerical Simulations: Computer simulations of the behavior of celestial objects and phenomena using numerical methods.
• Astrophysical Observation: Observing celestial objects and phenomena using telescopes and other instruments.
• Astrophysical Processes: The physical and chemical processes that occur in celestial objects and phenomena, such as nuclear fusion and star formation.
• Astrophysics: The branch of astronomy deals with the physical properties and behavior of objects in space, such as stars, galaxies, and black holes.
• Astroparticle Physics: The study of subatomic particles and radiation in space, such as cosmic rays and gamma rays.
• Astroparticle detectors: Specialized instruments designed to detect subatomic particles and radiation in space.
• Astroparticle physics experiments: Experiments that investigate the properties of subatomic particles and radiation

#B

• Black Hole: An object with such strong gravity that nothing can escape from it, not even light.
• Binary Star: Two stars that orbit each other closely.
• Big Bang Theory: The leading explanation for the origins of the universe, which states that the universe began as a hot, dense, and infinitely small point, and has been expanding and cooling ever since.
• Blackbody Radiation: A theoretical concept that describes the spectrum of light emitted by a perfect absorber and emitter of radiation.
• Baryon: A type of subatomic particle that includes protons and neutrons.
• Blue Shift: A phenomenon where the light from an object moving towards us appears bluer than it would if the object were at rest.
• Binary Planet: A pair of planets that orbit each other closely.
• Brown Dwarf: A celestial object with a mass between that of a large planet and a small star.
• Boltzmann’s Constant: A physical constant that relates the average kinetic energy of particles in a gas to the temperature of the gas.

#C

• Cosmic Background Radiation: The faint afterglow of the Big Bang, detected as faint microwave radiation throughout the universe.
• Cosmic Microwave Background: The afterglow of the Big Bang is a faint glow of light that fills the universe.
• Cosmic Rays: High-energy particles that travel through space and can be detected on Earth.
• Cosmic Expansion: The increase in the distance between galaxies over time caused by the expansion of space itself.
• Cosmic Distance Ladder: A series of methods used to measure distances to objects in the universe, starting with nearby objects and working out to more distant ones.
• Cosmic Background Explorer (COBE): A satellite launched in 1989 to study cosmic microwave background radiation.
• Cosmic Inflation: A theory that explains the uniformity and flatness of the universe by postulating that the universe underwent a brief period of exponential expansion in its earliest moments.
• Cosmic String: A theoretical one-dimensional object that could form due to a symmetry-breaking phase transition in the early universe.
• Cosmic Horizon: The farthest distance from which light has had time to reach us, beyond which we cannot see.
• Cosmic Ray Shower: A cascade of secondary particles produced by interacting a high-energy cosmic ray with the Earth’s atmosphere.
• Cosmic Time: The elapsed time since the Big Bang, measured in billions of years.
• Calibration: The process of adjusting a measurement instrument to meet a known standard.
• Celestial sphere: An imaginary sphere with the Earth at its center, used to describe the positions of objects in the sky.
• Chandra X-ray Observatory: A satellite launched in 1999 to study X-ray emission from high-energy sources such as black holes and supernovae.
• Closed universe: A model of the universe in which the total density is greater than the critical density, resulting in a finite, closed space.
• Cluster of galaxies: A group of galaxies that are gravitationally bound together.
• Comet: A small, icy body that orbits the Sun and has a characteristic tail.
• Cosmic rays: High-energy particles that travel through space and can be detected on Earth.
• Cosmic background radiation: The afterglow of the Big Bang, a faint glow of light that fills the universe.
• Cosmic evolution: The study of how the universe has changed over time.
• Cosmic microwave background: The afterglow of the Big Bang is a faint glow of light that fills the universe.
• Cosmic ray shower: A cascade of secondary particles produced by the interaction of a high-energy cosmic ray with the Earth’s atmosphere.

