Each MODIS instrument has three doors that protect the internal components from contamination, damage, and in some cases help the instrument's self-calibration processes. The Solar-view door (Solar Diffuser Door, or SDD) covers the opening in MODIS that faces the Sun. This door also has an optical screen on it that operates independently of the door to allow variable amounts of solar energy to reach the Solar Diffuser. The SDD and screen also cover the opening to the Solar Diffuser Stability Monitor (SDSM). The SDD and screen can both be open, both be closed, or have the door open and the screen closed. This last position is the most common because it allows a compromise between contamination control and calibration. This door is also made in a honeycomb pattern to keep the overall weight low.
Each of the door assemblies were latched closed for launch, then opened in orbit by energizing the High Output Paraffin (HOP) actuators. Energizing the HOP actuators caused the release of captured torsion springs, which in turn rotated the latch arms to the open position. Each of the latches were used only once while in orbit, but have a re-settable feature that allowed them to be tested before launch. Stepper motor/gearhead sets with redundant windings were used to drive each door assembly to the open position and can be used again to close the doors when MODIS is directed by the spacecraft to go into the “safe mode.” All doors have failsafe mechanisms to provide fault-tolerant door operation.
The NAD weighs 3kg. The door is constructed of Kevlar, the outer surface of which is coated with S-13GP/LO white thermal control paint, and the inner surface with z-306 black paint. The NAD has two latches to anchor the door closed, and in case of a problem with the primary motor/gearhead, a mid-span failsafe linkage with stored spring energy.
The SVD weighs 7kg, and is coated with the same paint as the NAD.
The SDD (including the panels, actuator/latch assemblies, and thin aluminum screen) weighs 1.9kg.