Let's start off with the smaller, humbler one, Planck. Its main purpose is to measure what's known as the "Cosmic Microwave Background" or CMB. This is a type of radiation (read: light, except you can't see it) which is everywhere, but very very weak. It was released quite close to the beginning of the universe, so studying it is a way to understand what sort of physical processes were happening then. Planck is going to make what is called an all-sky map of the CMB. This means it will spin around and around, taking pictures of the sky in strips, until it's looked at everything.
Surely this has been done before, you say. Yes, it has, but not at high enough resolution to answer all the questions scientists have about it. And, as this webcomic illustrates (see panel 2), unlike in the movies, when you order someone to enhance an image that is already at the limit of its resolution...you can't. You could try and "interpolate between pixels", but in this case, that would be more commonly known as "making things up".
The aim of Planck, then, is to take the resolution of the all-sky maps of the CMB from the images on the left to the image on the right.
|Old maps of the CMB||Planck's map of the CMB (predicted|
More detail == more information == better understanding of the birth of the universe.
Herschel, on the other hand, is like the Hubble telescope, except a lot bigger. Its 3.5 m mirror eclipses Hubble's by more than a meter. It also operates in the infrared - again, a region of light that we can't see with our eyes. Its size and the type of light it uses means that it will be able to peer far into deep space to see very distant objects. It will also be able to see closer, but rather cold objects - at least when compared with stars - such as planets that aren't in our solar system.
Both of these spacecraft represent a remarkable technological achievement, as they operate at tenths of a degree above absolute zero. But they're in space, you protest, or at least my mum did. Surely it's not difficult to be cold in space. Actually, it's surprisingly difficult. Space, in our solar system, is about 3 degrees Kelvin. Spacecraft also generate quite a bit of heat, just from being powered on. Herschel & Planck are cryo-cooled by liquid helium. This means they only work as long as the supply of liquid helium is there. These missions have strictly limited lifetimes, like Replicants. Most spacecraft can keep going as long as they have enough power to keep their instruments on. Herschel & Planck are also going to be quite far from the Earth, beyond the reach of the space shuttle (see below). They can't be serviced by humans once they run out of liquid helium. Shuttle missions are also expensive, which means that it'll likely be cheaper to build new robotic spacecraft and send them out to replace Herschel & Planck than to try and refill their cooling systems.
But I'm getting ahead of myself. The launch is today. Good luck to them both, and their teams!
ETA: You can follow these spacecraft on Twitter: Herschel & Planck. ESA also has a Flickr account here, from which I poached that lovely image at the top.
ETA 2: The launch has been a success. The spacecraft have separated, are in communication with the Earth and are on their way to their orbits around the L2 point illustrated above. Yay!