Space Glossary of
terms, abbreviations, and acronyms relating to astronomy, solar
system, rocketry and space technology.
Accretion Disk: A relatively flat,
rapidly rotating disk of gas surrounding a black hole, a newborn
star, or any massive object that attracts and swallows matter.
Accretion disks around stars are expected to contain dust
particles and may show evidence of active planet formation. Beta
Pictoris is an example of a star known to have an accretion
Aeronomy: The study of the atmosphere of
a planet, with particular attention to the composition,
properties and motion of atmosphere constituents.
Altitude: Height in space of an object or
point relative to sea level or ground level.
Antimatter: The 'opposite' to ordinary
matter. For every particle of ordinary matter there is an almost
identical antiparticle of antimatter: protons and antiprotons;
electrons and positrons. The particle's mass is exactly the same
as its antiparticle's mass, but their electrical charges and
other fundamental properties are opposite. When a particle
meets its antiparticle, they annihilate each other.
Angstrom (Å): A unit of length equal to
Aphelion: The point on a planet's
elliptical orbit at which it is furthest from the Sun.
Apogee: The most distant point from Earth
on a satellite's orbit.
Asteroid: A small solar system object
composed mostly of rock. Many of these objects orbit the Sun
between Mars and Jupiter. Their sizes range anywhere from 33
feet (10 meters) in diameter to less than 620 miles (1,000 km).
Asteroid Belt: A region of space between Mars and Jupiter
where the great majority of asteroids is found.
Astrometry: The branch of astronomy
concerned with measuring the positions of celestial bodies, such
as stars and galaxies, and their real and apparent motions.
Astrophysics: The part of astronomy
dealing with the physics and chemistry of astronomical objects
Astronaut: A person who travels in space.
Astronomy: The study of the universe and
the celestial bodies and phenomena that reside in it, including
their composition, history, location, motion, positions,
dimensions, distribution, energy and evolution.
Astrophysics: The part of astronomy that deals
principally with the physics and chemistry of stars, stellar
systems and interstellar material.
Astronomer: Scientist who observes and
studies planets, stars and galaxies.
Atom: The smallest particle of an element
that exhibits the chemical properties of the element.
Atmosphere: The gas that surrounds a
planet or star. The Earth's atmosphere is made up of mostly
nitrogen, while the Sun's atmosphere consists of mostly
Astronomical Unit (AU): The average
distance between the Earth and Sun (about 150 million kms - 93
million miles). This unit of length is commonly used for
measuring the distances between objects within the solar system.
Aurora: A phenomenon produced when the
solar wind (made up of energized electrons and protons) disturbs
the atoms and molecules in a planet’s upper atmosphere. Some of
the energy produced by these disturbances is converted into
colourful visible light, which shimmers and dances. Auroras have
been seen on several planets in our solar system. On Earth,
auroras are also known as the 'Northern Lights' (Aurora
Borealis) or 'Southern Lights' (Aurora Australis), depending on
in which polar region they appear.
Big Bang: A broadly accepted
theory for the origin and evolution of our universe. The theory
says that the observable universe started roughly 15 billion
years ago from an extremely dense and incredibly hot initial
Binary Stars: Binary stars are
two stars that orbit around a common centre of mass. An X-ray
binary is a special case where one of the stars is a collapsed
object such as a white dwarf, neutron star, or black hole, and
the separation between the stars is small enough so that matter
is transferred from the normal star to the compact star star,
producing X-rays in the process.
Black Hole: A region of space
containing a huge amount of mass compacted into an extremely
small volume. A black hole’s gravitational influence is so
strong that nothing, not even light, can escape its grasp.
Bok Globules: Dark clouds of dust and gas
that are typically condensing to form stars, named after
astronomer Bart Bok who studied them.
Brown Dwarf: A kind of 'failed' star: a
small and opaque object whose mass is not sufficient to start,
in its core, the nuclear reaction to transform hydrogen into
helium. A brown dwarf cannot therefore produce enough energy to
shine as a star. A brown dwarf's mass is not more than 0.08
Cassegrain Telescope: Popular design for
large, two-mirror reflecting telescopes in which the primary
mirror has a concave parabolic shape and the secondary mirror
has a convex hyperbolic shape. A hole in the primary allows the
image plane to be located behind the large mirror.
Cassini Division: A gap in Saturn's rings
that divides the outer set from the inner set of rings.
Cassini-Huygens Mission: The joint NASA/ESA spacecraft
studying Planet Saturn and its moons and carried onboard ESA's
Huygens Probe. Named after Jean Dominique Cassini (1625-1712),
the Italian-French astronomer who discovered several Saturnian
satellites and the magnificent rings of Saturn, and Christiaan
Huygens (1629-1695), who discovered Titan in 1655.
Celestial: Of or relating to the sky or
visible objects in the sky, like the Moon, Sun, planets, comets,
asteroids, stars, and galaxies.
Celsius (Centigrade): A temperature scale
on which the freezing point of water is 0° C and the boiling
point is 100° C.
