Find here an executive summary of the first NEOShield-2 periodic report provided to and accepted by the European Commission.

Executive Summary

Context & overall objectives

As a result of modern observing techniques thousands of near-Earth objects (NEOs) have been discovered over the past 20 years and the reality of the impact hazard has been laid bare. Even relatively small impactors can cause considerable damage: the asteroid that exploded over the Russian city of Chelyabinsk in February 2013 had a diameter of only 18 m yet produced a blast wave that damaged buildings and caused injuries to some 1500 people [Fig. 1]. The potentially devastating effects of an impact of a large asteroid or comet are now well recognized. Can we protect our civilisation from the next major impact?

In contrast to other natural disasters, such as earthquakes and tsunamis, the impact of an asteroid discovered early enough can be predicted and prevented. Following on from the original NEOShield project (FP7), the objectives of NEOShield-2 included improvement of the targeting accuracy and relative velocity of a kinetic impactor spacecraft to deflect a small asteroid, and development of autonomous spacecraft control systems to facilitate navigation close to a low-gravity, irregularly shaped asteroid. Scientific objectives included astronomical observations of NEOs and the analysis of archival data (radar, infrared, spectroscopy, etc.), complemented by modelling and computer simulations, to improve our understanding of their physical properties and how a NEO would respond to a deflection attempt (for a more detailed Executive Summary including the illustrations see below).

Work performed & main achievements

Cover Sheet of NEOShield-2 Periodic Report 2

We have carried out detailed investigations of key technologies vital to the exploration and deflection of NEOs [Fig. 2], including autonomous guidance, navigation, and control systems for a spacecraft in the final approach and proximity phases to an asteroid for the purposes of in-situ science such as surface observations and setting down a lander module, and for a kinetic impactor spacecraft to maximize the targeting accuracy. A harmonized verification approach [Fig. 4] for those technology developments was established leading to an independent validation of all three scenarios to TRL 5-6 by extensive test campaigns. Furthermore, an innovative low-cost kinetic-impactor deflection demonstration concept called NEOTωIST [Fig. 3] has been developed. We have also demonstrated techniques for precise and rapid NEO orbit determination [Fig. 5] and developed mechanisms for the collection of material samples from the surface of a NEO [Fig. 6].

Astronomical observations [Fig. 7] of selected NEOs have been carried out for the purposes of broadening our knowledge of their mitigation-relevant physical properties, concentrating on the smaller sizes of most concern for mitigation purposes, and increasing the list of suitable candidate targets [Fig. 8 & 9] for deflection test missions. Statistical analyses of recently published NEO survey data have led to a novel means of estimating asteroid thermal inertia. Results suggest that the density and thermal conductivity of near-surface material increases rapidly with depth [Fig. 10], providing support for the kinetic impactor as a viable and effective deflection concept. Enhanced computer modelling and simulations in support of a NASA-ESA kinetic impactor study have provided insight into the post-impact ejecta evolution and fate [Fig. 11], which is crucial for the identification of safe locations for an observing spacecraft during and after a kinetic impactor deflection attempt.

Our study of the requirements for future research and international actions, in collaboration with the UN-mandated Space Mission Planning Advisory Group, has identified 11 areas requiring continued or increased effort at the present time. High on the priority list are the development and execution of deflection test missions on real asteroids and technologies for remotely-sensed physical characterization of small NEOs. Our results could form the basis of a European strategy for future mitigation-related endeavours.

Progress beyond the state of the art & expected potential impact

Apart from the direct and tangible progress summarized above, we have demonstrated that a large international team of scientists and engineers, brought together by the FP7 and H2020 programmes, can work closely and effectively together to bring about significant advances in the complex and diverse fields relevant to the NEO impact threat. The efficiency of the team increased with time as the partners developed greater mutual understanding and respect. The results of our work and the nature of the impact hazard underline the need for resources to be made available in this field beyond the horizons of short-term project funding. The full socio-economic impact of our work cannot be realised unless the momentum built up since 2012 during the course of the NEOShield-2 and the original NEOShield projects is continued. The NEO impact hazard is a permanent problem, which can only be tackled by permanent effort.

Finally, the wider societal implications of our work lie in easing public concern over the impact hazard, and demonstrating that the scientific and space-engineering communities are abreast of the problem and have a good chance of successfully deflecting a dangerous NEO should one threaten the Earth in the near future.

 

More detailed Executive Summary (incl. illustrations)

Summary of the context and overall objectives of the project

Context

Fig. 1: The trail left by the Chelyabinsk bolide (15 February 2013, 08:03:54). The left part of the image shows two contra-rotating vortices formed by heating and buoyancy effects in the horizontal cylinder of air in which kinetic energy of the asteroid was deposited. The asteroid had a diameter of only 18 m yet produced a blast wave in the atmosphere that damaged thousands of buildings and caused injuries to some 1500 people. The high altitude of the airburst (> 20 km) and the shallow entry angle (about 17° from the horizon) combined to prevent a potentially far worse outcome. (Credit: Nikita Plekhanov; Wikimedia Commons.)

Collisions of celestial objects with the Earth have taken place frequently over geological history and major collisions of asteroids and comets with the Earth will continue to occur at irregular, unpredictable intervals in the future. As a result of modern observing techniques and directed efforts thousands of near-Earth objects (NEOs) have been discovered over the past 20 years and the reality of the impact hazard has been laid bare. Even relatively small impactors can cause considerable damage: the asteroid that exploded over the Russian city of Chelyabinsk in February 2013 had a diameter of only 18 m yet produced a blast wave that damaged buildings and caused injuries to some 1500 people (see Fig. 1). The potentially devastating effects of an impact of a large asteroid or comet are now well recognized.

Asteroids and comets are considered to be remnant bodies from the epoch of planet formation. Planet embryos formed in the protoplanetary disk about 4.5 billion years ago via the accretion of dust grains and collisions with smaller bodies (planetesimals). A number of planet embryos succeeded in developing into the planets we observe today; the growth of other planet embryos and planetesimals was terminated by catastrophic collisions or a lack of material in their orbital zones to accrete. Most asteroids are thought to be the fragments of bodies that formed in the inner Solar System and were subsequently broken up in collisions.

