The Planck mission was devised to collect and characterise radiation from the Cosmic Microwave Background (CMB) using sensitive radio receivers operating at extremely low temperatures. These receivers determined the black body equivalent temperature of the background radiation and were capable of distinguishing temperature variations of about one microkelvin. These measurements have been used to produce the best maps of anisotropies in the CMB radiation field.
The Planck spacecraft was 4.2 metres high and had a maximum diameter of 4.2 metres, with a launch mass of around 1.9 tonnes. The spacecraft comprised a service module, which housed systems for power generation and conditioning, attitude control, data handling and communications, together with the warm parts of the scientific instruments, and a payload module. The payload module consisted of the telescope, the optical bench, with the parts of the instruments that needed to be cooled - the sensitive detector units - and the cooling systems.
The Telescope and Instruments
The Planck telescope was an off-axis tilted Gregorian design with a primary mirror measuring 1.9 m × 1.5 m and with a projected aperture of 1.5 m diameter. The 1.1 m × 1.0 m secondary mirror focused the collected light onto the two scientific instruments:
- LFI (Low Frequency Instrument), an array of radio receivers using high electron mobility transistor mixers
- HFI (High Frequency Instrument), an array of microwave detectors using spider bolometers equipped with neutron transmutation doped germanium thermistors
Planck has provided a map of the Cosmic Microwave Background (CMB) field at all angular resolutions greater than 10 arcminutes and with a temperature resolution of the order of one part in 106. The simultaneous mapping of the sky at a wide range of frequencies has enabled scientists to separate the Galactic and extragalactic foreground radiation from the primordial cosmological background signal.
The Questions Planck is Answering
The questions that Planck is answering include:
- What are the (more precise) values of fundamental cosmological parameters such as the Hubble constant?
- Can it be shown conclusively that the early Universe passed through an inflationary phase?
- What is the nature of the dark matter that dominates the present Universe?
Planck has been carried into space by an Ariane 5 ECA launcher on 14 May 2009 and within two months after the launch reached its orbit around L2. For reasons of cost effectiveness, ESA decided to launch Planck together with Herschel, an infrared space telescope. The two spacecraft separated soon after launch and operated independently.
Planck's operational orbit was located 1.5 million km away from the Earth in a direction diametrically opposite the Sun, at the second Lagrange point of the Sun-Earth system (L2). The spacecraft operated in a Lissajous orbit around the L2 point with an average amplitude of about 400 000 km.
Planck operated beyond its nominal operational lifetime of fifteen months from the end of the Calibration and Performance Verification Phase; it was turned off on 23 October 2013, after nearly 4.5 years soaking up the relic radiation from the Big Bang and studying the evolution of stars and galaxies throughout the Universe's history.
The Cosmic Background Explorer (COBE) was launched on 18 November 1989. COBE determined that the CMB exhibits anisotropies at a level of one part in 105 and showed that the CMB spectrum matched that of a black body with a temperature of 2.725 K ± 2 mK.
The Wilkinson Microwave Anisotropy Probe (WMAP) was launched on 30 June 2001 and has made measurements of the CMB enabling the creation of a map of the anisotropies with much higher spatial and temperature resolution and improved accuracy compared to the COBE results.