Contents at a Glance
- 1 Summary of the context and overall objectives of the project
- 2 Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far
- 2.1 Reference Mission Definition & GNC Technology Validation I/F
- 2.2 GNC of Impacting S/C mission
- 2.3 GNC of Reconnaissance S/C mission
- 2.4 GNC of Sample Return S/C mission
- 2.5 Sampling Device” and “Instruments suitable for NEO in-situ analysis
- 2.6 Real-time Test Facility & Test Campaigns
- 2.7 Orbit determination and monitoring
- 2.8 NEO potential mission targets
- 2.9 NEO Observations and data reduction/analysis
- 2.10 NEO Physical Properties: Data analysis and modelling
- 2.11 Requirements for Future Research and International Actions
- 3 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)
Summary of the context and overall objectives of the project
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 Figure 1). The potentially devastating effects of an impact of a large asteroid or comet are now well recognized.
Figure 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.)
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 14000 (http://neo.ssa.esa.int/risk-page); over 1600 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 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.”
In continuation of the former NEOShield project the objectives of NEOShield-2 broadly reflect the above findings and recommendation by thoroughly addressing 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 are integrated into a coherent programme of research and technology development. Building on NEOShield experience, the project is investigating 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 and surface observation 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 are being 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, are being further explored in NEOShield-2. The current focus is 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 will be enhanced to explore the effects of large spin rates, shattered and rubble-pile structures, and mineralogy on an object’s response to a deflection attempt.
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 so far 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.
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 Figure 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 agreed within the consortium during the Reference Mission Definition and Requirement Review (MDRR) conducted within the first months of the project.
Figure 2: Graphical illustration of the NEOShield-2 addressed mission scenarios and activities.
In subsequent activities the reference mission definitions are 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.
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 will occur through 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. The proposed approach will be 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 is to secure 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 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 Figure 3).
Figure 3: NEOTωIST scenario with sub-satellite constellation observing impactor collision with Itokawa; two Chasers (lower left) and the Fly-by module (upper centre). (Image: Airbus DS, 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 is 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 comprises 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 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.)
- 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 and methods by the team (correspondence and teleconferences)
- Down selection of observation goals and identification of open questions
GNC of Impacting S/C mission
Starting from the status achieved during the former NEOShield project the GNC subsystem design for the Impacting S/C mission has been updated and improved as well as the used simulator has been advanced. Further work has been done on the navigation & image processing algorithms, and first GNC design simulations and sensitivity analysis have been performed.
GNC of Reconnaissance S/C mission
In view to achieve the WP5 objectives, the whole set of proposed functions and scenarios were developed, engineered and tested (under nominal conditions), and it enables:
- Robust and safe approach/arrival to an asteroid with coarse knowledge of its ephemerides and shape (mean radius of a highly irregular shape, as currently baselined).
- Stable maintenance of relative position (between S/C and asteroid) at inertial hovering point (5km), as immediate subsequent operations after arrival, during a period of 12h until ground takes over. The knowledge of the asteroid shape is still assumed coarse (ellipsoid that fits the irregular shape).
- Stable and high performance body-fixed hovering for a duration of 1.8h (i.e., half the rotation period of the selected target asteroid 2001 QC34), enabling mission tasks related to surface observation and preparation for descent/landing tasks, and tested under extreme operational range (sun phase angle from -90 to 90 degrees).
The documentation containing design and current validation results has been provided to the consortium. The overall description provided leads to the conclusion that the WP5 goals are being met for this reporting period, and that the beneficiary is widening its knowledge and availability of integrated systems to perform approach and short-term operations in the vicinity of asteroids (being the most demanding with small asteroids). Moreover, the technology is scalable to other planetary bodies and moons.
In the particular case of small NEOs with highly irregular shapes addressed within NEOShield-2, the results are very promising in order to accomplish the objectives of the Reconnaissance GNC technology development by the end of the project.
GNC of Sample Return S/C mission
An analysis and review of Sample Return mission concepts has been performed in order to define a reference Sample Return mission, including definition of general strategies and requirements for the Sample Return spacecraft GNC. The architectural design of the vision-based GNC system in line with the mission and the requirements has been defined. Support to the definition of an End-to-End Performance Verification and Validation Tests Plan to reach TRL 5-6 has been given and the development of the GNC system (including Image Processing) and of the Real World environment (for validation purpose) in Matlab/Simulink has been thoroughly addressed. Moreover verification & validation of the related GNC requirements through execution of tests in Model-in-the-Loop environment has been performed and the results have been documented.
