A single unit of information; the singular of data 2 Any numerical or geometrical quantity or set of such quan-

tities that may serve as a reference or base for other quanti- ties. When the concept is geometric, the preferred plural form is datums, as in the statement “Researchers used two geodetic

datums in the analysis of the experiment.”

Dawes, William Rutter

(1799–1868) British Amateur Astro-

nomer, Physician William Dawes made precision measure- ments of binary star systems. In 1850 he independently discovered the major inner C ring (or crêpe ring) of Saturn. Astronomers often call this particular Saturnian ring the crêpe ring because of its light and delicate appearance.

See alsoBINARY STAR SYSTEM; SATURN.

Dawn

The Dawn mission, which is scheduled to launch in June 2006, is part of NASA’s Discovery Program, an initia- tive for highly focused, rapid-development scientific space- craft. The goal is to understand the conditions and processes during the earliest history of the solar system. To accomplish this mission the Dawn scientific spacecraft will investigate the structure and composition of two minor planets, 1 Ceres and 4 Vesta. These large main belt asteroids have many contrasting characteristics and appear to have remained intact since their formation more than 4.6 billion years ago.

Dawn is a mission that will rendezvous and orbit the asteroids Vesta and Ceres, two of the largest protoplanets. Ceres and Vesta reside in the extensive zone between Mars

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and Jupiter with many other smaller bodies in a region of the solar system called the main asteroid belt. The objectives of the mission are to characterize these two large asteroids with particular emphasis being placed on their internal structure, density, shape, size, composition, and mass. The spacecraft will also return data on surface morphology, the extent of craters, and magnetism. These measurements will help scien- tists determine the thermal history, size of the core, role of water in asteroid evolution, and which meteorites found on Earth came from these bodies. For both asteroids the data returned will include full surface imagery, full surface spectro- metric mapping, elemental abundances, topographic profiles, gravity fields, and mapping of remnant magnetism, if any.

The top question that the mission addresses is the role of size and water in determining the evolution of the planets. Scientists consider that Ceres and Vesta are the right two celestial bodies with which to address this important ques- tion. Both asteroids are the most massive of the protoplanets in the asteroid belt—miniature planets whose growth was interrupted by the formation of Jupiter. Ceres is very primi- tive and wet, while Vesta is evolved and dry. Planetary scien- tists now suggest that Ceres may have active hydrological processes leading to seasonal polar caps of water frost. Ceres may also have a thin, permanent atmosphere—an interesting physical condition that would set it apart from the other minor planets. Vesta may have rocks more strongly magne- tized than those on Mars, a discovery that would alter cur- rent ideas of how and when planetary dynamos arise.

As currently planned, the spacecraft will launch from Cape Canaveral on a Delta 7925H expendable rocket in late June 2006. After a four-year heliocentric cruise, Dawn will reach Vesta in late July 2010 and then go into orbit around the minor planet for 11 months. One high-orbit period at 700 km of altitude is planned, followed by a low orbit at an altitude of 120 km. Upon completion of its rendezvous and orbital recon- naissance of Vesta, the Dawn spacecraft will depart this minor planet in early July 2011 and fly on to Ceres. The spacecraft will reach Ceres in mid-August 2014, where it will again go into 11 months of an orbital reconnaissance mission. Both a high-orbit scientific investigation of Ceres at an altitude of 890 km and a low-orbit study at an altitude of 140 km are current- ly planned. The Ceres orbital reconnaissance phase of this mis- sion will end in late July 2015. Aerospace mission planners anticipate that 288 kg of xenon will be required to reach Vesta and 89 kg to reach Ceres. The hydrazine thrusters will be used for orbit capture. Depending on the remaining supply of on- board propellants and the general health of the Dawn space- craft and its complement of scientific instruments, mission controllers at the Jet Propulsion Laboratory (JPL) could elect to continue this robot spacecraft’s exploration of the asteroid belt beyond July 2015.

The Dawn spacecraft structure is made of aluminum and is box-shaped with two solar panel wings mounted on oppo- site sides. A parabolic fixed (1.4-m-diameter) high-gain dish antenna is mounted on one side of the spacecraft in the same plane as the solar arrays. A medium-gain fan beam antenna is also mounted on the same side. A 5-m-long magnetometer boom extends from the top panel of the spacecraft. Also mounted on the top panel is the instrument bench holding the

cameras, mapping spectrometer, laser altimeter, and star trackers. In addition, there is a gamma-ray/neutron spectrom- eter mounted on the top panel. The solar arrays provide 7.5 kW to drive the spacecraft and the solar electric ion propul- sion system.

