Planck scanning the sky

Some of the key questions Planck will answer are:

• Will the Universe continue its expansion forever, or will it collapse into a 'Big Crunch'?

• What is the age of the Universe?

• What is the nature of the so-called 'dark matter' (which may account for more than 90% of the total amount of matter in the Universe but that has never been detected directly)?

• What is the nature of dark energy (a hypothetical form of energy that may account for the Universe’s expansion at an accelerating rate)?

	Simulation of cosmic ray background, as Planck would see it

Why is the 'first light' of the Universe detected today as microwaves? When the 'first light' CMB was released, the Universe was much smaller than it is now. As a consequence, the waves of that primeval light were much more compressed, that is, their frequency was very high. The Universe has expanded since then, so the waves of that light have stretched, or the frequency of the CMB waves now is much lower than it used to be. They are classified in the 'microwave' range.

Planck is designed to 'see' the microwaves and, in practice, it will detect them by measuring temperature. That temperature is already known to be about 2.7K (which is very cold, about –270°C, near absolute zero). It has been measured to be 2.726K all over the sky to three decimal figures. This degree of accuracy in the measurement may seem good enough, but much more precise measurements are needed. Scientists know, from previous observations, that slightly hotter or colder 'patches' appear in the sky (different by one part in 100 000). Again, this may seem like a small difference, but these differences in temperature are nothing less than the imprints left in the CMB by the primeval 'seeds' of today's huge concentrations of matter — the galaxies and galaxy clusters for example.

The information Planck has to gather lies in the pattern formed by these slightly hotter and colder regions, called 'anisotropies' or 'inhomogeneities'. As a consequence, the Planck detectors will have to be highly sensitive and will have to work at temperatures very close to the absolute zero, otherwise their own emission of heat will spoil the measurements.

Planck will be launched in tandem with ESA' Herschel space telescope. Together they will study different aspects of the 'cold' cosmos.
 
 

Planck on display

Spacecraft
 
The Planck spacecraft, weighing about 1900 kilograms at launch, is 4.2 metres high and has a maximum diameter of 4.2 metres.

Planck will carry a telescope with a 1.5-metre primary mirror. The telescope will focus radiation from the sky onto the payload, two highly sensitive detectors called the Low Frequency Instrument and the High Frequency Instrument.

The Low Frequency Instrument (or LFI) is an array of 22 tuned radio receivers that will be operated at –253°C. These receivers will work grouped in four frequency channels, centred between 30 and 70 GHz. They are based on devices called 'HEMTs' (High Electron Mobility Transistors), and work just like transistor radios. The transistors amplify the signal collected by the antenna (the telescope), and the amplified signal is then converted to a voltage. In a normal radio, the detected signal would then be passed on to a speaker, but in Planck it will instead be stored in a computer for later analysis.

The High Frequency Instrument (or HFI) is an array of 52 bolometric detectors, which work by converting radiation to heat. The amount of heat is then measured by a tiny electrical thermometer, the signal from which is converted to a temperature by a computer. The HFI detectors will work in six frequency channels centred between 100 and 857 GHz. They are operated at –272.9°C (only one tenth of one degree above absolute zero). To achieve that temperature a complex system of on-board refrigerators is used, each of which uses a different technology to provide a successively colder temperature.

The Planck telescope and instruments are placed on top of an octagonal service module. A baffle surrounds the telescope and instruments to prevent straylight from the Sun and the Moon to spoil the detection of the microwave radiation. The baffle is also used to effectively radiate to cold space the heat generated by the focal plane units of the scientific payload, and to provide to the instrument coolers a cold and stable background environment of about -223ºC (or 50ºK). Inside the service module are the computers and subsystems that allow the spacecraft to function, and to compress the raw data signals from the instrument detectors. At the base of the service module sits a flat, round solar panel for generating electricity from sunlight to power the spacecraft, and to protect the whole spacecraft from direct solar radiation.

In order to achieve its scientific objectives, Planck' detectors have to operate at very low and stable temperatures. The spacecraft is therefore equipped with the means of cooling the detectors to levels close to absolute zero (-273.15ºC), ranging from about -253ºC to only a few tenths of a degree above absolute zero.
 
 

	Planck's cruise to L2

Journey
 
Planck will be launched on an Ariane-5 from the Guiana Space Centre, Kourou, French Guiana, in early 2009. It will be launched together with ESA's Herschel spacecraft, in a dual launch configuration.

About 2.5 hours after launch Planck will separate from Herschel and, in less than six months, the satellite will reach its final orbit. This is located at 1.5 million kilometres away from the Earth around a point in space called 'L2', or Second Lagrangian Point. This is far enough away to avoid the undesirable emission of heat from the Earth, the Moon and the Sun which would cause too much interference in the measurements.

Planck' routine science observations at L2 will last 15 months, allowing two sky surveys. The mission could in principle be further extended, depending on the resources still available for the instruments cooling.
 
 
History
 
Planck was formerly called COBRAS/SAMBA (Cosmic Background Radiation Anisotropy Satellite and Satellite for Measurement of Background Anisotropies).

This awkward former name had historical roots, since the mission grew out of a pair of proposals with similar objectives. The two proposed payloads were eventually merged into one mission.

COBRAS/SAMBA was studied by ESA in 1994, and later together with industry in 1995–96. The latter study was the basis for the selection of COBRAS/SAMBA as the third medium-sized mission (M3) in ESA's Horizon 2000 scientific programme.
 
 
Partnerships
 
The Prime Contractor for the Planck satellite is Alcatel Alenia Space (Cannes, France). It leads a consortium of industrial partners with the Alcatel Alenia Space industry branch in Torino, Italy, responsible for the Service Module. ESA and the Danish National Space Centre (Copenhagen, Denmark, funded by the Danish Natural Science Research Council) are responsible for the provision of Planck’s telescope mirrors, manufactured by EADS Astrium (Friedrichshafen, Germany). There is also a host of subcontractors spread throughout Europe, with a few more in the USA.

The Low Frequency Instrument was designed and built by a Consortium (led by the Istituto di Astrofisica Spaziale e Fisica Cosmica (IASF) in Bologna, Italy) of scientists and Institutes from Italy, Finland, the United Kingdom, Spain, The United States, Germany, The Netherlands, Switzerland, Norway, Sweden and Denmark.

The High Frequency Instrument (HFI) was designed and built by a Consortium (led by the Institut d'Astrophysique Spatiale (CNRS) in Orsay, France) of scientists and Institutes from France, the United States, the United Kingdom, Canada, Italy, Spain, Ireland, Germany, The Netherlands, Denmark and Switzerland.

Many funding agencies contributed to the LFI and HFI instruments hardware; the major ones are: CNES (F), ASI (I), NASA (USA), PPARC (UK), Tekes (FIN), Ministerio de Educación y Ciencia (Spain), and ESA.
 
 
Last update: 26 June 2008