Glossary
30 terms
Atmosphere
The layer of gases surrounding a planet, held by gravity.
Example: Earth's atmosphere thins rapidly with altitude.
Atmospheric entry
The process of a vehicle entering a planet's atmosphere.
Example: Atmospheric entry began at about 120 km altitude.
Attitude control system
The systems that orient a spacecraft and maintain or change its pointing.
Example: The attitude control system kept the telescope locked on target.
Atmospheric drag
Resistance from air that slows a vehicle or satellite.
Example: Atmospheric drag decays low orbits unless reboosted.
Attitude
A vehicle's orientation in three-dimensional space.
Example: Attitude was held sun-pointing during the coast.
Aerodynamic heating
Heating caused by compressing and rubbing air at high speed.
Example: Aerodynamic heating glowed the heat shield white-hot.
Ablative
A material that sheds heat by charring and eroding away.
Example: An ablative coating protected the capsule during reentry.
Ablation
The process of a material eroding to carry away heat.
Example: Ablation slowly consumed the heat shield as planned.
Apogee
The point in an orbit farthest from Earth.
Example: The satellite fired its engine at apogee to raise its perigee.
Aerospike
A nozzle that uses ambient pressure as part of its expansion surface, efficient across altitudes.
Example: Aerospike engines promise better performance from sea level to vacuum.
Aphelion
The point in a solar orbit farthest from the Sun.
Example: Earth reaches aphelion in early July.
Apoapsis
The farthest point from the central body in any orbit.
Example: The spacecraft coasted near apoapsis before its insertion burn.
Airlock
A compartment that allows passage between pressure and vacuum without depressurizing the cabin.
Example: The crew cycled the airlock before the spacewalk.
Actuator
A device that moves a control surface or engine to effect commands.
Example: Actuators gimbaled the engine for steering.
Ascending node
The point where an orbit crosses the reference plane moving from south to north.
Example: The ascending node defines where the ground track starts heading north.
Antenna
A device that transmits or receives electromagnetic waves for communications.
Example: A high-gain antenna relayed science data to Earth.
Ablative cooling
Using a material that chars and erodes to absorb heat without passing it inward.
Example: Ablative cooling sacrifices liner material to protect the engine.
Aerodynamic control surface
A movable surface steering a vehicle through airflow.
Example: The vehicle's control surfaces became effective in thicker air.
Argument of perigee
The angle from the ascending node to perigee, locating the orbit's near point.
Example: Adjusting the argument of perigee shifts where the satellite dips lowest.
Aerocapture
Using atmospheric drag to enter orbit around a planet in one pass.
Example: Aerocapture would save propellant for Mars orbit insertion.
Aerobraking
Repeatedly dipping into an atmosphere to lower an orbit gradually.
Example: The orbiter used aerobraking to circularize its orbit.
Abort
Terminating a launch or flight for safety reasons.
Example: The flight computer triggered an abort after an engine anomaly.
Accelerometer
A sensor measuring linear acceleration.
Example: Accelerometers detected the onset of deceleration.
Angle of attack
The angle between a body's axis and the oncoming airflow.
Example: The vehicle modulated angle of attack to steer the entry.
Autonomous flight
Flight controlled by onboard logic without real-time human input.
Example: The vehicle flew autonomously through staging.
Auto-sequencer
The computer that executes the final countdown steps.
Example: The auto-sequencer aborted the count after a redline.
Autopilot
A system that automatically controls a vehicle's flight without human input.
Example: The autopilot held the vehicle on its commanded trajectory.
Astrobiology mission
A mission seeking conditions or signs of life elsewhere.
Example: The astrobiology mission sampled a lakebed for organic matter.
Atmospheric model
A mathematical description of atmospheric density and temperature.
Example: Atmospheric models predict drag for reentry and decay.
Attitude determination
Computing a spacecraft's orientation from sensor data.
Example: Attitude determination fused star trackers and gyros.