Instrument Host Overview                                                  
    ========================                                                  
      This description was provided by Dr. Michael A'Hearn, the               
      principal investigator for the Deep Impact mission.                     
                                                                              
      The Deep Impact Impactor vehicle is a complete spacecraft with a        
      CCD-based camera and telescope called the Impactor Targeting            
      Sensor (ITS) for both scientific imaging and auto-navigation, a         
      complete attitude control subsystem using hemispherical                 
      resonator gyros, a star tracker, and a complete, hydrazine-based        
      propulsion system.  Electrical power is supplied by an internal         
      battery since the expected operating lifetime is very short (24         
      hours).                                                                 
                                                                              
      The Impactor is mechanically and electrically attached to the           
      Flyby spacecraft and both launch together and fly together until        
      one day before the impact event.  On July 3, 2005, the impactor         
      is released and pushed away from the Flyby at a distance of             
      864,000 km from comet Tempel 1.  The auto-navigation system             
      takes control and maneuvers the Impactor to ensure an impact on         
      an illuminated portion of the cometary nucleus.  On July 4,             
      2005, the impactor hits the nucleus at a relative speed of 10.2         
      km/s.  The 360-kg Impactor releases 19 gigajoules of kinetic            
      energy to excavate a crater on the surface of the nucleus.              
      The estimated time of impact is 05:44:36 UT (Earth-received             
      time 05:52:02 UT), as reported by A'Hearn et al. (2005)                 
      [AHEARNETAL2005A].                                                      
                                                                              
      About half of the mass of the Impactor is copper, a noble metal,        
      which minimizes chemical reactions that could lead to species           
      that contaminate the spectrum with bright lines. About half this        
      copper is in a spherical cap at the 'front' of the impactor.            
      This cap is made of chamfered discs that are internally hollowed        
      much like a lightweight telescope mirror to reduce the density          
      by a factor two or so from the bulk density of copper.                  
                                                                              
      Data taken by the ITS are white-light images and are used by the        
      onboard software for auto-navigation and transmitted to the             
      Flyby over an S-band link at 60 kilobits per seconds (kbps).            
      Attitude control and minor trajectory corrections are performed         
      using a small hydrazine propulsion subsystem.                           
                                                                              
      In the last minute before impact, long after the last trajectory        
      maneuver, the project expects the optics to be destroyed by dust        
      impacts, either by sandblasting by small particles or by the            
      impact of a large particle that cracks the primary mirror of the        
      ITS telescope.  If the optics survive, the last image will have         
      a resolution of 20-cm per pixel.                                        
                                                                              
      System Requirement Specifications for the Impactor are:                 
                                                                              
        Image Data Volume       : Approximately 17 megabytes                  
        Pointing Accuracy       : 2 milliradian 3-sigma                       
        Pointing Knowledge      : 150 microradian, 3 axes, 3-sigma            
        Targeting Accuracy      : 300 m 3-sigma with respect to the           
                                  center of brightness of the nucleus         
        Telecom Band            : S-Band                                      
        Data Rate to Flyby      : 64 kbps                                     
        Command Rate            : 16 kbps                                     
        Energy Storage          : 2.8 kilowatt-hr for 24 hr mission           
        Propulsion              : 25 m/s delta-velocity                       
                                                                              
      For further descriptions of the impactor spacecraft, the                
      anticipated flight data, and the cratering experiment, see              
      A'Hearn, et al. (2005) [AHEARNETAL2005], Blume (2005)                   
      [BLUME2005], Hampton, et al. (2005) [HAMPTONETAL2005], Klaasen,         
      et al. (2005) [KLAASENETAL2005], Richardson, et al. (2005)              
      [RICHARDSONETAL2005], and Schultz and Ernst (2005)                      
      [SCHULTZ&ERNST2005].