SpaceOps02 Pre T3 24

Rosetta Nominal and Backup Operational Scenarios to Land on the Comet WirtanenF. Dufour, J. Bernard, P. Gaudon, CNEST. Ceolin, S. Kerambrun, CS-SI: 

Rosetta Nominal and Backup Operational Scenarios to Land on the Comet Wirtanen F. Dufour, J. Bernard, P. Gaudon, CNES T. Ceolin, S. Kerambrun, CS-SI Space Ops 2002 Houston, TX, USA 9 - 12 October 2002

Contents: 

Contents 1. Introduction 2. Rosetta Mission 3. Cometary Environment Modeling 4. Landing Phase 5. Descent Maneuvers 6. Numerical Results 7. Conclusion

1. Introduction: 

1. Introduction ESA mission that aim at a rendezvous with the comet 46P/Wirtanen First ever landing of a probe on a comet to perform is-situ measurements of the cometary materials CNES is in charge of the landing maneuvers We must propose and validate a robust scenario to land on the comet in the best conditions of precision and terminal velocity The landing trajectory will be under the influence of complex forces: gravitational forces greatly dependent on the comet nucleus shape, aerodynamic forces induced by the outgassing. Backup landing scenarios must also be prepared to cope with a failure of the nominal release mechanism.

2. Rosetta Mission: 

2. Rosetta Mission Launch: January 2003 Rendezvous with Wirtanen: November 2011 Landing on Wirtanen: October 2012

3. Cometary Environment Modeling 3.1. A Complex Task: 

3. Cometary Environment Modeling 3.1. A Complex Task Lack of information concerning the comet’s shape and environment At this stage, we can only count on astronomical observations done during previous perihelion passes The one-year long observation period should greatly improve our knowledge of Wirtanen Till then, we have to make many assumptions about Wirtanen’s physical characteristics

3. Cometary Environment Modeling 3.2. Comet Nucleus Shape: 

3. Cometary Environment Modeling 3.2. Comet Nucleus Shape Forces acting in the vicinity of a comet are strongly linked with the shape of the nucleus At first, we assumed simple ellipsoidal shapes (a sphere or and ellipsoid) But a precise analysis requires more realistic nucleus shapes Muinonen’s statistical model allows very complex shapes

3. Cometary Environment Modeling 3.3. Comet Attitude: 

3. Cometary Environment Modeling 3.3. Comet Attitude The attitude of the comet is defined by the orientation of the main inertia axes of the nucleus The poles’ motion is derived from the rotation of these axes At least 3 ways to define the attitude : 1- polar axis fixed in the reference frame and nucleus motion restricted to a constant rotation around that axis 2- the 3 Euler angles that define the attitude evolve linearly with the time 3- the attitude is precisely interpolated from observation data Up to now, we assumed a fixed polar axis and a constant rotation

3. Cometary Environment Modeling 3.4. Gravitational Forces (1/2): 

3. Cometary Environment Modeling 3.4. Gravitational Forces (1/2) Gravitational forces are one of the 2 main acting forces in the vicinity of a comet 3 methods have been used to model the gravitational force field : 1- Spherical Harmonic Expansion divergence problems with very elongated bodies 2- Ellipsoidal Harmonic Expansion no sign of divergence outside the smallest fitting ellipsoid but a divergence possible between ellipsoid and nucleus

3. Cometary Environment Modeling 3.4. Gravitational Forces (2/2): 

3. Cometary Environment Modeling 3.4. Gravitational Forces (2/2) 3- Polyhedric Potential polyhedron with multiple facets the volume integral can be replaced by a surface integral only valid for a constant density the iso-potential curves follow closely the shape of the nucleus no divergence problem Comet density = 0.75 g/cm3 Gravitational constant = 45.28 m3/s2

3. Cometary Environment Modeling 3.5. Aerodynamic Forces (1/2): 

3. Cometary Environment Modeling 3.5. Aerodynamic Forces (1/2) Aerodynamic forces are not negligible because of the outgassing that increases close to the Sun (3 AU) An outgassing model was used to analyze these aerodynamic forces : Sun in the equatorial plane lander characteristics : sphere of 48 cm of radius mass of 100 kg

