Asteroid
A small rocky body orbiting the Sun, mainly in the main belt between Mars and Jupiter. Missions like Psyche or OSIRIS-REx study their composition to understand the formation of the Solar System.
Astronomy glossary
Simplified definitions of key terms in space exploration and astronomy, illustrated by the missions and spacecraft in the TRACKER-1 catalog.
53 terms
7 categories
18 letters
All terms
A
Astrobiology
The discipline studying the origin, evolution and distribution of life in the universe. Martian missions (Perseverance) and the study of subsurface oceans (Europa) fall under astrobiology.
Atmosphere
The gaseous envelope surrounding a planet. Studying exoplanet atmospheres (via transit spectroscopy) is a major goal of James Webb: detecting H₂O, CO₂, CH₄ to assess habitability.
Aurora
A luminous phenomenon observed near the poles, caused by the interaction between solar wind particles and the upper atmosphere. Green auroras (oxygen) are most common; red and purple are rarer.
B
Black hole
A region of space where gravity is so intense that nothing, not even light, can escape. Telescopes (Chandra for X-rays, Event Horizon Telescope for direct imaging) study supermassive black holes at galaxy centers.
C
CNSA
China National Space Administration, the Chinese space agency founded in 1993. Operates the Tiangong station and the Zhurong rover on Mars, and develops a crewed lunar program.
Comet
A small icy body that, as it approaches the Sun, releases gas and dust forming a coma and a tail. Comets are remnants of the primitive Solar System. The Rosetta probe studied comet 67P/Churyumov-Gerasimenko.
Coronagraph
An instrument that blocks a star light to reveal faint objects around it (exoplanets, dust disks). Used by space telescopes to directly image exoplanets.
Cruise (interplanetary transit)
The travel phase of a probe between launch and arrival at destination. BepiColombo cruised to Mercury for 7 years, using gravity assists to save fuel.
D
Dark matter
An invisible component of the universe: it neither emits nor absorbs light, but its gravity reveals its presence (galaxy rotation, gravitational lensing). It accounts for about 85% of the matter in the universe. Its nature remains unknown, with candidates ranging from WIMPs to axions. Experiments like LZ hunt it deep underground, missions like Euclid map it from space.
E
Earth orbit
A spacecraft trajectory around Earth. Low orbit (400 km) hosts the ISS; geostationary orbit (36,000 km) hosts telecom satellites; Lagrange point L2 hosts observation telescopes.
Eclipse
The occultation of one celestial body by another. Solar eclipses (Moon in front of Sun) and lunar eclipses (Earth blocking Sun from Moon) are observable with the naked eye, with precautions.
ESA
European Space Agency, the European space agency founded in 1975, grouping 22 member states. Participates in major programs (JWST, ISS) and leads its own missions (Gaia, Euclid, JUICE) from ESOC center in Germany.
Exoplanet
A planet orbiting a star other than the Sun. Detecting and characterizing exoplanets (atmosphere, size, temperature) is a major field of modern astrophysics, notably with the James Webb telescope.
G
Galaxy
A vast system of stars, gas, dust and dark matter bound by gravity. Our galaxy, the Milky Way, contains about 200 billion stars. Telescopes like Hubble and Webb observe distant galaxies to understand the evolution of the universe.
H
Habitability
The capacity of an environment to harbor life. In astrophysics, we look for necessary conditions: liquid water, energy source, chemical elements (C, H, N, O, P, S). Mars, Europa, Titan are habitability targets.
I
Infrared
A wavelength longer than visible light, invisible to the naked eye. Infrared penetrates dust clouds, revealing forming stars and distant galaxies. James Webb primarily observes in infrared.
Interferometer
An instrument combining light from several telescopes to achieve a resolution equivalent to a telescope the size of their separation. The Event Horizon Telescope is an interferometer linking observatories worldwide.
ISRO
Indian Space Research Organisation, the Indian space agency founded in 1969. Achieved the lunar landing of Chandrayaan-3 in 2023 and launched its solar mission Aditya-L1.
J
JAXA
Japan Aerospace Exploration Agency, the Japanese space agency founded in 2003. Conducts exploration missions (Hayabusa2, SLIM) and operates the ISS Kibo module.
Jupiter
The largest planet in the Solar System, a gas giant. Jupiter has nearly 100 moons including Europa, which hides a liquid water ocean beneath its ice, a major target in the search for habitability.
L
Lagrange point
Stable positions around two massive bodies where a third object can remain fixed. Point L2 (1.5 million km behind Earth) is ideal for telescopes: James Webb and Euclid are stationed there.
Lander
A spacecraft designed to land softly on a celestial body surface and conduct fixed measurements. Unlike a rover, a lander does not move after touchdown. Examples: InSight, Chandrayaan-3.
Launch
The phase where a launcher (rocket) propels a spacecraft out of the atmosphere. Launches occur from dedicated bases (Cape Canaveral, Kourou, Baikonur). The launch window depends on orbital alignments.
M
Magnetosphere
The region around a planet where the magnetic field deflects the solar wind. Earth magnetosphere protects life from radiation and channels particles toward the poles, creating auroras.
Mars
The fourth planet of the Solar System, nicknamed the Red Planet due to iron oxide on its surface. Mars is the prime target for robotic exploration: rovers, orbiters and landers search for past traces of life and study its climate.
Microgravity
A state where gravity effects are nearly zero, as in Earth orbit. Microgravity enables experiments impossible on Earth: combustion, biology, fluid physics. The ISS is the main laboratory.