Charge-Coupled Device (CCD): An
electronic detector that records visible light from stars and
galaxies to make photographs. These detectors are very sensitive
to the extremely faint light of distant galaxies. They can see
objects that are 1,000 million times fainter than the eye can
see. CCDs are electronic circuits composed of light-sensitive
picture elements (pixels), tiny cells that, placed together,
resemble mesh on a screen door.
Chromosphere: Layer of the Sun's
atmosphere located above the photosphere and below the corona.
It is about 10 000 km deep and consists mainly of ionised
hydrogen, helium and calcium. Its temperature ranges from 6000
°C at the photosphere boundary to 100 000 °C at the boundary
with the corona. Its low density means it is usually only
visible during a total solar eclipse.
Coma: The cloud of gas and dust that
forms around a comet's nucleus. This cloud is created when the
solar wind strikes the surface of the nucleus.
Comet: A ball of rock and ice, often referred to as a
'dirty snowball'. Typically a few kilometres in diameter, comets
orbit the Sun in paths that either allow them to pass by the Sun
only once or that repeatedly bring them through the solar system
(as in the 76-year orbit of Halley's Comet). A comet’s
'signature' long, glowing tail is formed when the Sun’s heat
warms the coma or nucleus, which releases vapours into space.
Comet Nucleus: The core of a comet, made up of ice, dirt
Comet Shoemaker-Levy 9 (SL-9): A comet that became
gravitationally bound to Jupiter, colliding with the planet in
July 1994. Prior to entering the planet’s atmosphere, the comet
broke into several distinct pieces, each with a separate coma
Constellation: A geometric pattern of
bright stars that appears grouped in the sky. Ancient observers
named many constellations after gods, heroes, animals, and
mythological beings. Leo (the Lion) is one example of the 88
Corona (plural: Coronae): The outermost
layer of the atmosphere of a star, including the Sun. The corona
is visible during a solar eclipse or when special adapters or
filters are attached to a telescope to block the light from the
star’s central region. The gaseous corona extends millions of
kilometres from the star’s surface and has a temperature in the
millions of degrees.
Coronal Mass Ejection (CME): An outward
eruption of billions of tonnes of material thrown into space
from the Sun's corona. Caused by magnetic storms on the Sun. The
most energetic of all solar explosions, they send material at
high speed (200 - 400 km/s) into space, and have a great
influence on the magnetosphere. They are most frequent around
the time of solar maximum.
Cosmic Rays: High-energy atomic particles
that travel through space at speeds close to the speed of light;
also known as cosmic-ray particles.
Cosmology: The investigation of
the origin, structure, and development of the universe,
including how energy, forces, and matter interact on a cosmic
Crater: A bowl-shaped depression
caused by a comet or meteorite colliding with the surface of a
planet, moon, or asteroid. On geologically active moons and
planets (like Earth), craters can result from volcanic activity.
Dark Energy: A mysterious force
that seems to work opposite to that of gravity and makes the
universe expand at a faster pace.
Dark Matter: Matter that is too
dim to be detected by telescopes. Astronomers infer its
existence by measuring its gravitational influence. Dark matter
makes up most of the total mass of the universe.
Density: The ratio of the mass of an
object to its volume. d=m/V
Deep Space Network (DSN): NASA network of
radio telescopes used to communicate with spacecraft operating
far from the Earth. It includes three 70 m antennas located at
Goldstone (California), near Madrid (Spain) and near Canberra
Diffuse: Widely spread or scattered; not
Doppler Shift: The change in observed
frequency due to relative motion between source and observer.
Earth: The third planet from the Sun and
one of four terrestrial planets in the inner solar system.
Earth, the only planet where water exists in large quantities,
has an atmosphere capable of supporting myriad life forms. The
planet is 150 million kms (93 million miles) away from the Sun.
Earth has one satellite — the Moon.
Electric Propulsion: Advanced propulsion
system, as used on certain satellites (SMART-1), employing solar
electric power to ionise and expel a propellant (such as Xenon
gas) at high speed, causing it to move forwards. Compared with
conventional chemical propulsion systems, electric propulsion
requires less propellant to perform similar operations.
El Niño: Warm-water conditions off the
western tropical coasts of the Americas, occurring irregularly
but usually around Christmastime, caused by weakening trade
winds and causing depleted fisheries, heavier-than-normal rain
in the central and eastern Pacific and drought in the western
Electromagnetic Radiation: A broad band
of energy that consists of radio waves, microwaves, infrared
rays, visible light, ultraviolet rays, extreme ultraviolet rays,
X-rays, and gamma rays.
Electromagnetic Spectrum: The range of
all wavelengths of radiation.
Electron: A very lightweight, negatively
charged part of an atom.
Element: A substance that is made of
atoms with the same chemical properties, and which cannot be
decomposed chemically into simpler substances.
Elliptical Orbit: An orbit which
describes an ellipse or oval shape.