The Dawn spacecraft is the first purely scientific NASA space exploration mission to be powered by ion propulsion. The ion propulsion technology for this mission is based on the

Deep Space 1 spacecraft ion drive and uses xenon as the pro-

pellant that is ionized and accelerated by electrodes. The xenon ion engines have a thrust of 90 millinewtons (mN) and a spe- cific impulse of 3100 s. The spacecraft maintains attitude con- trol through the use of 12 strategically positioned 0.9 N thrust hydrazine engines. The spacecraft communicates with scientists and mission controllers on Earth by means of high- and medi- um-gain antennas as well as a low-gain omnidirectional anten- na that uses a 135-watt traveling wave tube amplifier.

In summary, the goal of the Dawn mission is to help sci- entists better understand the conditions and processes that took place during the formation of our solar system. Ceres and Vesta represent two of the few large protoplanets that have not been heavily damaged by collisions with other bod- ies. Ceres is the largest asteroid in our solar system and Vesta is the brightest asteroid—the only one visible with the unaid- ed eye. What makes the Dawn mission especially significant is the fact that Ceres and Vesta possess striking contrasts in composition. Planetary scientists now speculate that many of these differences arise from the conditions under which Ceres and Vesta formed during the early history of the solar system. In particular, Ceres formed wet and Vesta formed dry. Water kept Ceres cool throughout its evolution, and there is some evidence to indicate that water is still present on Ceres as either frost or vapor on the surface and possibly even liquid water under the surface. In sharp contrast, Vesta was hot, melted internally, and became volcanic early in its develop- ment. The two large protoplanets followed distinctly different evolutionary pathways. Ceres remains in its primordial state, while Vesta has evolved and changed over millions of years.

See alsoASTEROID;ELECTRIC PROPULSION.

deadband

In general, an intentional feature in a guidance and control system that prevents a flight path error from being corrected until that error exceeds a specified magni- tude. With respect to the space shuttle Orbiter Vehicle, for example, the attitude and rate control region in which no Orbiter reaction control subsystem (RCS) or vernier engine correction forces are being generated.

See also ATTITUDE CONTROL SYSTEM; SPACE TRANS- PORTATIONSYSTEM;VERNIER ENGINE.

deboost

A retrograde (opposite-direction) burn of one or more low-thrust rockets or an aerobraking maneuver that lowers the altitude of an orbiting spacecraft.

See alsoAEROBRAKING;ROCKET;THRUST.

debris

Jettisoned human-made materials, discarded launch vehicle components, and derelict or nonfunctioning space- craft in orbit around Earth.

See alsoSPACE DEBRIS.

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decay (orbital)

The gradual lessening of both the apogee and perigee of an orbiting object from its PRIMARY BODY. For

example, the orbital decay process for artificial satellites and space debris often results in their ultimate fiery plunge back into the denser regions of Earth’s atmosphere.

See alsoORBITS OF OBJECTS IN SPACE;SPACE DEBRIS.

decay (radioactive)

The spontaneous transformation of one radionuclide into a different nuclide or into a different energy state of the same nuclide. This natural process results in a decrease (with time) in the number of original radioac- tive atoms in a sample and involves the emission from the nucleus of alpha particles, beta particles, or gamma rays.

See also NUCLEAR RADIATION; RADIOACTIVITY; RADIOISOTOPE.

deceleration parachute

A parachute attached to a craft and deployed to slow the craft, especially during landing. Also called a drogue parachute.

See alsoDROGUE PARACHUTE.

deci-

(symbol: d) A prefix meaning multiplied by one-tenth (10–1_{); for example, decibel.}

See alsoDECIBEL; SI UNITS.

decibel

(symbol: dB) 1. One-tenth of a BEL. 2. A dimen-

sionless measure of the ratio of two POWER levels, sound intensities, or voltages. It is equal to 10 times the common logarithm (that is, the logarithm to the base 10) of the power ratio, P2/P1. Namely,

n (decibels) = 10 log10(P2/P1)

Since the bel (B) is an exceptionally large unit, the decibel is more commonly encountered in physics and engineering.

declination

(symbol: δ) For an object viewed on the celes- tial sphere, the angular distance north (0° to 90° positive) or south (0° to 90° negative) of the celestial equator. Declination is a coordinate used with right ascension in the equatorial coordinate system—the most commonly used coordinate sys- tem in astronomy.

