Space Glossary: 30 Terms Explained Simply

Space news is full of jargon. Here are the terms you will meet most often, explained in plain English.

Webb's primary mirror being tested: NASA/MSFC/David Higginbotham, Public domain

Term	Meaning
Astronomical unit (AU)	The average distance from Earth to the Sun, about 150 million km.
Light-year	The distance light travels in a year, about 9.46 trillion km. It measures distance, not time.
Orbit	The curved path of one object around another, shaped by gravity.
Perihelion / aphelion	The closest and farthest points of an orbit around the Sun.
Perigee / apogee	The closest and farthest points of an orbit around Earth.
Gravity assist	Using a planet's gravity to change a spacecraft's speed and direction without fuel.
Dwarf planet	A round body orbiting the Sun that has not cleared its orbital neighbourhood, like Pluto or Ceres.
Moon (natural satellite)	A natural body that orbits a planet or other object.
Asteroid	A small rocky or metallic body orbiting the Sun, mostly in the main belt.
Comet	An icy body that releases gas and dust, forming a coma and tails, near the Sun.
Meteor / meteorite	A meteor is the streak of light from space debris burning up; a meteorite is a piece that reaches the ground.
Kuiper Belt	A region of icy bodies beyond Neptune, including Pluto.
Oort Cloud	A vast, distant shell of icy bodies thought to surround the solar system; source of long-period comets.
Heliopause	The boundary where the Sun's solar wind meets interstellar space.
Solar wind	A stream of charged particles flowing out from the Sun.
Corona	The Sun's hot outer atmosphere, visible during a total solar eclipse.
Exoplanet	A planet orbiting a star other than the Sun.
Habitable zone	The range of distances from a star where liquid water could exist on a planet's surface.
Tidal heating	Heat produced inside a moon or planet as gravity repeatedly flexes it.
Tidally locked	When an object always shows the same face to the body it orbits, like our Moon.
Retrograde	Moving or spinning in the opposite direction to most objects in the solar system.
Lagrange point	A spot where gravity from two large bodies balances, letting a spacecraft hold position, like Webb at L2.
Rover / lander / orbiter	A rover drives on a surface, a lander stays where it lands, an orbiter circles a world.
Flyby	A mission that passes a world without stopping or orbiting.
Sample return	A mission that collects material and brings it back to Earth.
Equinox / solstice	Equinoxes have roughly equal day and night; solstices are the longest and shortest days.
Eclipse	When one body moves into the shadow of another, like the Moon blocking the Sun.
Magnitude	A measure of brightness; smaller numbers mean brighter objects.
Biosignature	A sign that could indicate past or present life, needing careful confirmation.
Cryovolcano	An ice volcano that erupts water, ammonia or methane instead of molten rock.

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