Emission: The release of electromagnetic
radiation (e.g., light, X-rays, gamma rays) from excited atoms
Escape Velocity: The minimum speed needed
to escape the gravitational attraction of a celestial body and
enter space. The Earth's escape velocity is 11.2 km/s.
Extinction: Dimming of light
(attenuation) from a celestial body produced by the Earth’s
atmosphere or by interstellar material.
Extrasolar Planet: A planet orbiting a
star other than the sun.
Equinox: The date when the sun is
directly overhead at the equator (0° latitude). On this day, at
all places on the globe, night and day are of equal length (12
hours each). The spring or vernal equinox (for the northern
hemisphere) occurs on about March 21. The fall or autumnal
equinox (for the northern heisphere) occurs on or about
European Space Agency (ESA): An
intergovernmental organisation with a mission to provide and
promote - for exclusively peaceful purposes - the exploitation
of space science, research & technology, and space applications.
The 15 ESA Member States are: Austria, Belgium, Denmark,
Finland, France, Germany, Ireland, Italy, Norway, the
Netherlands, Portugal, Spain, Sweden, Switzerland and the United
Event Horizon: The spherical outer
boundary of a black hole. Once matter crosses this threshold,
the speed required for it to escape the black hole’s
gravitational grip is greater than the speed of light.
Exobiology: The study of biological
processes that have or could have evolved away from the Earth.
Extraterrestrial: An adjective that means
“beyond the Earth.” The phrase “extraterrestrial life” refers to
possible life on other planets.
Extravehicular: Outside the spacecraft;
activity in space conducted by astronauts (EVA = extravehicular
Fahrenheit: A temperature scale on which
the freezing point of water is 32° F and the boiling point is
Flyby Spacecraft: A spacecraft that
travels past a celestial object. Frequently, such a spacecraft
is unmanned and takes images of the object.
Galaxy: A collection of stars, gas, and
dust bound together by gravity. The smallest galaxies may
contain only a few hundred thousand stars, while the largest
galaxies have thousands of billions of stars. The
Milky Way Galaxy contains our
solar system. Galaxies are classified or grouped by their shape.
Round or oval galaxies are elliptical galaxies and those showing
a pinwheel structure are spiral galaxies. All others are called
irregular because they do not resemble elliptical or spiral
Gamma-ray Astronomy: Field of astronomy
which studies very energetic processes. Gamma-ray astronomers
observe the formation of new elements in space (nucleosynthesis),
very dense objects such as neutron stars or black holes, active
galaxies, supernovae and the most powerful explosions in the
Universe. Because gamma rays are absorbed by the Earth's
atmosphere, gamma-ray astronomy has to be done from space.
Gamma-Ray Burst (GRB): A brief, intense,
and powerful burst of gamma rays, the highest-energy,
shortest-wavelength radiation in the electromagnetic spectrum.
These bursts emanate from distant sources outside our galaxy and
last only a few seconds. They are the brightest and most
energetic explosions known.
Gamma Rays: Light with the shortest
wavelengths and the highest energies and frequencies in the
electromagnetic spectrum; also called gamma radiation.
Ganymede: One of Jupiter’s largest moons.
Ganymede, the largest satellite in our solar system, is about
5300 kilometres (3300 miles) wide and larger than the planet
Gas Giant: A large planet with a small, rocky core and a
deep atmosphere composed mostly of hydrogen and helium. Our
solar system contains four gas giants: Jupiter, Saturn, Uranus,
and Neptune. This group is also known as Jovian planets.
Gaseous Nebula: A glowing cloud of gas in interstellar
space. The cloud of gas may be either an emission nebula, which
absorbs ultraviolet light from nearby stars and re-radiates
visible light, or a reflection nebula, which reflects light off
of its dust particles.
General Theory of Relativity: A theory Einstein developed
to explain how gravity influences space and time. Its
fundamental principle is the equivalence of gravitational and
inertial forces. General relativity is a geometric theory which
states that gravity causes space-time to curve. This curvature
affects the motion of objects in space-time. General relativity
explains the bending of light by massive objects or the nature
of black holes.
Geocentric: An adjective meaning “centered on the Earth.”
Most early civilizations had a geocentric view of the universe.
Geosynchronous Orbit: Also known as geostationary. An
orbit in which an object circles the Earth once every 24 hours,
moving at the same speed and direction as the planet’s rotation.
The object remains nearly stationary above a particular point,
as observed from Earth.
Giotto Mission: ESA's first deep space probe.
Made close fly-bys of comets Halley (1986) and Grigg-Skjellerup
(1992) and obtained the first close range pictures of a comet
nucleus. Other experiments measured the magnetic field, charged
particle distribution, dust and gas composition and
Globular Cluster: A collection of
hundreds of thousands of old stars held together by gravity.
Globular clusters are usually spherically shaped and are often
found in the halos of galaxies. Each star belonging to a cluster
revolves around the cluster’s common center of mass.
Gravitational Lens: A massive object that
magnifies or distorts the light of objects lying behind it. For
example, the powerful gravitational field of a massive cluster
of galaxies can bend the light rays from more distant galaxies,
just as a camera lens bends light to form a picture.
