The MESSENGER team is assembling the spacecraft in a cleanroom to protect the science instruments and other sensitive components from contamination.
The assembly process actually produces shavings from drilling holes, debris from fitting insulation blankets, and small particles carried in by workers.
Engineers routinely inspect and clean the spacecraft; in this picture they are cleaning out the Launch Vehicle Adaptor, which will house four scientific instruments (see May 1 image). The anti-static mat under the spacecraft cuts down on static electric discharges and makes it easier to find screws and other small parts that may be dropped during assembly.
Later this week the first science instrument, the Mercury Atmospheric and Surface Composition Spectrometer (MASCS), will be installed in the Launch Vehicle Adapter - a major milestone for the MESSENGER project.