As they started on their way to L2, the Planck and Herschel satellites were observed from Earth. The ESA “Optical Ground Station” on Tenerife, which is one of the stations that tracks ESA satellites and monitors their progress, observed them a few hours after launch and made this animation. At this time – around 21:30 GMT on 14 May 2009, just over 8 hours after launch – they were 100,000 km from Earth. That’s already a quarter of the distance to the Moon, but only around 1/15th of their final distance from Earth – L2 is 1,500,000 km from Earth. They were also seen by the Faulkes Telescope in Australia. In both images, there are 3 moving dots. The two brighter ones are Herschel and Planck while the fainter one, which is quite close to Planck, is the SYLDA 5 fairing which separated the two spacecraft in the rocket.
Soon the spacecraft will be much, much further away, and much more difficult (if not impossible) to see – at a few metres across they’re right on the lower limit of the smallest near-Earth objects observed. Their brightness will primarily be due to the reflectivity of the solar panels. Since these are pointed towards the Sun, and therefore almost at Earth, it might work out now and again as Herschel and Planck move in their orbits. I haven’t crunched the numbers to work out if this is possible (exercise for the reader?), but it should certainly be a challenge.