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.
For the development of the sampling device, the focus has been on the development of a first prototype sampling and collection device. The first step involved a review of NEO material sample collection concepts and analysis of the technical requirements for the sampling device from deliverable D3.2. Based on the requirements, a technical approach for the sampling device was generated and a powder-actuated bolt impactor was determined to be the best solution. The sampling device under development is capable of collecting both, loose material (e.g. dust particles) and breaking hard material (e.g. rock) for collection. The goal is to collect the two different materials (regolith layer material on the surface of an asteroid and broken rock material) into separate chambers. The regolith and the broken material are collected by a flushing process with nitrogen gas. First tests were performed already with a powder-actuated bolt-setting tool from HILTI (see Figure 4). The best geometry of the bolt impactor was tested, determined and the amount of broken material was measured.
Figure 4: First tests with a powder-actuated bolt-setting tool from HILTI to measure the broken material.
In the subsequent phase the design of the sample device has been developed. Detailed evaluations and laboratory tests of many subsystems were necessary to assemble the device, for example a functional triggering mechanism, the necessary energy to ignite the blasting caps, flushing process, etc. Based on these results the first mechanical prototype of the sampling device has been produced and first tests were scheduled for May 2016 in order to develop the sampling mechanism into an autonomous sampling device.
Real-time Test Facility & Test Campaigns
Requirements for the Real-time Test Facility have been derived from the analysis of the respective reference mission definition documents for all related scenarios to be reproduced in the test facilities as well as from the commonly elaborated GNC E2E Performance Verification and Validation Plan and the information exchanged during the interface meetings performed with the corresponding work package leads.
The design of the Real-time Test Facility for the optical and robotics facilities has been derived from the requirements. These two facilities are based on the tailoring of the existing laboratories at GMV premises to the scenarios of the three GNC systems. Trade-off analyses have been performed and specific HW components have been selected, including space representative HW cameras, lenses, IP computers, etc. The HIL architectures are based on the re-use of the main components of the PIL test bench (following the step-wise incremental chain MIL-PIL-HIL). The HW components identified in the design phase have been procured and the integration of them in the laboratories is being performed.
Preliminary integration tests and proof of concept for the HIL optical stimulation has been conducted, the HIL optical laboratory has been adapted for the Reconnaissance spacecraft GNC scenario, integrating the required HW and SW components of the facility. Simulated images of the scenario from the HW camera within the HIL optical laboratory (in open loop) have been produced and provided to the interfering partners for assessment and cross-check.
Orbit determination and monitoring
A survey, trade-off, and reference concept definition has been performed. An initial definition of requirements applicable to Orbit determination and monitoring (ODM) (and further breakdown to absolute / relative referencing) has been established, a survey of state-of-the-art techniques for orbit determination and monitoring has been conducted to evaluate the accuracy achievable with radio science measurements, and various solutions have been addressed and 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 initially analysed with respect to operational scenario, estimation strategy, and projected performance.
In context of the algorithm development and validation the orbit determination filter algorithms are understood to 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 currently being developed and fine-tuned. For validation purposes, an End-to-End simulator has been set up which includes high-fidelity models of the dynamics, environmental perturbations, and measurements. The ODM algorithms will ultimately be validated with simulated mission data which cover cases generated by Monte-Carlo runs (as well as analysis of boundary/critical conditions particularly relevant for the mission) with the aim of testing their functionality in an increased-realistic mission environment (TRL 5).
NEO potential mission targets
A NEOShield-2 NEO Properties Portal (NEOPP, http://neoshield.net/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. 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.
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. 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 and with approximate rendezvous delta-v’s < 6 km/sec. If fast spins with P>3 hours are ruled out, this still leaves ~10 potential targets.
However this number remains a reflection of lack of data, and it is likely that a large fraction of the ~400–500 NEOs of suitable absolute magnitude are in reality suitable mission targets. As NEOShield-2 progresses it is expected that this list will grow both through own observations and external efforts such as the NEOWISE mission, the UKIRT NEO programme and others. Importantly, orbit refinement of almost all potential targets will occur during the next 10 years with operation of PS1 and LSST telescopes making this activity to a dynamical and ongoing process.
