ATK was the prime contractor for the probe Bus and probe Carrier for the Time
History of Events and Macroscale Interactions during Substorms program (THEMIS).
A NASA Medium Class Explorer mission, THEMIS consists of a constellation of five
microsatellites carrying identical suites of electric field, magnetic field and
particle instruments. THEMIS launched Feb. 17, 2007, from Cape Canaveral, Fla.,
aboard a Delta II launch vehicle. The University of California at Berkeley was
the mission integrator and instrument provider.
The five THEMIS probes are identical in design and can be placed in any
THEMIS orbit. This provides robustness to the constellation design and allows
for any one of the probes to be placed in any orbit. The probe bus has a number
of driving requirements that dictate its design and layout, including:
Although some of these requirements are encountered in many spacecraft
designs, in combination, they make for an extremely challenging design. The
major subsystem designs and how they achieve the mission objectives are
described in the following sections.
Structure and mechanical subsystemThe THEMIS probe bus
structure provides mechanical support for all other subsystems and consists of
ultra-lightweight panels constructed of composite graphite epoxy facesheets and
an aluminum honeycomb core. In addition, there are corner panels and a center
tube that houses the axial electrical field instruments (UCB provided) comprised
of layers of composite materials. All panels have embedded fittings of either
titanium and/or aluminum that have been machined to minimize mass.
The primary structure must withstand launch loads and also the extreme
temperature swings during early-orbit operations and when entering the Earth
eclipse. Extensive analysis and development testing was performed on the new
composite elements of the structure. These environments are simulated via
vibration testing and panel level thermal cycling at the subsystem level before
delivery of the probe structure to integration and test. The mass of the entire
structure and mechanical subsystem, including mounting hardware, is 15 kilograms
and represents approximately 19.5 percent of the probe's dry mass (without fuel)
and 12 percent of the probe's wet mass.
Reaction Control SubsystemThe Reaction Control Subsystem
provides the actuators to change probe velocity, inertial attitude and spin
rate, and consists of two fuel tanks, tangential and thrust engines (four), a
pressurant tank, latch valves, pyro valves, and miscellaneous hardware. The RCS
holds up to 49 kilograms of fuel in two tanks that were specially made and
qualified for THEMIS. These tanks are made of high strength steel (inconel) and
are supported by the bottom and top panels via integral polar fittings.
The tanks are highly optimized for mass. The unique feature of the RCS is the
combination of a pressurization system that enhances the capability of the
system once on orbit. Once the fuel in the system is depleted by approximately
25 percent, a command is sent to the probe initiating a pyro valve firing that
repressurizes the tank system. This design feature provides more performance to
the system by increasing the pressure within the fuel tanks.
Two thrust (axial) engines provide 4.4 Newtons of thrust, allowing for major
orbit changes of the probe. Two tangential engines of the same size provide spin
control and/or lateral thrust to the probe. The entire RCS weighs only 12
kilograms without fuel and is approximately 15 percent of the probe dry
Attitude Control SubsystemThe Attitude Control Subsystem
provides the telemetry and command capability for spin rate and attitude control
in conjunction with the RCS. The THEMIS probes, when released from the probe
carrier and the launch-vehicle third stage, are spin stabilized, which infers
motion stability via spinning. The nominal rate is 16 revolutions per
In order to achieve spin stabilization, the probes are configured to have
their center of mass closely aligned to the geometrical thrust axis. This
alignment is accomplished through painstaking placement of components and by
adjusting spin balance masses before launch.
Once the probes are released, spinning, the ACS enables the probes to
maintain spin stability throughout the life of the mission. The ACS monitors the
spin rate and attitude of the probe once it separates from the launch-vehicle
third stage. The ACS is assisted by the onboard instrument magnetometer that
provides Earth magnetic field measurements.
THEMIS probes must be stable for a wide range of configurations (multiple
mass property changes) due to instrument boom deployments and fuel depletion.
The ACS major bus components are the miniature sun sensor that enables
estimation of spin rate from sun crossing times, and solid-state inertial
reference unit assembly that measures angular rate in the other two axes. This
ACS telemetry is linked to the ground via the communication subsystem ,where it
is processed and a set of commands is generated to be linked back to the probe
to command the RCS as required. The ACS Bus components together (excluding the
instrument magnetometer) weigh only 0.6 kilograms.