Gravity: A physical force that appears to
exert a mutual attraction between all masses. It is proportional
to the mass of the object. In Einstein's Theory of General
Relativity, it is explained as a curvature of space-time.
Gravity Assist: An effect through which
an orbiting object, such as a spacecraft or a comet, gains or
loses speed by virtue of the gravitational might of a planet or
other celestial object that it passes. For example, the Cassini
spacecraft in its journey to Saturn used a gravity assist from
Earth to increase its velocity by about 36,000 km per hr (22,300
miles per hour).
Gravitational Wave: Ripple in the
structure of space-time which may occur individually or as
continuous radiation. According to Einstein's Theory of General
Relativity, they are emitted when extremely massive objects
experience sudden accelerations or changes of shape. They travel
through space at the speed of light. Gravitational waves remain
Great Red Spot: A circulating
storm located in Jupiter’s upper atmosphere. The storm, which
rotates around the planet in six days, is the width of two to
three Earths. Galileo first observed the spot in the 17th
Gyroscope: A disc with a heavy
rim mounted in such a way that its axis of rotation can adopt
any position. Once the disc is spinning, the rotation axis
remains fixed with reference to fixed stars, which makes the
gyroscope useful for determining movement away from a fixed
course. Three gyroscopes rotating about perpendicular axes help
to maintain the orientation of a spacecraft in space by
detecting rotation about any of the axes and initiating a
mechanism to correct it.
Habitable Zone: A region around
a star where planets with liquid water may be present. A planet
on the near edge of the habitable zone would have a surface
temperature slightly lower than the boiling point of water. A
planet on the distant edge of the habitable zone would have a
surface temperature slightly higher than the freezing point of
Hale-Bopp Comet: The brightest
comet to appear in the night sky for many decades. Discovered by
Alan Hale and Thomas Bopp on 22 July 1995, it reached perihelion
on 1 April 1997 and was a naked eye object for many months. Its
nucleus appears to be very large, about 40 km across.
Halley's Comet: The most famous periodic comet. Its
aphelion is beyond the orbit of Neptune, but it returns to the
inner Solar System every 76 years. Named after the 17th century
British scientist, Edmond Halley, who first recognised its
regular pattern of reappearances. Studied by a fleet of
spacecraft during its 1986 apparition, including ESA's Giotto.
Heat-Sink: A region or structure
that can absorb and hold large quantities of heat.
Heat Shield: A thick layer that
protects from heat.
Heliocentric: With the Sun in
the centre. Our Solar System is heliocentric.
Helioseismology: The study of the Sun's interior by
measuring oscillations (ripples) as they appear at the surface.
The oscillations are caused by sound waves that originate at
different depths inside the Sun. By measuring their travel times
and the distance they travel, scientists can study conditions in
the Sun's interior.
Heliosphere: The volume of space around the Sun which
contains the charged particles and magnetic field carried in the
solar wind. Its outer boundary (which has never been crossed) is
called the heliopause and marks the beginning of interstellar
space. This is thought to be 50 - 100 AU from the Sun.
Humidity: The presence of water
vapour in air.
Impact Crater: A large
depression on a moon or a planet. An impact crater is created
when an asteroid, a comet, or a meteorite strikes the moon or
the planet with great force.
Impact Event: A collision between two solar system bodies
that releases exceptionally large amounts of energy. Some
examples are the 1908 Siberian Tunguska impact by a comet or an
asteroid and the asteroid that struck Earth 65 million years
ago, which may have led to the extinction of the dinosaurs and
other species of the Cretaceous-Tertiary era.
Inflation: The theory that the
universe expanded very rapidly shortly after the Big Bang.
Interplanetary Space: The region
of space surrounding our Sun. Asteroids, comets, Earth, and the
solar wind are examples of things occupying interplanetary
Interstellar: Located between or among
Interstellar Dust: Microscopic solid
grains, composed largely of common elements such as graphite,
Interstellar Gas: Sparse gas in
Interstellar Medium (ISM): The gas and
dust between stars, which fills the plane of the Galaxy much
like air fills the world we live in. For centuries, scientists
believed that the space between the stars was empty. It wasn't
until the eighteenth century, when William Herschel observed
nebulous patches of sky through his telescope, that serious
consideration was given to the notion that interstellar space
was something to study. It was only in the last century that
observations of interstellar material suggested that it was not
even uniformly distributed through space, but that it had a
Interstellar Reddening: The interstellar
dust scatters blue light more effectively than red light - which
means that most of the light that reaches us through the
interstellar dust is reddish.
Io: The innermost of Jupiter’s four large
moons. Due to Jupiter's gravitational might, Io is geologically
active; its surface is peppered with volcanoes that send
sulphurous eruptions into its thin atmosphere. Io appears to
have the most active volcanoes in the solar system.
