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

to Timation and eventually the Navstar-GPS (Global
Positioning System).
SECS (Special Experimental
Communications System)
See GLOMR.
SEDS (Small Expendable-tether Deployer System)
An experiment developed by the Marshall Space Flight
Center and Tether Applications Company of San Diego,
California, to demonstrate the use of a space tether to
de-orbit a small payload. Two SEDS missions have been
flown to date. Each involved the deployment of a 20-km
tether at the downward end of which was the de-orbit
mass. The principle is that the tether acts as a thruster,
causing the lower satellite to hover at an altitude below
that which the orbital angular rate would allow, thus
causing the orbit to decay. SEDS-1 and SEDS-2 were
both successful, although the SEDS-2 tether was severed
after five days by a micrometeroid or a piece of space
debris. NASA has successfully deployed three SEDS
Class (Small Expendable Deployer System) probes:
SEDS-1 and -2 were 20 km missions, and PMG (Plasma
Motor Generator) was a 500-m mission. All tethered systems
were attached to a spent Delta second stage. Not
only were tether models verified, it was demonstrated
that a probe can be downwardly deployed from a very
long tether. (See table, “SEDS Missions.”)
Launch
Vehicle: Delta 7925
Site: Cape Canaveral
Mass: 25 kg
SEDS Missions
Spacecraft Launch Date Orbit
SEDS-1 + deployer Mar. 30, 1993 195 × 705 × 34°
SEDS-2 + deployer Mar. 10, 1994 350 km (circular)
SEDS (Students for the Exploration and
Development of Space)
Founded in 1980 at the Massachusetts Institute of Technology
and Princeton, a chapter-based pro-space organization
at high schools and universities around the world.
The chapters are entirely student run, and each chapter is
independent and coordinates its own local activities.
Nationally, SEDS runs a scholarship competition and
design contests and holds an annual international conference
and meeting in late summer.
SELENE (Selenological and Engineering Explorer)
A Japanese mission to study the origin and evolution of
the Moon and to help develop the techniques needed for
Sergeant 371
future lunar exploration and utilization. Following the
many trips to the Moon in the 1960s and 1970s, our nearest
celestial companion in space fell out of spaceexploration
favor. Russia’s Luna 24 was the last lunar
lander, a quarter of a century ago. SELENE will give scientists
and engineers around the world the most recent
and probably the best assessment yet of the potential for
future human utilization, and even habitation, of the
Moon. It is scheduled for launch in 2003.
semi-major axis
Half the maximum diameter of an ellipse; in other
words, the distance from the center of the ellipse to one
end.
semi-synchronous orbit
An orbit set at an altitude of 20,650 km. Satellites in this
orbit revolve around the Earth in exactly 12 hours.
sensor
A device that converts an input signal into a quantity that
can be measured by another instrument.
Sentinels
A proposed future mission in NASA’s Living with a Star
initiative. Sentinels will monitor the entire Sun using a
fleet of spacecraft stationed at the stable Lagrangian
points in Earth’s orbit. Their observations will reveal precisely
how solar disturbances propagate and evolve, and
will vastly improve the accuracy of space weather predictions.
separation
In multistage space vehicles, the point at which a burnedout
stage is discarded and the remaining vehicle continues
on its course.
separation rocket
A small, usually solid-fuel, motor that helps separate a
spent stage from the rest of a multistage spacecraft. The
separation rocket provides extra thrust to slow the spent
stage and to enable the spacecraft to move away from it.
Sergeant
A U.S. Army single-stage, surface-to-surface ballistic missile
designed to replace the Corporal. First deployed in
September 1962, the Sergeant was powered by a solidfueled
motor. Although designed as a weapon capable of
delivering a conventional or nuclear warhead, it played a
significant part in the early American space program. The
second stage of the Jupiter C, which launched America’s
first satellite, was a ring of 11 scaled-down Sergeant motors