Power SubsystemThe Power Subsystem provides power to all
electrical components and consists of body mounted solar arrays and a
lithium-ion battery made of multiple battery cells. The THEMIS probe is highly
efficient in power usage, with approximately 36.85 watts required in full
science mode for a 24-hour orbit, including a three-hour eclipse and a 30-minute
transmitter turn-on. That is less than a 40-watt home light bulb. The capability
for that orbit at the mission end of life is 40.35W.
The THEMIS probe has eight solar arrays that provide power generation for any
orientation of the probe. There are two arrays mounted on the bottom and top
decks, and there are four side panels. The arrays use high-efficiency (more than
27 percent) cells that are bonded to the composite substrates. The side panels
are also the primary structure, which adds to their design complexity because
they have to transfer loads between the top and bottom decks.
The solar arrays are also unique. In order to reduce charging effects
(minimal exposed insulators), all of the cover glass must be electrically
grounded to a common ground on each panel. This is accomplished by bonding a
highly conductive grid onto the panels following cell placement. Power is stored
onboard by a Lithium-ion battery that maintains the probe power during eclipse,
which can last up to three hours. The battery is lightweight and provides up to
12 Amp hours of power capacity. The major power subsystem components weigh
approximately 10.3 kilograms and represent about 13 percent of the total probe
bus dry mass.
Communication SubsystemThe Communication Subsystem
provides communication between the probe and the ground stations. The subsystem
consists of an S-band transponder and S-band antenna mounted to the center boom
structure. The transponder is lightweight and converts the radio-frequency
signal from the ground into a digital signal to the bus avionics unit.
The transponder also performs the reverse operation, where it takes the
digital signals from the BAU and converts it to RF. It then transmits this RF
signal to the antenna where it is radiated to the ground.
The THEMIS S-band antenna consists of six receiver/transmit stack patch
antennas and a power divider. These antennas are extremely lightweight and must
have a conductive surface in order not to build up surface charge as the probe
travels through the space plasma environment. The total mass of the
communication subsystem is 3.2 kilograms and represents 4 percent of the probe
Bus Avionics UnitThe BAU provides numerous functions for
the probe bus and contains the flight computer for the satellite. The BAU
provides for the processing of all the data handling, internal communication
interface, instrument electrical interface and power control for the probe bus.
The BAU contains five modules, with the top module containing a
radiation-hardened main processor (coldfire processor operating at 16.78 MHz).
This module performs all the onboard processing and data handling. It contains
64 MB of bulk memory and supports a 2.1 Mbps data rate interface with the
The BAU hosts the RTEMS real-time operating system and the application
control and data-handling software for the probe bus. The second module is a
communication module that interfaces with the transponder and other modules
within the BAU. The remaining modules are power modules that control the
distribution of power on the probe and provide the function of energy balance by
balancing the power from the solar arrays and battery. The BAU is extremely
lightweight at 3 kilograms and consumes, on average, less than 7 watts.
Thermal Control SubsystemThe THEMIS probe Thermal
Subsystem is a hot-biased design that uses solar heat input to elevate component
temperatures, allowing survival at all sun-aspect angles and three-hour eclipses
with minimal heater power consumption (less than 12 watts orbit average). The
hot-biased thermal design includes external coatings with high solar
absorbance-to-emittance ratios, such as vapor-deposited gold and high-efficiency
multi-layer insulation blankets to minimize heat loss from the hydrazine
Reaction Control System (temperature requirement of greater than 5 C).
Probe Carrier and Separation SystemThe THEMIS
constellation of five probes was deployed from a probe carrier mounted to the
third stage of a Delta II rocket spinning nominally at 16 rotations per minute.
This is a significant engineering challenge in the design of the separation
system and the probe carrier. However, the stability of the probe during
separation is crucial in order to avoid collisions between probes and/or the
The separation system facilitates an unobstructed and stable separation of
the probes by moving quickly away from the separation plane and imparting a low
tip-off rate (rotation) to the probes. The top probe deploys first and the lower
four probes deploy simultaneously three seconds later. The deployment is
triggered by the third stage of the Delta II.
The probe carrier is predominantly aluminum alloy, is weight-optimized and
includes a patch panel that manifolds all of the umbilical electrical and
control-circuit cabling from the probes to the launch vehicle. The separation
system was extensively analyzed and tested to properly characterize its
performance and to verify all of the mechanical parameters that drive the
overall probe and probe carrier system clearance verification analysis.
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