As well as the official ESA ground stations which are tracking Planck and Herschel, a group of (mainly) Spanish astronomers have been observing them with optical telescopes. They’ve also identified a few other pieces of the Ariane 5 upper stage travelling along with them, as well as the Sylda adapter which separated them within the rocket fairing.
On their “Images” page, you can see some light curves – which show how the brightness of the objects varies with time – for Herschel, Planck, Sylda, and a few other fragments. The vertical axis, labelled “R”, is the brightness in astronomical units of “magnitudes“. For comparison, the faintest stars visible with the naked eye are around magnitude 6, and a higher number means fainter – I’m sure this made sense to Ptolemy and Hipparchus. In fact, the scale is logarithmic, so a magnitude of 17 is about 10,000 Ifainter than magnitude 6, putting them at around the same brightness as a small asteroid or a larger Kuiper Belt Object.
It’s interesting to see that Herschel brightens by about 2.5 magnitudes (about a factor of 10) at one point. There is one very plausible explanation for this: the timing coincides pretty much with the orbital manoeuvre which Herschel executed on 18th May, which must have oriented the solar panels to a more favourable angle for reflecting sunlight back to Earth. Planck and the other fragments show smooth gradual declines in brightness over time as they get further from Earth.
So here’s where I make a prediction, just as any scientist should. I predict that Sylda and the fragments will eventually fall back to Earth and burn up, so they might brighten a little. They’re almost certainly in elliptical orbits so may vary slightly as their distance from Earth changes, and also if they’re spinning or tumbling at all. [I've been corrected by Bill Gray (see comment), who informs us that Sylda and the fragments will end up in heliocentric orbits - i.e. orbiting the Sun]. Planck’s brightness should stabilise when it reaches its final orbit around L2, as its angle realtive to the Earth and Sun should stay pretty much constant. I predict that Herschel, however, will show slight variations in brightness as it slews to point at various objects around the sky – though it has to keep its solar panels pointed somewhere towards the Sun. Whether the variations will be large enough to observe with telescopes on Earth remains to be seen.
If you want to see how far Planck and Herschel are from Earth, then you can use the JPL Horizons catalogue. It’s somewhat self explanatory, but the output can look a little technical. Change the “Target Body” and search for “Herschel” or “Planck”. You also have to make sure that the time span covers the range you want. For teh table output, I recommend selecting “Obsrv range and rng rate” and “One-Way Light-Time” – though there are many to choose from (though not all applicable). You can set some of the units in the “optional” section. When you hit “Generate ephemeris” you’ll get a table showing the numbers you’ve requested, and even a table explaining what they all mean. There are a few options if you want to export the results to a file your favourite spreadsheet programme can read, so you can make your own plots. You can play with a few other things too, such as setting the observer location to L2 (by putting “@392″ in the “Lookup Named Location” box). It’s not certain that the orbital parameters used are exactly what Planck and Herschel will actually use, but they’re probably not too far off.