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

February 28, 1959, a Thor-Agena placed Discoverer 1
into the first polar orbit ever achieved by a human-made
object. An Agena A carried Discoverer 14 into orbit on
August 18, 1960, and sent it back to Earth 27 hours later
to become the first satellite recovered in midair after reentry
from space. The Agena had primary and secondary
propulsion systems. The main engine had a thrust of
about 70,000 newtons (N), while the secondary was used
for small orbital adjustments. Both engines used liquid
propellants and (from the Agena B on) could be restarted
in orbit.
aging
The main problem facing future interstellar voyagers is
the immense distances involved—and consequently the
inordinate lengths of time required to travel—between
even neighboring stars at speeds where relativistic effects
do not come into play. For example, at a steady 16,000
km/s—over 1,000 times faster than any probe launched
from Earth has yet achieved—a spacecraft would take
about 80 years to cross from the Sun to the next nearest
stellar port of call, Proxima Centauri. No astronauts embarking
on such a voyage would likely live long enough
to see the destination, unless they boarded as children.
Volunteers might be hard to find. This problem of limited
human life span and extremely long journey times
led, earlier this century, to the suggestion of generation
starships and suspended animation.
agravic
A region or a state of weightlessness.
AIM (Aeronomy of Ice in the Mesosphere)
A proposed NASA mission to investigate the causes of the
highest altitude clouds in Earth’s atmosphere. The number
of clouds in the mesosphere, or middle atmosphere,
over the Poles has been increasing over the past couple of
decades, and it has been suggested that this is due to the
rising concentration of greenhouse gases at high altitude.
AIM would help determine the connection between the
clouds and their environment and improve our knowledge
of how long-term changes in the upper atmosphere
are linked to global climate change. It has been selected
for study as an SMEX (Small Explorer) mission.
air breakup
The disintegration of a space vehicle caused by aerodynamic
forces upon reentry. It may be induced deliberately
to cause large parts of a vehicle to break into smaller
parts and burn up during reentry, or to reduce the impact
speed of test records and instruments that need to be
recovered.
airlock 13
Air Force Flight Test Center
A U.S. Air Force facility at Edwards Air Force Base, California.
The Test Center includes the Air Force Rocket
Propulsion Laboratory (formed in 1952 and previously
known as the Air Force’s Astronautics Laboratory), the
Air Force Propulsion Laboratory, and the Air Force
Phillips Laboratory, which is the development center for
all Air Force rocket propulsion technologies, including
solid-propellant motors and liquid-propellant fuel systems
and engines.
Air Force Space Command (AFSPC)
A U.S. Air Force facility located at Peterson Air Force
Base, Colorado. Among its responsibilities have been
or are BMEWS (Ballistic Missile Early Warning System),
DSCS (Defense Satellite Communications System),
FLSATCOM (Fleet Satellite Communications
System), GPS (Global Positioning System), and NATO
satellites.
air-breathing engine
An engine that takes in air from its surroundings in order
to burn fuel. Examples include the ramjet, scramjet, turbojet,
turbofan, and pulse-jet. These contrast with a
rocket, which carries its own oxidizer and thus can operate
in space. Some vehicles, such as space planes, may be
fitted with both air-breathing and rocket engines for efficient
operation both within and beyond the atmosphere.
airfoil
A structure shaped so as to produce an aerodynamic reaction
(lift) at right angles to its direction of motion. Familiar
examples include the wings of an airplane or the
Space Shuttle. Elevators, ailerons, tailplanes, and rudders
are also airfoils.
airframe
The assembled main structural and aerodynamic components
of a vehicle, less propulsion systems, control guidance
equipment, and payloads. The airframe includes
only the basic structure on which equipment is mounted.
airlock
A chamber that allows astronauts to leave or enter a
spacecraft without depressurizing the whole vehicle. The
typical sequence of steps for going out of a spacecraft in
orbit is: (1) the astronaut, wearing a spacesuit, enters the
airlock through its inner door; (2) the airlock is depressurized
by transferring its air to the spacecraft; (3) the
inner door is closed, which seals the spacecraft’s atmosphere;
(4) the airlock’s outer door is opened into space,
and the astronaut exits. The reverse sequence applies
when the astronaut returns.