End-Of-Life Disposal Concepts for Lagrange-Point and Highly Elliptical Orbit Missions

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1 End-Of-Life Disposal Concepts for Lagrange-Point and Highly Elliptical Orbit Missions Executive summary of the main study and the study extension Version February 2015 ESA/ESOC contract No /13/F/MOS ESA/ESOC Technical Supervisor: Markus Landgraf and Florian Renk Technical Responsible: Camilla Colombo, University of Southampton Camilla Colombo, Francesca Letizia, Stefania Soldini Elisa Maria Alessi, Alessandro Rossi, Linda Dimare Massimiliano Vasile, Massimo Vetrisano, Willem van der Weg University of Southampton SpaceDyS University of Strathclyde 1

2 Executive summary Libration Point Orbits (LPOs) and Highly Elliptical Orbits (HEOs) are often selected for astrophysics and solar terrestrial missions as they offer vantage points for the observation of the Earth, the Sun and the Universe. No guidelines currently exist for LPO and HEO missions end-of-life; however, as current and future missions are planned to be placed on these orbits, it is a critical aspect to define a sustainable strategy for their disposal, with the objective to avoid interference with protected regions. Indeed, LPOs or HEOs lie in a highly perturbed environment; moreover, due to their challenging mission requirements, they are characterised by large-size spacecraft. Therefore, the uncontrolled s/c on manifold trajectories could re-enter to Earth or cross the protected regions. In the framework of the ESA/ESOC contract No /13/F/MOS End-Of-Life Disposal Concepts for Lagrange-Point and Highly Elliptical Orbit Missions, a detailed analysis of possible disposal strategies for LPO and HEO missions was performed [1, 2]. The study was done by the consortium led by the University of Southampton in collaboration with SpaceDyS and the University of Strathclyde. A list of missions is delivered, covering all the ESA missions currently operating on LPO and HEO and future missions that are currently in post phase-b1. Based on the available debris mitigation requirement documentation and the mission parameters, the requirements and constraints for the disposal are defined. Then, a preliminary analysis of the possible disposal strategies is presented [1, 3]. Six ESA missions currently (or in the future) operating on LPO and HEO are selected as test case scenarios: Herschel, Gaia, SOHO and Lisa Pathfinder as missions in LPO, and INTEGRAL and XMM-Newton as missions in HEO. In order to keep the general validity of this study to different LPOs and HEOs, the search for optimal trajectories for disposal is not limited to the on-board propellant; rather, a limit on the deltav of 150 m/s is initially considered for LPO, while for HEO 2/3 times the available delta-v on-board is used. A parametric analysis was performed to define optimal disposal strategies (in terms of time and delta-v) for different starting dates and orbital conditions for the disposal. The manoeuvre is optimised considering the constraints on the available fuel at the end-of-life. For each mission the disposal strategies are analysed, in terms of optimal window for the disposal manoeuvre, manoeuvre sequences, time of flight and disposal characteristics, such as re-entry conditions or the hyperbolic excess velocity at arrival in case of a Moon impact. The disposal strategies proposed and designed are: HEO disposal through Earth re-entry [4], HEO disposal through injection into a graveyard stable orbit, HEO disposal through transfer to a LPO, HEO disposal through Moon capture [5], LPO disposal through Earth re-entry [6], LPO disposal towards a Moon impact [5], LPO disposal towards the inner or the outer solar system, LPO disposal towards the outer solar system through solar radiation pressure [7]. For each strategy, the mission scenario, the simulation framework and the requirements and constrains for the detailed strategy analysis are defined. On the basis of the operational cost, complexity and demanding delta-v manoeuvres, some disposal options were later discarded via discussion with ESA. Those disposal solutions, namely, HEO disposal through transfer to a LPO, HEO disposal through Moon capture, LPO disposal to another planet, LPO disposal through capture at the Moon, could be anyway considered as starting point for future studies. 2

3 As a further step, the optimal trajectories for each mission scenario and for each disposal strategy are then refined with a high fidelity model of the dynamics and an uncertainty analysis on the initial parameters and the spacecraft parameters is performed. Finally, a trade-off is made considering technical feasibility (in terms of the available on-board resources and delta-v requirements), as well as the future sustainability of the disposal and the collision probability in the protected regions. General recommendations are drawn in terms of system requirements and mission planning. In light of the objective of sustainability, it appears reasonable to postulate a permanent removal of the hardware from the space environment as a main objective for the end-of-life strategy. For HEO missions this can be achieved by a controlled or semi-controlled re-entry into the Earth atmosphere, when this is allowed by the on-board delta-v. Alternatively, long-term stability orbits should be selected as graveyard. The effect of luni-solar perturbations can be exploited to enhance the disposal manoeuvre. In particular INTEGRAL can be disposed into a re-entry trajectory by giving a manoeuvre that decrease the nominal eccentricity [4] (Figure 1a); in this way the effects of luni-solar perturbations will enhance the oscillation in eccentricity, therefore the orbit will enter into the Earth atmosphere, when the maximum eccentricity is attained. In the case of XMM-Newton the available on-board was not sufficient to allow a re-entry in the next 30 years, therefore, a graveyard orbit is considered with a limited perigee variation (Figure 1b). In this case an analysis of longer timespan should be performed to evaluate possibilities of re-entry. v Figure 1. HEO mission disposal in the eccentricity-perigee angle phase space (red: nominal orbit, cyan: disposal trajectory). A) INTEGRAL disposal on 08/08/2014 through Earth re-entry and b) XMM-Newton graveyard disposal on 20/04/2016. For LPO missions, the feasibility of a controlled re-entry to the Earth depends on the operational orbit and the spacecraft capabilities at the end-of-life [6]. An Earth re-entry can be considered a feasible option for Herschel and SOHO-like missions with a time of flight of less than a year and the re-entry angle can be selected for fulfilling on-ground risk requirements. The Δv requirements are consistent with the fuel available at the end-of-life for Herschel and SOHO. The same conclusions can be drawn for Gaia-like missions: there exist almost zero-cost solution with a time of flight of less than a year and the probability of collision within the protected regions can be considered negligible (Figure 2a). If a re-entry is not possible, a permanent removal from the space environment can be achieved by lunar impact. Trajectories legs deriving from the unstable invariant manifolds leaving the 3