#E

• Eclipse: the blocking or obscuring of one celestial body by another, such as a solar eclipse, where the Moon blocks the Sun’s light from reaching the Earth, or a lunar eclipse, where the Earth blocks the Sun’s light from reaching the Moon.
• Extraterrestrial: coming from or located outside of the Earth
• Exoplanet: a planet that orbits a star outside of our solar system
• Escape velocity: the minimum velocity needed for an object to escape the gravitational pull of a celestial body, such as the Earth’s escape velocity of 11.186 km/s
• Electric propulsion: a method of propelling a spacecraft by accelerating ions using electric fields rather than chemical reactions as in traditional propulsion systems.
• Event horizon: the boundary around a black hole beyond which no matter or radiation can escape its immense gravitational pull

#F

• Fermi Paradox: the apparent contradiction between the high probability of the existence of extraterrestrial civilizations and the lack of evidence for or contact with such civilizations.

#G

• Galaxy: a large collection of stars, gas, dust, and dark matter held together by gravity.
• Gamma-ray: a form of electromagnetic radiation with the highest energy and shortest wavelength, often emitted by the most energetic phenomena in the universe, such as supernovae and black holes.
• Geostationary orbit: an orbit that matches the Earth’s rotation, allowing a satellite to remain in the same location relative to the Earth’s surface.
• Gravitational lensing: the bending of light by the gravity of a massive object, such as a galaxy or cluster of galaxies, which can be used to study distant objects and phenomena such as dark matter and dark energy.

#H

• Habitable zone: the region around a star where conditions are suitable for liquid water to exist and potentially support life as we know it.
• Heliosphere: the region of space around the Sun that is affected by the solar wind and the solar magnetic field, extending far beyond the orbit of Pluto.
• Hydrogen atom: the simplest atom, consisting of one proton and one electron. Hydrogen is the most abundant element in the universe, making up about 75% of its total elemental mass.
• Hyperbolic orbit: an orbit where the spacecraft’s speed is greater than the escape velocity of the celestial body it is orbiting.

#I

• Inertia: the property of an object to remain at rest or in uniform motion in a straight line unless acted upon by an external force.
• Interstellar: located or occurring between stars
• Ion: an atom or molecule that has lost or gained one or more electrons, giving it a net electrical charge.

#K

• Kepler Space Telescope: NASA’s space telescope that launched in 2009 has made many groundbreaking discoveries, such as the detection of exoplanets, the study of dark matter and dark energy, and the observation of distant galaxies and star formation.

#L

• Light-year: the distance light travels in one year, about 9.461 x 10^12 kilometers or 5.878 x 10^12 miles.
• Luminosity: the total amount of energy emitted by a star or other celestial object per unit of time.

#M

• Magenta: the color of a planet that is close to a red star, which causes the planet to heat up and the sky to turn magenta
• Meteor: a small rock or metal that burns up as it enters the Earth’s atmosphere, creating a streak of light in the sky known as a shooting star or meteor.
• Meteoroid: a small piece of rock or metal in space, ranging in size from a grain of sand to a boulder.
• Milky Way: the galaxy that contains our solar system, a barred spiral galaxy visible as a band of light in the night sky
• Magnitude: A measure of the brightness of a celestial object, typically used in astronomy.
• Massive Compact Halo Object (MACHO): A hypothetical type of dark matter composed of brown dwarfs, black holes, or other massive objects that do not emit enough radiation to be detected easily.
• Meteor: A small rock or metal that burns up as it enters the Earth’s atmosphere, creating a bright streak of light in the sky.
• Meteorite: A meteor that survives its passage through the Earth’s atmosphere and lands on the surface of the Earth.
• Milky Way: The galaxy in which our solar system is located is named for the hazy band of light stretching across the night sky.
• Moon: A natural satellite that orbits a planet or other celestial body.
• Milky Way: the galaxy where our solar system is located
• Microgravity: The condition in which the gravitational force is so small that it can be ignored for certain experiments or calculations.
• Magnetic field: A region of space where a magnetic force can be detected, typically generated by the movement of electrically charged particles.
• Main-belt Asteroids: A group of asteroids that orbit the Sun between Mars and Jupiter.
• Magnetosphere: The region surrounding a planet or other celestial body in which charged particles are affected by the body’s magnetic field
• Mass: The measure of the amount of matter in an object.
• Meson: A type of subatomic particle made up of a quark and an antiquark.
• Microgravity: The condition of weak gravitational force, such as that experienced by objects in orbit around a planet or other celestial body. 
• Milky Way: The galaxy in which our solar system is located. -Moon: A natural satellite that orbits a planet. 
• M-theory: A theory in physics that attempts to unify the five different versions of superstring theory. 
• Multiverse: The hypothetical set of multiple possible universes, including our own, comprise all of reality. 
• Myopia: A condition in which objects that far away appear blurry, caused by the eye’s lens not focusing light correctly.

