Glossary

Absolute zero: The coldest temperature it is physically possible to reach, at -273.15oC, though no one has ever reached all the way down to absolute zero. The coldest temperature ever reached is less than a billionth of a degree above abolute zero.  The Kelvin temperature scale is measured from abolute zero.

All-Sky Survey: Planck's mission is to observe the entire sky.  It does this in around seven months, as that's how long it takes it to swing round the Sun as it orbits L2 and see the whole sky.  One such seven month period is called an "All-Sky Survey".  Planck will perform two surveys as part of its primary mission, and then hopefully go on to perform more until its helium runs out.

Black Body: A black body is the (rather misleading) name for an object that is a perfect absorber and emitter of radiation. It does not have to be black!  It is the term which describes an object that emits radiation with a characteristic spectrum, peaking at a specific wavelength.  The CMB looks like a perfect black body with a temperature of about 3 K, with emission peaking at a wavelength of light around 1 mm.  The Sun looks like an almost perfect black body with a temperature of 6000 K, with emission peaking at around 500 nm (in the middle of the visible light spectrum, somewhere near green).  The Sun looks white, or at least yellow, because it has relatively large amounts of the rest of the visible spectrum in it.  The atmosphere also scatters blue light (shorter wavelength) more than red light, which changes the apparent colour ever so slightly.

Bolometer: A detector used in many mm-wave instuments to measure radiation.  A bolometer works by detecting tiny increases in its temperature when it is heated by radiation.

Cosmic Microwave Background (CMB for short): The afterglow of the Big Bang, showing the Universe as it was around 400,000 years after the Big Bang.  The CMB appears as an almost constant black body with a temperature of 3 K.  Tiny fluctuations, at the level of 1 part in 100,000, contain information about the age, structure and evolution of the Universe.

Cryogenics: Cryogenic systems are systems used to cool something down to very cold temperatures.  At these low temperatures, elements which are gases at room temperature become liquids and then, at lower temperatures, solids.  For example, Nitrogen (the main ingredient in air) becomes a liquid below 77 K (-196oC) and a solid below 63 K (-210oC).


Image credit: Wikipedia Commons
Electromagnetic Radiation: The technical term for light is electromagnetic radiation.  This is because light can be thought of as waves of electricity and magnetism oscillating together as they travel through space.  An analogy is a taught string being shaken at one end, creating a wave which travels along it.  Electromagnet radiation covers an enormous spectrum, or range of wavelengths, from radio waves with a wavelength of miles, all the way up to harmful gamma raysproduced in nuclear reactions.  The visible part of the spectrum, containing the rainbow colours of red through violet, is only a tiny part of it.  The individual colours can be seen by passing white sunlight through a prism, as shown on the left.

European Space Agency (ESA): The European Space Agency coordinates and facilitates European space missions.  It has several sites, including a launch site in Kourou, French Guiana.

First Light Survey: To make the final check that everything is working as planned, the first two weeks of routine operations are dedicated to a "First Light Survey", after which (assuming everything goes as planned) the first All-Sky Survey will start.  The First Light Survey runs from  13-27th August 2009.

Inflation: A very short period of very rapid expansion which is thought to have occurred in the first tiny fraction of the Universe.  There are many theories of inflation, with many different durations and expansion rates.  Inflation has not been proven, but does explain a lot of remarkable features of our Universe.

Kelvin: A temperature scale similar to the Centigrade scale, but which starts at abolute zero (-273.15oC).  It was developed by and named after Lord Kelvin and is used throughout physics and astronomy.  A change of 1 K is the same as a change of 1oC, so 0oC = 273 K and 100oC = 373 K.  Room temperature is around 300 K.

Kourou: The location of one of the European Space Agency's space centres, officially called Guyana Space Centre.  Located in French Guiana, the launch site is close to the Equator.  This makes it ideal for launching satellites into equatorial orbits, which is where Planck will go before travelling to L2, the second Lagrangian Point.

Lagrangian Point (or Lagrange Point): A point where a small body (such as a satellite) can remain stationary in the orbit of two large bodies (such as the sun and the earth).  This is because the gravitational pull of the two larger bodies exactly cancel each other out at these points.  There are five Lagrangian Points in any system, with Planck travelling to the second point, called L2.  This point is around 1.5 million km (1 million miles) from Earth in the opposite direction to the Sun.  Since L2 is an unstable (like a ball on top of a hill, it is easy for a small object to fall away), Planck will instead orbit around the point in a "Lissajous Orbit" (a twisted ellipse shape). Image of L2
Image credit: ESA

Max Planck (1858-1947): Commonly referred to as the father of quantum theory, Max Planck was one of the most influential physicists of the twentieth century. His work to relate the energy and frequency of electromagnetic radiation earned him the 1918 Nobel Prize in Physics.  This theoretical work led to the theory of a black body.  The CMB is an excellent black body, which is why the Planck satellite is named after Max Planck.

Micron: A micron, or micrometre, is a thousandth of a millimetre.  The abbreviation is μm.

Millimetre-wave light: Millimetre-wave light (or mm-wave for short) is electromagnetic radiation with a wavelength between 1 and 10 mm.  It sits between sub-millimetre light and microwaves in the electromagnetic spectrum.  The emission from the CMB peaks in this region of the electromagnetic spectrum.

Nanometre: A nanometre is a millionth of a millimetre.  The abbreviation is nm.

Polarisation: The polarisation of light, or electromagnetic radiation, describes the direction in which it oscillates.  A light source generally emits millions of waves all oscillating in different directions, so overall there is no preferred polarisation.  This is called "unpolarised light".  However, some sources do have a preferred direction, so there will be more waves oscillating, say, up-down than left-right.  Flat surfaces, such as roads or mirrors, reflect some polarisations slightly more than others, and so unpolarised sunlight can become slightly polarised.  This is why polarised sunglasses are efficient at blocking sun light reflected off roads or water.

Sub-millimetre light: Sub-millimetre light is electromagnetic radiation with a wavelength of between 300 microns (0.3 mm) and 1 millimetre.  It sits between far-infrared and millimetre-wave light in the electromagnetic spectrum.

SYLDA 5: The adapter that separates the Planck and Herschel satellites within the Ariane 5 rocket fairing.  The name is a French acronym which stands for "Système de Lancement Double Ariane 5".

Wavelength: Light can be thought of as an electromagnetic wave.  Like a wave on a string, the electromagnetic wave has a "wavelength", which is the distance it travels in one oscillation.  For example, visible light has a wavelength between 400 and 800 nanometres, while infra-red light has a wavelength between 800 nm and around 300 microns.  The wavelength of millimetre radiation is, unsurprisinly, around a millimetre, while sub-millimetre light has a wavelength a little under a millimetre.
Image credit: Wikipedia Commons