Note : Sabina Raducan is an impact simulation expert for both DART mission & Hera mission
1. ----------------------------
Title - The role of asteroid strength and porosity in impact momentum transfer
By - SD Raducan et al, 2017 (Davison, Collins)
Summary - For a porosity of 20%, which is the current best estimate for the Didymos system, crater radii ranged from 6m to 15m (crater diameter : 12 m to 30 m). While these are somewhat larger than previous estimates of crater size for the DART impact, that assumed higher target strengths, they lie between ... which appear to be the most appropriate analogs for porous rocky asteroid surfaces.
2. ----------------------------
Title - The role of asteroid strength, porosity and internal friction in impact momentum transfer
By - SD Raducan et al, 2019 (Davison, Collins, Luther)
Summary - Crater diameter : 9 m to 36 m (range is now broader than what the above paper mentioned, previous year), strength regime, beta of 2-4, iSALE2D shock physics code is accurate, Dimorphos is a rubble-pile. See extracts at page bottom.
3. ----------------------------
By - SD Raducan et al, 2020 (Davison, Collins)
Summary - Crater diameter : 9 - 36 m. iSALE shock physics code was used to simulate the DART impact into 3 different target scenarios in the strength regime :
- a homogeneous, porous target (single color)
- 2 layer target, with a porous weak layer overlying a stronger bedrock (2 color bands)
- target with exponentially decreasing porosity with depth (2 color gradient)
See extracts at page bottom.
4. ----------------------------
Title - Asteroid-scale consequences of the DART impact: the role of target strength and heterogeneity
By - SD Raducan & Martin Jutzi, 2021
Summary -
5. ----------------------------
By - SD Raducan & Martin Jutzi, LPSC 2021
Summary - remix of paper 4
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Title - Ejecta distribution and momentum transfer from oblique impacts on asteroid surfaces
By - SD Raducan et al, March 2021 (Davison, Collins)
Summary - iSale3D, low angle impact sims
7. ----------------------------
By - Sabina Raducan et al, 2021
Summary - low strength-low gravity regime
8. ----------------------------
By - Sabina Raducan & Martin Jutzi, June 2022
Summary - If asteroid Dimorphos is homogeneous and weaker than 10 Pa, then DART impact on it may not lead to a cratering event, as originally anticipated. Rather, the impact may cause a global shape deformation / resurfacing of the asteroid.
9. ----------------------------
Title - Reshaping and ejection processes on rubble-pile asteroids by impacts
By - Sabina Raducan et al, Sept 6, 2022
Summary -
10. ----------------------------
By - Sabina Raducan et al, Jan 2022 (Martin Jutzi, Davisom, Angela Stickle, Collins)
Summary - Crater diameter : 6 - 26 m. Fig 5 shows max crater radius of 13 m (crater diameter of 26 m). Beta = 3.25, iSALE-3D
Crater depth-to-diameter ratio of d/D = 0.33
Circular crater with a radius of R = 3 m (crater diameter of 6 m)
While a weaker target translates to longer transient crater growth times, ranging from few minutes for a 50 Pa target, up to 2 h for a cohesionless target. Therefore, the strength of the target is not expected to influence the projectile geometry’s effect on the ejecta cone.
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By - Masatoshi Hirabayashi et al, Jun 2022 (Fabio Ferrari, Martin Jutzi, SD Raducan)
Summary - Rainey et al. (2020) suggested a rough estimate of the DART impact-driven crater size as 2 m diameter for a high-strength case and 20 m diameter or more, for a low-strength case. The development of an impact crater permanently changes Dimorphos's shape.
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By - Fabio Ferrari et al, July 2022 (SD Raducan, Martin Jutzi)
Summary -
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Title - Boulder exhumation and segregation by impacts on rubble-pile asteroids
By - Ormo et al, Sept 15, 2022 (SD Raducan, Martin Jutzi)
Summary -
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Title - Spacecraft Geometry Effects on Kinetic Impactor Missions
By - Michael Owen et al, Sept 20, 2022 (Sabina Raducan)
Summary -
15. ----------------------------
Title - Predictions for the Dynamical States of the Didymos System before and after the Planned DART Impact
By - Derek Richardson et al, July 2022 (Sabina Raducan, Angela Stickle, Yun Zhang)
Summary -
Crater diameter : 32 m (for surface strength of 1 kPa, 100 m for 0 Pa)
16. ---------------------------- Post-impact paper
Title - Momentum Transfer from the DART Mission Kinetic Impact on Asteroid Dimorphos
By - Andrew Cheng et al, March 2023 (Sabina Raducan, Angela Stickle, Martin Jutzi, Patrick Michel, Yun Zhang)
Summary -
For a Dimorphos bulk density range of 1,500 to 3,300 kg/m3, we find that the expected value of the momentum enhancement factor, β, ranges between 2.2 and 4.9, depending on the mass of Dimorphos. If Dimorphos and Didymos are assumed to have equal densities of 2,400 kg/m3, β= 3.61. These β values indicate that significantly more momentum was transferred to Dimorphos from the escaping impact ejecta than was incident with DART. Therefore, the DART kinetic impact was highly effective in deflecting the asteroid Dimorphos.