NEO Observations and data reduction/analysis
While we are not yet at half of the NEOShield-2 project duration, within the NEO observation campaigns undertaken we already acquired/analysed a good clump of data to characterize the small NEA population. Our 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.net/neopp), which has been deployed to support the mission analysts as well as the observers and observation campaigns through an advanced prioritization algorithm.
- Eight observing runs have been carried out at TNG (6 runs) and LBT (2 runs) telescopes to acquire the photometric colours of a total of 64 targets. Data have been analysed for 44 of them, and the corresponding taxonomic type derived. Collaboration has been established with the Observatorio Nacional (Brazil) to make use of the OASI telescope, to acquire phase curves of NEAs. Five objects have been observed so far, with the data analysed and the corresponding absolute magnitude H derived for four of them.
- Light-curves of 13 NEAs have been taken at OHP and PDM telescopes, where additional astrometric data have been also acquired for 12 asteroids (these data could help to enhance our understanding of the Yarkovsky effect). The light-curve and spectrum of 2004 BL86 have been also obtained at the IRTF telescope. Efforts are in progress to make use of the photometric data acquired by NASA’s Kepler space telescope to obtain further light-curves of asteroids and NEAs in particular.
- Fifteen out of the 30 observing nights of our GTO programme at the ESO-NTT telescope have been carried out: reflectance spectra of 100 small NEAs have been acquired. Data have been analysed for 69 of them, and the corresponding taxonomic type derived (suggesting that the primitive, carbonaceous objects are more common within the smaller NEA population). An analysis of the available literature of spectroscopic data of the PHA population has also been carried out, to identify those particularly hazardous objects requiring a special attention in the near future.
- The thermal inertia of NEAs (3200) Phaethon and (1685) Toro has been constrained based on newly derived shape models and thermophysical modelling of WISE data. The results obtained for Phaeton suggest a very coarse-grained surface. A novel strategy is also proposed (and successfully tested on Toro) to constrain the thermal inertia of asteroids with retrograde rotations and no 3-D models, assuming a spherical shape (e.g., for objects coming from the n6 secular resonance).
In summary, NEOShield-2 new observations already more than doubled the available data for what concerns the surface composition (taxonomy) of small NEAs, and several objects of particular interest have been identified and/or studied. A number of observing runs are already foreseen for the next 1.5 years at several worldwide telescopes. Novel strategies are being developed by the related partners for the rotational and thermophysical modelling of extended samples of NEAs.
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 (refer to Figure 5, left). 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 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 Figure 5, right). Further study of these results promises greater insight into connections between the thermal and other physical properties of NEOs.
Figure 5: (left): η versus radar albedo for M- and X-type main-belt asteroids. A very significant correlation of η with radar albedo is evident; supporting the idea that η can be used as a proxy for thermal parameter, which in turn is very sensitive to metal content. (right): Plot of thermal inertia (Γ, SI units) versus spin period for NEOs. Thermal inertia values are those derived from thermophysical modelling.
In our simulation work to date we have concentrated on the complex problem of the fate of ejecta produced by a kinetic impactor, motivated by the fact that this information is needed to determine safe positions for a spacecraft observing the event. We have studied the evolution of ejecta in the case of the Double Asteroid Redirect Test (DART) impact on the secondary of the binary NEO (65803) Didymos in 2022, in the framework of the AIDA space project under study at ESA and NASA. A detailed dynamical model for the simulation of an ejecta cloud from a binary asteroid has been developed that synthesizes all relevant forces based on the analysis of the mechanical environment. A full-scale simulation has been performed using parameters specified by the AIDA mission (e.g., mass and speed of the projectile). We have confirmed that the solar tide and solar radiation pressure are the major sources of perturbation of the ejecta. We show that the motion of ejecta near Didymos is dominated by the binary gravity or solar radiation pressure, depending on the ejecta size. The violent period of ejecta evolution lasts for the first few hours after impact. The risk appears to be largely mitigated in the two weeks post impact (refer to Figure 6). While no region near Didymos (e.g., <10 km from the binary’s centre of mass) is guaranteed to be empty of ejecta, the high-speed streams that comprise the greatest flux of kinetic energy show rapid dissipation as the ejecta cloud spreads.
