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Dawn Dictionary


Achondrite ear icon
A meteorite consisting of igneous minerals and no chondrules, very similar to terrestrial igneous rocks

Albedo ear icon
The ratio of the amount of light reflected from a surface to the amount of incident light

Aperture ear icon
The effective diameter of the primary mirror or lens of a telescope

Aphelion ear icon
The point in a planetary orbit that is at the greatest distance from the Sun

Apparent Magnitude ear icon
The brightness of a star as it appears to the eye or to the telescope, as measured in units of magnitude—the brighter the star, the smaller the apparent magnitude.

Asteroid ear icon
(also called a “minor planet”) A small solar system object in orbit around the sun composed mostly of rock. Many of these objects orbit the sun between Mars and Jupiter. Their size can range anywhere from 100 meters in diameter to almost 1000 kilometers.
- Principal asteroids – Ceres, Pallas, Juno and Vesta
- C-type asteroids are carbon-rich, very dark, reflect only 3%-9% of sunlight.
- S-type asteroids are comprised of metallic nickel-iron mixed with iron- and magnesium-silicates.
- V-type asteroids contain more pyroxene than S-type asteroids.

Asteroid belt ear icon
A region of space lying between Mars (1.5 AU) and Jupiter (5.2 AU), where the great majority of the asteroids are found

Astronomical Unit (AU) ear icon
The mean distance between the Earth and the Sun: 149,598,500km; the preferred unit for distances within the Solar System

Axis of rotation ear icon
The theoretical straight line through a celestial body, around which it rotates

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Bode’s Law ear icon
This law describes a mathematical method for calculating planetary distances which states that the distance to the nth planet is 0.4 + (0.3)n Astronomical Units. Bodes law works surprisingly well out to Uranus.

Brightness Magnitude ear icon
The apparent brightness of a star is called the apparent magnitude and that is what is measured by a telescope: how much energy the star puts into the telescope's collecting area per second.

Brightness ear icon
Refers to the amount of light coming from an object

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Canon law ear icon
The body of ecclesiastical law by which a Christian church is governed.

Celestial body ear icon
A solid object found in space; in the heavens

Celestial Police ear icon
A group of astronomers looking for Kepler’s missing planet between Jupiter and Mars

Ceres ear icon
Largest of the known asteroids, and the first to be discovered (by Piazzi in 1801)

Chondrite ear icon
A meteorite containing chondrules, which are small, spherical silicate objects on the order of a millimeter in size that formed in the solar nebula before the asteroids were formed

Chondrules ear icon
A crystallized sphere of rocky material found in chondrite meteorites.  Named after the Greek for seeds, they are essentially tiny igneous rocks that formed in the solar nebula.

Circumscribe ear icon
To draw a line around; to encircle

Classical planets ear icon
Mercury, Venus, Mars, Jupiter and Saturn are known as “the classical planets

Collodian ear icon
A viscous solution used in coating photographic films; the wet collodion process produced plates that had a much higher sensitivity than was previously available

Comet ear icon
A small solar system object orbiting the Sun consisting of ice, dust and gas that form a coma and sometimes a visible tail whenever they orbit close to the Sun

Comet debris ear icon
Clumps of dust, ice and rocky material that are released from comets as they approach the sun

Concave ear icon
Hollowed or rounded inward like the inside of a bowl

Concave lens ear icon
A lens whose interior surface in hollowed or curved inward, away from the viewpoint, making the interior of the lens thinner than the outer edges

Convex ear icon
Curved or rounded outward like the exterior of a sphere or circle

Convex lens ear icon
A lens whose interior surface is curved outward, toward viewpoint, making the interior of the lens thicker than the outer edges

Convex - profile inversion ear icon
A modeling technique for producing a three-dimensional shape from a two-dimension convex profile

Copernican system ear icon
A heliocentric model of the universe; the hypothesis that the earth and the other planets orbit the sun

Coulomb ear icon
A measure of the amount of electric current, being the quantity transferred in one second by a current of one ampere. 1 coulomb = 1 A s.

Crust ear icon
The outer part of a planet, moon, or asteroid composed essentially of crystalline rocks—generally the crustal composition is different from the bulk composition of the planet (adapted from

Cyclic ear icon
Moving or recurring in cycles or periods

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Dawn Discovery Mission ear icon
The ninth of NASA’s Discovery Program missions will investigate two of the largest protoplanets in the main asteroid belt, Vesta and Ceres.

Declination (DEC) ear icon
Declination is measured in degrees, and refers to how far above the imaginary"celestial equator" an object is (like latitude on the Earth).