As a result of collisions, subtle thermal effects and the very strong gravitational field of Jupiter, small main-belt asteroids can drift into certain orbital zones from which they may be ejected under the influence of Jupiter into the inner Solar System. The population of NEOs is thought to consist mainly of such objects, together with an unknown smaller number of old, inactive cometary nuclei. At the time of writing the number of known NEOs exceeds 16000 (http://neo.ssa.esa.int/risk-page); over 1700 of these are so-called potentially hazardous objects (PHOs), i.e. those having orbits that can bring them within 7.5 million kilometres of the Earth’s orbit and are large enough (diameter = 100 m) to destroy a large city or urban area and kill millions of people if they were to impact the Earth. Smaller objects can also present a significant threat: the Chelyabinsk event is a very recent example (see above); a somewhat larger object caused the Tunguska event of 1908 in Siberia, in which an area of over 2000 square km (greater than metropoles like Berlin, Paris or London) was devastated and some 80 million trees felled. The Tunguska event is thought to have been due to the airburst of an object with a diameter of 30 – 50 m at a height of 5 – 10 km. The estimated impact frequency of NEOs on the Earth depends on size. The impact frequency increases with decreasing size due to the size distribution of the asteroid population: there are many more small objects than large ones. Current, albeit uncertain, statistical knowledge of the NEO size and orbital distributions indicates that NEOs with diameters of 50, 100, 300 m, for example, impact roughly every 1000, 10,000, and 70,000 years, respectively.

The known NEO population contains objects with a confusing variety of physical properties. Some NEOs are thought to be largely metallic, indicative of material of high density and strength, while some others are carbonaceous, of lower density, and less robust. A number of NEOs appear to be evolved cometary nuclei that are presumably porous and of low density but otherwise with essentially unknown physical characteristics. In terms of large-scale structure NEOs range from monolithic slabs to re-accumulated masses of collisional fragments (so-called rubble piles) and binary systems (objects with moons). More than 50 NEOs in the currently known population have been identified as binary or ternary systems and many more are probably awaiting discovery.

The phenomenon of collisions in the history of our Solar System is a fundamental process, having played the major role in forming the planets we observe today. Collisions of asteroids and comets with the Earth have taken place frequently over geological history and probably contributed to the development of life. In contrast, later impacts of asteroids and comets most likely played a role in mass extinctions. NEOs present a scientifically well-founded threat to the future of our civilisation. While past impacts have probably altered the evolutionary course of life on Earth, and paved the way for the dominance of mankind, we would now rather not remain at the mercy of this natural process.

Can we protect our civilisation from the next major impact?

In contrast to other natural disasters, such as earthquakes and tsunamis, the impact of an asteroid discovered early enough can be predicted and prevented. The partners in the NEOShield-2 and former NEOShield project are confident the basic technology necessary to prevent an impact is available now. But how do we implement it and what do we need to know about the hazardous object to maximize our chances of success? Preventing a collision with a NEO on course for the Earth would require either total destruction of the object, to the extent that remaining debris posed little hazard to the Earth or, perhaps more realistically, deflecting it slightly from its catastrophic course. In either case accurate knowledge of the object’s mass would be of prime importance. In order to mount an effective mission to destroy the object, knowledge of its density, internal structure, and strength would also be highly desirable. Deflection of the object from its course would require the application of an impulse or continuous or periodic thrust, the magnitude and positioning of which may depend on the mass and its distribution throughout the (irregularly-shaped) body, the surface characteristics, and the spin vector, depending on the strategy deployed. It is crucial to ensure that the deflection operation does not simply move the object to another hazardous trajectory. Mitigation planning takes on a higher level of complexity if the Earth-threatening object is a rubble pile or binary system.

In the case of an object with a diameter of about 50 m or less, the best course of action may be to simply evacuate the region around the predicted impact point, assuming there would be sufficient advance warning (however, only a small fraction of the asteroids in this size category have been discovered to date). For objects larger than 50 m a number of mitigation strategies may be considered, depending on circumstances. The NASA Report to Congress, “Near-Earth object survey and deflection. Analysis of alternatives” (http://www.nasa.gov/pdf/171331main_NEO_report_march07.pdf, 2007) on the surveying and deflection of near-Earth objects concluded that nuclear devices offer the most effective means of applying a deflecting force to an asteroid. While they may offer the only feasible solution in desperate circumstances, e.g. in the case of very little advanced warning, it is clear that the geopolitical issues associated with launching nuclear devices and testing them in space seriously compromise the practicability of this technique. The NASA report concluded that the most effective non-nuclear option is the kinetic impactor, which involves applying an impulsive force to the asteroid by means of a large mass in the form of a spacecraft accurately guided to the target at a high relative velocity. The gravity tractor is a “slow-pull” approach that may require a long period of time to achieve the required amount of deflection, but is a promising technique for cases in which there are many years of advance warning, the target NEO is relatively small, and/or a very slight, precise deflection is required to prevent an impact on the Earth.

The report of the National Research Council of the US “Defending Planet Earth: Near-Earth Object Surveys and Hazard Mitigation Strategies: Final Report” (http://www.nap.edu/catalog/12842/defending-planet-earth-near-earth-object-surveys-and-hazard-mitigation, 2010) contains the following findings and recommendation:

  • “Finding: Mitigation of the threat from NEOs benefits dramatically from in-situ characterization of the NEO prior to mitigation, if there is time to do so.”
  • “Finding: Kinetic impactors are adequate to prevent impacts on Earth by moderately sized NEOs (many hundreds of meters to 1 kilometer) with decades of advance warning. The concept has been demonstrated in space, but the result is sensitive to the properties of the NEO and requires further study.”
  • “Recommendation: If Congress chooses to fund mitigation research at an appropriately high level, the first priority for a space mission in the mitigation area is an experimental test of a kinetic impactor along with a characterization, monitoring and verification system, such as the Don Quijote mission that was previously considered, but not funded, by ESA. This mission would produce the most significant advances in understanding and provide an ideal chance for international collaboration in a realistic mitigation scenario.”

Objectives

In continuation of the former NEOShield project the objectives of NEOShield-2 broadly reflected the above findings and recommendation by thoroughly addressed activities to “access technologies and characterisation for near-Earth objects” as a logical step in preparation for a deflection demonstration mission.

