The Complete Book of Spaceflight: From Apollo 1 to Zero Gravity

exosphere
The tenuous outermost layers of a planetary atmosphere
that merge into the interplanetary medium.
exotic matter
A hypothetical kind of matter that has both a negative
energy density and a negative pressure or tension that
exceeds the energy density. All known forms of matter
have positive energy density and pressures or tensions that
are always less than the energy density in magnitude. In a
stretched rubber band, for example, the energy density is
100 trillion times greater than the tension. A possible
source of exotic matter lies in the behavior of certain vacuum
states in quantum field theory (see Casimir effect). If
such matter exists, or could be created, it might make possible
schemes for faster-than-light (FTL) travel such as stable
wormholes and the Alcubierre Warp Drive.
expansion ratio
The ratio between the area of the combustion chamber
exit and the area of the nozzle exit. A large expansion
ratio improves the performance of an engine in a vacuum
since the exhaust is expanded further, thus converting
potential energy into kinetic energy. However, at sea level
a high expansion ratio can result in flow separation,
which can drastically reduce or eliminate the net thrust of
the engine.
expendable launch vehicle (ELV)
A launch system that cannot be recovered for use on subsequent
missions.
Experimental Rocket Propulsion Society (ERPS)
A nonprofit liquid fuel rocket engine design and test
team based in the San Francisco Bay Area. Its current
projects include a monopropellant rocket to launch a
payload to 100 km; a small two-man suborbital vehicle; a
9-kg to low Earth orbit, single-stage-to-orbit demonstrator;
a rocket-pack; and a rocket-assist module for high
performance gliders.
Exploration of Neighboring Planetary Systems
(ExNPS)
One of the key elements of NASA’s Origins Program. It
is intended to be a long-term program of scientific investigation
and technological development with the goal of
detecting and probing Earth-like planets around nearby
stars. The responsibility for identifying a specific plan
for ExNPS has been assigned to JPL (Jet Propulsion Laboratory).
Explorer
A long and ongoing series of small American scientific
spacecraft. Explorer 1, the first successful American
Explorer 131
satellite, was launched on January 31, 1958, and discovered
the Van Allen belts. Explorer 6 took the first
photo of Earth from space. Subprograms of the
Explorer series have included ADE (Air Density
Explorer), AE (Atmosphere Explorer), BE (Beacon
Explorer), DME (Direct Measurement Explorer), EPE
(Energetic Particles Explorer), GEOS (Geodetic Earth
Orbiting Satellite), IE (Ionosphere Explorer), and IMP
(Interplanetary Monitoring Platform). Among the more
recent Explorers are IUE (International Ultraviolet
Explorer) and COBE (Cosmic Background Explorer).
Subcategories of the modern Explorer program include
SMEX (Small Explorer), MIDEX (Medium-class
Explorer), UNEX (University-class Explorer), and Missions
of Opportunity, while the themes of the program
are the Astronomical Search for Origins and Planetary
Systems, the Sun-Earth Connection, and the Structure
and Evolution of the Universe. (See table, “Explorer
Spacecraft.”)
Explorer From left to right: William Pickering, director of the
Jet Propulsion Laboratory; James Van Allen, from the University
of Iowa; and Wernher von Braun hold a full-size model of
America’s first satellite, Explorer 1, following a successful
launch on January 31, 1958. NASA