Rosetta LAP Operations at the Target Comet

ESA's Rosetta mission is currently en route to comet 67P/Churyumov-Gerasimenko, where it will arrive in 2014. Among the instruments onboard, the Langmuir probe instrument LAP of the Rosetta Plasma Consortium can be expected to be particularly sensitive to early activation of the comet, as it can detect small plasma density changes even at the very low densities typical of the solar wind plasma. By running various operational modes, LAP can also characterize the denser plasmas expected to appear as the coma develops, and monitor the plasma dynamics around the comet all through the mission. In order to exploit these possibilities from the start of operations at the comet, it is essential to define useful operational modes in advance. The objective of this project was to prepare for LAP operations at the comet by simulating LAP measurements in the plasma environment expected at the comet, and for what can be regarded as typical operational scenarios for the spacecraft itself. To do so, results of an ESA-sponsored modeling activity at the International Space Science Institute formed the background, combined with a simulation of the LAP measurements. The results show that for heliocentric distances less than 2 AU, the LAP probe currents are appropriate to follow the dynamics of the plasma, but to extract the electron density and temperature dependence so to give an absolute value of the former we need other instruments, e.g. MIP designed to measure the electron density and temperature, in particular inside the contact surface of the comet, if the electron temperature indeed is as low as predicted by the models. The results also show that Rosetta has a good chance of occasionally being located close to the contact surface which creates a very good possibility to investigate the physics of this hitherto almost unexplored boundary.