The project work packages have been integrated into a coherent programme of research and technology development. Building on NEOShield experience, the project investigated in more detail key technologies crucial to space missions to explore and deflect NEOs, including autonomous guidance, navigation, and control systems to allow increased targeting accuracy and relative velocity of a kinetic impactor spacecraft into a small (e.g. 50-300m diameter) asteroid, to facilitate autonomous approach/arrival including surface observation and landing close to a low-gravity, irregularly shaped asteroid, demonstrate techniques for precise and rapid NEO orbit determination, review existing and currently developed instruments suitable for NEO in-situ analysis, and develop mechanisms for the collection of material samples. Moreover astronomical observations of selected NEOs have been carried out for the purposes of broadening our knowledge of their mitigation-relevant physical properties, concentrating on the smaller sizes of most concern for mitigation purposes, and increasing the list of suitable candidate targets for deflection test missions. On the scientific front, statistical analyses of recently published NEO survey data, which have already been very successful in the course of NEOShield research, have been further explored in NEOShield-2. The current focus concentrated on detailed analyses of relevant individual objects (e.g. potentially hazardous objects) on the basis of published data from different observing techniques (radar, infrared, spectroscopy, etc.) and new data obtained during the course of NEOShield-2. Furthermore, modelling work and computer simulations have been enhanced to support the design of a kinetic impactor demonstration mission (AIDA space mission project under study at ESA and NASA) focussed on the post-impact ejecta evolution and fate to determine safe locations for an observing spacecraft during and after a kinetic impactor deflection attempt.

Conclusion

As a result of modern observing techniques thousands of near-Earth objects (NEOs) have been discovered over the past 20 years and the reality of the impact hazard has been laid bare. Even relatively small impactors can cause considerable damage: the asteroid that exploded over the Russian city of Chelyabinsk in February 2013 had a diameter of only 18 m yet produced a blast wave that damaged buildings and caused injuries to some 1500 people [Fig. 1]. The potentially devastating effects of an impact of a large asteroid or comet are now well recognized. Can we protect our civilisation from the next major impact?

In contrast to other natural disasters, such as earthquakes and tsunamis, the impact of an asteroid discovered early enough can be predicted and prevented. Following on from the original NEOShield project (FP7), the NEOShield-2 contributions to the global effort building up a viable deflection capability against hazardous near-Earth objects included substantial elements, such as the improvement of the targeting accuracy and relative velocity of a kinetic impactor spacecraft to deflect a small asteroid, development of autonomous spacecraft control systems to facilitate navigation close to a low-gravity, irregularly shaped asteroid, and demonstration of techniques for precise and rapid NEO orbit determination before and after a deflection attempt.

Scientific results included astronomical observations of NEOs concentrating on their mitigation-relevant physical properties increased the available data on the physical properties (e.g. surface composition/taxonomy) of small NEAs by a factor of a few and thus enlarged the list of suitable candidate targets. Moreover the analysis of archival data (radar, infrared, spectroscopy, etc.), complemented by modelling and computer simulations, improved our understanding of their physical properties and how a NEO would respond to a deflection attempt, finally leading to a novel means of estimating asteroid thermal inertia. Results suggest that the density and thermal conductivity of near-surface material increases rapidly with depth, providing support for the kinetic impactor as a viable and effective deflection concept.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

The following is a brief description of highlights and results achieved from the technical and scientific work packages following the logical structure of the project: starting with the reference mission definition activities, the technical results from developing autonomous guidance, navigation, and control systems, the developed techniques for precise and rapid NEO orbit determination or the assessment of sample return technologies and development of a sampling device through target selection for deflection demonstration missions, to the results from the scientific work packages concerned with astronomical observations on multiple telescopes, the related data analysis, the derivation of NEO physical properties, modelling work and computer simulations, and finally the requirements for Future Research and International Actions.

Fig. 2: Graphical illustration of the mission scenarios addressed by NEOShield-2, which are based on existing mission designs, e.g. the NEOShield kinetic impactor deflection demonstration mission or the NEOTωIST impactor-only concept of which the Flyby Module is shown. Within NEOShield-2 reference missions and scenarios were defined in the context of technology development for deflection demonstration and sample return missions.

Reference Mission Definition & GNC Technology Validation I/F

Based on existing mission designs, i.e. the NEOShield Kinetic Impactor deflection demonstration mission or the NEOTωIST Impactor only concept within NEOShield-2 the definition of reference mission and scenario requirements for targeted technology development for mitigation demonstration and sample return missions has been performed (refer to Fig. 2) for the addressed mission scenarios). Two separate sets of reference mission descriptions and requirements have been documented in deliverables D3.1 “Reference mission definition: Mitigation Demonstration” and D3.2 “Reference mission definition: Sample Return”. Both deliverables have been collaboratively elaborated and agreed within the consortium during the Reference Mission Definition and Requirement Review (MDRR) conducted within the first months of the project.

In subsequent activities the reference mission definitions have been used to technically guide and validate against the technology development activities in WP4, 5, 6, 7 and 8 of the project. In context of the “GNC Technology Validation I/F” task, established to harmonize the undertaken technology development and verification/validation process for the different GNC missions as an independent validation instance towards the European Commission, numerous discussions interface and review meetings between all relevant parties have been held finally leading to a commonly established and agreed test plan (refer to Fig. 4).

Fig. 4: Harmonized verification and validation process applied to the guidance, navigation and control (GNC) technology developments within NEOShield-2 leading to an independent validation towards the European Commission of all three GNC scenarios to TRL 5-6 by extensive test campaigns. The three different reference mission scenarios and requirements were used to technically guide and validate against the technology development activities for the kinetic impactor mission, the reconnaissance mission and the surface landing mission objectives. Numerous conversations, interface and review meetings between all relevant parties were held finally leading to a commonly established and agreed test plan, which then resulted in extensive test campaigns successfully completed throughout the project. Raising the level of technology readiness of those GNC techniques relevant for missions to small NEOs up to TRL 5-6 is a crucial prerequisite for conducting such missions in the near future and, as a first step, for undertaking deflection test missions.

