Glossary of Space Exploration Terminology

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H
H2O2
Hydrogen Peroxide.
Hatch
Door or doorway, usually hermetically sealed.
Heat shield
A device which protects people or equipment from heat, such as a shield in front of a reentry capsule.
Helio-
Prefix referring to the Sun.
Heliocentric
Centered on the Sun.
Heliopause
The boundary theorized to be roughly circular or teardrop-shaped, marking the edge of the Sun's influence, perhaps 100 AU from the Sun.
Heliosphere
The space within the boundary of the heliopause, containing the Sun and solar system.
HEO
High Elliptical Orbit.
Hertz
A unit of frequency equal to one cycle per second, named after Heinrich Hertz.
Heterosphere
The Earth atmosphere above 105 km altitude where species-wise concentration profiles establish due to diffusive equilibrium, with N2 dominance below 200 km, O dominance from 200 to 600 km, and He dominance as of 600 km altitude.
HGA
High-Gain Antenna.
High-energy particle detector
A device for measuring the energy spectra of trapped energetic electrons, and the energy and composition of atomic nuclei.
High-gain antenna
A dish-shaped spacecraft antenna principally used for high rate communication with Earth This type of antenna is highly directionally and must be pointed to within a fraction of a degree of Earth.
Hohmann transfer orbit
An interplanetary trajectory in which a spacecraft is launched into an elliptical solar orbit whose perihelion (inner planet) or aphelion (outer planet) reaches the orbit of the target planet on the opposite side of the Sun. Uses least propellant.
Homosphere
The Earth atmosphere below 105 km altitude where complete vertical mixing yields a near-homogeneous composition of about 78.1% N2, 20.9% O2, 0.9% Ar, and 0.1% CO2 and trace constituents. The homopause (or turbopause) marks the ceiling of the homosphere. The homosphere can be broadly divided into three distinct regimes: the troposphere (0 to 12 km), the stratosphere (12 to 50 km) and the mesosphere (50 to 90 km).
Horizon
The line marking the apparent junction of Earth and sky.
Horizon scanner
A scanner which automatically seeks the horizon for purposes of a spacecraft's orientation and control, e.g. one that detects the sharp discontinuity in infrared intensity at the outer edge of the Earth's tropopause.
Hour angle
The angular distance of a celestial object measured westward along the celestial equator from the zenith crossing.
HTP
High Test Peroxide.
HTPB
Hydroxy-terminator polybutadiene. A polymeric fuel binder.
Hydrazine
A rocket fuel which burns spontaneously with nitric acid or nitrogen tetroxide. Can also be used as a mono-propellant: when passed through an iridium-bearing catalyst, it decomposes at high temperature into constituent gases of ammonia, nitrogen and hydrogen. Used in small thrusters for orbit modification and attitude control of spacecraft. Also see MMH and UDMH.
Hydrosphere
The water on or around the surface of a planet.
Hydyne
A rocket fuel comprised of 60% UDMH and 40% diethylene-triamine.
Hyperbolic
A trajectory path to a planet shaped like a hyperbola.
Hypergolic
A term applied to an oxidizer and a fuel which ignite spontaneously with each other.

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Rocketry & Space Technology

Glossary courtesy of Robert Braeunig's
Rocketry & Space Technology website.