DDA 2015 – High precision comet trajectory estimates: the Mars flyby of C/2013 A1 (Siding Spring)

This is one of a series of notes taken during the 2015 meeting of the AAS Division on Dynamical Astronomy, 3-7 May, at CalTech. An index to this series (all the papers presented at the meeting) is here.

Session: Dynamics of Small Solar System Bodies

Davide Farnocchia (JPL, CalTech)


The Mars flyby of C/2013 A1 (Siding Spring) represented a unique opportunity for imaging a long-period comet and resolving its nucleus and rotation period. Because of the small encounter distance and the high relative velocity, the goal of successfully observing C/2013 A1 from the Mars orbiting spacecrafts posed strict requirements on the accuracy of the comet ephemeris estimate. These requirements were hard to meet, as comets are known for being highly unpredictable: astrometric observations can be significantly biased and nongravitational perturbations significantly affect the trajectory. Therefore, we remeasured a couple of hundred astrometric positions from images provided by ground-based observers and also observed the comet with the Mars Reconnaissance Orbiter’s HiRISE camera on 2014 October 7. In particular, the HiRISE observations were decisive in securing the trajectory and revealed that nongravitational perturbations were larger than anticipated. The comet was successfully observed and the analysis of the science data is still ongoing. By adding some post-encounter data and using the Rotating Jet Model for nongravitational accelerations we constrain the rotation pole of C/2013 A1.


  • Observations
    • 140,000 km close approach
    • HiRISE FoV: 4×4 mrad $\rightarrow 280$ km
    • post-conjunction updates: positions kind of all over the place
      • center of light is not coincident with position
      • PSF is diffuse, not starlike
      • most astrometry was from amateurs
      • non-grav perturbations
        • previously unknown for this comet
      • use observations fromMRO!
        • provided good constraints on non-grav params
  • post-flyby
    • astrometric predictions sucked!
    • rotating jet model
      • spin pole
      • thrust angle between jet and pole
      • superposition of two jets
      • avg over rotation
      • $\rightarrow$ better trajectory
      • $\rightarrow$ spin pole direction

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