Absorption
Decrease in intensity of radiation, when it crosses a material medium,
as a consequence of an interaction between the radiation and the material
medium.
see also: science section
-> 
spectrum
Accretion (disk, zone)
Process whereby small particles of matter accumulate and create larger
bodies under the influence of their mutual gravitational attraction or
as a result of chance collisions.
see also: science section
->
solar system formation
| Aphelion The point on a planet or on an asteroid's elliptical orbit at which it is furthest from the Sun. |
 |
Apocentre
The point on a spacecraft's orbit at which it is furthest away from the
body it is orbiting.
Apogee
The most distant point from Earth on a satellite's orbit.
Arcmin, arcsec
The size of an object in the sky can be measured by the angle that it
covers when viewed from Earth. The full circle has 360 degrees. An arcmin
is 1/60 of a degree; an arcsec is 1/60 of an arcmin or 1/3600 of a degree.
The diameter of the full Moon is about one-half of a degree or 30 arcmin.
Asteroid
The
word asteroid means "star-like", even if these minor bodies
of the Solar System don't emit light on their own, but are visible only
because they reflect sunlight. The size of asteroids range from dust particles
to significant bodies hundreds of miles in diameter (Ceres, the largest
observed is 913 km of diameter). Globally, the total mass of all the asteroids
is less than that of the Moon.
Asteroids are found in different places in the solar system: most of them
orbit around the Sun, grouped in the main belt, which is a region located
between Mars and Jupiter, roughly 2-4 AU from the Sun. These asteroids
are further divided into families. Others, are farther objects, with highly
unpredictable orbits, such as the Trojans, which lie on the orbit of Jupiter,
or such as the Centaurs, in the very outer solar system. Asteroids that,
for some dynamical mechanism, closely approach the Earth are named Near
Earth Asteroidsand form a class of particular interest.
Asteroid belt
Region between the orbits of Mars and Jupiter which is populated by billions
of asteroids.
Astrometry
The branch of astronomy concerned with measuring the positions of celestial
bodies, such as stars and galaxies, and their real and apparent motions.
Astronomical Unit (AU)
1 Astronomical Unit corresponds to the distance separating the Earth from
the Sun. 1AU=150 million km.
Blueshift
When a distant object moves toward the observer the lines in its spectrum
shift to shorter (bluer) wavelengths. This is because of the apparent
compression of the wave of light. As a result of this compression the
wavelength shortens and thus shifts towards the blue side of the electromagnetic
spectrum. The blueshift of an astronomical object is an indication of
the speed at which this object is approaching the observer.
Icy body which orbits the Sun. Thought to be leftover planetesimals
from the formation of planets in the outer Solar System. The small, solid
nucleus consists of water and other ices coated with dark organic compounds.
As the nucleus approaches the Sun, it vapourises, creating a coma and
two main tails. These tails - one made of gas and one of dust - may stream
million of kilometres into space, and almost always point away from the
Sun. Some `dead' comets, which no longer display a coma or tails, resemble
asteroids.
Crater
Basin-shaped
depression in the surface of a planet or moon. May be caused by a comet
or asteroid impact, or by a volcanic eruption. Usually circular as seen
from above, impact craters often have a raised rim formed from material
(ejecta) thrown out by the collision. Larger impact craters have central
mountain peaks. Volcanic craters may contain lava which is supplied from
subsurface magma. They are surrounded by material produced during previous
volcanic eruptions.
On the left, Barringer Crater or Meteor Crater in Arizona, one of the
most famous craters on Earth.
Dispersion
Scattering of an electromagnetic wave as light is split into its constituent
colours by a prism or diffraction grating.
see
also: science section -> spectrum
Doppler shift
The change in observed frequency due to relative motion between source
and observer.
Dust (cosmic, interstellar)
Tiny particles in space (in the Solar System, around and among the stars...).
Typically a few angstroms in size. Dust is produced in many diverse processes,
such as supernovae explosions or asteroid collisions. The dust hampers
the view of optical telescopes - to which it is opaque - but can be detected
with infrared telescopes. The dust around a star absorbs the star's light
and re-emits it as infrared light; therefore, dusty regions (e.g., star-forming
regions) are best studied with infrared telescopes.
Ecliptic plane
The plane of Earth's orbit and the planets' orbit around the Sun. Projected
onto the celestial sphere it is the path the Sun takes across the sky
and because it is the plane of the Solar System, the planets always stay
close to it in their passages across the sky.
Electromagnetic radiation - spectrum
Electromagnetic
radiation, or light, can be considered to be composed of particles (photons)
or waves. The electromagnetic spectrum is the complete range of wavelengths
of the electromagnetic radiation. The properties of each radiation depend
on its wavelength: longer waves are less energetic than shorter waves
- photons with long wavelength have less energy than short-wavelength
photons. Electromagnetic radiation is usually described as bands of radiation
of similar wavelength, e.g., infrared, radio waves, microwaves, gamma
rays, X-rays...