#N

• NASA: National Aeronautics and Space Administration. It is an independent agency of the U.S. federal government responsible for the civilian space program and aeronautics and space research.
• Near-Earth Object (NEO): A celestial object whose orbit brings it close to the Earth’s orbit. This includes asteroids and comets.
• Neutron Star: A type of star formed from a massive star’s collapsed core. They are extremely dense and have extremely strong magnetic fields.
• Nova: A type of star that suddenly increases its brightness by a factor of 10 to 100 before gradually returning to its original brightness over a period of weeks to months.
• Nuclear Rocket: A rocket that uses nuclear reactions to heat a fluid, which is then expelled from a nozzle to produce thrust.
• Null Point: In space physics, it is a point in space where the magnetic field strength is zero.
• Navigation: The process of determining the position and direction of a spacecraft or other moving object.
• Near-Infrared: A region of the electromagnetic spectrum with wavelengths slightly longer than visible light, but shorter than microwaves.
• New Horizons: A NASA spacecraft that was launched in 2006 to study Pluto and other objects in the Kuiper Belt.
• Nitrogen: A colorless, odorless, and tasteless gas that makes up about 78% of the Earth’s atmosphere. It is used as a pressurizing agent in some spacecraft propulsion systems.
• Non-Return-To-Launch-Site Abort: An abort mode for a spacecraft in which the vehicle would land downrange from its launch site in the event of a launch emergency.
• Northrop Grumman: A major American aerospace and defense company that designs, develops, and manufactures a wide range of products and systems for both military and civilian applications, including space technology.

#O

• Oberth Effect: A principle in celestial mechanics that states that the energy required to accelerate a spacecraft is less the faster it is moving. This effect is named after the German scientist Hermann Oberth.
• Orbital Debris: Man-made debris in Earth orbit, such as defunct satellites, rocket stages, and fragments from collisions.
• Orion (Spacecraft): A NASA spacecraft currently being developed as a part of the Artemis program. It is designed to take astronauts to the Moon and other deep-space destinations.
• Orbits: The path that an object in space takes around another object due to the gravitational pull of the latter.
• Ozone: A molecule of three oxygen atoms in the Earth’s stratosphere. It acts as a shield against harmful ultraviolet radiation from the Sun.
• On-Orbit Servicing: The process of maintaining, repairing, or upgrading a satellite while it is still in orbit.
• Oppositions: The point in a planet’s orbit where it is closest to the Earth and appears brightest in the sky. It occurs when the planet is opposite the Sun in the sky as viewed from Earth.
• Optics: The branch of physics deals with light’s properties and how it interacts with matter, including the design and construction of telescopes and other optical instruments used in space exploration.
• Outgassing: The release of gases trapped within materials used in space hardware, such as spacecraft or rocket stages.
• Overlay: A method of superimposing one image or map onto another used to compare or combine data from different sources.
• OV (Orbital Vehicle): A spacecraft designed to operate in orbit around a planet or other celestial body.