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Ecliptic ear icon
The mean plane of the Earth's orbit around the Sun

Elastic Collision ear icon
A collision between two particles which conserves the total kinetic energy and momentum of the system

Electromagnetic Radiation ear icon
"Waves" of electrical and magnetic "disturbance", radiated as visible light, radio waves, or any other manifestation of the electromagnetic spectrum

Electron Volt (eV) ear icon
A unit of energy used to indicate the energy of a charged particle. 1 eV is the energy gained when an electron is accelerated by a potential of one volt. 1 electron volt = 1.6 x 10-12 ergs

Elliptical ear icon
Shaped like an ellipse, a plane curve in which the sum of the distances of each point along its periphery from two points - its "foci" - are equal

Emission spectrum ear icon
A spectrum of energy released from a source, usually in the form of electromagnetic radiation

Emit (also see Light) ear icon
To send forth or to send out electromagnetic radiation

Emulsion ear icon
A physical mixture of two immiscible liquids. One example is oil and vinegar in your salad dressing. Another example is suspending silver salts in a medium for coating plates and films.

Ephemeris (Ephemerides; plural) ear icon
A table that gives the positions of objects in the sky at various times

Epicycle ear icon
Circular orbit of a body round a point that is itself in a circular orbit round a parent body—such a system was formulated to explain some planetary orbits in the Solar System before they were known to be elliptical

Epithermal Neutron ear icon
A neutron that has undergone energy loss due to scattering collisions, but is not in thermalequilibrium with the regolith (E = ~0.3 eV to ~600 keV)

Evolutionary path ear icon
The path by which a body changes from one initial state to its final state. For example, many complex or differientiated asteroids develop from simpler states.

Exposure ear icon
To lay open to the influences of climate or other external forces

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Fast Neutron ear icon
A neutron ejected at high kinetic energy in a nuclear reaction (E = ~103 eV to 6 MeV)

Fusion Crust ear icon
Fusion crust is the outer covering that a meteorite has acquired as a result of the melting of its surface layer as it passes through the atmosphere. The compression of the Earth's atmosphere heats the surface of the meteoroid to its melting point. Most of the melted material is ablated (removed): what remains resolidifies as the fusion crust. The thickness of these crusts can range from less than one millimeter up to several millimeters. Typically, the leading side of the meteoroid will have a thinner crust than the trailing side. Fusion crusts can be light or dark in color depending on the amount of iron in the meteorite minerals.

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Gamma rays ear icon
Very energetic EMR photons with energies between 1 MeV–¬10 GeV
Inelastic collision; a collision between two particles in which part of their kinetic energy is transformed into another form of energy (gamma rays)—the total amount of energy remains the same

Geocentric ear icon
Having the Earth at the center.

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Heliocentric ear icon
Having the Sun at the center.

Hubble Space Telescope (HST) ear icon
NASA launched the Hubble Space Telescope in 1990. It orbits around the Earth every 97 minutes about 600 kilometers above the Earth, above the distorting effects of the atmosphere. Hubble’s instruments include cameras and spectrographs, using mirrors to focus and magnify light.

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Igneous ear icon
In the case of meteorites the term “igneous” refers to a rock formed by solidification of a molten rock or metal. Examples of igneous meteorites include the iron meteorites, and basaltic meteorites (Howardites and Eucrites) thought to come from the asteroid Vesta.

Impact basin (also impact crater) ear icon
A circular depression on the surface of a planet, moon, asteroid, or other celestial body. Craters are typically caused by meteorite impacts. In complex craters formed by large impacts, a central peak or peak ring is caused by rebounding crustal rock after the impact. In the center of some impact craters on Earth, a crater lake accumulates; but some crater lakes occur in volcanic craters, as well.

Inclination ear icon
In astronomy, the angle between one plane and another. The (equatorial) inclination of a planet is the angle between the plane of its equator and that of its orbit. The inclination of the orbit of a planet in the Solar System other than Earth is the angle between the plane of that orbit and the ecliptic.

Inelastic Collision ear icon
A reaction involving a change in the kinetic energy of the system, as in ionization, excitation, or capture; or a process which changes the energy level of the system.

Ion ear icon
A charged particle consisting of an atom, or group of atoms, that has either lost or gained electrons.

Ionization ear icon
Loss or gain by an atom of one or more electrons, by which process the atom becomes an ion and instead of being neutral, has a charge: positive if it has lost an electron, negative if it has gained one.

Ion propulsion ear icon
A system in which the atoms of the propellant are given an electrical charge by removing electrons. The charged atoms—ions—are then accelerated with electric or magnetic fields and emitted at very high velocity, imparting a thrust to the spacecraft. The high velocity of the propellant makes this technology very efficient. On Dawn, xenon atoms are ionized and accelerated with an electric field which is produced with a voltage between two grids. This design is inherited directly from Deep Space 1 (DS1), which was the first interplanetary mission to use ion propulsion.

IPS ear icon
Acronymn for Ion Propulsion System. (See ion jet propulsion above.)

Inscribe ear icon
To draw on figure within another so as to have as many incidences as possible.