See also CELESTIAL SPHERE; EQUATORIAL COORDINATE SYSTEM; KEPLERIAN ELEMENTS.

Deep Impact

The objectives of NASA’s Deep Impact mis- sion are to rendezvous with Comet P/Tempel 1 and launch a projectile into the comet’s nucleus. Instruments on the space- craft will observe the ejecta from the impacting projectile, much of which will represent pristine material from the inte- rior of the comet; the crater formation process; the resulting crater; and any outgassing from the nucleus, particularly the newly exposed surface. This project was successfully accom- plished in early July 2005.

The spacecraft consists of a 350-kg cylindrical copper impactor attached to a flyby bus. The launch mass of the flyby bus and impactor is 1,010 kg. The spacecraft is a box- shaped honeycombed aluminum framework with a flat rect- angular Whipple shield (a space debris shield) mounted on one side to protect components during the close approach

to a comet. Mounted on the framework are one high- and one medium-resolution instrument, each of which consists of an imaging camera and an infrared spectrometer that will observe the ice and dust ejected by the comet’s nucleus when the impactor strikes. Scientists believe that much of the ejected matter will be exposed to outer space for the first time in more than 4 billion years. The medium-resolu- tion camera has a field of view (FOV) of 0.587 degrees and a resolution of 7 m/pixel at a distance of 700 km. This instrument also supports navigation and collects context images. The high-resolution camera has a FOV of 0.118 degrees and a resolution of 1.4 m/pixel at a distance 700 km. The infrared spectrometers cover the wavelength range from 1.05 to 4.8 micrometers (µm) with FOV of 0.29 degrees (in the high-resolution mode) and 1.45 degrees (in the low-resolution mode). The total flyby bus instrument payload has a mass of 90 kg.

The flyby spacecraft measures approximately 3.2 by 1.7 by 2.3 m, is three-axis stabilized, and uses an onboard hydrazine propulsion system. The flyby bus communicates with scientists on Earth via X-band (8,000 MHz) through a 1-meter-diameter parabolic dish antenna mounted on a 2-axis gimbal. Communications between the Deep Space impactor and flyby spacecraft take place via S-band. The S-band por- tion of the radio frequency spectrum ranges from 1,700 to 2,300 MHz. The maximum data rate will be 400 kbps. A spacecraft electric power level of 620 W at the time of the comet encounter will be provided by a 7.5-square-meter solar array and stored in a small NiH₂battery.

The projectile is made of copper so it will be easily iden- tifiable in the spectra after the projectile is largely vaporized and mixed in with the comet ejecta on impact. The impactor is equipped with an impactor targeting sensor, an imager that provides knowledge for autonomous control and targeting, and a cold-gas attitude control system.

NASA used a Delta II rocket to launch Deep Impact from Cape Canaveral on January 12, 2005. The spacecraft traveled in a heliocentric orbit and rendezvoused with Comet P/Tempel 1 in July 2005. As presently planned, Deep Impact will be about 880,000 km from the comet on July 3, 2005 and moving at a velocity of 10.2 km/s relative to the comet. The projectile was released at this point, and shortly after release the flyby spacecraft executed a maneuver to slow down (at 120 m/s) relative to the impactor. About 24 hours after release, on July 4, 2005, the impactor struck the sunlit side of the comet’s nucleus. At the relative encounter velocity of 10.2 m/s, the impactor formed a crater roughly 25 meters deep and 100 meters wide, ejecting material from the interior of the nucleus into space and vaporizing the impactor and much of the ejecta. The flyby spacecraft was approximately 10,000 km away at the time of impact and began imaging 60 seconds before impact. At 600 seconds after impact the spacecraft was about 4000 km from the nucleus, and obser- vations of the crater began and continued until closest approach to the nucleus, at a distance of about 500 km. At 961 seconds after impact, imaging ended when the spacecraft reoriented itself to cross the inner coma. At 1,270 seconds the crossing of the inner coma was complete, and the spacecraft

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In document Encyclopedia of Space and Astronomy (Page 182-184)