It took one and a half months to arrive at its destination, 1.43 million kilometres from the Earth. But now Planck is there, in orbit around the second Lagrange point (L2). From here, for at least two years starting in August, it will travel back in time to the origin of the Universe by listening to the Cosmic Microwave Background (CMB). The CMB is the residual echo of the Big Bang, or, more precisely, the first light emitted 380 thousand years after the Big Bang, about 14 billion years ago.
In mid-June (while travelling towards L2) Planck activated the Italian instrument on board, the LFI (Low Frequency Instrument), and now it will activate the HFI (High Frequency Instrument). Planck’s ‘orbital insertion’ manoeuvre at L2 commenced on Thursday 2 July at 13.15 (Italian time) and the satellite completed the manoeuvre automatically that night using its on-board rockets. Planck now has a relative velocity with respect to the Earth of 1010 km/h and together with L2 and our planet it will orbit the Sun at the speed of 29.5 km per second.
To ‘collect’ the echo of the CMB with its on-board instruments, Planck has to work in the microwave region at the lowest possible temperatures, in order to eliminate any negative influences that could contaminate the signal. Planck’s two cooling systems are therefore fundamental; the passive system that takes the on-board temperature to -230 degrees Centigrade and the active system that lowers it further to -273.05 degrees Centigrade (just 0.1 degrees above absolute zero). The sensors on Planck measured these temperatures on 3 July, making the European satellite the coldest object in the Universe. The 74 detectors that cover the nine frequency channels that Planck works in need these conditions to look for the thermal variations in the CMB with a sensitivity equal to a millionth of a degree. More or less like being on Earth and measuring the heat generated by a rabbit on the Moon.