The GNC validation and demonstration activities are decisive for raising the level of technology readiness of guidance, navigation and control techniques relevant for missions to NEOs to TRL 5-6, a prerequisite for conducting such missions in the future and for undertaking mitigation demonstrations. Validation and demonstration occurred through the E2E GNC performance verification and validation definition and test plan (GNC E2E Performance Verification and Validation Plan, D3.3), monitoring of the GNC activities and TRL assessment. This approach has been performed under the lead and guidance of GNC experts at ADS-DE who are however not involved in the respective GNC development for the specific missions. This method moreover ensured the achievement of the targeted objective of significant rise of readiness level for enabling GNC technologies, whilst providing efficient coordination and guidance of the technical activities in a transparent and impartial manner.

 

In form of a mini-project the novel deflection demonstration concept NEOTωIST has been introduced to explore a promising implementation option for a low-cost and high-value demonstration mission of the kinetic impactor concept (see Fig. 3, Drube et al. 2016, Engel et al. 2016).

Fig. 3: The innovative, affordable deflection demonstration concept NEOTωIST was introduced as an attractive demo-mission option. The NEOTωIST scenario (Drube et al. 2016, Engel et al. 2016) includes a kinetic impactor plus a sub-satellite constellation observing the impactor collision with Itokawa, two “chasers” (lower left), and the fly-by module (upper centre). Assessment of the mission rationale led to consolidation of the concept and provided insight into key technical challenges. (Image: Airbus, JAXA, SunSpace, SERPENS)

The mission rationale and its value proposition have been assessed during an organized workshop and its aftermath, as well as the derivation of observation requirements and therefore needed observation techniques. This was essential to perform in a next step high level concept trade-offs to consolidate the reference mission concept and to further examine the key technical challenges that must be resolved for successful mission development. This comprised the following subset of activities:

  • Discussion and analysis of information value of different observation options that are part of the mission
  • As part of this generation of a detailed memorandum analysing all factors contributing to uncertainty in the ground based observation of the mission impact event
  • Conduction of a detailed Post Mitigation Risk Analysis (PMIRA) for the NEOTωIST mission scenario and publication of its results in paper: “Dealing with uncertainties in asteroid deflection demonstration missions: NEOTωIST” (Eggl et al. 2015)
  • Mapping of observation functionalities to different elements of the mission
  • Survey of potential observable features and possible observation techniques for each observable
  • Ranking and consolidation of observation goals including required methods by the team (email correspondence and teleconferences)
  • Down selection of observation goals and identification of open questions

GNC of Impacting S/C mission

The GNC subsystem design for the Impacting S/C mission has been updated and improved with regard to the former NEOShield project. Further work has been done on simulator advancement, the navigation & image processing algorithms, and first GNC design simulations and sensitivity analysis have been performed.

A full impacting GNC system prototype including image processing which has demonstrated remarkable performance to TRL5 and above has been developed, featuring the technical capability to hit even small, poorly lit asteroids with high accuracy. The developed solution is expected suitable to a wide range of asteroid targets and mission condition without major modifications. This prototype can thus be considered as a state of the art reference for impacting spacecraft GNC and be re-used as basis for mission development in case a demonstration mission is decided or an actual threat occurs. Considering it is presently a prototype an industrialization process is necessary to ensure full compliance with space mission standards.

GNC of Reconnaissance S/C mission

The developed, engineered and tested GNC subsystem technologies for a Reconnaissance S/C mission comprise the GNC/IP technologies SW (as part of GNC/AOCS subsystem modes) up to TRL 5-6, for the “Close Approach”, “Arrival Inertial Hovering for 6h” and “Body-Fixed Hovering” mission scenarios. The achieved results with the particular applicability for small NEOs with highly irregular shapes are ready for exploitation in ESA projects/programmes at development phases A/B1/B2 concerning space exploration missions to celestial bodies like asteroids, comets, planets and/or moons.

The developed GNC/IP technologies were developed for small and highly irregular bodies, with coarse prior knowledge, providing confidence its suitability for applications with less-irregular bodies (like planets and moons).

The developed GNC/IP technologies SW are directly applicable to:

  • Robust and safe “Close Approach” & “Arrival Inertial Hovering for 6h”: autonomous operations to ease the ground processing delays for interplanetary missions approaching/ arriving to small irregular bodies, other planetary bodies and moons.
  • Stable and high performance “Body-Fixed Hovering”: for missions requiring surface observation, landers release and/or preparation of (but not including) descent & landing operations tested under extreme operational range (sun phase angle from -90 to 90 degrees).

GNC of Sample Return S/C mission

Vision based autonomous GNC and IP technology for a Sample Return S/C mission scenario to land on a small asteroid has been designed and developed according to the required autonomy, robustness and landing accuracy needs, which were the main drivers for this activity. The landing phase starts from a body-fixed position in the close vicinity of the asteroid and is then supported by an autonomous guidance, navigation, and control subsystem to safely land on its surface. This GNC scenario introduced the need of fast navigation update frequency (of 1 Hz) requiring utilization of HW acceleration for the IP implementation to deal with it. Critical functions have been identified, developed and carefully tested as part of the extensive real-time hardware-in-the-loop end-to-end performance verification and validation activities including successful TRL assessment to a rating of TRL5-6.

Real-time Test Facility & Test Campaigns

The existing optical and robotics Real-time Test Facilities at GMV premises have been tailored for the scenarios of the three GNC systems according to the validation requirements derived from the GNC End-to-End Performance Verification and Validation Plan commonly elaborated by the respective team members.

Fig. 5: A precise and rapid NEO orbit determination & monitoring (ODM) technique was implemented and validated, estimating the state of the NEO around the Sun based on absolute referencing (Earth → spacecraft) and relative referencing (spacecraft → NEO). Corresponding ODM algorithms are developed, fine-tuned and for validation purposes, an end-to-end simulator has been set up which includes high-fidelity models of the dynamics, environmental perturbations, and measurements. Validation of the ODM algorithms by Monte-Carlo simulations of a reference mission scenario with simulated mission data in an increased-realistic mission environment (TRL 5) has successfully taken place, as well as analysis of boundary/critical conditions particularly relevant for the mission.

The specific HW components have been selected and procured including space representative HW cameras, lenses, IP computers, etc. while a stepwise development and test approach ensured the re-use of the PIL test benches main components for the different HIL architectures. Subsequently the test equipment has been integrated into the test benches and the facilities have been calibrated, in order to conduct the planned test for the GNC mission scenarios.

