DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

(NASA) initiative to test and prove new technologies
by flying them on missions similar to
science missions of the future; thus, the missions
are technologically driven. Included in
them are Deep Space 1 (propelled by xenon
ions), launched October 24, 1998; Deep Space
2 (which was to probe beneath the surface of
Mars), launched January 3, 1999 and lost on
Mars on December 3, 1999; Deep Space 3 (with
telescopes flying in formation), scheduled for
launch in 2005; Deep Space 4 (a voyage to the
heart of a comet); Earth Orbiter 1 (with an advanced
land imager); and Earth Orbiter 2 (with
a laser wind instrument on the Space Shuttle).
These missions will return results promptly to
future users about whether or not the technologies
work in space. The missions are high
risk because they incorporate unproven technologies,
probably without backup, that require
flight validation.
Newtonian A type of reflecting telescope invented
by Isaac Newton, with a small flat secondary
mirror mounted in front of the primary
mirror, to deflect rays approaching a focus out
one side of the support tube, where they are
viewed using a magnifying lens (eyepiece).
Newtonian cosmology A collection of models
of the universe constructed by the rules of
Newtonian mechanics and hydrodynamics. A
Newtonian model describes the universes via the
evolution of a portion of a fluid, assuming the
fluid moves in 3-dimensional Euclidean space.
The historically first such model (Milne and Mc-
Crea, 1934) demonstrated that some basic predictions
of the Friedmann–Lemaître cosmological
models could have been deduced much earlier
from Newtonian physics. However, Newtonian
models are of limited value only. Descriptions
constructed in general relativity theory
automatically imply the laws of propagation
of light in the universe, and the geometry
of space. The Newtonian models say nothing
about geometry and cannot describe the influence
of the gravitational field on the light rays.
Moreover, they are unable to describe gravitational
waves (which simply do not exist in Newtonian
physics).
© 2001 by CRC Press LLC
Newtonian gravity
Newtonian fluid A viscous fluid in which
shear stress depends linearly on the rate of shear
strain. In tensor notation, the flow law is σ ′ ij =
2µ˙εij , where σ ′ ij is deviatoric stress, ˙εij is the
rate of strain, and constant µ is the viscosity. A
solid such as a rock deforms like a Newtonian
fluid as a result of diffusion creep at elevated
temperatures.
Newtonian gravitational constant The constant
G = 6.673 × 10−11m3kg−1s−2 , which
enters the Newtonian force law:
F = Gm1m2 ˆrr −2
where F is the attractive force, m1 and m2 are
the masses, r is their separation, and ˆr is a unit
vector between the masses.
Newtonian gravitational fields The region
in which one massive body exerts a force of attraction
over another massive body according to
Newton’s law of gravity.
Newtonian gravity The description of gravity
due to Newton: The attractive gravitational
force of point mass m1 on point mass m2 separated
by displacement r
F =−ˆrGm1m2/r 2 ,
where G is Newton’s gravitational constant
G = 6.67 × 10 −11 m 3
/ kg sec 2
= 6.67 × 10 −8 cm 3
/ g sec 2
.
In Newtonian gravity forces superpose linearly,
so the force between two extended bodies
is found by summing (integrating) the vector
forces between infinitesimally small masses in
the two bodies. Newton proved that the force on
a point mass due to a spherical extended mass is
as if all the extended mass were concentrated at
its center.
One can also show that the force on a point
mass m1 due to a second mass is given by m1
times the gradient of the potential, defined in
general by integration over the extended mass:
with
F =−m1▽φ
φ =
GρdV ′ /r ′ ,
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