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Light ear icon
The radiant energy that enables organs of vision to perform the function of sight—more accurately called luminous energy. (see also Electromagnetic Radiation).
- Emitted - An emanation of light from a light-giving body, such as the Sun
- Infrared light - Electromagnetic radiation with wavelengths too long to be detectable by the human eye, yet able to be seen by an infrared camera.
- Reflected Light - Light that is purely returned back from or bounced off an object.
- Visible Light - Electromagnetic radiation with wavelengths of or close to those detectable by the human eye.

Light Curve ear icon
A plot of the amount of light detected from an object (i.e., the apparent magnitude) as a function of time. Light curves provide evidence of eclipsing binaries, variable stars, and track the progress of nova and supernova explosions. Asteroid light curves can give evidence of the asteroid's shape, or how its surface properties vary.

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Magnification ear icon
The effect of an optical system on the apparent angular size of an object—an increase in angular size occurs if the magnification factor is greater than 1

Magma ear icon
Molten rock material within a melted planet or asteroid from which igneous rock results by cooling (adapted from

Magnitude ear icon
An astronomical unit of brightness—originally corresponding to the eye's response to starlight—the magnitude system is logarithmic, with 5 magnitudes corresponding to a factor of 100 in brightness. To further confuse things larger magnitudes correspond to fainter objects.

Mantle ear icon
The part of a planet or asteroid between the crust and the core

Metamorphic ear icon
A metamorphic rock contains minerals that have recrystallized after melting. In meteorites, the melting may occur due to internal heat on the parent body—heat from radiogenic decay—or heat from impacts.

Meteor ear icon
The flash and trail of light that we see in the night sky caused by a meteoroid passing through the atmosphere

Meteorite ear icon
An object that passes through the Earth's atmosphere and is too large to be destroyed before it hits the surface—meteorites may in some way be connected with asteroids.

Meteoroid ear icon
An interplanetary piece of matter larger than dust, but smaller than an asteroid or dwarf planet

Minor planet ear icon
(see Asteroid) Technically, anything that is not classified as a planet is a minor planet, so any small body in the solar system that primarily orbits the Sun that is not characterized as a satellite of another planet. Pluto is a minor planet, for example.

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Nebulosity ear icon
A hazy, cloudy, or misty characteristic of matter in a gaseous or finely divided state

Neutron capture ear icon
A process in which a neutron is absorbed by a nucleus to produce an excited nucleus that transitions to ground state by emission of one or more gamma rays

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Observatory ear icon
A building equipped for observation of natural phenomena, as in meteorology or astronomy

Opposition ear icon
A configuration of the Sun, Earth and a planet or asteroid in which the apparent geocentric longitude of the planet or asteroid differs by 180 degrees from the apparent geocentric longitude of the Sun. During opposition, the planet is closest to full for that particular orbit.

Orbit ear icon
The path in space followed by a celestial body

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Patent ear icon
A writing securing to an inventor the exclusive right to make, use, and vend his invention

Period of revolution ear icon
The interval of time required to complete one revolution in an orbit

Photochemical ear icon
Materials in which the absorption of light leads to a chemical reaction

Photographic plate ear icon
The flat surface on which the emulsion containing light-sensitive chemicals is placed; after exposing to light, the emulsion forms a negative of the object photographed.

Photometry ear icon
The measurement of light. Specifically refers to the procedure of highly accurate measuring of the apparent magnitudes of astronomical objects. In general, astronomers measure only a portion of the wavelength spectrum when they do photometry. Different types of photometry are defined by the portion of the wavelength that they examine. Differential photometry, or "UBV Photometry", measures the light within three standard regions defined by filters. These are Ultraviolet, Blue and Visual (hence UBV). There are many different photometry systems and standards.

Photomultiplier tube ear icon
A vacuum encapsulated photocathode from which electrons are ejected by the photoelectric effect followed by multiple cathodes from which many additional electrons are emitted in a cascade—when finally collected, the original single electron may have generated a pulse of over one million electrons; first used in astronomy in the early 1950s in a process known as differential photometry

Planetary orbit ear icon
The path in space followed by a planet

Planet ear icon
A large object formed from the solar nebula; the disk-shaped cloud of gas and dust surrounding a protostar. The classification of a planet by type (such as giant, dwarf, terrestrial, jovian, super Earth, etc.), depends on the body's intrinsic and orbital properties.

