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

140 flux
it also supported AFSATCOM (Air Force Satellite Communications
System), which is a vital component of the
American strategic nuclear capability.
flux
A measure of the energy or number of particles passing
through a given area of a surface in unit time. Luminous
flux is the rate of flow of energy in the form of photons.
Particle flux is the number of particles (for example, in the
solar wind) passing through a unit area per second. Magnetic
flux is a measure of the strength of a magnetic field
perpendicular to a surface.
flux density
The energy in a beam of radiation passing through a unit
area at right angles to the beam per unit time.
fly-away disconnects
Umbilical or other connections to the various stages of a
space vehicle that disconnect from the vehicle at liftoff,
either mechanically or by the actual rising of the vehicle
off the launch pad.
flyby
A type of space mission in which the spacecraft passes
close to its target but does not enter orbit around it or
land on it.
flyby maneuver
See gravity-assist.
Fobos
Two nearly identical Soviet spacecraft intended to
explore Mars from orbit and land probes on Phobos, the
larger (but still very small) Martian moon. One failed
completely, the other partially. The mission involved
cooperation with 14 other nations including Sweden,
Switzerland, Austria, France, West Germany, and the
United States (which contributed the use of its Deep
Space Network for tracking). As well as onboard instruments,
each vehicle carried a lander designed to carry
out various measurements on Phobos’s surface. Phobos
2 also carried a second, smaller “hopper” lander
designed to move about using spring-loaded legs and
make chemical, magnetic, and gravimetric observations
at different locations. Phobos 2 operated according to
plan throughout its cruise and Mars orbital-insertion
phases, gathering data on the Sun, interplanetary medium,
Mars, and Phobos. However, contact with the
probe was lost on March 27, 1989, two months after it
had entered Martian orbit and shortly before the final
phase of the mission during which the spacecraft was to
approach within 50 m of Phobos’s surface and release its
landers. The cause of the failure was found to be a malfunction
of the onboard computer. Contact with Phobos
1 had been lost much earlier, while the spacecraft
was en route to Mars. This failure was traced to an error
in the software uploaded to the probe, which had deactivated
its attitude thrusters. This caused a loss of lock on
the Sun, resulting in the spacecraft orienting its solar
arrays away from the Sun, thus depleting the batteries. It
remains in solar orbit.
Launch
Date: July 7, 1988 (Fobos 1); July 12, 1988 (Fobos 2)
Vehicle: Proton
Site: Baikonur
Mass (at Mars): 2,600 kg
FOBS (Fractional Orbital Bombardment System)
An orbital variant of the Soviet R-36 intercontinental ballistic
missile, known in Russia as the R-360 (see “R”
series of Russian missiles) and intended to deliver
nuclear warheads from space. The Western designation
“FOBS” refers to the fact that although the payloads
reached orbit, they were intended to reenter over their
targets prior to completing a full circuit of the Earth.
Four suborbital FOBS tests were carried out between
December 1965 and May 1966. These were followed by
more tests through 1971, delivering 5-ton payloads into
low Earth orbit. FOBS is a direct ancestor of the modernday
commercial Tsyklon launch vehicle.
Fontana, Giovanni da (c. 1395–1455)
An Italian author of a sketchbook, Bellicorum instrumentorum
liber (Book of War Instruments), published in 1420,
in which appear various rocket-propelled devices including
a cart designed to smash through enemy strongholds
and a surface-running torpedo for setting fire to enemy
ships.
footprint
(1) In the case of a communications satellite, a map of
the signal strength showing the power contours of equal
signal strengths as they cover the Earth’s surface. (2) In
the case of a spacecraft lander, the area within which the
vehicle is targeted to touch down.
force
In mechanics, the cause of a change in motion of an
object. It is measured by the rate of change of the object’s
momentum.