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 Space-view
door (SVD) covers the opening that faces the vast reaches
of space. This door has two functions; to limit contamination
and to prevent EarthÈs energy from reaching the Radiative
Cooler. The inner surface of this door is designed to
limit the intermediate-stage radiatorÈs field of view to cold
space to avoid data contamination from other radiant energy
sources. Like the NAD, the outside of the SVD is painted white
for thermal control. In order to keep this large doorÈs weight
down, the panels were made using a honeycomb pattern.
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.