Space Exploration Glossary: 40 Core Terms, Clearly Defined

Space news runs on acronyms. This page defines the ones that turn up repeatedly across Starliner.net articles and in mainstream coverage of current missions. Each definition links to the longer article where the term actually comes up, so you can go as deep as you need to.

A

Aphelion: The point in an orbit around the Sun where a body is farthest from the Sun. For a Mars-bound spacecraft on a Hohmann transfer, aphelion is the point where it reaches Mars's orbit. Contrast perihelion.
Apogee: The point in an Earth orbit farthest from Earth. Launch vehicles and transfer stages raise apogee as an intermediate step before committing to a final orbit. Contrast perigee.
Artemis: NASA's programme to return humans to the Moon and establish a sustained lunar presence. Covered in detail on the Moon and Artemis II pages.
Astronomical Unit (AU): The average distance between the Earth and the Sun, about 150 million km. Used as a convenient ruler for Solar System distances: Mars orbits at about 1.5 AU, Jupiter at about 5.2 AU, Pluto at about 39 AU.

B

Biosignature: A chemical, isotopic, or morphological pattern that is hard to explain without invoking biology. In astrobiology, biosignatures are the targets of mission instrument design. No confirmed biosignature has ever been detected outside Earth. See also the search for alien life.

C

Cislunar: The region of space between the Earth and the Moon, including all the orbits that do not go beyond the lunar orbit. Most Artemis-era architecture is cislunar.
CLPS: Commercial Lunar Payload Services, NASA's programme that pays commercial vendors a fixed price to deliver instruments and small payloads to the lunar surface. Firefly's Blue Ghost and Intuitive Machines' IM-1/IM-2 missions are CLPS deliveries.
Coronal Mass Ejection (CME): A large burst of plasma and magnetic field ejected from the Sun's corona. Earth-directed CMEs drive geomagnetic storms and are the main space-weather hazard to satellites and power grids. See the Sun.

D

Delta-v (Δv): The change in velocity a manoeuvre requires, measured in m/s or km/s. Delta-v is the universal currency of orbital mechanics: every destination has a delta-v cost, and every vehicle has a delta-v budget set by its propellant and specific impulse. Launching to low Earth orbit costs about 9.4 km/s of delta-v; a lunar landing and return adds several more.
Deep Space Network (DSN): The global array of large parabolic antennas NASA uses to communicate with deep-space spacecraft. See Deep Space Network.

E

ECLSS: Environmental Control and Life Support System. The hardware that keeps a crewed spacecraft's atmosphere breathable, its water drinkable, and its temperature survivable. See Life Support Systems.
Ecliptic: The plane of Earth's orbit around the Sun. The orbits of most Solar System bodies lie close to the ecliptic; missions that deliberately leave the ecliptic (like Solar Orbiter or Ulysses) do so to see the Sun from above or below.
EMU: Extravehicular Mobility Unit — a spacesuit rated for external use in vacuum. The term specifically refers to NASA's current ISS suit, but is often used generically. See Spacesuits.
EVA: Extravehicular Activity — any activity performed in a spacesuit outside a spacecraft, whether a planned spacewalk, an emergency repair, or a lunar-surface excursion.

G

Gateway: The small crew-tended space station being assembled in a near-rectilinear halo orbit around the Moon as part of Artemis. See the Moon.
Geostationary Orbit (GEO): A circular orbit at about 35,786 km altitude above Earth's equator, where the orbital period equals Earth's rotational period. A satellite in GEO stays fixed over the same point on the ground, which is why most broadcast and weather satellites live there. Contrast LEO.

H

HLS: Human Landing System. The vehicle that actually puts Artemis astronauts on the lunar surface. See Human Landing Systems.
Hohmann Transfer: The minimum-energy transfer between two coplanar circular orbits, using two impulsive burns. It is slow but propellant-cheap. Most interplanetary mission trajectories are variations on Hohmann transfers with added gravity assists.

I

International Space Station (ISS): The crewed low-Earth-orbit laboratory operated jointly by NASA, Roscosmos, ESA, JAXA, and CSA. In continuous operation since 1998; currently planned for retirement around 2030. See Orbital Habitats.
Specific Impulse (Isp): A measure of rocket propellant efficiency, expressed in seconds. Higher Isp means more thrust per unit of propellant consumed per second. Chemical rockets hit 300–450 s; ion thrusters hit 3,000–5,000 s; nuclear thermal hits about 900 s.
ISRU: In-Situ Resource Utilisation. The practice of using materials found at a destination (lunar ice, Martian CO₂, regolith oxygen) instead of launching everything from Earth. Essential to any long-term crewed presence beyond low Earth orbit.

L

Lagrange Point: One of five equilibrium points in a two-body system (such as Earth-Sun or Earth-Moon) where a small spacecraft can hold position relative to the two larger bodies. See Lagrange Points.
Low Earth Orbit (LEO): Any orbit with altitude below about 2,000 km. Most satellites, the ISS, Starlink, and human crew vehicles all operate in LEO. Launching to LEO is by far the most-flown space activity. Contrast GEO.

M

Mascon: A concentration of mass in a planetary body's interior that locally increases the gravitational pull at the surface above. The Moon has several, mapped by the GRAIL mission, and they affect lunar orbit stability.

N

Near-Rectilinear Halo Orbit (NRHO): A highly elliptical orbit around the Earth-Moon L2 point, used by the Gateway programme. Stable with modest station-keeping, with continuous line of sight to Earth and good access to the lunar south pole. See Lagrange Points.

O

Orbital Period: The time it takes for an orbiting body to complete one full orbit. For the ISS, about 93 minutes. For the Moon around Earth, about 27.3 days. For Mars around the Sun, about 687 Earth days.

P

Perigee: The point in an Earth orbit closest to Earth. Contrast apogee.
Perihelion: The point in a solar orbit closest to the Sun. Parker Solar Probe's perihelion is inside the solar corona. Contrast aphelion.
Permanently Shadowed Region (PSR): A region near a lunar pole that never receives direct sunlight. PSRs are cold enough to trap water ice for billions of years. They are the primary targets for lunar ISRU. See the Moon.
Power and Propulsion Element (PPE): The Lunar Gateway's first module: a solar-electric propulsion module that holds Gateway in its NRHO. See ion propulsion.

R

Regolith: The loose, fragmented surface layer of a rocky body, formed over billions of years by impacts, thermal cycling, and solar-wind bombardment. Lunar regolith is abrasive and electrostatically charged, which is why it is an operational hazard for any surface mission.

S

SLS: Space Launch System — NASA's super heavy-lift rocket used to launch Orion on Artemis missions. Its first stage is hydrogen-oxygen with solid rocket boosters. See the technology hub.
Solar Cycle: The roughly 11-year rise and fall in the Sun's magnetic activity, sunspot count, and flare frequency. Cycle 25 peaked around 2024–2025. See the Sun.

T

Trans-Lunar Injection (TLI): The burn that takes a spacecraft from Earth orbit onto a trajectory to the Moon. Roughly 3 km/s of delta-v from LEO. Artemis II's ICPS performed its TLI after on-orbit checkout.
Trans-Mars Injection (TMI): The burn that takes a spacecraft from Earth orbit onto a Mars-bound trajectory, typically a Hohmann-like transfer. Higher delta-v than TLI, and only available during Mars launch windows roughly every 26 months.

Last reviewed on 2026-04-24.