DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

decibel
universe and . indicates the time derivative. This
definition assumes an approximately isotropic
universal expansion (as is observed). The quantity
˙R is related to the Hubble parameter: H0 =
˙R/R.
decibel A dimensionless measure of the ratio
of two powers, P1 and P2, that is equal to 10
times the logarithm to the base 10 of the ratio
of two powers (P1/P2). The units expressed this
way are one-tenth of a bel and are referred to as
decibels. The power P2 may be some reference
power. For instance, in electricity, the reference
power is sometimes taken as 1 milliwatt (abbreviated
to dBm).
declination In terrestrial magnetism, at any
given location, the angle between the geographical
meridian and the magnetic meridian; that is,
the angle between true north and magnetic north
is the declination. Declination is measured either
east or west as the compass needle points
to the east or west of the geographical meridian.
East is taken as the positive direction. Lines of
constant declination are called isogonic lines,
and the one of zero declination is called the agonic
line. See dip, magnetic. In astronomy, an
angle coordinate on the celestial sphere corresponding
to latitude, measured in degrees north
or south of the celestial equator.
decollement A near horizontal detachment
zone between distinct bodies of rocks.
deep(-focus) earthquake Earthquakes at
depths ranging from about 300 to 700 km that
occur along the Wadati–Benioff zone, which is
inclined from a trench toward a continental side
beneath the subduction zone of an oceanic plate.
Fault plane solutions with down-dip compression
are dominant. Since a deep(-focus) earthquake
takes place under high-pressure conditions,
where friction is large, it is difficult to
explain its generation mechanism by frictional
sliding processes such as those for a shallow
earthquake. Recent laboratory experiments indicate
that shear melting, a self-feedback system
of phase transformations of olivine and
fault growth, and brittle fracturing due to pore
pressure are possible generation mechanisms of
deep(-focus) earthquakes.
© 2001 by CRC Press LLC
Deep Space 1 (DS1) A New Millennium
spacecraft launched October 24, 1998. It is the
first mission under NASA’s New Millennium
Program to test new technologies for use on future
science missions. Its objective is to test
12 advanced technologies in deep space to lower
the cost and risk to future science-driven missions
that use them for the first time. Among
these technologies are a xenon ion propulsion
system (performing beyond expectations), autonomous
navigation, a high-efficiency solar array,
and a miniature camera/spectrometer. By
December 1, 1998, DS1 had accomplished
enough testing to satisfy the technology validation
aspects of the minimum mission success
criteria and is well on its way toward meeting
maximum criteria.
It carried out a flyby of the near-Earth asteroid
1992 KD on July 28, 1998 at an altitude of
10 km. The primary mission ended on September
18, 1999. It is now on a new trajectory to
encounter Comets Wilson–Harrington and Borrelly.
Deep Space 2 Two microprobes that were
onboard the Mars Polar Lander spacecraft
launched on January 3, 1999 and lost in the
landing on Mars on December 3, 1999. The primary
purpose of the Mars Microprobe Mission
was to demonstrate key technologies for future
planetary exploration while collecting meaningful
science data (thus, it was named Deep Space
2). In this case, the scientific objectives were
to determine if ice is present below the Martian
surface; to characterize the thermal properties
of the Martian subsurface soil; to characterize
the atmospheric density profile; to characterize
the hardness of the soil and the presence of any
layering at a depth of 10 cm to 1 m. See Mars
Microprobe, Deep Space 1.
Deep Space Network (DSN) The NASA
Deep Space Network is a world-wide network
of large antennas with the principal function
of maintaining communications with spacecraft
beyond the moon’s orbit. The three main
tracking complexes are in Goldstone, California
(U.S.), near Canberra (Australia), and Madrid
(Spain).