Instrument Host Overview      ========================The Dawn spacecraft is an ion-propelled spacecraft capable ofvisiting and stopping at multiple targets in the Main Asteroid Belt.Most of the spacecraft design was provided by Orbital SciencesCorporation (Orbital), and the Jet Propulsion Laboratory (JPL)provided the ion propulsion system (IPS) and parts of theelectrical power system and telecommunications system.  Orbitalwas responsible for overall assembly, including integration ofinstruments, system-level tests, and launch operations.  JPL isresponsible for mission operations.The IPS is an expanded version of the system operated extensively onDeep Space 1 (DS1).  The spacecraft design benefits from inheritancefrom previous projects at Orbital and the JPL.  The mechanical design isbased on Orbital's Star-2 series, and the avionics draw from theLEOStar-2 series.There is significant redundancy throughout the design, and themission can be accomplished in the presence of most crediblesingle faults.A complete description of the Dawn spacecraft may be found inRayman et al. (2006), from which most of the information in this filewas taken.      System Description      ==================Ion Propulsion System---------------------The core of the structure is a graphite composite cylinder.The hydrazine and xenon (Xe) tanks mount inside the cylinder, whichprovides a load path directly to the launch vehicle interface.  TheXe tank, titanium overwrapped with composite, has a capacity of 425kg.  To provide reliability in expending that much propellant, thespacecraft carries three ion thrusters.  The thruster design isqualified for a throughput of about 150 kg.  Each 30-cm diameterthruster is mounted to a two-axis gimbal to allow for migration ofthe flight system's center of mass during the mission and to allowthe attitude control system (ACS) to use the IPS to control attitudewhen the IPS is thrusting.  The IPS system includes two sets ofinterface and control electronics and two power processing units,although no more than one thruster will be operated at a time.Attitude Control System and Reaction Control System---------------------------------------------------In its normal cruise mode, ACS estimates attitudes and rates with astar tracker.  The estimation is augmented with mechanical gyros insome ACS modes, including during science data acquisition.  The startrackers and the gyros are mounted on the +z panel to minimizemechanical alignment errors between the sensors and FC and VIR.Coarse Sun Sensors (CSS), providing 4-pi steradian coverage, are usedfor rough attitude determination and for fault protection.  Allattitude sensors are fully redundant.ACS controls the articulation of the solar arrays and the basebodyto assure that the arrays are normal to the Sun-spacecraft line.ACS can be commanded to use the reaction control system (RCS) orreaction wheels to control the attitude.  Either type of actuatorcan be used in combination with the IPS for attitude control duringIPS thrusting.The hydrazine-based RCS is used by ACS for direct control of attitudeor for desaturation of the reaction wheels.  In addition, somehydrazine is allocated for a contingency orbit-control maneuver atthe asteroids in the event there is insufficient time to achieve therequired delta-v with the low-thrust IPS.  To date, no scenarios forthis contingency have been identified.  The titanium hydrazinetank capacity is 45.5 kg.  RCS has two independent strings, each withsix 0.9 N thrusters.  Each string has pairs of thrusters pointed in+x, -x, and -z, with the last pair used for the contingency trajectorycontrol.Electrical Power System and Thermal Control System--------------------------------------------------Because of Dawn's uniquely high delta-v requirements, much of thespacecraft design is driven by the resulting need to maximize thepower available to the IPS.  When the flight system is at Ceres ata heliocentric range of 3 AU, the electrical power system (EPS) hasto provide sufficient power to operate the IPS.  Therefore thespacecraft has two large solar arrays which together provide 10.3 kWat 1 AU and 1.3 kW at Dawn's maximum heliocentric distance of 3 AU(Ceres' aphelion).  Each of the two solar arrays has 18 square metersof InGaP/InGaAs/Ge triple-junction cells.  The arrays are articulatedaround their long axis.Because the IPS requires high-voltage power, the flight system hastwo power buses.  The EPS delivers most of the IPS power at80-140 V, and the rest of the flight system (including some IPScomponents) receives power at 22-35 V.  A 35 Ah NiH2 battery,sized to provide energy during launch, supplements the low-voltagebus during IPS thrusting to cover transients in the flight system'spower consumption.Apart from the IPS, the thermal control system (TCS) is the largestconsumer of power, requiring 200 W during IPS thrusting at 3 AU.Ammonia based heat pipes embedded in the y panels aid in distributingthe heat, and louvers reduce thermal power requirements under coldconditions.  Because the solar arrays are always maintained normalto the Sun-spacecraft line, the sun will be in the plane of the ypanels.  Most components are mounted on these panels to take advantageof the relatively uniform thermal environment.Command and Data Handling System--------------------------------The Command and Data Handling System (CDHS) is based on a RAD6000processor running sofware written in C running in VxWorks.  CDHSalso provides 8 Gb of mass memory for engineering and scientificdata.  The system receives telemetry from all ACS sensors and includesdrivers for all ACS actuators except the IPS thruster gimbals.  Mostcomponents communicate over a Mil-Std-1553 data bus, although GRaNDuses a RS-422 serial interface.The flight software incorporates virtual machine language (VML),currently in use on Mars Odyssey, Mars Reconnaissance Orbiter,and the Spitzer Space Telescope.  VML allows conditionals and complexlogic in command sequencing.  VML is used for all commandingduring the mission, although in order to rely on only the most basicfunctions, fault protection does not use it.CDHS supports uplink data rates from 7.8 b/s to 2.0 kb/s and downlinkdata rates from 10 b/s to 124 kb/s using a rate 1/6 turbo code,implemented as recommended by the Consultative Committee for SpaceData Systems.Telecommunications System-------------------------The telecommunications system (Telecom) operates in X-band for uplinkand downlink.  It is based on the Small Deep Space Transponder,validated on DS1 and used on the majority of NASA missions beyond themoon since Mars Odyssey.  Radiating 100 W, the travelling wave tubeamplifiers are identical to those in use on Mars ReconnaissanceOrbiter.  An unarticulated 1.52 m high gain antenna (HGA) is on thespacecraft's +x axis, and near-hemispherical (7 dBi) low gainantennas (LGAs) are aligned with +x, +z, and -z.  The system isconfigured to always use only one of the four antennas at a time.Instrument Mounting Geometry----------------------------The figure below represents the Dawn spacecraft with the -y panelfacing the viewer.  GRaND and the FCs are mounted on the +z paneland VIR is mounted on the -x panel.  All instruments are mountedso the centers of their fields of view are aligned with thespacecraft's +z axis.  The high gain antenna is mounted to the+x panel.  One IPS thruster is mounted at the -z panel and the othertwo are on their own panels, one canted towards -x and the other toward+x.  The thrusters are about 48 degrees from -z.  The two solar panelsextend from the +y and -y panels.                               +X                           /\  High Gain Antenna                  -------------------                   \                /                    \              /                   --------------------------/\                ___|                        |  \          GRaND |  |                        |_/_   Ion propulsion                |  |                        |   |  thrusters                |__|                        |   |       Z          _|            o           |   |    -Z              FC |_|                        |   |                   |                        |___|                   |                        | \                   |                        |  /                    -------------------------\/               VIR |         |                   |_________|                               -X      References      ==========Rayman, M.D., T.C. Fraschetti, C.A. Raymond, and C.T. Russell, Dawn:A mission in development for exploration of main belt asteroids Vestaand Ceres, Acta Astronautica 58, 605-616, 2006.