For scientists engaged with large complex projects like Rosetta, there is always a delightful period early on when, unbound by practical realities, it is possible to dream. And so it was that at one time the scientists were thinking about having a lander with the capability to hop around a comet’s surface. In this way it would be possible to make measurements from different parts of the comet.

Interestingly, this unplanned opportunity presented itself on 12 No-vember 2014, when Philae landed not once but three times on Comet 67P/Churyumov-Gerasimenko.

The Ptolemy instrument on Philae is a compact mass spectrometer designed to measure the composition of the materials making up 67P/C-G, with a particular focus on organic molecules and mineral compo-nents. Earlier in 2014, Ptolemy had collected data at distances of 15,000, 13,000, 30, 20, and 10 km from the comet, while Philae was still attached to Rosetta.

This short excerpt is from an ESA blog from the Ptolemy team, you can read more at the following link.