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

306 ONERA (Office National d’Études et de Recherches Aérospatiale)
reaching—the ultimate speed limit of light. The table
(“Round-Trip Times Assuming an Acceleration of 1g”)
shows some of the dramatic possibilities for lengthy
excursions in a 1g spacecraft. These figures assume equal
periods of acceleration and deceleration at 1g on both the
outgoing and return legs of the journey.
ONERA (Office National d’Études et de
Recherches Aérospatiale)
The French national aerospace research center. Since its
creation in 1946 it has worked on all the major French
and European aeronautical and space programs, including
Mirage, Concorde, Airbus, and Ariane. It operates a
number of major laboratories and wind tunnel facilities.
one-way light time (OWLT)
The time taken for an electromagnetic signal to travel one
way between Earth and a spacecraft or another body in
the Solar System.
OPAL (Orbiting PicoSat Launcher)
A small Stanford University satellite, launched from the
JAWSAT payload adaptor on January 26, 2000, and
which in turn deployed six even smaller picosatellites.
Two of these picosatellites were tethered and built by the
Aerospace Corporation for ARPA (Advanced Research
Project Agency) research, three (Thelma, Louise, and
JAK) were built by Santa Clara College, and one (Stensat)
was provided by radio amateurs. OPAL had a mass of 23.1
kg and measured approximately 20 cm in each direction.
Operation Paperclip
See Paperclip, Operation.
Orbcomm
The first commercial venture to provide global data and
messaging services; it became operational in 1998 and
uses a constellation of 26 satellites in low Earth orbit. At
full capacity, the Orbcomm system can handle up to 5
million messages from users utilizing small portable terminals
to transmit and to receive messages directly to the
satellites. The system is controlled from Orbcomm’s
headquarters in Dulles, Virginia.
orbit
The curved path an object follows under the gravitational
influence of another body. A closed orbit, such as that followed
by a satellite going around Earth, has the shape of
a circle or an ellipse. A satellite in a circular orbit travels
at a constant speed. The higher the altitude, however, the
lower the speed relative to the surface of the Earth. Main-
taining an altitude of 35,800 km over the equator, a satellite
is said to be in geostationary orbit. In an elliptical
orbit, the speed varies and is greatest at perigee (minimum
altitude) and least at apogee (maximum altitude).
Elliptical orbits can lie in any plane that passes through
Earth’s center. A polar orbit lies in a plane that passes
through the North and South Poles; in other words, it
passes through Earth’s axis of rotation. An equatorial
orbit is one that lies in a plane passing through the equator.
The angle between the orbital plane and the equatorial
plane is called the inclination of the orbit. As long as
the orbit of an object keeps it in the vacuum of space, the
object will continue to orbit without propulsive power
because there is no frictional force to slow it down. If part
or all of the orbit passes through Earth’s atmosphere,
however, the body is slowed by aerodynamic friction
with the air. This causes the orbit to decay gradually to
lower and lower altitudes until the object fully reenters
the atmosphere and burns up. An open orbit is one in
which a spacecraft does not follow a closed circuit
around a gravitating body but simply has its path bent
into the shape of a parabola or a hyperbola. 210
orbit decay
A gradual change in the orbit of a spacecraft caused by
the aerodynamic drag of a planet’s outer atmosphere and
other forces. The rate of orbit decay rises as the spacecraft
falls and encounters increasing atmospheric density,
eventually resulting in reentry.
orbital curve
The trace of an orbit on a flattened map of Earth or
another celestial body about which the spacecraft is orbiting.
Each successive orbit is displaced by the amount of
rotation of the body between each orbit.
orbital cycler
An economical method of travel within the Solar System
proposed by Buzz Aldrin. It turns out that there are stable
orbits that cross the orbits of both Earth and Mars.
Aldrin has suggested placing a large permanent station in
such an orbit. Travelers would embark on the station
when it passed Earth and disembark as it passed Mars.
The energy cost for the traveler is simply the cost to make
the rendezvous at each end.
orbital elements
Six quantities that are used to describe the motion of an
orbiting object, such as that of a satellite around the
Earth. They are: (1) the semi-major axis, which defines
the size of the orbit; (2) the eccentricity, which defines
the shape of the orbit; (3) the inclination, which (in the