Orbit determination and monitoring

An extensive assessment and definition of requirements applicable to Orbit determination and monitoring (ODM) (and further breakdown to absolute / relative referencing) has been performed, a survey of state-of-the-art techniques for ODM has been conducted evaluating the accuracy achievable with radio science measurements. Various solutions have been reviewed in terms of projected performance, applicability, design constraints, heritage and TRL. Candidate alternatives have been generated for ODM, and an extensive trade-off analysis has been conducted leading to the selection of a reference concept which has been analysed with respect to operational scenario, estimation strategy, and projected performance.

Development and validation the orbit determination filter algorithms aim at estimating the state of the NEO around the Sun based on absolute referencing (Earth → spacecraft) and relative referencing (spacecraft → NEO). Corresponding ODM algorithms are developed, fine-tuned and for validation purposes, an End-to-End simulator has been set up which includes high-fidelity models of the dynamics, environmental perturbations, and measurements (refer to Fig. 5). Validation of the ODM algorithms by Monte-Carlo simulations of a reference mission scenario with simulated mission data in an increased-realistic mission environment (TRL 5) has successfully taken place, as well as analysis of boundary/critical conditions particularly relevant for the mission.

Fig. 6: Images of the sampling device. a) Top view; b) Bottom view; c) Inside view with lid removed; d) Inside view with flaps removed. The design of the sampling device, supported by a powder-actuated bolt impactor, is driven by the technical requirements of a sample return scenario. It is capable of collecting both loose material (e.g. dust particles) from the upper surface as well as breaking hard material (e.g. rock) from subsurface layers. Sample collection takes place by means of nitrogen gas being flushed into separate chambers. Detailed evaluations and extensive laboratory tests were performed on multiple prototypes with asteroid like materials in appropriate environments.

Sampling Device and Instruments suitable for NEO in-situ analysis

An analysis and review of Sample Return mission landing scenarios has been performed in order to define an appropriate reference Sample Return mission, including definition of general strategies and requirements for the sampling operations of the Sample Return spacecraft. Moreover an investigation on existing technology and identification of necessary technology development for NEO in-situ analysis and sample return to the Earth’s surface has been addressed including a survey on sampling mechanisms and on in-situ analyses instruments.

The developed sampling device, supported by a powder-actuated bolt impactor, is designed along the technical requirements derived for the sample return scenario. It is capable of collecting both, loose material (e.g. dust particles) from the upper surface as well as breaking hard material (e.g. rock) from subsurface layers and collect it by a flushing process with nitrogen gas into separate chambers (see Fig. 6). Detailed evaluations and extensive laboratory tests were performed subsequently with multiple prototypes and on asteroid like materials in the corresponding or equivalent environments to validate the device functionality and needed performance.

Fig. 7: A large body of NEO observational data was acquired and analysed for the purposes of characterizing primarily the small NEA population. In summary, the new NEOShield-2 observations increased the available data on the physical properties (e.g. surface composition/taxonomy) of small NEAs by a factor of a few. Several objects of particular interest have been identified and/or studied in more detail, and novel strategies have been developed for the modelling of physical properties. About 170 small NEAs have been observed in the framework of the GTO programme at the ESO-NTT telescope recording reflectance spectra, and deriving the taxonomic classification for 137 of them (whose size distribution is shown here above) Photometric colours of more than 230 asteroids have been acquired using many observing runs at the TNG, Asiago and LBT telescopes, resulting in taxonomic classifications of those objects. Collaboration with the Observatorio Nacional (Brazil) using their OASI telescope and observations with the Campo Imperatore telescope in Italy led to acquisition of phase curves for about 50 NEOs. 64 NEA light-curves have been taken through the collaboration with the OASI telescope in Brazil. A new analysis of thermal-infrared Wide-Field Survey Explorer (WISE) data of NEAs and Mars Crossers has been performed, and a new thermophysical model has been developed to derive shapes and thermal properties such as size, albedo and thermal inertia simultaneously. Furthermore, more than 30 precovery detections of PHAs and our observed spectroscopic targets have been successfully obtained in archival Pan-STARRS images.

NEO Observations and data reduction/analysis

A large body of data has been acquired and analysed in the frame of the undertaken NEO observation activities to characterize in particular the small NEA population. The targets have been selected mostly based on the Physical Properties Priority List (P3L) feature of the established NEOShield-2 NEO Properties Portal (NEOPP, http://neoshield.eu/neopp), developed within the project to coordinate the observation campaigns through an efficient prioritization algorithm.

Photometric colours of more than 200 asteroids have been acquired using manifold observing runs at TNG (see Fig. 7), Asiago and LBT telescopes resulting in taxonomic classifications of those objects. Collaboration with the Observatorio Nacional (Brazil) using their OASI telescope leads to acquisition of phase curves for 29 NEOs, while approximately 60 phase curves have been derived with the Itacuruba and the Campo Imperatore telescopes in Italy. Further 43 NEA light-curves have been taken through the collaboration with the OASI telescope in Brazil. 70 small NEAs have been observed completing the GTO programme at the ESO-NTT telescope recording reflectance spectra. In context of thermal infrared data, an original analysis of WISE data of NEAs and Mars Crossers has been performed, and a new thermophysical model has been developed to derive the asteroid shape and thermal properties such as size, albedo and thermal inertia simultaneously. Moreover more than 30 precovery detections of PHAs and our observed spectroscopic targets have been successfully obtained in archival Pan-STARRS images.

In summary, NEOShield-2 new observations increased by a factor of a few the available data for what concerns the physical properties (e.g. surface composition/taxonomy) of small NEAs. Several objects of particular interest have been identified and/or studied in more detail, and novel strategies have been developed for the modelling of the investigated physical properties.

Fig. 9: The NEOShield-2 NEO Properties Portal (NEOPP, http://neoshield.eu/neopp) has been deployed to support observers and mission analysts by facilitating access to the tools and data produced by the project. In the context of “NEO potential mission targets” the online tool provides a dynamical web-interface (the so- called accessibility Mission Opportunity Tables), which can be filtered by mission requirements such as mission scenario, departure date or required delta-V. Therefore realistic mission profiles are computed as soon as newly discovered asteroids or new data become available. The accessibility of an NEO is a time-independent quantity obtained by computing the total velocity change (delta-V in km/s) needed to transform an initially zero inclination circular 1 AU orbit into one identical to that of the object (evaluated through a classical Hohmann transfer). The mission opportunities table relies on ballistic trajectories computed within a given time span and within a certain range of mission requirements (e.g. available delta-V, launch date, mission duration). The table lists minimum delta-V (cheapest) and minimum duration (quickest) alternatives.