These bands of radiation roughly correspond to the range of wavelengths
which can be detected by different instruments. Only a small fraction
of the entire range of electromagnetic radiation can be detected by the
human eye: visible light, or what in everyday-life is referred to simply
as light. The human eye cannot detect wavelengths longer than those of
the visible light, such as those of infrared light, microwaves (wavelengths
of centimetres), or radio waves (wavelengths of metres). Wavelengths shorter
than visible light cannot be seen either: ultraviolet light, X-rays, gamma
rays (the most energetic). Electromagnetic radiation can be described
in terms of wavelength (L), measured in metres (m), or frequency (f),
measured in hertz (Hz). The relationship between these two is given by:
f = L/c where c= speed of light.
see
also: science section -> spectrum
eV - electron Volt
In the description of light, the most convenient unit of energy
to use is the electron volt, abbreviated eV. The electron volt is the
energy gained by an electron that moves across a positive voltage of one
volt (V). For example 1.5 electron volts is the energy gained by an electron
moving from a negative metal plate to a positive plate which are connected
to the terminals of a common 1.5 volt "C" batter.
see also: science section
->
light
Elliptical orbit
An orbit which describes an ellipse or oval shape.
Energy
The capacity of a body or system to do work. In the metric measurement
system, the unit of energy is the Joule, which is the work produced by
a force of 1 Newton moving over a distance of 1 metre.
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
Reduction in the intensity of electromagnetic radiation received from
a celestial body (e.g. a star) as a result of scattering and absorption
by intervening material (e.g. dust).
Field of view (FOV)
The full angular extent of the sky being viewed by an instrument.
Fly-by
The passage of a spacecraft near a planet, a moon or an asteroid.
Frequency
The rate at which a wave oscillates: the number of full cycles performed
by the wave in a second. See also Electromagnetic radiation and Wavelength.
see also: science section
->
light
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
Natural 'slingshot' effect which increases a spacecraft's speed and changes
its direction of flight. It occurs when a spacecraft gains energy as it
flies close to a planet or moon. At the same time, the planet loses a
tiny amount of momentum, which causes its orbital speed to be fractionally
reduced.
Ground segment
All the facilities and systems required on Earth to control and operate
a space mission.
Hertz (Hz)
A measure of frequency. It is the number of oscillations per second of
a vibrating system.
Hydrocarbons
A group of chemical compounds composed only of carbon and hydrogen.
Infrared light is part of the radiation spectrum, between
0.7-30 micrometers.
But why is it so important? Infrared light turns out to be particularly
useful for studying the surfaces of planets, including our own. This is
because infrared light interacts with molecular and crystalline structures.
As a result, materials made of different kinds of molecules or crystals
reflect or absorb different wavelengths of infrared light. So when we
can't visit a place to identify its rocks or plants, we take pictures
of it in the infrared and study them to figure out what kinds of minerals
and molecules are present. Scientists split infrared light into three
categories: near infrared, with wavelengths from 0.7 to 1.3 micrometers
(or microns), mid infrared, with wavelengths from 1.3 to 3 microns, and
thermal infrared, with wavelengths from 3 to 30 microns. If you look at
the spectral irradiation curve above, you'll notice that the Sun produces
more near IR than mid IR light, and produces a relatively small amount
of thermal IR (small enough that it isn't even shown on the graph). However,
the sun produces enough thermal IR for us to feel its radiation as heat.
Chlorophyll--the green protein in plants that allows photosynthesis--is
very highly reflective in the near infrared, so satellites that detect
near infrared light are used to map vegetation health and abundance. Minerals
show a lot of variation in the mid infrared, so satellites that map geologic
features are designed to see variations in how much mid-infrared light
is reflected at different wavelengths.
Thermal infrared is a slightly different case from near and mid infrared,
because any object that has a temperature above absolute zero emits thermal
infrared radiation. What that means is that you don't need to have a light
source, like the Sun, to see an object in thermal infrared wavelengths.
We can't see thermal IR, but our skin can detect it. When we say that
something is hot, that is another way of saying that it is emitting a
lot of thermal infrared radiation. We make special thermal light bulbs,
called "heat lamps," that emit thermal IR--you may have seen
these in fast food restaurants. Have you ever noticed that heat lamps
are redder than ordinary lights? (Why do you think that is?) Some animals,
like pit vipers, have special sensory organs that allow them to "see"
warm-blooded animals by detecting the thermal IR that they emit. Studying
the thermal wavelengths that a material emits, and how those wavelengths
change with changes in atmospheric temperature, is another way of determining
the composition of the surface of a planet.
see also: science section
->
spectra
Instrument
Apparatus capable of registering information with a precise objective.
A science spacecraft can carry several instruments such as cameras, spectrographs,
magnetometers, gas analysers, etc.
Nm - Nanometre
A unit of distance in the metric scale and is abbreviated as nm. One nanometer
(nm) equals one thousand millionths of a meter (m) or 1 nm = 10-9 m) and
red light has a wavelength of 680 nanometers.
Perihelion |
|
