All of the functionality of the equipment in this laboratory must be condensed into an instrument the size of a shoebox.

At the Open University, the Ptolemy team is interested in measuring the isotope ratios of Hydrogen, Carbon, Nitrogen and Oxygen. The ratios of heavy to light isotopes (e.g. ¹⁸O/¹⁶O in H₂O) act as “fingerprints”, allowing scientists to determine where a sample of water came from. At the Open University, such measurements are made using Gas Chromatography and Mass Spectrometry techniques, using instruments that usually fill a room, and can be made very precisely.

Existing measurements of isotope ratios within comets, however, have been made by observation of the coma at great distance, and the measurements are rather imprecise. If the stable isotope ratios of a comet could be measured much more precisely, then it may be possible to determine whether the water currently on Earth has a similar fingerprint to that in comets. If so, this would suggest that comets delivered to the early Earth the chemical elements necessary for life to evolve.

From the outset of the Rosetta Mission, it has been clear that the spacecraft will be taking a one-way trip. It cannot carry enough fuel for a return journey, and so it cannot bring samples from the comet nucleus back to Earth to be measured in the University’s laboratories. So instead, the Open University scientists have miniaturised their analytical system, in effect to take the laboratory to the comet. The result of this effort is the Ptolemy instrument, which is the size of a small shoebox and weighs less than 5 kg, but contains much of the functionality of its Earth-bound counterparts. Once delivered to the comet surface, Ptolemy will carry out the same experiments that that scientists would like to perform on Earth.