 

NEO potential mission targets

A NEOShield-2 NEO Properties Portal (NEOPP, http://neoshield.eu/neopp) has been deployed to support the observers and as well the mission analysts to disseminate the tools and the data produced by the project (see Fig. 9). In context to “NEO potential mission targets” the online tool provides with a dynamical web-interface the so called Mission Opportunity Tables, which can be filtered by mission requirements like mission scenario, departure date or needed delta-V. Therefore realistic mission profiles are computed as soon as newly discovered asteroids or new data become available.

The evaluation of the accessibility of the potential NEO targets and the computation of realistic mission profiles displayed in the NEO Properties Portal, represent two major steps toward selecting suitable target NEOs for exploration and mitigation/deflection (demonstration) missions. The accessibility of an NEO is a time-independent quantity obtained by computing the total velocity change (delta-V in km/s) needed to transform an initially zero inclination circular 1 AU orbit into one identical to that of the object (evaluated through a classical Hohmann transfer). The mission opportunity table relies on ballistic trajectories computed within a given time span and within a certain range of mission requirements (e.g. delta-V available, launch date, mission duration). They represent the first two steps that must be performed in order to compute more complex (e.g. including gravity assist trajectories) and therefore time-consuming mission profiles. Thus, focussing on lower delta-V objects astronomers can observe among the thousands of known NEAs, those are more likely to be selected as targets for a corresponding space mission.

Fig. 8: The filtering of suitable candidate targets for scientific exploration or deflection demonstration missions took into account the NEO physical properties inferred from the observational work. The most fundamental of these are albedo and taxonomic class, which allow the true diameter and mineralogy to be estimated. Since the original NEOShield project (under FP7) there has been a substantial increase in the number of sub-km NEOs for which mitigation-relevant observational results are available, many of which were observed within NEOShield-2. There are now > 30 probable silicate NEOs with diameters between 100m and 300m. Restricting selection to NEOs with delta-v < 6 km/sec and excellent physical characterization, there are 7 silicate NEOs and 2 carbonaceous/primitive NEOs suitable for all classes of proposed missions. Extending the required delta-v to 7 km/sec adds another 4 silicate NEOs and 1 carbonaceous/primitive NEO. Finally, relaxing the characterisation requirement by allowing uncertain orbits, taxonomies, or binaries gave a further 7 silicate and 8 carbonaceous/primitive NEOs. Overall, there now exists a good selection of well characterized sub-km silicate NEOs which could be used for impact mitigation tests or other in-situ exploration missions.

For appropriate “NEO potential mission targets” selection the NEO physical properties need to be taken into account as well and the current and on-going measurements of NEO physical properties have been considered and inspected in that respect (refer to Fig. 8). The most fundamental of these are albedo and taxonomic class, which alone or in combination allow the true diameter and mineralogy to be estimated. Since the original NEOShield project (under FP7) there has been a substantive increase in the number of sub-km NEOs with mitigation-relevant observational studies, many of which are due to the continuing work within NEO observations undertaken within NEOShield-2. There are now > 30 probable silicate NEOs with diameters between 100m and 300m. Restricting selection to NEOs with delta-v < 6 km/sec and excellent physical characterization, there were 7 silicate NEOs and 2 carbonaceous/primitive NEOs suitable for all classes of proposed missions. Extending the required delta-v to 7 km/sec adds another 4 silicate NEOs and 1 carbonaceous/primitive NEO. Finally, relaxing the characterisation requirement by allowing uncertain orbits, taxonomies, or binaries gave a further 7 silicate and 8 carbonaceous/primitive NEOs. Overall, there now exists a good selection of well characterized sub-km silicate NEOs which could be used for impact mitigation tests or other in-situ exploration missions.

Fig. 10: (left) Plot showing the rapid increase of thermal inertia with depth for NEOs. The skin depth is roughly the depth to which the diurnal (day-night) thermal wave penetrates. (right) The plot shows the difference in thermal inertia between samples of C-type and M-type asteroids observed by WISE. The M types have significantly higher thermal inertia on average. On the basis of published data, primarily from the NASA Wide-field Infrared Survey Explorer satellite telescope (WISE), we have found strong indications that a thermal-model parameter, η, related to an asteroid’s surface temperature distribution, correlates with thermal inertia and radar albedo. We have also shown that relatively high rotation rates contribute to the higher η values of many metal-rich asteroids. Our findings enhance and explain the scientific significance of the η parameter with regard to identifying potentially metal-rich, and therefore possibly relatively dense, asteroids. We have derived a means to estimate asteroid thermal inertia from considerations of parameters influencing the temperature distribution on the surface of an asteroid. Our novel thermal-inertia estimator has revealed an interesting and unexpected tendency for slowly-rotating asteroids to have higher thermal inertia, which can be explained in terms of rapidly increasing thermal properties, such as density and thermal conductivity, with depth in the topmost porous surface layers. These results may influence the choice of an appropriate value of thermal conductivity for calculation of the Yarkovsky effect, e.g. in calculations of impact probabilities of potentially hazardous objects. Our results suggest that in general large chunks of solid rock are present just tens of centimetres below the surface of a NEO, which in turn implies that the kinetic impactor method of hazardous NEO deflection may be more effective in general than has been assumed to date.

