Planck was launched on 14 May 2009 on an Ariane 5 along with ESA’s Herschel infrared observatory. The mission was designed to study the Cosmic Microwave Background (CMB), the relic radiation from the Big Bang, with an accuracy defined by fundamental astrophysical limits.
At launch, the Herschel-Planck combination measured approximately 11 m high and 4.5 m wide, with a weight of about 5.7 tonnes. They separated soon after launch and headed into different orbits. The two spacecraft operated independently.
If Planck has been placed in orbit around Earth, heat from our planet, the Moon and the Sun would have interfered with its instruments, reducing their sensitivity. Instead, the telescope orbited the second Lagrange point of the Earth-Sun system (L2), a point in space located 1.5 million km from Earth. L2 has the important property that a spacecraft there can stay fixed in the Earth-Sun system and is situated on Earth’s night-side.
It was an excellent location for Planck: the satellite avoided unwanted emission from the Earth, Moon and Sun, which would otherwise confuse the signal from the CMB. Because Earth and the Sun are in the same general direction, it also offered good sky visibility for astronomical observations. In addition, this orbit kept Planck outside Earth’s radiation belts, which could have disturbed observations.
For more information, see L2, the second Lagrangian point.
It took Planck about 60 days to enter its final operational orbit around L2. The observatory settled into an orbit that resembles a halo around L2, with an average amplitude of