Figure 6: Snapshots of the time evolution of the ejecta cloud near Didymos (view size ~4.6 km). The binary and heliocentric orbits are marked with solid lines of green and purple colour, respectively. Fictitious large particle sizes are adopted for visual enhancement, and the accreted particles onto Didymos are coloured in green.
Requirements for Future Research and International Actions
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 are monitoring the activities of other organisations (e.g. ESA, NASA) and Chinese, Japanese, Russian, etc. groups working in the impact hazard and mitigation fields. A primary aim is to analyse and report on the effectiveness of international coordination and funding of mitigation activities and develop a European strategy for future research and mission-related endeavours in this field.
In continuation of our efforts in the originally NEOShield project (where we reviewed the status of existing international mitigation-relevant political activities as well as the adequacy and networking of emergency management agencies worldwide to analyse the relevancy of the existing activities to cope with a NEO impact emergency and to identify issues requiring further attention in order to build an internationally-agreed global response roadmap) within NEOShield-2 we now focused on the smaller – in size but not importance – entities involved in the global response roadmap. In particular, 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. We defined three areas to discuss i) the emergency situation; ii) international cooperation; and iii) prevention and preparedness. A few countries (1/3) replied to our mail, and only seven (Austria, Cyprus, Czech Republic, Finland, Spain, Sweden, and United Kingdom) discussed the questions listed in our mail. Looking at the low number of received replies we could deduce a low interest of structures and organizations responsible for civil protection in the NEO hazard problem. On the other hand, some civil protection authorities expressed a high concern about the problem and moreover, the will for an eventual future collaboration. In summary, we find the following areas are in need of improvement:
- Distribution of information on the impact-hazard problem to national civil protection authorities. 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).
A further aim of our NEOShield-2 work in this task is to monitor worldwide activity in the field of the impact hazard and highlight areas in which further scientific research and technical development work is necessary. This initiative covers efforts to identify and update the most effective means of deflecting a NEO in the light of improving scientific understanding and technological capabilities, reduce the risk of a NEO deflection attempt failing, and facilitate more accurate predictions of the possible consequences of a deflection attempt (which may succeed, only partially succeed, or fail completely) and/or an impact on the Earth.
To date a list of the major activities in this field by the country/region in which relevant work is currently taking place has been compiled and details of on-going work in which there would appear to be a need for further emphasis in the future have been collected. The results of our work to date have been presented at meetings of the SMPAG (Washington DC, Nov. 2015; Vienna, Feb. 2016).
Technical aspects concerning post-2017 developments and activities for NEO deflection missions in particular emphasizing the Kinetic Impactor concept are being monitored, analysed and supported where possible. This comprises the assessment of possible mission profiles and required technologies to implement such a deflection mission, either a deflection demonstration or an actual deflection mission in response to a real NEO threat. First activities in this context have been initiated already and the collection of inputs is ongoing.
The results are going to be documented in a dedicated “Technology development plan”, which will report on deflection mission related technology readiness levels and most pressing measures to advance them 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.
- 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.
- A novel low-cost kinetic impactor deflection demonstration concept called NEOTωIST, based on changing the spin rate of the NEO Itokawa. 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.
- Improved 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.
- 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.
- The astronomical observations carried out by NEOShield-2 already 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. Moreover a number of observing runs are already foreseen for the next 1.5 years at several worldwide telescopes to continue our significant contribution to the physical characterisation of the NEO population leading to an increased list of suitable candidate targets for deflection test missions. Novel strategies are being developed by the related partners for the rotational and thermophysical modelling of extended samples of NEOs.
- The identification and characterization of several suitable target NEOs for exploration and mitigation/deflection (demonstration) missions on the basis of pre-computed trajectories is an ongoing process dynamically considering the list of known NEOs, 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.net/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. 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.
- An agreement to future migrate all data, made publicly available through the “NEO Properties Portal” internet server, to the ESA’s NEO Coordination Centre has been concluded to ensure public availability of the related project results when the project has been ended.
- Our work on the statistical analysis of NEO physical properties and observables like surface temperature distribution and radar albedo our findings 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.
- 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. 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.