 

NEO Physical Properties: Data analysis and modelling

On the basis of published data, primarily from the NASA Wide-field Infrared Survey Explorer satellite telescope (WISE), we have found strong indications that a thermal-model parameter, η, related to an asteroid’s surface temperature distribution, correlates with thermal inertia and radar albedo. We have also shown that relatively high rotation rates contribute to the higher η values of many metal-rich asteroids. Our findings enhance and explain the scientific significance of the η parameter with regard to identifying potentially metal-rich, and therefore possibly relatively dense, asteroids. We have derived a means to estimate asteroid thermal-inertia from considerations of parameters influencing the surface temperature distribution on the surface of an asteroid. Our novel thermal-inertia estimator has revealed an interesting and unexpected tendency for slowly-rotating asteroids to have higher thermal inertia, which can be explained in terms of rapidly increasing thermal properties, such as density and thermal conductivity, with depth in the topmost porous surface layers (see Fig. 10). These results may influence the choice of an appropriate value of thermal conductivity for calculation of the Yarkovsky effect, e.g. in calculations of impact probabilities of potentially hazardous objects. Our results suggest that in general large chunks of solid rock are present just tens of centimetres below the surface of a NEO, which in turn implies that the kinetic impactor method of hazardous NEO deflection may be more effective in general than has been assumed to date.

Fig. 11: The dynamical fates of ejecta with different ejection speeds from Didymoon’s surface. The maps show the distribution of the 7 types of dynamical fates pictured against the launching sites of the sampled particles, each for a fixed ejection speed: (a) 6.0 cm/s (b) 10.0 cm/s (c) 16.0 cm/s (d) 26.0 cm/s (e) 34.0 cm/s and (f) 38.0 cm/s. The left of each figure faces the primary of Didymos (From Yu & Michel 2017). A campaign of validation and inter-comparison of codes for computer simulations of impacts on asteroids was started, which will continue after NEOShield-2 with other groups in Europe and the US. We have studied the fate of small ejecta particles resulting from a spacecraft impact into the moon of the asteroid Didymos (“Didymoon”) in the framework of the NASA-ESA AIDA project, providing estimates of the timescale of the risk for an observing spacecraft from considerations of solar tides, the gravity field of the binary system, and solar radiation pressure. Our studies of the dependency of ejecta fate on launching sites and speeds reveal the detailed proportions of the ejecta that are either orbiting, escaping or re-accreting on the primary/secondary at the end of the considered timescale, as a function of the ejection speed. Our full-scale simulations of the ejecta cloud released from 6 hypothetical impact sites show that the cloud evolution can be divided in two periods: a first violent period (<10 hr) with fast re-accretion or loss of the ejecta from the system, and a second period more sensitive to the launching site than the first one. Moreover, due to solar radiation pressure there is a size-sorting effect which leads to efficient expulsion from the system of small dust-size ejecta (<1 mm) for all considered launch sites and material types.

A campaign of validation and inter-comparison of codes for computer simulations of impacts on asteroids was started, which will continue after NEOShield-2 with other groups in Europe and the US. We have studied the fate of small ejecta particles resulting from a spacecraft impact into the moon of the asteroid Didymos (“Didymoon”) in the framework of the NASA-ESA AIDA project, providing estimates of the timescale of the risk for an observing spacecraft from considerations of solar tides, the gravity field of the binary system, and solar radiation pressure. Our studies of the dependency of ejecta fate on launching sites and speeds reveal the detailed proportions of the ejecta that are either orbiting, escaping or re-accreting on the primary/secondary at the end of the considered timescale, as a function of the ejection speed. Our full-scale simulations of the ejecta cloud released from 6 hypothetical impact sites show that the cloud evolution can be divided in two periods: a first violent period (<10 hr) with fast re-accretion or loss of the ejecta from the system, and a second period more sensitive to the launching site than the first one. Moreover, due to solar radiation pressure there is a size-sorting effect which leads to efficient expulsion from the system of small dust-size ejecta (<1 mm) for all considered launch sites and material types (see Fig. 11).

 

Requirements for Future Research and International Actions

Continuing our efforts in the original NEOShield project we investigated the effectiveness of impact disaster planning and the ability of individual nations of the European Union to cope with a serious NEO impact warning. We contacted 28 members of the European union, asking for information/statements on the role and preparedness of their national civil protection authority(ies) (and other relevant national bodies) in the case of an asteroid impact alarm. In summary, we find the following areas are in need of improvement:

  • An increase in asteroid impact risk awareness is necessary as a first step to updating the national risk assessments/registers of individual countries.
  • Dissemination of knowledge about NEOs to the general public.
  • The information exchange between national authorities and the scientific community.
  • Early notification of the danger and the information exchange between different organizations.
  • Taking decisions and acting on uncertain forecasts.
  • Predictions of the impact circumstances (e.g. location).

Through our representation in the UN-mandated International Asteroid Warning Network (IAWN) and the Space Mission Planning Advisory Group (SMPAG) (DLR and CNRS), which meet twice per year, we are acquiring first-hand knowledge of, the plans for international coordinated activities as they develop. NEOShield-2 personnel have monitored the activities of other organisations (e.g. ESA, NASA) and Chinese, Japanese, Russian, etc. groups working in the impact hazard and mitigation fields, in order to study the effectiveness of international coordination and funding of mitigation activities and help to develop a European strategy for future research and mission-related endeavours in this field. The NEOShield-2 work has been carried out in close cooperation with the UN-mandated Space Mission Planning Advisory Group (SMPAG). The personnel involved in the work are members of the German delegation to SMPAG. Despite significant progress over the past decade or so, we feel that much more needs to be accomplished before the international community can feel adequately protected from a potentially catastrophic asteroid impact. We have identified 11 areas requiring continued or increased effort at the present time, including the development and execution of test missions to enable deflection concepts to be tried out on real asteroids, remotely-sensed physical chara­cterization of small NEOs, and the development of new deflection techniques for small NEOs.

A further requirement is clarification of the legal implications of international actions to deflect an asteroid. At the February 2016 session of SMPAG it was agreed to establish a working group to discuss and make proposals on the legal issues relevant to the execution of NEO deflection missions, both for test purposes and in an emergency situation, and associated aspects of planetary defence. The group, which consists of space law experts and scientists/engineers from more than 10 countries, came together for its first meeting at the UN in Vienna during the COPUOS session, February 2017. Two members of the group, including the coordinator, are NEOShield-2 personnel.