Io Plasma Torus: A bagel-shaped region of trapped sulphur
ions around Jupiter that originates from the surface of Io, one
of Jupiter’s moons. Gravitational tidal forces between Jupiter,
other Galilean moons, and Io cause tidal friction in Io’s
interior, producing geysers that spew sulphur at tremendous
speeds. Some of the sulphur ions leave Io’s surface and become
trapped around Jupiter.
Ion: An atom that has become electrically
charged by the addition or loss of one or more electrons.
Ionization: The process by which ions are
produced, typically by collisions with other atoms or electrons,
or by absorption of electromagnetic radiation.
Ionized gas: A gas that has been ionized
so that it contains free electrons and charged particles, a
plasma if it is electrically neutral overall.
Ionosphere: A region of the Earth’s upper
atmosphere where solar radiation ionizes the air molecules. This
region affects the transmission of radio waves and extends from
50 to 400 km (30 to 250 miles) above the Earth's surface.
Jovian Atmosphere: The atmosphere
surrounding the giant, massive planet Jupiter. The Jovian
atmosphere is composed primarily of hydrogen (90 percent) and
helium (10 percent). Other minor ingredients include water,
hydrogen sulphide, methane, and ammonia.
Jovian Planets: The planets Jupiter, Saturn, Uranus, and
Neptune. They are called Jovian planets because of similarities
in their composition and location. This group is also known as
the “giant planets,” the “gas planets” and, when grouped with
the planet Pluto, the “outer planets.”
Jovian Winds: The hurricane-force, high-velocity motion
of gas molecules in Jupiter’s atmosphere. The wind speed
increases as one travels deeper into Jupiter’s atmosphere. The
various patterns of atmospheric winds are easily identified in
Jupiter’s upper cloud layer.
Jupiter: The fifth planet from the Sun and the largest
planet in our solar system, twice as massive as all the other
planets combined. Jupiter is a gaseous planet with a very faint
ring system. Four large moons and numerous smaller moons orbit
the planet. Jupiter is more than five times the Earth’s distance
from the Sun. It completes an orbit around the Sun in about 12
Kelvin Scale: The temperature scale most
commonly used in science, on which absolute zero is the lowest
possible value. On this scale, water freezes at 273 K and boils
at 373 K.
Kepler’s Laws: Three laws, derived by 17th century German
astronomer Johannes Kepler, that describe planetary motion.
Kepler’s First Law: The orbits of planets are ellipses,
with the Sun at one focus. Therefore, each planet moves in an
elliptical orbit around the Sun.
Kepler’s Second Law: An imaginary line connecting any
planet to the Sun sweeps over equal areas in equal intervals of
Kepler’s Third Law: The square of any planet’s orbital
period is proportional to the cube of its mean distance from the
Kourou: French-operated launch site in
French Guiana. Location at 5° N latitude provides
fuel-efficient, near equatorial launches.
Lagrange Points: Points in the vicinity of two massive
bodies (such as the Earth and the Moon) where each others'
respective gravities balance. There are five points and are
labelled L1 through L5. L1, L2 and L3 lie along the centreline
between the centres of mass between the two masses; L1 is on the
inward side of the secondary, L2 is on the outward side of the
secondary; and L3 is on the outward side of the primary. L4 and
L5, the so-called Trojan points, lie along the orbit of the
secondary around the primary, sixty degrees ahead and behind of
L1 through L3 are points of unstable equilibrium; any
disturbance will move a test particle there out of the Lagrange
point. L4 and L5 are points of stable equilibrium, provided that
the mass of the secondary is less than about 1/25.96 the mass of
the primary. These points are stable because centrifugal
pseudo-forces work against gravity to cancel it out.
Light Year: The distance that light can
travel one year in a vacuum, which is about 5.8 trillion miles
or 9.5 trillion kms.
Local Bubble: A hot, low density region
containing the Sun and a few other nearby stars.
Local Group: A small cluster of more than
30 galaxies, including the Andromeda galaxy, the Magellanic
Clouds, and the Milky Way galaxy.
Long-Period Comet: A comet having an orbital period
greater than 200 years and usually moving in a highly
elliptical, eccentric orbit. Comets have orbits that take them
great distances from the Sun. Most long-period comets pass
through the inner solar system only once. Hale-Bopp is an
example of a long-period comet.
Luminosity: The amount of energy radiated
into space every second by a celestial object, such as a star.
It is closely related to the absolute brightness of a celestial
Lunar Eclipse: A darkening of the Moon,
as viewed from Earth, caused when our planet passes between the
Sun and the Moon.
Magellanic Clouds: The Magellanic Clouds
are two dwarf irregular galaxies. Known as the Large Magellanic
Cloud (LMC) and the Small Magellanic Cloud (SMC), the galaxies
are in the Local Group. The closer LMC is 168,000 light-years
from Earth. Both galaxies can be observed with the naked eye in
the southern night sky.
