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

460 Vanguard
fired from here in December 1958. Since then, Vandenberg
has acquired pads and support facilities for launching
other rockets, including versions of the Atlas, Delta,
Titan, and Scout. Because of its location, with open water
to the south, it is America’s premier launch site for polar
missions. It was also originally intended to serve as a
launch center for polar Space Shuttle missions, and construction
of Shuttle launch facilities at VAFB was begun.
However, this was permanently halted after the Challenger
disaster and the decision of the Department of
Defense to use expendable launch vehicles for highpriority
polar military payloads.
Vanguard
See article, pages 461–462.
VASIMR (Variable Specific Impulse
Magnetoplasma Rocket)
A plasma engine that has been under development by a
NASA astronaut for the past two decades. Astronaut
Franklin Chang-Diaz began work on the rocket technology
in 1979, when he was a graduate student at the Massachusetts
Institute of Technology. Since late 1993,
Chang-Diaz and colleagues have continued work on the
engine at the Advanced Space Propulsion Laboratory at
the Johnson Space Center. Unlike conventional rocket
engines, which ignite a mix of fuel and oxidizer to generate
thrust, VASIMR uses a series of magnetic fields to create
and accelerate plasma, or high-temperature ionized
gas. The process begins when neutral hydrogen gas is
injected into the first of three magnetic cells. That cell
ionizes the gas, stripping away the sole electron from
each hydrogen atom. The gas then moves into the central
magnetic cell, where radio waves, generated in a manner
similar to a microwave oven, heat the gas to more than
50,000°C, turning it into a high-temperature plasma. The
plasma is then injected into the last magnetic cell, a magnetic
nozzle, which directs the plasma into an exhaust
that provides thrust for the engine. A key advantage of
the engine is that its specific impulse—a measure of the
velocity of its exhaust—can be varied in flight to change
the amount of thrust. The specific impulse of the engine
can be turned down to provide additional thrust during
key portions of a mission, then turned up during cruise
phases to improve efficiency. If VASIMR is successfully
developed, it could cut in half the time needed for travel
to Mars. By firing continuously, accelerating during the
first half of the flight, and then turning to deaccelerate
the spacecraft for the second half, VASIMR could send a
human spacecraft to Mars in just over three months. In
addition, VASIMR would enable such a mission to abort
to Earth if problems developed during the early phases, a
capability not available to conventional engines. A scale
version of the VASIMR engine could fly in space as early
as 2004 as part of the Radiation and Technology Demonstrator
spacecraft.
VCL (Vegetation Canopy Lidar)
A spacecraft that will use a radarlike imaging technique
called lidar (light detection and ranging) to map the
height of the world’s forests to an accuracy of one meter.
VCL will shine five laser beams onto the forest’s canopy.
By reconstructing the reflected light, the satellite will produce
an accurate map of both the heights of the trees and
the topography of the underlying terrain. The data will
be used to calculate the density and mass of the world’s
vegetation, enabling scientists to monitor the capacity of
forests to absorb carbon dioxide. In addition, the satellite
will provide the first accurate estimate of how much carbon
is being released into Earth’s atmosphere through
the burning of tropical rainforests. VCL is a mission of
NASA’s ESSP (Earth System Science Pathfinder) program;
a launch date has yet to be decided.
vector
A quantity having both magnitude and direction; examples
include velocity, acceleration, and force. Vectors
are added when, for instance, movement takes place in a
frame of reference that is itself moving (for example, as
when a swimmer tries to cross a flowing river). Vectors are
added like arrows, end to end, so that in the case of two
vectors, the sum is the vector from the tail of the first to
the tip of the second.
vector thrust
A steering method in which one or more combustion
chambers are mounted on gimbals so that the direction
of the thrust force (thrust vector) may be tilted in relation
to the center of gravity of the missile to produce turning.
Vega 1 and 2
Two identical Soviet spacecraft, each carrying a Venus
lander and a Halley’s Comet flyby probe. The spacecraft
released balloons into the atmosphere of Venus and landers
onto the surface before flying on to a rendezvous
with Halley in March 1986. The twin Vegas passed 8,900
km and 8,000 km, respectively, from the comet’s nucleus,
returning photographs and analyzing ejected gas and
dust. (See table, “Vega Missions,” on page 463.)
Launch vehicle: Proton
Mass: 4,000 kg