Aerospace and Electronic Systems - April 2019 - 10

The Lunar IceCube EM-1 Mission: Prospecting the Moon for Water Ice
Attitude control models were developed that informed the
ACS design, resulting in the use of three 50 milli-Newton
meter second (mNms) reaction wheels, a precise star
tracker and a series of sun sensors. Mission operations
will be made relatively complicated by these attitude control maneuvers, but the mission design based on the ACS
models developed closes for all mission phases.

SPACECRAFT SYSTEMS
Figure 5.
Attitude control and orientation of Lunar IceCube during the
science orbit.

products, and interface with the Mission Operations Center (MOC) located at MSU [6]-[8]. The final science orbit
has the following characteristics:
100 km periapsis altitude at equatorial crossing.
Drives argument of periapsis at ascending node.
High inclination (70-90).
Overlap of groundtracks for calibration.
Drives Semimajor axis (period of 7 hrs).
5000 km apoapsis altitude.

ATTITUDE MODELING AND CONTROL
The Lunar IceCube mission requires an intricate choreography of the attitude control systems (ACS), including
on-board determination and control systems and mission
operations to perform complex attitude control maneuvers
during all phases of the mission. During the science orbit,
these maneuvers include: 1) orienting the spacecraft's
instrument Lunar Nadir for science observations, 2) orienting toward anti-Lunar Nadir for instrument calibration,
3) pointing the antenna systems toward the Earth vector
for communications, 4) keeping the solar arrays pointed
toward the sun vector, and 5) orienting the spacecraft to
optimize thermal management, all while avoiding "keep
out zones" for the star tracker and IR spectrometer. ACS
maneuvers during a typical science orbit are shown below
in Figure 5.
Since the spacecraft contacts the ground approximately once every three orbits, the ACS command
sequence must be carefully constructed, and the spacecraft
must act autonomously between communication sessions.
The spacecraft will enter a safe mode if a scheduled communication session is missed. These requirements along
with requirements to dynamically adjust thrust vectors
and attitude orientations during the cruise phase and to
maintain a challenging low perigee science orbit, have
represented a challenge for the Flight Dynamics team.
10

BIRCHES INSTRUMENT
The BIRCHES is a compact (1.5U, 3.2 kg, 10-15 W
including cryocooler) point spectrometer with a compact
cryocooled 1 Megapixel HgCdTe focal plane array for
broadband (1-4 mm) measurements, achieving sufficient
SNR (>400) and spectral resolution (10 nm) through the
use of a linear variable filter assembly to characterize and
distinguish important volatiles (water, H2S, NH3, CO2,
CH4, OH, organics) and mineral bands. BIRCHES is a
miniaturized, actively cooled version (via a closed system
600-mW Cryo-Cooler) of the OVIRS instrument on
OSIRIS-Rex. The instrument has built-in flexibility, using
an on-orbit adjustable 4-sided iris, to maintain the same
spot size regardless of variations in altitude (by up to a
factor of 5) or to vary spot size at a given altitude, as the
application requires. Compact instrument electronics
have been developed which feature a reprogrammable
PRO-ASIC III FPGA which can be easily reconfigured
to support different read out and averaging schemes.
The BIRCHES instrument has been designed to survive
the rigors of the vibration during launch, the temperature
variations experienced during the mission, and the radiation environment on the way to and at the moon and is an
evolutionary step in compact and efficient instrument
design. The instrument will provide hyperspectral measurements of volatile-related features in the 1-4 micron region,
especially the 3 micron band, consisting of several waterrelated features. Previous lunar orbiting near IR spectrometers did not provide measurements beyond 3 microns
BIRCHES has heritage from OSIRIS-Rex, OVIRS and
New Horizons LEISA. BIRCHES represents a compact
version of a family of simple, versatile, and low-cost
instruments, adapted as OSIRIS Rex OVIRS is adapted,
from the GSFC-developed LEISA. LEISA-derived instruments represent (lower cost, mass, volume, power) concepts in spectrometer design made possible by largeformat infrared detectors combined with advances in thinfilm technology in the form of nondispersive thin film
filters, called wedged or linear variable etalon filters
(LVE or LVF), as wavelength selection elements.
Such spectrometers represent a great reduction in optical
and mechanical complexity and volume compared to
the conventional grating, prism, or Fourier transform

IEEE A&E SYSTEMS MAGAZINE

APRIL 2019

Aerospace and Electronic Systems - April 2019

Table of Contents for the Digital Edition of Aerospace and Electronic Systems - April 2019