Glossary - Autonomous and Autonomic Systems - page 247

Momentum dumping A procedure for spacecraft controlled by reaction
or momentum wheels, whereby angular momentum is removed from the
wheel system to prevent the wheels from saturating (a circumstance where
wheel speeds can be run up to maximum values). Angular momentum
dumping utilizes other attitude actuators to shift or remove angular mo-
mentum from the wheels (e.g., by applying current to magnetic coils,
thereby generating a spacecraft counter-torque by means of coupling to
the geomagnetic field).
Nadir vector In the context of the gravitational field of a massive body,
the nadir at a given point is the direction along the force of gravity at
that point. For Earth-orbiting spacecraft, the vector is directed toward
the geocenter. See Zenith.
Nadir-pointer A spacecraft whose primary science instrument is directed
along the nadir vector.
Nanospacecraft A small spacecraft characterized as having a weight of
about 10 kg or less, a cylindrical diameter about 30 cm or less, and a low
cost.
Networked science mission See distributed science mission.
Occultation A geometric condition when the view of the target of a space-
craft sensor or science instrument is obstructed by a celestial body.
Orbit acquisition Achieving mission orbit.
Orbit decay For Earth-orbiting spacecraft, decrease in altitude due to at-
mospheric drag.
Orbit determination Computation of the orbit of a spacecraft (or celestial
body) in inertial space.
Orbit determination accuracy A quantitative measurement of the valid-
ity of the computed spacecraft orbit.
Orbit dynamics The study of the motion of the spacecraft's center of mass.
Orbit elements A set of parameters specifying the size, shape, and orienta-
tion of the orbit in inertial space, as well as the location of the spacecraft
at a given time (the epoch time). An example is the osculating Keplerian
elements.
Orbit generator An algorithm-based computational means of predicting
the future position and velocity of body in orbit. Commonly, the algorithm
can also be used to compute orbits into the past from a given set of orbital
elements.
Orbit maneuver A commanded change in the spacecraft's orbit, accom-
plished by producing a net thrust force on the spacecraft, generally by
firing a thruster. If applied to null errors in the actual orbit relative to the
desired orbit, the maneuver is called a station-keeping maneuver .
Orbit normal A vector perpendicular to the orbital plane. The normal vec-
tor obtained by the right hand rule , with the fingers curled in the direction
of spacecraft orbital motion, is positive orbit normal . The opposite vector
is negative orbit normal .