4 LPO in the Sun (Earth + Moon) Circular Restricted Three Body Problem (CR3BP) and the stable manifolds of a LPO around L 2 in the Earth Moon CR3BP are connected into a single trajectory. The Sun (Earth + Moon) CR3BP has as primaries the Sun and the Earth Moon barycentre. Connection between the two models is accomplished via the use of Poincaré sections and then optimised with a high-fidelity ephemerides model [5]. Trajectories with longer transfer time generally have a lower cost in Δv (Figure 2b). If the lunar impact disposal is performed in line with a sustainable conduct of avoiding heritage sites and sites of high scientific interest, it can be considered more sustainable than the semi-permanent solution of using a parking orbit. Figure 2. LPO mission disposal: A) Herschel disposal through Erath re-entry coloured as function of the time of flight in days from the LPO to the Earth. B) SOHO disposal to Moon impact: time of flight as function of the time of departure from the LPO in MJD2000. Solutions are colour-coded according to the Δv cost in m/s. In the case a Sun-parking disposal orbit is selected, the zero velocity curves need to be closed with a manoeuvre, considering additional margin to counteract the perturbations due to other bodies and solar radiation pressure. A preliminary study in the CRTBP is performed for SOHO, Herschel and Gaia. Then, the disposal design is performed in the high-fidelity ephemerides model for Gaia and Lisa Pathfinder missions for accounting the effects of all the perturbations [2] (Figure 3). Moreover, an uncertainty analysis is performed to study the sensitivity to manoeuvre errors and the effect of solar radiation pressure [8]. As the available on-board Δv for Gaia is high, the heliocentric disposal is robust to uncertainties on the manoeuvre in terms of hazard to return in the Earth vicinity, while in the case of disposal through Earth re-entry and error in the last manoeuvre could affect the footprint on ground. In the case of Lisa Pathfinder heliocentric disposal, instead, due to the limited Δv onboard it is not guaranteed that all the solutions of the uncertainty analysis do not return into the Earth vicinity, therefore some correction manoeuvres should be considered. 4

5 Figure 3. LPO heliocentric disposal. A) Gaia heliocentric disposal: minimum distance from Earth as function of the angular position of the Earth + Moon when the disposal is initiated. B) Lisa Pathfinder disposal: minimum distance from the Earth as a function of the time of departure (solutions colour coded according to their ΔV cost). References [1] Colombo C., Letizia F., Soldini S., Lewis H., Alessi E. M., Rossi A., Vasile M., Vetrisano M., Van der Weg W., End-of-life disposal concepts for lagrange-point and highly elliptical orbit missions, Final Report, ESA/ESOC contract No /13/F/MOS, [2] Colombo C., Letizia F., Soldini S., Alessi E. M., Rossi A., Dimare L., Vasile M., Vetrisano M., Van der Weg W., End-of-life of gaia mission and lisa pathfinder mission: Heliocentric disposal and Earth re-entry options, Final Report study extension, ESA/ESOC contract No /13/F/MOS, [3] Colombo C., Alessi E. M., Weg W. v. d., Soldini S., Letizia F., Vetrisano M., Vasile M., Rossi A., Landgraf M., End-of-life disposal concepts for libration point orbit and highly elliptical orbit missions, Acta Astronautica, 2014, in press. doi: /j.actaastro [4] Colombo C., Letizia F., Alessi E. M., Landgraf M., End-of-life Earth re-entry for highly elliptical orbits: The INTEGRAL mission, Proceedings of the 24 th AAS/AIAA Space Flight Mechanics Meeting, Santa Fe, New Mexico, 2014, AAS [5] Weg W. J. v. d., Vasile M., Sun Earth L 1 and L 2 to Moon transfers exploiting natural dynamics, Celestial Mechanics and Dynamical Astronomy, Vol. 120, N. 3, 2014, pp doi: /s [6] Alessi E. M., The reentry to Earth as a valuable option at the end-of-life of libration point orbit missions, under review Advances in Space Research, [7] Soldini S., Colombo C., Walker S., Landgraf M., Libration-point orbit missions disposal at the end-of-life through solar radiation pressure, Proceedings of the 2 nd International Academy of Astronautics Conference on Dynamics and Control of Space Systems, (DYCOSS), Rome, Italy, 2014, IAA-AAS-DyCoSS [8] Vetrisano M., Van der Weg W., Vasile M., Navigating to the Moon along low-energy transfers, Celestial Mechanics and Dynamical Astronomy, Vol. 114, N. 1-2, 2012, pp doi: /s