Glossary — E
10 terms
Equatorial orbit
An orbit whose inclination is near zero degrees, lying in the plane of Earth's equator.
Example: Geostationary satellites must first reach an equatorial transfer orbit.
Eccentricity
A measure of how much an orbit deviates from a perfect circle, ranging from zero for circular to nearly one for parabolic.
Example: A highly elliptical transfer orbit has an eccentricity close to 0.9.
Expedition
A long-duration crew stay aboard a space station.
Example: The expedition lasted six months aboard the orbital lab.
Earth observation
A mission monitoring Earth's surface, weather, or environment.
Example: Earth-observation missions track climate change.
Expander cycle
A cycle that vaporizes fuel in the nozzle to spin the turbopump without a gas generator.
Example: Expander-cycle engines use waste heat to drive their pumps.
EVA
Extravehicular Activity: work performed by an astronaut outside a spacecraft.
Example: The extravehicular activity replaced a failed pump on the station.
Error signal
The difference between desired and actual state in a control loop.
Example: The error signal drove corrective steering.
Exhaust velocity
The speed at which propellant mass leaves the nozzle, a key efficiency term.
Example: Exhaust velocity appears in the rocket equation as the effective speed.
Exosphere
The outermost, extremely thin layer of the atmosphere.
Example: The exosphere fades into the vacuum of space.
Epoch
The reference time for which a set of orbital elements is valid.
Example: Stale orbital elements need a fresh epoch for accurate propagation.