#P

• Payload: The portion of a spacecraft or launch vehicle carrying the equipment or instruments intended to accomplish the mission objectives.
• Pegasus Rocket: A small orbital launch vehicle developed by Orbital Sciences Corporation (now Northrop Grumman Innovation Systems) that is air-launched from a carrier aircraft.
• Perigee: The point in an orbit where a spacecraft or other object is closest to the body it is orbiting.
• Photon: A unit of light energy.
• Planetary Science: The study of planets, moons, and other objects in our solar system and in other star systems.
• Propellant: A substance used to create thrust in a rocket or other propulsion system.
• P-POD (Poly-Picosatellite Orbital Deployer): A small satellite deployment system that is used to deploy CubeSats from the International Space Station and other spacecraft.
• PSLV (Polar Satellite Launch Vehicle): An Indian rocket developed by the Indian Space Research Organization (ISRO) that is used to launch satellites into polar orbits.
• Pulsejet: A type of jet propulsion in which fuel is rapidly ignited and burned in pulses to produce thrust.
• Planetary Protection: The practice of preventing the contamination of other planets and moons by terrestrial organisms and vice versa during space exploration missions.
• Planetary Transit: The passage of a planet in front of a star as seen from a particular viewpoint, such as from Earth.
• Polar orbit: An orbit around a planet or other celestial body in which the satellite passes over the poles of the body.
• Propulsion: The technology and methods used to accelerate or decelerate a spacecraft or other object in space.
• PSR (Precision Solar Array): A type of solar array that uses precise pointing to maximize the amount of sunlight it receives.
• PTC (Passive Thermal Control): The use of passive means, such as insulation and heat pipes, to regulate the temperature of a spacecraft or its instruments.
• P-POD (Poly-Picosatellite Orbital Deployer): A small satellite deployment system that is used to deploy CubeSats from the International Space Station and other spacecraft
• Pyramid (a solar sail): A type of solar sail in which the sail is shaped like a pyramid to increase its stability and controllability
• P-band: A radio frequency band used for radar and other types of remote sensing, with a wavelength of about 1 meter

#R

• Radial Velocity: The speed at which an object moves away from or towards an observer, as measured by the Doppler shift of its light or other radiation.
• Radio Frequency (RF): A range of electromagnetic frequencies used for communication, navigation, and other purposes.
• Radiometer: An instrument that measures the intensity of electromagnetic radiation in various wavelength bands.
• RCS (Reaction Control System): A system of small thrusters used to control the orientation and movement of a spacecraft in space.
• Re-entry: The process of a spacecraft or other object returning from space and entering a planet’s atmosphere or other celestial body.
• Reflector: A surface or device that reflects or bounces back light or other radiation.
• Remote sensing: The measurement or detection of an object or phenomenon from a distance using instruments such as cameras, radar, or lidar.
• Retrograde: The opposite of prograde, a movement or rotation that is in the opposite direction to the rotation of a planet or other celestial body.
• Rocket: A vehicle or device that uses thrust to move through the air or space.
• RCS (Reaction Control System): A system of small thrusters used to control the orientation and movement of a spacecraft in space.
• Rover: A type of vehicle designed to move over the surface of a planet or other celestial body, typically used for exploration and scientific research.
• RPM (Revolutions Per Minute): A measure of an object’s number of rotations or revolutions in a minute.
• RTG (Radioisotope Thermoelectric Generator): A type of power source that uses the heat generated by the decay of radioactive isotopes to generate electricity.
• RTLS (Return-to-Launch-Site): A type of abort mode in which a spacecraft or launch vehicle returns to its launch site in the event of a problem during launch.

