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
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 SDD (including the panels, actuator/latch assemblies,
and thin aluminum screen) weighs 1.9kg.