The COmetary SAmpling and Composition experiment COSAC is one of the two 'evolved gas analysers' (EGAs) on board the Rosetta-Lander. Whereas the other EGA, Ptolemy, aims mainly at accurately measuring isotopic ratios of light elements, the COSAC is specialised on detection and identification of complex organic molecules.

It is, like all Lander experiments, an ambitious undertaking, and could be described as an effort to analyse in situ, mainly with respect to the composition of the volatile fraction, cometary matter nearly as well and accurately as could be done in a laboratory on Earth or, in other words, it can be regarded as an attempt to bring a laboratory to the surface of the nucleus and make it work there, in part automatically, in part under remote control. Considering that the 'laboratory' equipment must be extremely low in mass, power consumption, and cost but high in efficiency, resolution, sensitivity, and reliability, that it will be used first more than 10 years after assembly, and that the working environment on the nucleus will be rather harsh, the COSAC enterprise is even more challenging.

The fact that this experiment can, due to the Rosetta Lander rotatability, conduct analyses and investigations at different spots of the landing site and, aided by the drill, take samples for analysis from a depth up to at least 0.2 m, adds possibilities which would not even have existed for the cometary sample return mission originally considered.

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