A dimming light - Planck at perihelion
As Planck moves in its orbit, its distance to the Sun changes slightly. Not by much - just around 3%. The time at which Planck is closest to the sun, called "perihelion", is 2200GMT on 7th Jan 2009, when it will be around 148,600,000 km from the Sun. After that, Planck starts to get a little further away, until it reaches its furthest point, or "aphelion" in July, when it will be around 153,600,000km from the Sun. The names "perihelion" and "aphelion" are from the Greek words peri and apo, meaning "near" and "from", and the Greek word for the Sun: helios.
The distance to the Sun does not affect Planck greatly, and the inner workings are well isolated. However, just a 3% change in distance means around 7% change in the solar energy which reaches Planck from the Sun. This will have very small effects, such as changing the power generated by the solar panels, or causing the outer parts of the spacecraft to warm up slightly.
Moving towards and away from the Sun is by no means unique to Planck. All planets, moons and other bodies - including satellites - move in elliptical orbits, and the Earth is no exception. The Earth's perihelion was on 4th January this year, when it was 147,500,000km from the Sun, and in around 6 months time it will be 152,000,000km from the Sun. Just as Planck is warmed up, the Earth is every so slightly warmer than it would otherwise be. It is only by tiny a amount, and is much smaller than the normal variations in temperature cause by weather. Obviously, there's no weather on Planck, so the temperature changes are easier to measure!
Since the L2 point is linked closely with the Earth, it moves relative to the Sun as well. It is this motion of the L2 point which is the main reason for Planck moving closer and farther from the Sun. the details of its orbit around the L2 point mean that Planck's perihelion is shifted slightly from Earth's.
Planck's sister satellite, Herschel, will experience its own perihelion on 15th January.
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