#S

• S/C (Spacecraft): A vehicle or machine designed to operate in space, including satellites, probes, and manned spacecraft.
• Solar Array: A collection of solar cells or other devices that convert sunlight into electricity to power a spacecraft or other device.
• Solar Constant: The amount of solar radiation that reaches Earth’s outer atmosphere on a surface perpendicular to the Sun’s rays, equal to approximately 1361 W/m².
• Solar Sail: A type of propulsion that uses the pressure of sunlight to push a spacecraft through space.
• Solar System: The collection of celestial bodies that orbit the Sun, including the eight planets, their moons, and other smaller objects such as asteroids and comets.
• Spectrometer: An instrument that separates light or other radiation into its different wavelengths, allowing the composition or properties of an object to be studied.
• SRM (Solid Rocket Motor): A rocket propulsion that uses solid fuel.
• Satellite: An object artificially placed into orbit around the Earth or other celestial body.
• Solar System: The celestial bodies that orbit around the Sun, including the Sun, planets, dwarf planets, moons, asteroids, comets, and other small bodies.
• Spectroscopy: The study of the interaction of electromagnetic radiation with matter, often used to analyze the properties of stars and galaxies.
• Star: A luminous, spherical ball of gas that generates energy through nuclear fusion.
• Supernova: A powerful explosion occurs when a star runs out of fuel, and its core collapses.
• Spacecraft: A vehicle or machine designed to fly in outer space.
• Space Debris: Man-made objects in space that no longer serve a useful purpose.
• Space Shuttle: A reusable spacecraft designed to transport people and cargo to and from Earth’s orbit.
• Space Station: An artificial structure placed in orbit around the Earth or other celestial body, used for research and other purposes.
• Space Telescope: A telescope placed in orbit around the Earth to observe distant celestial objects.
• Sub-Orbital Flight: A spaceflight in which the vehicle reaches space but does not go into orbit.
• Solar Flare: A sudden and intense burst of radiation from the sun.
• Space Weather: The environmental conditions in space that can affect the Earth and its inhabitants, such as solar flares and coronal mass ejections.
• Synchronous orbit: An orbit in which an object takes the same amount of time to complete one orbit as the object around which it is orbiting.
• Space Propulsion: The technology used to move a spacecraft through space.
• Space Navigation: The process of determining the position and velocity of a spacecraft in space.
• Space Radiation: High-energy particles and electromagnetic radiation that exists in space.
• Space Suit: A suit worn by astronauts to provide a pressurized and temperature-controlled environment while in space.
• Space Walk: An extravehicular activity (EVA) in which an astronaut exits a spacecraft and performs tasks outside.
• Singularity: A point in space-time at which the laws of physics as we know them to break down.
• Solar Wind: A stream of charged particles that flows out from the Sun, affecting the motion of planets and other bodies in the Solar System.
• Sputnik: The first artificial satellite to orbit the Earth, launched by the Soviet Union in 1957.
• Star Cluster: A group of stars bound together by gravity and formed from the same gas cloud.
• Stellar Evolution: The process by which stars form, evolve, and eventually die.
• Supercluster: A large structure of galaxies that are held together by gravity and contains many galaxy clusters.
• Space Elevator: A proposed structure that would extend from the Earth’s surface to a point in space, allowing for easy transportation of people and cargo to and from orbit.
• Space Mining: The extraction of resources from celestial bodies, such as asteroids or the Moon.
• Space Colonization: The establishment of permanent human settlements in space.
• Space Tourism: The business of providing trips to space for paying customers.
• Space Weathering: The process by which the surfaces of celestial bodies are affected by exposure to the space environment, including micrometeoroids, radiation, and temperature fluctuations.
• Spacecraft Bus: The main structural component of a spacecraft to which other subsystems are attached.
• Spacecraft Power System: The system that provides power to a spacecraft, typically through solar panels or batteries.
• Spacecraft Telemetry: The transmission of data from a spacecraft to a ground station, used to monitor and control the spacecraft.
• Spacecraft Attitude Control System: The system that controls the orientation of a spacecraft in space.
• Spacecraft Thermal Control System: The system that regulates the temperature of a spacecraft to ensure its components and instruments function properly.
• Spacecraft Propulsion System: The system that provides thrust to a spacecraft, allowing it to change its velocity and direction in space.
• Spacecraft Guidance, Navigation, and Control System: The system that allows a spacecraft to navigate and control its flight path in space.

#T

• Telemetry: The process of measuring and transmitting data from a spacecraft or other remote source to a ground station or other location.
• Telescope: An instrument that uses lenses or mirrors to collect and focus light, allowing distant objects to be viewed and studied.
• Thrust: The force that propels a spacecraft or other object through space.
• Tracking: The process of following or monitoring the movement of a spacecraft or other object.
• Transponder: A device that receives and retransmits a signal, typically used for communication or navigation.
• Triaxial: A device or system with three axes of motion or rotation.
• TLE (Two-Line Element Set): A type of orbital data that describes the position and velocity of an object in space using two lines of text.
• Tracking and Data Relay Satellite System (TDRSS): A system of satellites and ground stations that provides communication and tracking services for spacecraft in low Earth orbit.
• Thermal Control System: A system of devices and materials used to regulate the temperature of a spacecraft or its components.
• Thrust-to-Weight Ratio: The ratio of the thrust of a propulsion system to the weight of the vehicle or object it is propelling.
• Tether: A cable or rope connects two or more spacecraft or other objects in space.
• Tracking Station: A ground-based facility that tracks and communicates with spacecraft and other objects in space.
• Transit: The passage of a celestial body across the face of another, as seen from a third location.
• Transit Time: The time it takes for a spacecraft or other object to travel from one location to another.
• Trans-Lunar Injection (TLI): The maneuver that propels a spacecraft from Earth’s orbit toward the Moon.
• Transit Method: A method of detecting exoplanets by observing the small dip in a star’s brightness as the planet transits across the star’s face.
• Triangulation: A method of determining the position of an object by measuring the angles between it and two or more other points.
• Tidal Force: The force exerted on a celestial body by the gravity of another body, causing a stretching and squeezing effect that can cause tides.
• Thermal Protection System (TPS): A system of materials and devices used to protect a spacecraft from the heat of reentry into the Earth’s atmosphere.
• Titan: The largest moon of Saturn and the second-largest in the Solar System, with a thick atmosphere and a surface that features lakes and rivers of liquid methane and ethane.
• Thermal vacuum testing: A type of test that simulates the thermal and vacuum conditions a spacecraft or component will experience in space.
• Time Dilation: The effect by which time appears to pass more slowly in a stronger gravitational field or at higher speeds.