Magnetic Field: A region of space in which magnetic
forces may be detected or may affect the motion of an
electrically charged particle. As with gravity, magnetism has a
long-range effect and magnetic fields are associated with many
Main Sequence: The stable phase of a
star's lifetime, when outward pressure from internal fusion
process using hydrogen for fuel is balanced by the inward force
of self-gravitation. This phase is usually the longest phase of
a star's lifetime. Our Sun is a main sequence star.
Mars: The fourth planet in the solar
system and the last member of the hard, rocky planets (the inner
or terrestrial planets) that orbit close to the Sun. The planet
has a thin atmosphere, volcanoes, and numerous valleys. Mars has
two moons: Deimos and Phobos.
Marshall Space Flight Center (MSFC): NASA center
overseeing the research, development, and implementation of
three primary areas essential to space flight: reusable space
transportation systems, generation and communication of new
scientific knowledge, and management of all space lab
activities. Located in Huntsville, Alabama, the center aided in
the design, development, and construction of the Hubble Space
Megaparsec (MPC): Equals one million
parsecs (3.26 million light-years) and is the unit of distance
commonly used to measure the distance between galaxies.
Mercury: The closest planet to the Sun. The temperature
range on Mercury’s surface is the most extreme in the solar
system, ranging from about 400° C (750° F) during the day to
about –200° C (–300° F) at night. Mercury, which looks like
Earth’s moon, has virtually no atmosphere, no moons, and no
Microgravity: Microgravity is a term
commonly applied to a condition of free-fall within a
gravitational field in which the weight of an object is reduced
compared to its weight at rest on Earth.
Meteor: A meteoroid which is in the
process of entering Earth's atmosphere. It is called a
meteorite after landing.
Meteorite: The remains of a meteoroid that plunges to the
Earth’s surface. A meteorite is a stony or metallic mass of
matter that did not completely vaporize when it entered the
Meteoroid: A small, solid object moving through space. A
meteoroid produces a meteor when it enters the Earth’s
atmosphere or another planet.
Micrometeoroid: A very small meteoroid
with a diameter of less than a millimeter. Micrometeoroids form
the bulk of the interplanetary solid matter scattered throughout
the solar system.
Milky Way Galaxy: The Milky Way, a spiral
galaxy, is the home of Earth. The Milky Way contains more than
100 billion stars and has a diameter of 100,000 light-years.
Molecule: A combination of two or more
atoms bound together electrically; the smallest part of a
compound that has the properties of that substance.
Molecular Cloud: A relatively dense, cold
region of interstellar matter where hydrogen gas is primarily in
molecular form. Stars generally form in molecular clouds.
Molecular clouds appear as dark blotches in the sky because they
block all the light behind them.
Moon: A large body orbiting a planet. On
Earth’s only moon, scientists have not detected life, water, or
oxygen on this heavily cratered body. The Moon orbits our planet
in about 28 days.
NASA (National Aeronautics And Space
Administration: A Federal agency created on July 29, 1958
after President Dwight Eisenhower signed the National
Aeronautics and Space Act of 1958. NASA coordinates space
exploration efforts as well as traditional aeronautical research
Naval Research Laboratory (NRL):
The US Navy's corporate laboratory, which conducts a
broadly-based multidisciplinary program of scientific research
and advanced technological development directed toward maritime
applications of new and improved materials, techniques,
equipment, system, and ocean, atmospheric, and space sciences
and related technologies.
Nebula: A cloud of gas and dust located
between stars and/or surrounding stars. Nebulae are often places
where stars form.
Neptune: The eighth planet and the most
distant giant gaseous planet in our solar system. The planet is
30 times the Earth’s distance from the Sun, and each orbit takes
165 Earth years. Neptune is the fourth largest planet and has at
least eight moons, the largest of which is Triton. Neptune has a
ring system, just like all the giant gaseous outer planets.
Neutrino: A neutral, weakly interacting elementary
particle having a very tiny mass. Stars like the Sun produce
more than 200 trillion trillion trillion neutrinos every second.
Neutrinos from the Sun interact so weakly with other matter that
they pass straight through the Earth as if it weren’t there.
Neutron Star: An extremely compact ball
of neutrons created from the central core of a star that
collapsed under gravity during a supernova explosion. Neutron
stars are extremely dense: they are only 10 km or so in size,
but have the mass of an average star (usually about 1.5 times
more massive than our Sun). A neutron star that regularly emits
pulses of radiation is known as a pulsar.
Newtonian Reflector: A type of reflecting telescope whose
eyepiece is located along the side of the telescope. When light
enters the telescope, it reflects from the primary mirror to the
secondary mirror. The secondary mirror reflects the light at a
right angle through the side of the telescope to the eyepiece.
NOAA: National Oceanic and Atmospheric
Orion Nebula: A hot cloud of ionized gas
that is a nearby region of recent star formation, located in the
sword of the constellation of Orion.
Oort Cloud: Swarm of billions of comets
thought to surround the Solar System between 2000 and 20 000 AU
from the Sun. First proposed by E. Öpik in 1932 and later
developed by J. Oort in the 1950s. Its existence is based on
studies of long-period comet orbits, which seem to have their
aphelia in this zone.