Technical aspects concerning post-2017 developments and activities for NEO deflection missions in particular emphasizing the Kinetic Impactor concept have been monitored, analysed and supported where possible. This comprised identification and assessment of the most relevant mission profiles and required technologies to implement such an asteroid exploration, deflection preparation or even deflection mission, either a deflection demonstration or an actual deflection mission in response to a real NEO threat. A summary of the mission scenarios together with the required technologies and their current readiness level has been prepared in a dedicated “Technology development plan” leading to recommendations for most pressing measures to advance them in order to enable a European strategy for future mitigation-related endeavours.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

We consider the main results from the NEOShield-2 project to be:

  • Detailed definition of characteristic reference mission scenarios applicable to different NEO threat, deflection demonstration and in-situ characterization scenarios (see Fig. 2).
  • Significant advances in development of autonomous guidance, navigation, and control systems as key technologies to explore and deflect NEOs. It allows for increased targeting accuracy and relative velocity of a kinetic impactor spacecraft into a small (e.g. 50-300m diameter) asteroid and facilitates approach/arrival including surface observation close to and landing onto low-gravity, irregularly shaped asteroids. A harmonized common approach for GNC validation and demonstration activities and transparent success criteria to raise & evaluate the level of technology readiness has been applied to all three GNC technology development scenarios (refer to Fig. 4).
  • A novel low-cost kinetic impactor deflection demonstration concept called NEOTωIST, based on changing the spin rate of the NEO Itokawa (see Fig. 3). In its cheapest form, the concept requires only one spacecraft, the impactor, since the change in spin rate of the asteroid, and therefore the momentum transfer efficiency, can be measured via ground-based lightcurve observations. However, as the mission rationale and its value proposition assessment shows, the scientific value is largely increased by a small impact observing flyby module and CubeSat chasers for close-up ejecta cloud observations. Essential high level concept trade-offs to consolidate the technical concept and further examine the key technical challenges are ongoing.
  • Fast and precise techniques for NEO orbit determination allowing in short-term accurate quantification and validation of deflection attempts and its effects on the NEOs orbit also for deflection mission scenarios where no observations from Earth are possible (refer to Fig. 5).
  • Identification of most promising sampling strategies as key aspect for all future sample-return missions targeting solar system bodies. Conducting an in depth analysis of existing and currently developed instruments suitable for NEO in-situ analysis and development including breadboard tests of a promising sampling device for the collection of material samples (see Fig. 6).
  • The astronomical observations carried out by NEOShield-2 more than doubled the available data for what concerns the surface composition (taxonomy) of small NEOs of most concern for mitigation/deflection purposes, and several objects of particular interest have been identified and/or studied in parallel (refer to Fig. 7). This significantly contributes to the physical characterisation of the NEO population leading to an increased list of suitable candidate targets for deflection test missions. Novel strategies have for rotational and thermophysical modelling of extended samples of NEOs have been established.
  • The identification and characterization of several suitable target NEOs for exploration and mitigation/deflection (demonstration) missions (see Fig. 8) on the basis of pre-computed trajectories is an automatized ongoing process dynamically considering the list of known NEOs (refer to Fig. 9), which is enlarged by about 3-8 new objects per day (!) due to the respective rate of daily new NEO discoveries.
  • Establishment of an online NEOShield-2 “NEO Properties Portal” (NEOPP, http://neoshield.eu/neopp) disseminating the data, tools and results produced by the project in open access to the public and supporting the observers as well as the mission analysts (see Fig. 9). The “NEO Properties Portal” contains observational data, derived physical properties, observation status & priority lists, and mission opportunities tables to suitable target NEOs for exploration and mitigation/deflection (demonstration) missions. All data available at the NEOPP will be migrated to the ESA’s NEO Coordination Centre in order to ensure public availability of the related project results when the project has been ended.
  • Our work and findings on the statistical analysis of NEO physical properties and observables like surface temperature distribution and radar albedo enhance and explain the scientific significance of the thermal-model parameter η with regard to identifying potentially metal-rich, and therefore possibly relatively dense, asteroids. We have confirmed an unexpected and very interesting dependence of thermal inertia on spin rate in the case of NEOs, such that thermal inertia appears to increase with increasing spin period (refer to Fig. 10). Our results are consistent with a very general picture of rapidly changing material properties in the topmost regolith layers (i.e. density and thermal conductivity increasing with depth). If large pieces of solid rock are present just tens of centimetres below the surface of a NEO, as our results suggest is the case in general, the kinetic impactor method of NEO deflection may be more effective in general than has been assumed to date.
  • In our simulation work to date the concentration to the complex problem of the fate of ejecta produced by a kinetic impactor has revealed that the violent period of ejecta evolution lasts for the first few hours after impact and that the risk for a spacecraft observing the event from a safe position appears to be largely mitigated in the two weeks post impact, allowing to leave its safe distant location for a second close-up scientific measurement phase after that timeframe (refer to Fig. 11). Depending on the ejecta size the motion of ejecta near the impacted asteroid is dominated by the gravity or solar radiation pressure and it has been confirmed, that the solar tide and solar radiation pressure are the major sources of perturbation of the ejecta dynamics.
  • The participation of several NEOShield-2 partners in the international UN-sanctioned SMPAG group is already leading to the use of results from NEOShield and NEOShield-2 deliverables for SMPAG tasks, such as consideration of mitigation/deflection mission types and technologies, reference mission design studies for different NEO threat scenarios, instruments and mission requirements for the characterisation of a threatening NEO, and the development of a coordinated strategy for future work on planetary defence.

In parallel to the direct results of the project funding there is a substantial inherent result worth to be mentioned and emphasized: The demonstration that a large international team of scientists and engineers, brought together by the European Commission’s research funding programme, can work closely and effectively together to make significant advances in the complex and diverse fields relevant to NEO impact threat mitigation. The efficiency with which the team has repeatedly tackled the complex issues inherent to this field has increased with time as the partners developed greater mutual understanding and respect. Resources should be made available beyond the horizons of short-term project funding to ensure the momentum built up during the course of NEOShield-2 (and the former NEOShield) does not go to waste, but rather the work of the NEOShield-2 partners can be continued on a long-term basis. The NEO impact hazard is a permanent problem, which can only be tackled by permanent effort.

Finally, the socio-economic impact and the wider societal implications of NEOShield-2 in continuation of NEOShield lie in easing public concern over the impact hazard, and demonstrating that the scientific and space-engineering communities are abreast of the problem and have a good chance of successfully deflecting a dangerous NEO should one threaten the Earth in the near future (as shown in Fig. 1).