Glossary — H
16 terms
HEO
Highly Elliptical Orbit: an orbit with high eccentricity whose apogee dwells over high latitudes for extended polar coverage.
Example: Molniya orbits are highly elliptical orbits designed to linger over northern latitudes.
Hypergolic propellant
A fuel and oxidizer pair that ignite spontaneously on contact.
Example: Hypergolic propellant is valued for reliable restart without ignition hardware.
Heat flux
The rate of thermal energy transferred per unit area.
Example: The heat flux at the stagnation point exceeded design limits.
Heat shield
A protective shield that absorbs and dissipates heat during atmospheric entry.
Example: The heat shield endured thousands of degrees during entry.
Hypersonic
Traveling at roughly Mach 5 or above.
Example: Hypersonic reentry generated a glowing plasma sheath.
Hold
A pause in the countdown to resolve a condition.
Example: Launch control called a hold to inspect a sensor reading.
High-gain antenna
A directional antenna concentrating power into a narrow beam.
Example: The high-gain antenna sent high-rate data to the Deep Space Network.
Hover slam
Igniting engines at the last moment for a zero-velocity landing, also called a suicide burn.
Example: The hover-slam ignited at the final moment to land.
Hold-down clamp
A mechanism holding the rocket to the pad until full thrust is verified.
Example: Hold-down clamps released once engines reached liftoff thrust.
Heliophysics mission
A mission studying the Sun and its influence.
Example: The heliophysics mission measured the solar wind.
Hohmann transfer
The most fuel-efficient two-burn transfer between two coplanar circular orbits.
Example: Geostationary delivery uses a Hohmann transfer to reach GEO altitude.
Horizon sensor
A sensor that detects the Earth's horizon for orientation.
Example: A horizon sensor provided an Earth-reference for pointing.
Hardware-in-the-loop
Testing with real hardware connected to a simulator.
Example: Hardware-in-the-loop verified the avionics interfaces.
Human spaceflight
The enterprise of sending humans into space.
Example: Human spaceflight demands robust life support and abort systems.
Hot staging
Igniting the next stage's engines before the previous stage separates.
Example: The vehicle used hot staging to avoid a thrust gap.
Hill sphere
The region around a body where its gravity dominates that of a larger parent body.
Example: A body must stay within Earth's Hill sphere to remain bound to Earth.