Today is a bit of sad day for the astrophysics community. The European Space Agency's space telescope, named Planck, was finally shut down after a little more than four years of collecting data.

The goal of Planck when it was launched back in 2009 was to provide higher quality data on the Cosmic Microwave Background. When the Universe was a mere 380,000 years old, the atoms that would form all matter cooled down to the point where they were no longer absorbing and emitting large numbers of photons, effectively making the Universe transparent for the first time in its history. The final emitted photons were in the microwave range of energies, and can still be detected today.

However the information gained from measuring the CMB has many uses. By studying its distribution, astrophysicists can calculate the shape of the Universe and its contents. Anywhere the matter clumped together, the microwave photons are slightly cooler than the rest of the cosmos. Areas with charged particles in bound states (ie clouds of atoms and ions) create different patterns in the CMB. All of these features have allowed an unprecedented level of understanding of the modern Universe as well as the early Universe.

The mission began closing down last August, when the satellite was moved slightly away from its operational orbit around the Sun–Earth ‘L2’ point towards a more distant orbit around the Sun where it can stay parked. In its final weeks, Planck has been preparing for its termination by closing activities and using up all of the remaining fuel before finally switching off the transmitter.

However it is not quite over for the physics community - much of the data from Planck has yet to be analyzed. 

So although it is a sad day, seeing the Planck project ended, but it is also a time to celebrate how much we have learned from its four and a half years in orbit. Enjoy your retirement Planck!

Note:This article was originally posted on October 24, 2013, but due to a computer glitch had to be reposted one week later.