Thruster plumes: Sources for high pressure and contamination at the payload location

The ESA comet mission Rosetta was launched in early March 2004. The goal of this mission is the rendezvous with comet 67P/Churyumov-Gerasimenko in 2014. The spacecraft will then accompany the comet toward the sun for about one year. During this close encounter the evolution of the nucleus and the coma will be monitored and analyzed. One of the instruments onboard is the comet pressure sensor. Two gauges are integrated in the sensor, which allow the measurement of the dynamic pressure and the total neutral particle density in the cometary coma. From these measurements the expansion velocity of the coma can be derived. Starting shortly after launch, during a time period of about two years, several tests and background measurements of the total particle density in the vicinity of the spacecraft were made. Some data were collected during the Rosetta thruster firing cycles, which show a huge pressure increase during thruster operation by several orders of magnitude above the background pressure. After a two-year flight duration, the background pressure around the spacecraft is about 2 x 10(-11) mbar, whereas values up to 1 x 10(-5) mbar hake been recorded during the thruster operation. Such high pressures might induce, in the worst case, high-voltage discharges as well as be responsible for a contamination layer on sensitive exposed surfaces of the payload.