The basic design philosophy for SWARM will be to take
advantage of previously developed and off-the-shelf components whenever
possible. This has a two-fold
advantage. First of all, these
technologies tend to be better developed with a longer performance baseline.
Secondly, they tend to be cheaper, since research and development costs are
reduced or shared.
To this end, SWARM will take full advantage of the
engineering and design work previously performed for the NEAR and DS-1 missions,
utilizing instruments and components that have already been flight-tested.
Details of the spacecraft design are broken down into four sections: instrumentation, propulsion, power, and C3 (Communications, Command, and Control).
In order to achieve the science objectives set forth earlier, SWARM will be equipped with the following scientific instruments, each based on those developed for the NEAR spacecraft (Anonymous, 1999c) :
Multi-Spectral Imager (MSI)
The Multi-Spectral Imager will be
used to determine characteristics such as the size, shape, and spin of Ceres.
It will also map the morphology and color properties of the surface.
Multi-Spectral Imager (Anonymous,
The MSI provides visible and near-infrared images of the asteroid surface. It incorporates f/3.4 refractive optics and an eight-position filter wheel covering the range from 450 to 1100 nm. The filter wheel is designed primarily to discriminate iron-containing silicate minerals, with one broadband filter for low-light imaging and optical navigation. The field-of-view (FOV) of 51.5 mrad × 39.4 mrad (2.95 × 2.26) is divided into 537 × 244 pixels (162 × 96 mrad), giving a 10 × 16 m resolution at 100 km. The CCD is a frame transfer unit with electronic shuttering (0 to 999 ms), and anti-blooming control. At 12-bits per pixel, an uncompressed image is 1.6 Mbits. Using a dedicated high-speed link to the Solid State Recorders, an image rate of 1 Hz can be sustained. Various forms of data compression are selectable by command, as is automatic exposure control. Using these latter options, or changing the filter between images, will lower the image rate. The instrument's total mass is 7.7 kg, including the electronics, and requires 6.92 watts of power to operate (Anonymous, 1999b) .
IR Spectrometer (IRS)
The Infrared Spectrometer (IRS) is designed to map the mineralogical composition of Ceres using the spectrum of reflected sunlight. Spectra measured will cover surface regions as small as 300 meters. Each spectrum has 64 spectral channels covering the near-infrared wavelength region, which allows identification of the key rock-forming minerals exposed on the asteroid surface. Together with high resolution and color imagery from the Multispectral Imager, NIS data will clarify the processes by which asteroids form and evolve.
Figure 3. IR Spectrometer (Anonymous, 1999d)
The IRS covers a 0.8 to 2.6 micrometer spectral range in 64 bins. This spectral range is achieved by dispersing the spectrum onto passively cooled Ge and InGaAs line array detectors. The IRS uses a 1- second integration time, and can produce spectra at a 1 Hz rate. A 1-D scan mirror allows the IRS's FOV to be boresighted with the MSI, or scanned more than 1.92 rad (110) away in 6.98 mrad (0.4) increments. Mirror scanning combined with spacecraft motion will be used to build up spectral images. A three-position slit mechanism allows two FOVs: 13.3 × 13.3 mrad (0.76 × 0.76) and 6.63 × 13.3 mrad (0.38 × 0.76). These provide spot sizes of 0.65 × 1.3 km or 1.3 × 1.3 km from a 100 km distance. The entrance slit can be closed altogether for dark count measurements. The IRS also carries a diffuse gold in-flight calibration target that can reflect sunlight into the spectrograph by correctly positioning the spacecraft and the scan mirror. Its total mass is 15.15 kg, including the electronics, and requires 15.1 watts of power to operate (Anonymous, 1999d) .
Laser Altimeter/Rangefinder (LAR)
The Laser Altimeter/Rangefinder (LAR) will use infrared laser pulses to
provide precision altimetry data. These data will accurately map Ceresí
topology, identify and characterize small-scale surface features, and precisely
determine overall volume and mass once they are combined with navigation data.
Figure 4. Laser Altimeter/Rangefinder (Anonymous, 1999a)
The LAR is a direct-detection single-pulse rangefinder. It uses a diode-pumped Nd:YAG laser transmitter, supplied by McDonnell Douglas, at a wavelength of 1.06 µm (1060 nm). The laser delivers a 15 mJ pulse of 12 ns duration. It has a maximum divergence of 300 mrad at the instrument's maximum range of 50 km. The resolution measured in test is better than 0.5 m. Repetition rates are selectable among 1/8 Hz, 1 Hz, 2 Hz, and 8 Hz (for a 2-second burst). An internal fiber optic calibration path is used for in-flight calibration. The laser rangefinder is boresighted with the MSI. Its total mass is 4.9 kg, and requires 15.1 watts of power to operate (Anonymous, 1999a) .
As shown in the appendix, the total delta-V required by
this mission, using patched-conic trajectories for a Hohmann-type transfer,
ranges from 8,970 to 10,003 meters per second.
Providing this delta-V using traditional chemical propellants would
require over 1,500 kg to propel a 500 kg-class spacecraft.