Milky Way
Our galaxy, a spiral galaxy containing the Sun and about 200 billion stars. Visible to the naked eye as a luminous band in a dark sky, the Milky Way is studied by missions like Gaia which maps stellar positions and motions.
Moon
Earth's natural satellite. The Moon is studied by orbiters and landers to understand its formation, its resources (water ice at the poles) and prepare crewed return.
Multi-messenger astronomy
An approach that observes the same celestial phenomenon with several different "messengers": light (all wavelengths), gravitational waves, neutrinos and cosmic rays. Each messenger carries information the others cannot provide. Landmark example: in 2017, a neutrino detected by IceCube led to the identification of the galaxy TXS 0506+056 as a source of high-energy particles, confirmed by NASA's Fermi gamma-ray telescope.
Multimessenger astronomy
A way of observing the universe by cross-referencing several "messengers" from the same event: light, gravitational waves, neutrinos and cosmic rays. Each messenger travels differently and tells part of the story. Example: in 2017, a neutrino detected by IceCube pointed to the same source as a gamma-ray flare seen by Fermi, a historic first.
N
NASA
National Aeronautics and Space Administration, the U.S. civilian space agency founded in 1958. Operates most major exploration missions (Mars, Jupiter, JWST) from its centers (JPL, Goddard, Johnson).
Nebula
A cloud of interstellar gas and dust. Nebulae are sites of star formation (emission nebulae) or remnants of dead stars (planetary nebulae, supernova remnants). Hubble and Webb produce iconic images of them.
Neutrino
A fundamental particle with almost no mass and no electric charge that passes through matter as if it were not there. The Sun produces astronomical numbers of them (about 65 billion per square centimeter per second at Earth), and the most violent phenomena in the universe produce them too. Because they are neither deflected by magnetic fields nor absorbed by dust, neutrinos keep the direction and energy of their source: they are direct messengers from places where light stays trapped. The IceCube detector hunts them inside a cubic kilometer of Antarctic ice.
R
Rover
A motorized vehicle that drives on the surface of another celestial body (Mars, the Moon) to explore terrain, collect samples and analyze soil composition. Examples: Perseverance, Curiosity, Zhurong.
S
Sample
Material collected from the surface of another celestial body and returned to Earth for laboratory analysis. Martian rovers (Perseverance) collect samples for future return.
Solar flare
A sudden explosion on the Sun surface releasing enormous amounts of energy. Classified from A (weak) to X (extreme). Major flares can disrupt communications and power grids.
Solar System
The system formed by the Sun and all bodies gravitationally bound to it: 8 planets, their moons, asteroids, comets and dwarf planets. Space probes have explored it since Voyager 1 and 2.
Solar wind
A continuous stream of charged particles emitted by the Sun at speeds of 300-800 km/s. The solar wind interacts with Earth magnetic field, generating auroras and potentially disrupting satellites and power grids.
Space probe
An uncrewed spacecraft sent to explore the Solar System. Unlike telescopes that observe from afar, probes travel to their target (planet, asteroid, Sun) to study it up close. Examples: Voyager, New Horizons, Parker Solar Probe.
Space station
An orbiting space habitat designed to host a human crew for extended periods. Stations serve as scientific laboratories in microgravity. Examples: International Space Station (ISS), Tiangong.
Space telescope
A spacecraft designed to observe the universe from Earth orbit or a Lagrange point, free from atmospheric distortion. Space telescopes observe in wavelengths inaccessible from the ground (infrared, ultraviolet, X-rays). Examples: Hubble, James Webb, Gaia.
Spectrometer
An instrument that decomposes light into its wavelengths to analyze the chemical composition, temperature and velocity of celestial objects. Found on most space telescopes (JWST, Hubble).
Spectroscopy
A technique analyzing light emitted or absorbed by an object to determine its chemical composition, temperature and velocity. Fundamental in astrophysics: each element leaves a unique signature in the spectrum.
Star formation
The process by which a cloud of gas and dust collapses under its own gravity to form a star. Nebulae are their cradle. James Webb observes protostars in infrared.
Subsurface ocean
A liquid water ocean hidden beneath the icy crust of a moon, kept liquid by tidal friction. Europa (Jupiter moon) and Encelade (Saturn moon) have them, making them major targets in the search for life.
Sun
The star at the center of our Solar System. The Sun is studied by probes that approach it (Parker Solar Probe) to understand the solar wind, eruptions and their impact on Earth (space weather).
Supermassive black hole
A black hole with a mass between millions and billions of times that of the Sun, typically located at a galaxy center. The Event Horizon Telescope imaged the one in galaxy M87 in 2019.
Supernova
The cataclysmic explosion of a dying star, releasing colossal energy. Supernovae synthesize heavy elements (gold, uranium) dispersed in the universe. Space telescopes observe their remnants.
T
Transit (exoplanet)
The passage of an exoplanet in front of its star, causing a slight detectable dip in brightness. Transit spectroscopy analyzes the exoplanet atmosphere during the passage. Detection method used by space telescopes.
U
Ultraviolet
A wavelength shorter than visible light. Ultraviolet reveals hot stars, active galaxies and planetary atmospheres. Hubble has long been the reference UV telescope.
W
WIMP
Weakly interacting massive particle, a hypothetical massive particle that barely interacts with ordinary matter. One of the leading dark matter candidates. The LUX-ZEPLIN detector, 10 tonnes of liquid xenon 1.5 km underground, is designed to detect it via rare collisions with xenon atoms.
X
X-rays
Very high-energy electromagnetic radiation, emitted by extreme phenomena (black holes, neutron stars, ultra-hot gas). The Chandra telescope observes the universe in X-rays.