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

deceleration
Slowing down, or negative acceleration.
decompression sickness
A disorder caused by reduced barometric pressure and
evolved or trapped gas bubbles in the body. It is marked
by pain in the extremities, occasionally leading to severe
central nervous system and neurocirculatory collapse.
Deep Impact
NASA Discovery Program mission to collide with a
comet and study material thrown out by the impact from
beneath the comet’s surface. If launched as planned on
January 6, 2004, Deep Impact will encounter comet Tempel
1 on July 4, 2005. The mission hardware consists of a
flyby spacecraft and a smart impactor that will separate
from the flyby probe 24 hours before collision. The 500kg
cylindrical copper impactor has an active guidance
system to steer it to its impact on the sunlit side of the
comet’s surface at a relative velocity of 10 km/s. Prior to
collision, the impactor will send back close-up images of
the comet. The impact itself will create a fresh crater,
larger than a football field and deeper than a seven-story
building. Two visible imaging systems on the flyby craft
will record the impact events and the subsurface
cometary structure, while two near-infrared imaging spectrometers
will determine the composition of the
cometary material. This is the first attempt to peer
beneath the surface of a comet to its freshly exposed interior
for clues to the early formation of the Solar System.
Images from the cameras on both the impactor and the
flyby craft will be sent to Earth in near real-time and be
made available on the Internet. The mission is under the
direction of principal investigator Michael A’Hearn of
the University of Maryland and is managed by JPL.
Deep Impact An artist’s rendering of the Deep Impact flyby
craft releasing the impactor, 24 hours before the impact event.
Pictured from left to right are comet Tempel 1, the impactor,
and the flyby craft. NASA
Deep Space Communications Complex (DSCC) 99
deep space
An imprecise term that may be used to refer to all space
beyond the Earth-Moon system or, alternatively, all
space beyond the orbit of Mars.
Deep Space 1 (DS1)
An experimental probe designed to test 12 advanced
spacecraft and science-instrument technologies that may
be used on future interplanetary missions. DS1 was powered
by a type of ion engine known as XIPS (xenon-ion
propulsion system). Such systems have been used for station
keeping by some satellites for a number of years, but
the DS1 ion drive was larger, more efficient, and worked
longer than any previously flown. In fact, DS1’s engine
accumulated more operating time in space than any other
propulsion system in the history of spaceflight. Among
the other devices and techniques successfully tested during
the probe’s primary mission were an autonomous navigation
system, a miniature camera and spectrometer, an
ion and electron spectrometer, a solar-energy concentrator
array, and experiments in low-power electronics.
DS1’s primary mission lasted two years and included a
flyby of the 3-km-wide asteroid Braille on July 29, 1999.
An extended mission culminating in an encounter with
comet Borrelly began in September 1999 but was soon
threatened by the failure of the craft’s most important
navigational instrument, its star tracker, which enabled
DS1 to orient itself relative to stellar patterns. Rather than
abandon the project, NASA engineers uploaded new software
to turn an onboard camera into a replacement star
tracker, despite major differences between the two devices.
It proved a valuable fix: on September 22, 2001, DS1 flew
past Borrelly’s nucleus at a distance of only 2,171 km,
snapping 30 or so superb black-and-white photos and collecting
data on gases and dust around the comet. The
spacecraft’s ion engine was turned off on December 18,
2001, but its radio receiver remains active in case future
generations want to contact the probe.
Launch
Date: October 24, 1998
Vehicle: Delta 7325
Site: Cape Canaveral
Mass, including propellant: 486 kg
Deep Space 2 (DS2)
See Mars Microprobe Mission.
Deep Space Communications Complex (DSCC)
Any one of the three Deep Space Network tracking sites
at Goldstone, California; near Madrid, Spain; and near
Canberra, Australia. These sites are spaced about equally