Orbit: The act of travelling around a
celestial body; or the path followed by an object moving around
a celestial body. For example, the planets travel around, or
orbit, the Sun because the Sun’s gravity keeps them in their
paths, or orbits.
Orbital Decay: Reduction in the size (and
period) of a satellite's orbit due to gravitational effects or
aerodynamic drag, eventually causing it to re-enter the
Orbit Maintenance: Keeping a spacecraft's
orbit within a given set of orbital parameters, the reference
orbit, over time. See orbit maintenance maneuvers, orbital
Orbit Maintenance Manoeuvres: Changes in
a spacecraft's velocity, usually through thruster firings,
designed for orbital maintenance.
Orbital Plane: The plane, passing through
a planet's centre of gravity, in which a satellite orbits.
Parsec: A unit of distance equal to 3.26
light years; 3.09 x 1013 km or 1.92 x 1013
miles. A parsec is the distance at which the radius of the
Earth’s orbit subtends an angle of one second of arc.
Periodic Comet: A comet in a closed,
elliptical orbit within our solar system. These comets typically
have orbital periods of less than 200 years. Many comets have
orbits that keep them in the inner solar system and allow their
trajectories to be calculated with great accuracy and precision.
Perhaps the best-known periodic comet is Halley’s comet, whose
orbital period is 76 years.
Phases: Regularly occurring changes in
the appearance of the Moon or a planet. Phases of the Moon
include new, full, crescent, first quarter, gibbous, and third
Planetesimal: A small body of rock and/or
ice, under 10 kms (6 miles) across, formed during the early
stages of the solar system. Planetesimals are the building
blocks of planets, but many never combined to form large bodies.
Asteroids are one example of planetesimals.
Plasma: A substance composed of charged particles, like
ions and electrons, and possibly some neutral particles. Our Sun
is made of plasma. Overall, the charge of a plasma is
electrically neutral. Plasma is regarded as an additional state
of matter because its properties are different from those of
solids, liquids, and normal gases.
Plasma: A gas consisting of equal numbers
of ionized atoms and electrons. A fourth state of matter - not a
solid, liquid, or gas. In a plasma, the electrons are pulled
free from the atoms and can move independently. The individual
atoms are charged, even though the total number of positive and
negative charges is equal, maintaining an overall electrical
Pluto: A dwarf planet whose small size
and composition of ice and rock resembles the comets in the
Kuiper Belt, a region beyond Neptune’s orbit where Pluto
resides. Pluto was considered the ninth planet until August
2006, when the International Astronomical Union reclassified it
as a dwarf planet. Pluto’s orbit is more elliptical than those
of the eight solar system planets.
Protoplanet: A small body that attracts
gas and dust as it orbits a young star. Eventually, it may form
a planetary body.
Protostar: A collection of interstellar gas and dust
whose gravitational pull is causing it to collapse on itself and
form a star.
Quasar: Abbreviation of "quasi-stellar
object". Quasars are unusually energetic objects which emit up
to 1,000 times as much energy as an entire galaxy, but from a
volume about the size of our solar system.
Radar: A system using beamed and
reflected radio-frequency radiation, usually microwave, to
detect objects and measure ranges.
Rocket: A missile or vehicle propelled by
the combustion of a fuel and a contained oxygen supply. The
forward thrust of a rocket results when exhaust products are
ejected from the tail.
A planet located in the inner solar system and made up mostly of
rock. The rocky planets are Mercury, Venus, Earth, and Mars.
This group is also known as terrestrial planets.
Rotation: The spin of an object around its central axis.
Earth rotates about its axis every 24 hours. A spinning top
rotates about its centre shaft.
Satellite: A small body which orbits a larger one. A
natural or an artificial moon. Earth-orbiting spacecraft are
called satellites. While deep-space vehicles are technically
satellites of the sun or of another planet or of the galactic
centre, they are generally called spacecraft instead of
Saturn: Sixth planet from the sun, a gas giant or
Jovian planet. It is famous for
its obvious ring structure. Saturn is almost ten times the
Earth’s distance from the Sun. The planet completes a circuit
around the Sun in about 30 Earth years. Saturn is the second
largest and the least dense planet in our solar system. The
planet has more than 21 moons, including Titan, the second
largest known moon in our solar system.
Shepherd Moons: Moons which
gravitationally confine ring particles.
Spectrometer: An instrument for examining
recorded and measured electromagnetic spectra.
Scattering: The change in the paths of
photons without absorption or change in wavelength.
Solar Cycle: The periodic changing of the
Sun’s magnetic field, which determines the number of sunspots
and the amount of particles emitted in the solar wind. The
period of the cycle is about 11 years.
Solar Eclipse: A phenomenon in which the
Moon’s disk passes in front of the Sun, blocking sunlight. A
total eclipse occurs when the Moon completely obscures the Sun’s
disk, leaving only the solar corona visible. A solar eclipse can
only occur during a new phase of the Moon.