This method, used by NASAís ìbig-scienceî missions such as
Galileo and Cassini, is incompatible with the current funding environment.
NSTAR Ion Thruster (DeFelice, 1999)
To overcome this limitation, SWARM will utilize a cluster of
4 NSTAR Ion thrusters to provide continuous thrust to the 1000 kg spacecraft for
the 600-day orbit planned. These
are the same thrusters successfully used by DS-1. Each thruster is capable of providing 92 mN of thrust at
maximum output, so SWARM will have a maximum thrust of 368 mN.
Approximately 570 kg of xenon (Xe) propellant will be carried onboard,
accounting for ~57% of the spacecraftís total mass of 1000 kg.
By using a continuous-thrust trajectory, SWARM will be able to travel to
Ceres using less than one-third the propellant needed for chemical propulsion
methods. This will be discussed
further in the section on trajectories.
The spacecraft will also be equipped with hydrazine thrusters for on-orbit maneuvering and attitude control, as well as reaction wheels to provide three-axis stabilization and pointing capabilities.
Long duration spacecraft, such as SWARM, typically get
their power from one of two sources: solar panels or radioisotope thermoelectric
generators (RTGs). The decision as
to which source to use is based on two parameters: power requirements and how
far from the Sun the spacecraft will travel. The usual rule-of-thumb is that
missions inside of Marsí orbit can use solar panels, while missions past Mars
must use RTGs, since the current generation of solar cells is incapable of
providing useful power levels at those distances.
Ceres orbits in the main asteroid belt, well past the orbit
of Mars, so solar power is unlikely to be sufficient to power the spacecraft.
Additionally, the NSTAR thrusters require approximately 2,300 watts of
power when at maximum thrust, which also rules out solar panels.
Therefore, SWARM will be equipped with three Cassini-class RTGs, each
producing 875 watts and weighing 56.2 kg, for a total of 2,625 watts at the
start of the mission, tapering off to approximately 2,100 watts at the end of
the primary mission. The RTGs will also provide a heat source for the
spacecraft, eliminating the need for instrument heaters.
The decision to use RTGs was not entered into lightly. The Cassini mission encountered a great deal of public outcry and protest over the use of RTGs (which contain plutonium). The concern was over the risk that the plutonium would be released into the environment if the spacecraft suffered a catastrophic failure during launch or accidentally re-entered the atmosphere during the planned fly-by maneuver. However, the actual risk is quite low, a fact that the public must be educated about from the initial mission press release onward.
Communications, Command, and Control (C3)
The C3 infrastructure used in SWARM is closely
modeled after that used by DS-1. The
goal was to make the spacecraft less dependent on ground control.
AutoNav, the autonomous navigation suite used by DS-1, will be modified to
perform command and control tasks onboard SWARM.
Throughout the cruise phase, about once per week, AutoNav
will acquire optical navigation images. It will turn the spacecraft and use the
Multi-Spectral Imager to take pictures of asteroids and stars, analyzing them
itself to determine its location. The apparent position of an asteroid relative
to the much more distant stars will allow AutoNav to calculate where SWARM is in
the solar system. The system will also command the ion propulsion system to
pressurize its xenon tanks for thrusting, and the spacecraft's attitude control
system (using either the hydrazine thrusters or the reaction control wheels) to
turn the spacecraft to thrust in the direction AutoNav directs. AutoNav will
also determine how much power to send to the ion propulsion system. To do this,
AutoNav will have to monitor the RTGs, since their power output drops as the
plutonium decays. Combining a
knowledge of how much thrusting has been accomplished, the gravitational forces
of the Sun and planets, and other information, AutoNav will calculate where the
spacecraft is headed. If it is not on course, AutoNav will determine how to
change the direction in which it points the ion thruster and the amount of
thrust to assure that it reaches its target (Rayman,
SWARM will use other technologies that were tested by DS-1
and designed to improve the engineering environment in deep space and reduce the
cost of space exploration missions. One such system is the Small Deep-Space
Transponder (SDST), which handles all communication between
mission control and the spacecraft. It
combines the spacecraft's receiver, command detector, telemetry modulation,
exciters, beacon tone generation, and control functions into one small, 3-kg
package. The transponder also
generates tones that allow the use of beacon monitor operations.
This helps reduce the large demand on NASA's deep space network (DSN).
The spacecraft determines its own health and need for human assistance.
It then selects one of four tones indicating its diagnostic findings,
transmitting the tone to Earth so that mission control can decide what, if any,
action is needed (Nelson,
SWARM will incorporate special low-power electronic
components to reduce power consumption. It
will also use multifunctional structure packaging technology to combine
load-bearing elements with electronic housings and thermal control, greatly
reducing the mass of the spacecraft's cabling and chassis.
In order to store science and engineering data for transmission to Earth, SWARM will be equipped with a radiation-hardened solid-state data recorder. Containing no moving parts, this storage device should prove to be more stable and energy efficient than the traditional onboard magnetic tape data recorder.
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