#U

• Ultraviolet (UV): A type of electromagnetic radiation with a wavelength shorter than visible light but longer than X-rays.
• Umbra: The innermost and darkest part of a shadow cast by an opaque object.
• Unmanned: A spacecraft or other vehicle that is not designed to carry or support human life.
• Up-link: The transmission of a signal or data from a ground station to a spacecraft or other remote location.
• Upper Stage: The upper portion of a rocket that provides propulsion for the final stages of a launch or for in-orbit maneuvers.
• Unmanned Aerial Vehicle (UAV): An aircraft that is flown without a human pilot on board.
• Unmanned Space Vehicle (USV): A spacecraft that is operated without a human crew on board.
• Ultraviolet Imaging Spectrometer (UVIS): An instrument that measures ultraviolet radiation from a celestial body.
• Unmanned Lunar Exploration Mission: A mission of robotic spacecraft to explore the Moon without a human crew.
• Unmanned Spacecraft: A spacecraft that does not have a human crew on board.

#V

• Van Allen Belts: The region of space surrounding Earth where charged particles are trapped by the planet’s magnetic field.
• Vega: A star in the constellation Lyra, used as a reference star for spacecraft navigation and as a target for interplanetary missions.
• Velocity: The speed and direction of an object’s motion.
• Venus: The second planet from the sun, known for its thick, toxic atmosphere and extremely high surface temperatures.
• Vernal Equinox: The point in the Earth’s orbit where it is closest to the sun, marking the start of spring in the northern hemisphere and fall in the southern hemisphere.
• Vibration Testing: The process of subjecting a spacecraft or component to various vibration frequencies to simulate the conditions it will experience during launch.
• VLBI (Very Long Baseline Interferometry): An astronomical observation technique that uses multiple radio telescopes spaced far apart to create a virtual telescope as large as the distance between them.
• VLT (Very Large Telescope): A group of four large telescopes located in Chile and operated by the European Southern Observatory.
• Volatiles: Substances that vaporize at relatively low temperatures, such as water, methane, or ammonia.
• Vibration Isolation System: A system that reduces or eliminates unwanted vibrations in spacecraft and other structures.
• Variable Star: A star whose brightness varies over time as a result of changes in its internal structure or external conditions.

#W

• Wavelength: The distance between two consecutive peaks or troughs in a wave.
• White Dwarf: A small, dense star that has exhausted the nuclear fuel in its core and cooled to a temperature at which it no longer generates significant heat.
• Weightlessness: The condition of being in freefall, where the object’s acceleration effectively cancels out the force of gravity.
• Wide Field Infrared Survey Telescope (WFIRST): A proposed space telescope that would survey the universe in infrared light, primarily focused on studying dark energy and exoplanets.
• World Space Observatory (WSO): A proposed space telescope that would study the universe in ultraviolet and infrared light, primarily focusing on studying stars and galaxies.
• WISE (Wide-field Infrared Survey Explorer) : A NASA infrared-wavelength astronomical space telescope launched in December 2009.
• Warp Drive: A hypothetical propulsion system that would allow a spacecraft to travel faster than the speed of light.
• Water Recovery System: A system used to collect and purify water from various sources, such as humidity in the air or wastewater, for use on a spacecraft or other remote location.
• Wild 2: A comet that NASA’s Stardust spacecraft visited in 2004.
• World Geodetic System (WGS): a standard for use in cartography, geodesy, and navigation, including GPS.