3. Cometary Environment Modeling 3.5. Aerodynamic Forces (2/2): 

3. Cometary Environment Modeling 3.5. Aerodynamic Forces (2/2) Aerodynamic forces in the same order of magnitude of gravitational forces in the vicinity of a small comet comet radius = 600 m So the aerodynamic forces can dominate the gravitational forces where ratio > 1

4. Landing Phase 4.1. Nominal Landing Scenario: 

4. Landing Phase 4.1. Nominal Landing Scenario 1- The spacecraft moves on the delivery orbit 2- The lander is released with the help of a separation maneuver 3- At least 1 ADS maneuver is planned during the descent (autonomous mode) 4- Comet landing according to the terminal constraints : no relative horizontal velocity a minimal vertical velocity

4. Landing Phase 4.2. Backup Landing Scenarios: 

4. Landing Phase 4.2. Backup Landing Scenarios No human intervention are possible during the descent because the lander will behave autonomously The occurence of a failure of one of the ADS maneuvers must be taken into account earlier at the trajectory planning phaseI If a failure occurs during the release maneuver we can still intervene with the help of the mechanical backup release mechanism If the umbilical cord between the lander and the orbiter is disconnected, we must launch another landing attempt in less than 4 hours no delivery orbit modification allowed but we can change the attitude of the orbiter If the lander is still connected to the orbiter, we can wait up to 48 hours to analyze the situation before a new attempt

5. Descent Maneuvers 5.1. Computation Method: 

5. Descent Maneuvers 5.1. Computation Method CNES objectives are to validate a robust landing scenario despite the uncertainties about the comet’s characteristics Instead of trying to solve the problem globally, we opted for a progressive approach : Many sensitivity analyses have been made to land on simple ellipsoidal shapes, assuming various comet sizes and densities, with and without outgassing More realistic comet shapes (Muinonen) have been tested afterwards The next step is to optimize the landing trajectories according to the terminal velocity constraints Finally, the optimal landing trajectories will be validated with the help of thorough Monte Carlo analyses

5. Descent Maneuvers 5.2. Trajectory Optimization: 

5. Descent Maneuvers 5.2. Trajectory Optimization We must solve a complex nonlinear control problem with constraints Performance index : terminal velocity components descent duration Control variables : delivery orbit separation maneuver ADS maneuvers Constraints included in the objective function (penalty functions) Direct optimization algorithms have been used (Nelder-Mead simplex)

5. Descent Maneuvers 5.3. Monte Carlo Analysis: 

5. Descent Maneuvers 5.3. Monte Carlo Analysis Because of the multiple uncertainties in the system, the optimal control maneuvers cannot be taken for granted without any further verification Every optimal landing trajectory will be validated with a Monte Carlo computation campaign Stochastic variables varied according to their probability distribution A statistical analysis of resulting landing trajectories will give the expectations of success of a specific scenario Finally, we will compare the landing scenarios and select the best one

6. Numerical Results 6.1. Landing on a Small Spherical Comet: 

6. Numerical Results 6.1. Landing on a Small Spherical Comet

6. Numerical Results 6.2. Importance of Aerodynamic Forces: 

6. Numerical Results 6.2. Importance of Aerodynamic Forces Aerodynamic forces can sometimes dominate gravitational forces (for a small comet) The resulting force on the lander will tend to throw it away from the comet Ignoring aerodynamic forces, an expected perfect landing can become a catastrophic miss We need precise outgassing data to evaluate that risk

7. Conclusion: 

7. Conclusion Aerodynamic forces should not be neglected in a comet landing they can throw the probe away from the comet but they can diminish the impact velocity Simple models are sufficient for preliminary mission analyses, but a more precise modeling is necessary to simulate a realistic trajectory The landing scenario have to be robust enough to ensure a safe landing despite the lack of knowledge about the force fields A failure can always occur and we have to be prepared to handle it ideally, the occurrence of a failure should be managed at the maneuver planning phase or a backup scenario must be implemented