The MIDAS instrument consists of a single mechanical unit of size 248 x 340 x 276 mm and a mass of 8 kg. The upper part contains the actual Atomic-Force-Microscope and the mechanisms that transport the collected dust particles to the sensors. The cone-shaped dust inlet sticks out of the outer surface of the spacecraft. The electronics are situated in the lower part of the casing. The entrance opening is directed into the dust stream from the comet during the gathering of dust particles. The shutter is opened for a specified time. The dust particles hit one of 64 specially coated facets around a target wheel. After the exposure time the wheel gets turned 180 degrees and is moved side ways, such that it ends up under one of the 16 sensors.
To start the probing by the needle of the Atomic-Force-Microscope the target has to be introduced. As soon as contact is found the linear scans across the target will start. The microscope consists of a row of 16 sensors that can be finely tuned around three axes through piezo electrical actuators. The maximal extent of motion is 100 µm parallel to the imaging surface and 10 µm perpendicular to it.
Each sensor consists of an approximately 0.5 mm long cantilever, which has a needle of approximately 10 µm length connected to its end in order to perform the etching procedure. The needle tip's radius is approximately 10 nm. This needle is moved over the target by a scanner. A piezo electrical sensor at the base of the cantilever obtains an electrical signal through the bending of the cantilever, which is used to control the scanning distance.
In the commonly used dynamic operation mode the cantilever is put into vibration near its resonance frequency of 90 Hz. During intermittent contact between the needle and the target, but also without contact, the amplitude and phase of the vibration changes through damping. However these parameters can be controlled remotely.
Structures up to approximately 5 µm can be investigated with this atomic force microscope. The apparent resolution is limited by the accuracy of the steering system and the shape of the needle. The resolution was tested and it was possible to measure structures of approximately 1 nm.
