ESA’s Planck spacecraft’s main goal is to study the Cosmic Microwave Background (CMB) – the relic radiation from the Big Bang. It will map the fluctuations in the CMB that became today’s clusters of galaxies. Released just 380,000 years after the Big Bang, the CMB is the oldest ‘light’ that can be seen in the Universe. Planck will see both nearby and distant clusters of galaxies and be able to use its microwave ‘eyes’ to study cold gas in our Galaxy, the Milky Way.