1. Rosetta Science Working Team Meeting #26 Working Group #1
ESTEC:
Vicente Companys

2. S/C and Comet dynamics estimation
Gravity model:
FD is performing analysis to establish a suitable gravity model for lander delivery. Options considered:
Spherical harmonic expansion
Agglomerate of mass points
Polyhedral shape of uniform density
Agglomerate of hexaedrons with uniform density
Spherical harmonic expansion adequate at few comet radius distance.
Polyhedral representation appears more adequate for close distance:
It allows a close form of the gravity potential
Can be reconstructed from the mass and shape
Can be corrected for non-uniformity by e.g. adding mass-cons…
Goes hand-in-hand with DTMs
Gravity model, data and I/F S/W will be provided as an end to end product.

3. S/C and Comet dynamics estimation
Digital terrain model:
An engineering DTM will be provided by FD to support operations (in particular lander delivery)
FD will select landmarks on the surface as features that can be described and unambiguously identified (e.g. “centre of crater appearing in certain location of a given NAVCAM image”).
A Comet body fixed reference frame will be defined by FD (e.g. origin = CoM, X axis towards 1st landmark, XY plane containing second landmark). This is the basis for exchanging any position information in the comet (e.g, to provide target for imaging or target for landing).
Catalogue of landmarks will be provided (position, identification, image).
DTM will be defined as data file containing set of vertexes and set of faces (face is an ordered list of vertexes forming a plane). Landmarks will be likely vertexes of the DTM.
Support S/W library will be provided (e.g. to compute surface point for direction).
Accuracy will be commensurate with NAVCAM capabilities (OSIRIS images may be fit in the model).
It shall be adequate for navigation towards the surface.
It will be not adequate e.g. to define the roughness of the surface (i.e. landing site selection must be based on other means).
It is part of the gravity potential model.

4. S/C and Comet dynamics estimation
OSIRIS/DTM:
It is understood that for science purposes a more accurate DTM will be available.
If based on different reference frame, transformation shall be provided.
Shall be checked for consistency. E.g. landmarks as estimated by FD shall be close to the surface for the OSIRIS DTM. Landing location shall be consistent with FD DTM.

5. S/C and Comet dynamics estimation
Estimation process:
Dynamics estimation is a process aimed at the determination of a whole set of parameters:
Comet gravity potential,
comet inertia properties,
comet position and velocity,
S/C position and velocity,
comet rotation parameters,
landmark positions.
Base measurements: range, Doppler, camera images.
Result: comet orbit file, comet orientation file, S/C orbit file, landmark positions, gravity potential and inertia properties.
Depending on the distance only a sub-set of parameters is estimated (at far approach only position and velocity information, later comet mass, rotation parameters, inertia properties…).
Status:
Prototype available. Gravity model being updated. It will be used in the next months as test bench for the approach phase.

6. Comet environment modelling
ESOC FD is implementing currently engineering model for CG gas and Dust environment.
The purpose of the model is to be used for operations, to predict disturbance force and torque, STR blinding by dust, dust coverage.
It is derived from ‘M. Mueller, RO-ESC-TA-5501, An Engineering Model of the Dust and Gas Environment of the Inner Coma of Comet P/Wirtannen”, which is being updated for CG.
It will compile full description of model equations, specifying e.g. densities and velocities of the different gas species and dust distribution.
Model description model will be delivered for review to the SWT by September 2009.
By 4th quarter of 2009 we intend to define a concept and prototype on the preparation for model in-flight update:
Starting point will be to define as example the measurement equation for NAVCAM (i.e. algorithm to predict NAVCAM images based on the model and on model parameters). This is the starting point to invert the model. Other instruments shall follow, with the help of PI teams.

7. S/C constraints for landing
FD will collect in a document a set of constraints for S/C safe operation around the comet. In particular for lander delivery.
Conflicts with precedent documentation shall be resolved. If not possible, most restrictive approach has to be taken.
In-flight experience may result in update of constraints.
So far identified:
S/C trajectory shall be collision-free and eclipse-free for TBD period. Period to be assessed as maximum possible outage in case of survival mode. This condition shall be fulfilled for the whole range of uncertain parameters (e.g. gas density). This will constraint the minimum distance and the speed for close fly-over.ring
We e.g. consider that entering a collision orbit prior to lander delivery is not acceptable.
S/C attitude excursions shall be commensurate with wheel capabilities.
STR outages by dust shall be limited. Trajectories shall be designed such that if control is interrupted the S/C drifts naturally away.
Coverage of surfaces (e.g. SA) by dust shall be limeted as to ensure S/C operation until nominal end of mission.
Loading of wheels by disturbance torques (including drag torques) shall be commensurate with the reaction wheel capabilities.