Solar Maximum: The midpoint in the solar
cycle where the amount of sunspot activity and the output of
cosmic particles and solar radiation is highest.
Solar Minimum: The beginning and the end of a sunspot
cycle when only a few sunspots are usually observed, and the
output of particles and radiation is normal.
Solar System: The Sun and its
surrounding matter, including asteroids, comets, planets and
moons, held together by the Sun’s gravitational influence.
Solar Wind: Streams of charged
particles flowing from the Sun at millions of kms an hour. The
composition of this high-speed solar wind may vary, but it
always streams away from the Sun. The solar wind is responsible
for the Northern and Southern Lights on Earth and causes the
tails of comets to point away from the Sun.
Solstice: The date when the sun
reaches its highest (summer solstice) or lowest (winter
solstice) noon-time point in the sky. On the summer solstice in
the northern hemispheres (on or about June 22), the subsolar
point is 23.5°N (Tropic of Cancer). On the summer solstice in
the southern hemisphere (on or about December 22) the subsolar
point is 23.5°S (Tropic of Capricorn).
Space Shuttle: A reusable U.S.
spacecraft operated by astronauts and used to transport cargo,
such as satellites, into space. The spacecraft uses rockets to
launch into space, but it lands like an airplane.
Specific Impulse: A measurement
of a rocket's relative performance. Expressed in seconds, the
number of which a rocket can produce one pound of thrust from
one pound of fuel. The higher the specific impulse, the less
fuel required to produce a given amount of thrust.
Star Tracker: A device that
continuously and automatically records the position and heading
of selected stars for spacecraft attitude control.
Sunspot: A region on the Sun’s
photosphere that is cooler and darker than the surrounding
material. Sunspots often appear in pairs or groups with specific
magnetic polarities that indicate electromagnetic origins.
Supernova: The explosive death
of a massive star whose energy output causes its expanding gases
to glow brightly for weeks or months. A supernova remnant is the
glowing, expanding gaseous remains of a supernova explosion.
Supernova Remnant: The remains of a
gigantic explosion marking the end of the life of either a
massive star or a white dwarf accumulating mass from a
Telescope: An instrument used to observe
distant objects by collecting and focusing their electromagnetic
radiation. Telescopes are usually designed to collect light in a
specific wavelength range. Examples include optical telescopes
that observe visible light and radio telescopes that detect
Temperature: A measure of the average of
random speeds of the microscopic particles in matter.
Terrestrial Planets: The four
planets of the inner solar system (Mercury, Venus, Earth, and
Mars) are called terrestrial planets because they are made up
mostly of rock.
Thruster: A control jet in a
spacecraft, usually using a monopropellant like hydrogen
peroxide or hydrazine.
Topography: The shape of a
surface, including its relief and the relative position of
Triton: The largest of Neptune’s
satellites. Triton has an atmosphere and is roughly the size of
Earth's moon. It has an “ice cap” of frozen nitrogen and methane
with “ice volcanoes” that erupt liquid nitrogen, dust, and
methane compounds from beneath its frozen surface.
Ultraviolet: A type of electromagnetic
radiation that is invisible and beyond the violet part of the
visible spectrum of light. It has shorter wavelengths (100 –
4000 Å) and carries higher energy than visible light.
Universe: The totality of space and time,
along with all the matter and energy in it. Current theories
assert that the universe is expanding and that all its matter
and energy was created during the Big Bang.
Uranus: The third largest planet in the solar system and
the seventh from the Sun. Uranus is 19 times the Earth’s
distance from the Sun and completes a circuit around the Sun in
about 84 Earth years. This gaseous, giant outer planet has a
visible ring system and over 20 moons, the largest of which is
Titania. Uranus is tipped on its side, with a rotation axis in
nearly the same plane as its orbit.
Van Allen Belt: A region containing charged particles
trapped in the Earth’s magnetic force field (magnetosphere). The
belt’s lower boundary begins at about 800 kms (496 miles) above
the Earth’s surface and extends thousands of kms into space.
Variable Star: A star whose luminosity (brightness)
changes with time.
Vandenberg: USAF-operated launch site on
the southern California coast. Location at 121° W longitude
provides easy polar and high-latitude launches over open ocean.
Venus: An inner, terrestrial (rocky)
planet that is slightly smaller than Earth. Located between the
orbits of Mercury and Earth, Venus has a very thick atmosphere
that is covered by a layer of clouds that produce a “greenhouse
effect” on the planet. Venus’s surface temperature is roughly
480° C (900° F), making it the hottest planet in the solar
X-Rays: Electromagnetic radiation with
very short wavelengths and very high energies and frequencies.
X-rays fall between gamma rays and ultraviolet radiation; also
called X-radiation or Roentgen ray.
X-ray Telescope: A special telescope used
to detect X-rays – high-energy electromagnetic radiation. The
high energy of X-rays means they will go through rather than
bounce off a “normal” telescope mirror. Instead, the mirrors are
arranged so the X-rays skip across them much like a stone skips
across the surface of a lake.
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