The COPS instrument consists of two sensors dedicated to measuring the neutral gas parameters around the comet Churyumov-Gerasimenko, mainly the total density and the radial flow.
The first sensor is a usual pressure gauge (called nude gauge) which gives a direct measurement of the total density ntot (or total pressure) around the comet. This paremeter is extremely important for safety reason, indeed if the total pressure exceeds a particular value ( mbar), for example during close approach phases, other sensors of ROSETTA (ROSINA, ALICE...) will be shut down. Furthermore, the knowledge of the total density will be useful for existing models of the inner coma.
The second sensor is an equilibrium chamber. A spherical chamber, containing a pressure gauge and whose opening is facing the comet, will measure the molecular flow that comes from the comet (usually called the "ram pressure"). This measurement can be considered as redundant with the first one, as far as safety is concerned, since it gives information about the dynamics of the flow by direct comparison. If the ram pressure and the total pressure are similar, then the flow is "quiet". In the expected conditions of temperature, pressure and speed of the gas based on present observations and experience from Halley, the ram pressure will only be a few times the total pressure.
The two sensors are placed at the extremity of booms. The three electronic boards are confined with a box. The weight is 1.54 kg.
The nude gauge is a Bayard- Alpert gauge. Free electrons emitted from a filament at the potential of +30 Volt are accelerated toward a cylindrical anode (grid or helix wire) which is kept at the potential of +150 Volt. Inside the cylindrical anode grid a very thin molybdenum wire (0.15 mm diameter) is mounted. This wire is called a collector and its voltage is maintained at 0 volt (electrical ground) .
Emitted electrons are orbiting around the collector, ionizing neutrals along their path, and eventually being trapped on the anode. The ions created are collected by the collector, the resulting current is proportional to the total density of the gas.
The equilibrium chamber
The equilibrium chamber is a sphere (60 mm diameter) with an opening (6 mm diameter) at its top and which stands on a hollow boom containing a pressure gauge.
The pressure gauge is an extractor type gauge. The principle is the same as for the nude gauge namely ionizing neutrals with electrons and collecting the created ions. The normal filament for the electron source is replaced by a microtip solid state electron emitter The microtips chip is placed on a ceramic with gilded tracks. Bonded wire is used to establish the different connections.