meetings - Space Flight Mechanics Committee

The Dawn spacecraft has recently spent over a year collecting direct science observations of
Vesta. While at Vesta, Dawn’s Orbit Determination (OD) team was tasked with
determination of the Vesta rotational rate, pole orientation and ephemeris, among other
Vesta parameters. This paper will describe the initial determination of the Vesta ephemeris
and frame using both radiometric and optical data, and the also describe the final results
using data from later in the Vesta mission, along with modeling and process refinements.
15:55 AAS Recovering the Gravity Field of Vesta from Dawn
13-348 Sami Asmar, NASA / Caltech JPL; Alex Konopliv, NASA / Caltech JPL; Ryan Park,
NASA / Caltech JPL; Carol Raymond, NASA / Caltech JPL
The Dawn mission to Vesta has completed a global solution of gravity measurements of
degree and order 20. When correlated with a shape model derived from imaging, these data
can constrain the interior structure from the core to the crust. Utilizes precision spacecraft
Doppler tracking and landmark tracking from framing camera images to measure the
gravity field, the solution also yields the spin-pole location and rotation. The second-degree
harmonics together with assumptions on obliquity or hydrostatic equilibrium determine the
moments of inertia and constrain the core size and density. J2 parameter shows
inconsistency with a homogenous density body.
16:15 AAS Orbit Determination of Dawn at Vesta with Image Constraints in GEODYN II
13-349 Frank Centinello, Massachusetts Institute of Technology; Erwan Mazarico,
Massachusetts Institute of Technology / EAPS; Maria Zuber, Massachusetts
Institute of Technology
The Dawn spacecraft conducted its mapping mission at the protoplanet Vesta from July
2011 to August of 2012. In this work, GEODYN II is used to compute the orbit of the
spacecraft using radiometric and image constraints. Image constraints are a new data type
for GEODYN and are particularly valuable for orbit determination in weak, non-uniform
gravity environments subject to non-conservative accelerations. For this work, orbits were
fitted using 3-way X-band radiometric measurements and image constraints. A definition of
how image constraints are used by GEODYN is provided followed by an analysis of
GEODYN performance with
16:35 AAS Spiraling Away from Vesta: Design of the Transfer from the Low to High
13-350 Altitude Dawn Mapping Orbits
John Smith, NASA / Caltech JPL; Daniel Parcher, NASA / Caltech JPL; Gregory
Whiffen, NASA / Caltech JPL
Dawn has successfully completed its orbital mission at Vesta and is currently en route to an
orbital rendezvous with Ceres in 2015. The longest duration and most complex portion of
the Vesta departure trajectory was the transfer from the low to high altitude science orbit.
This paper describes the design of this low-thrust trajectory optimized assuming a
minimum-propellant mass objective. The transfer utilized solar-electric ion propulsion
applied over 139 spacecraft revolutions about Vesta. Science drivers, operational
constraints, and robustness to statistical uncertainties are addressed. The 45-day transfer
trajectory was successfully implemented in early 2012.
23 rd AAS / AIAA Space Flight Mechanics Meeting Page 65