ESA - NEO Space Mission Preparation

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SIMONE is a unique interplanetary mission concept comprising a fleet of low-cost microsatellites that will individually rendezvous with a different Near Earth Object (NEO), each of a distinct spectral and/or physical type. In-situ science measurements taken by instruments on-board each spacecraft enable the wide diversity in the physical and compositional properties of the NEO population to be characterised in a highly cost-effective manner. Analysis of the in-situ measurement data from the SIMONE rendezvous missions will provide:

SIMONE can be realised by the use of microsatellite technology - this would be a world-first for an interplanetary mission. Conducting multiple NEO rendezvous missions with large conventional spacecraft would be prohibitively expensive. It is planned to deploy 5 of the SIMONE microsatellites to their intended rendezvous targets within the budget envelope of an ESA Flexi-mission. In order to achieve this target, a low-cost approach is needed and this is only attainable by using microsatellite technology.

The SIMONE microsatellites are based around a single spacecraft system design, configuration and payload, and a single ground segment, thereby significantly lowering recurring costs. "Piggyback" launch opportunities on the Ariane Structure for Auxiliary Payloads (ASAP) on Ariane-5 will be exploited in order to obtain low launch costs. Traditionally, launch costs are a significant cost driver for interplanetary missions because a dedicated deep space launch is usually required for direct injection onto an interplanetary trajectory. Instead, these launches will place each SIMONE spacecraft into a Geostationary Transfer Orbit (GTO).

From GTO, on-board propulsion is required to achieve an Earth escape trajectory, adjust the interplanetary trajectory and eventually rendezvous with the target NEO. This places high delta-velocity requirements on a spacecraft that has a tightly constrained mass budget. As a piggyback payload on an Ariane 5 launch, the mass of each spacecraft is limited to 120kg. Mission feasibility is assured by the use of efficient low-thrust solar electric propulsion in order to achieve the demanding rendezvous mission objectives with a low propellant mass consumption. The ion propulsion system is driven by lightweight, high power solar arrays that can be efficiently stowed within the limited launch volume.

	Mission Phase	Duration	Description
1	Launch and Early Orbit Phase (LEOP)	~3-4 days	Deployment from an Ariane-5 launch vehicle into GTO, and attainment of a stable 3-axis attitude control mode following ejection from the Ariane upper stage
2	Check Out Phase	~2 weeks	Complete system functional tests during visibility periods with the ground station.
3	Parking Orbit Phase	~9-12 months (depending upon time to optimum Escape Phase start)	From highly eccentric GTO, raising of the orbit above the proton & electron radiation belts into a safe (i.e. low radiation dose) near-circular "parking orbit" at 300,000km Wait for the Earth-Escape Phase to start.
4	Earth Escape Phase	~1 month	Expand and phase the orbit using the on-board ion propulsion system for a lunar swing-by gravity assist manoeuvre to lower delta-V and propellant consumption. Exceed Earth gravitational sphere of influence into a heliocentric orbit with an inclination similar to the target plane.
5	Rendezvous Phase	~22-33 months (depending upon the NEO target orbit)	Combination of phased long-duration low thrust arcs and coast (no thrust) arcs in heliocentric orbit. Follow an optimised transfer trajectory that ultimately arrives in vicinity of the target at a low approach velocity and matches the spacecraft's orbit with that of the target NEO orbit. Trajectory correction manoeuvres are determined based upon radio navigation techniques. Acquisition of the target NEO using the multispectral imager payload for optical navigation relative to the target. Long-range approach to the target until reaching a stand-off distance from 2000km down to ~200km ready for the Measurement Phase
6	Measurement Phase	~4 months (depending upon science data acquisition plan)	Intermediate proximity: Co-fly with the target NEO at ~100-200km range and take in-situ measurements with the payload instruments to determine size and shape. Close proximity: Close ballistic swing-bys of the spacecraft at a minimum distance of a few NEO radii (<10km) to determine gravity field and mass, using the radio science investigation and laser altimetry. Determine bulk density from shape, mass. "Imaging" swing-bys at <10km altitude to obtain high-resolution data on NEO surface features/composition.

The preliminary selection of the group of NEO targets for the SIMONE rendezvous missions was performed by the mission study team. A target selection scheme has been developed in order to filter the known NEO population by a number of key selection criteria and hence obtain a short list of candidates for rendezvous. The filter criteria include:

The two selected X-type objects could turn out to be either P, M or E types after further spectral follow-up observations are made. Thus, a good cross-section of different NEO spectral types can be characterised by the SIMONE rendezvous missions for risk assessment (C-types in particular are common but their basic properties are poorly understood) and science (detailed knowledge of primitive P or E-types, and metallic M-types). After C-type, M-type is a priority, but the nearest confirmed M-type metallic object is beyond the delta-V capability for a SIMONE rendezvous. Hence, a resonant orbit encounter strategy has been formulated for periodic brief flybys of the M-type asteroid Amun in case that neither of the selected X-types turns out to be an M-type object.

The NEO population contains a wide diversity of bodies with different physical properties relating to their origin and subsequent processing by solar radiation, cosmic ray and impact effects. Ground-based optical telescope observations have enabled a number of different types of NEO to be characterised by their unique spectral signatures. From the observations, physical properties such as size, shape, rotation, surface mineralogical type can be inferred. Radar observations can give excellent information on orbit, rotation state, surface topography, and binary nature, with the latter being useful to accurately derive object mass.

Whilst remote observations permit classification of general compositional types and determination of some physical properties, the range of uncertainty of this inferred physical information is often large and insufficient for impact risk assessment and mitigation purposes. Furthermore, some information simply cannot be collected from ground observations. Only dedicated rendezvous missions can provide the in-depth study of NEOs that is required to fully characterise their physical and compositional properties with high accuracy. A multiple rendezvous strategy to sample the different types of NEO at close range addresses critical gaps in our knowledge of the NEO population that would otherwise remain, even in the light of the various forthcoming missions. Filling these knowledge gaps is fundamental to how well we can define and counter the threat posed by NEOs to the Earth in the future.

The diversity of NEO physical properties (particularly bulk density) leads to great uncertainty in the magnitude of destruction that an object of a particular size would cause on Earth. The data provided by multiple rendezvous missions would allow better predictive tools to be established that would link key properties such as bulk density with parameters that can be derived from Earth-based or space-based remote observations. Such links are important in that they would allow a better initial characterisation of a potentially threatening object (in terms of size, mass and composition), thus influencing the decision regarding an effective mitigation approach.

Mitigation techniques may involve a slight deflection of a minor body by either a carefully planned impact (or series of impacts), long-term ablation of the surface, an explosive impulse, or long-term action of a low-thrust propulsive device. Understanding key properties of the surface and bulk materials of different types of NEOs and their response to heating or mechanical stress is thus crucial to the selection and development of effective mitigation techniques. Some techniques may be wholly ineffective for certain types of target object, such as highly porous or fractured bodies. Close-up reconnaissance of diverse NEOs is a logical precursor to the success of missions dedicated to executing specific mitigation strategies.

The multiple rendezvous philosophy echoes one of the recommendations of the report of the UK Government's Taskforce on Potentially Hazardous Near Earth Objects, namely that the case for mounting a number of co-ordinated rendezvous missions to visit different types of NEO should be explored. The recommendation goes on to suggest that they be based on relatively inexpensive microsatellites. This was the starting point for the SIMONE concept.

The primary mission objectives of the SIMONE mission at each different NEO are to determine (in priority order):