Planetoids ear icon
(see Asteroids)

Power ear icon
The magnifying capacity of a lens, expressed as the number of times it multiplies the diameter of an object

Protoplanets ear icon
Early stage in the formation of planets according to the theory by which planetary systems evolve through the condensation of gas clouds surrounding a young star

Ptolemaic system or Ptolemaic Model of the Universe ear icon
A geocentric model in which the Earth remained stationary as the other planets the Sun, the Moon and the stars orbited it on their spheres— was eventually replaced by the Copernican model

Pyroxene ear icon
A common metasilicate, chiefly of calcium and magnesium, usually in short, thick, prismatic crystals; next to feldspar the most frequent constituent of igneous rocks

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Reciprocity failure ear icon
The tendency for film emulsions to suffer a rapid fall-off in their ability to gather data after an initial surge of light

Reflection ear icon
The return of light or sound waves from a surface

Reflecting telescope ear icon
Telescope that uses mirrors to magnify and focus an image

Refraction ear icon
The bending of light as it passes from one medium to another

Refracting Telescope ear icon
Telescope that uses lenses to magnify and focus an image onto an eyepiece

Regmaglypts ear icon
Thumbprint-like indentations that are caused by parts of the meteorite melting and being sloughed off in its descent through Earth’s atmosphere

Regolith ear icon
The layer of rocky or icy debris and dust made by meteoritic impact that forms the uppermost surface of planets, moons and asteroids

Retina ear icon
The sensitive membrane of specialized cells in the eye which receives the image formed by the lens and is connected with the brain by the optic nerve

Right ascension (RA) ear icon
Right ascension is measured in hours of time. It is similar to longitude on the Earth. Astronomers have chosen the Vernal Equinox to define the starting point for the measurement of right ascension. The Vernal Equinox is the point where the Sun appears to cross the Celestial Equator at the beginning of spring. It is therefore one of the two points where the Ecliptic intersects the Celestial Equator.

Rotation ear icon
Of a single body in space, spinning on an axis; of a planetary system, rotation is generally planar in relation to the parent star

Rotation axis ear icon
(see Axis of rotation)

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Scintillator ear icon
A phosphor-containing radiation detector that absorbs a photon or ionizing particle

Semiconductor ear icon
A substance or material, typically crystalline, that conducts electricity better than an insulator, but not as well as a conductor, because it only allows current to flow under certain condictions. Common semiconductors are silicon, germanium, and gallium arsenide.

Semi-major axis ear icon
Half the length of the major axis of an ellipse; a standard element used to describe an elliptical orbit

Sensitivity ear icon
The capacity to respond to stimulation

Serendipity ear icon
Finding without seeking

Silver bromideear icon
A yellowish, odorless powder that darkens on exposure to light; is used in photographic colloids

Silver iodideear icon
A light yellow, odorless power that slowly darkens by light; is used in photographic colloids

Solar Nebula ear icon
The cloud of gas and dust that condensed and accumulated to form the Solar System.

Solar System ear icon
The complex of planets, asteroids, comets, and debris gravitationally bound to the Sun and including the particles and fields emitted from the sun and interacting with its components

Spacecraft ear icon
A vehicle that can travel in outer space

Speckle interferometry ear icon
A process that uses ground-based telescopes and computer technology to make highly detailed or high-resolution images of asteroids by clustering together loads of tiny “specks” to form a clearer picture

Spectra ear icon
The energies of the photons emitted or absorbed by an atom or compound are distinct. Photon energies are directly related to their frequencies and thus wavelengths, which dictate the colors in the spectrum. By observing the wavelength or frequency of a reflected or emitted photon, it is possible to determine which atoms or compounds comprise the body being observed. Spectra can be detected from the light reaching us from stars and planets, near or very distant, enabling us to identify the composition of the observed object.

Spectroscope ear icon
An optical instrument used for producing and observing spectra of light or other radiation

Spectroscopy ear icon
Spectroscopy is the study of the detailed features of a body's spectrum, accomplished by measuring the intensity of light emitted or reflected at as many different wavelengths as possible. The resulting spectrum of light allows us to locate emission or absorption lines which determine the object's composition, temperature, and properties—and for a distant, fast-moving object, even its velocity relative to Earth.

Sublimation ear icon
The process by which a substance transitions directly from the solid to the gas phase. By comparison, evaporation is the process by which a substance transitions from a liquid to a gas phase.

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Telescope ear icon
An optical instrument for making distant objects appear closer or larger

Theory ear icon
In science a theory is a verified hypothesis applicable to many related phenomena.

Thermal Neutron
ear icon
A neutron produced by fission and slowed by a moderator, so that it is in thermal equilibrium with its surrounding medium;  (E< ~0.3 eV)

Titius-Bode Law ear icon
The Titius-Bode Rule was first devised in 1772 and comprised the series 0 + 4/10, 3 + 4/10, 6 + 4/10, 12 + 4/10, 24 + 4/10 and so on. It was found to describe fairly accurately the distance in astronomical units of the then known planets from the Sun.

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Vesta ear icon
The brightest of all minor planets, at times approaching naked-eye visibility, was discovered by Olbers in 1807.

Volatiles ear icon
Substances that change into a vapor at relatively low temperatures

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