17. ---------------------------- Post-impact paper
Title - Successful Kinetic Impact into an Asteroid for Planetary Defense
By - Terik Daly et al, March 2023 (Sabina Raducan, Angela Stickle, Martin Jutzi, Patrick Michel, Yun Zhang)
Summary -
The shape model revealed Dimorphos to be an oblate spheroid with a volume equivalent diameter of 151 m (± 5 m). Dimorphos's shape is unusual relative to other near-Earth asteroids visited by spacecrafts and differs from other binary asteroid secondaries observed to date that have measured elongations. However, oblate secondaries show little or no measurable lightcurve amplitude, which biases the observational sample toward elongated secondaries.
18. ---------------------------- Post-impact paper
Title - Ejecta from the DART-produced active asteroid Dimorphos
By - Jian-Yang Li et al, March 2023 (Sabina Raducan, Angela Stickle, Martin Jutzi, Patrick Michel)
Summary -
Here we report the observations of the DART impact ejecta with the Hubble, from impact time T+15 minutes to T+18.5 days. Our observations reveal a complex evolution of ejecta, which is first dominated by the gravitational interaction between the Didymos binary system and the ejected dust and later by solar radiation pressure. The lowest-speed ejecta dispersed via a sustained tail that displayed a consistent morphology with previously observed asteroid tails thought to be produced by impact.
19. ---------------------------- Post-impact paper
Title - Physical properties of asteroid Dimorphos as derived from the DART impact
By - Sabina Raducan et al, Feb 2024
Summary -
In all impact scenarios simulated here, the DART impact does not produce a conventional impact crater but instead causes global deformation of the target. ESA’s upcoming Hera mission may find a reshaped asteroid rather than a well-defined crater.
My Note : There is no mention of crater diameter in this paper surprisingly. Even if the asteroid had deformed, it would happen only after the crater had reached its maximum possible size (width of its short axis, the side of an oblate shape). So for shape deformation to happen, the crater diameter would have to exceed 116 m, which is the thickness of the oblate shape. After this stage, the collapse could begin.
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SD Raducan et al, 2019 (No. 2)
The role of asteroid strength, porosity and internal friction in impact momentum transfer
Our simulations show that the ejecta produced from the impact can enhance the deflection by a factor of 2 to 4 (beta = 2 - 4).
An extensive number of laboratory impact experiments have been performed into various target materials to illuminate the ejection process and provide a quantitative measure of the mass-velocity distribution of ejected debris material.
Here, we use numerical simulations of strength-dominated impacts on small, low-gravity asteroids to investigate and quantify the effects of individual asteroid surface material properties, specifically strength, porosity and internal friction, on crater ejecta properties.
The ability to predict the crater size and hence, the ejecta production empirically, is also important in predicting the momentum transfer (beta). [ while the latter was predicted accurately enough, the constantly varying estimates / predictions on crater size, proves the claimed understanding to be bogus ]
To simulate vertical impacts on small asteroid surfaces, we used the iSALE2D shock physics code.
Due to the low strength of the target materials investigated, the craters grow many times larger than the projectile and over a long time-scale.
Having proved iSALE's ability to simulate impact ejecta with a reasonable accuracy, we then simulated impacts into idealised asteroid surfaces and measured the mass/velocity/launch-position distributions of ejecta. [ the claim of iSALE shock physics code being accurate is bogus, as seen from the constant changes to predicted crater size, every year (2019-2023) ]
It is most likely that the asteroid is a rubble-pile, covered by a thin layer of regolith. [ in 2019, 2 years before impact, they knew that Dimorphos was a rubble-pile asteroid & not one massive solid rock. ]
Observational studies suggest that Didymos is an S-type silicaceous asteroid system, and therefore the target asteroid material (Dimorphos) was considered to be made of weak porous basalt, which is a good approximation of the compositional structure of most asteroids.
Didymoon (Dimorphos) is approximately 160 m in diameter, with extremely low surface gravity, which means that the impact crater will most likely form in the so-called strength regime. This is because at such low gravity, the resistance to crater growth from the weight of the displaced target is small, while the resistance from the strength of the target material is very important even if the material is weak.
This suggests that it is the post-shock (damaged) strength of asteroid surface, rather than the intact (or pre-impact) strength, that controls impact ejecta behaviour. [ simplified : post-impact strength controls impact ejecta behaviour ]
In our simulations, the porosity of the asteroid varied between 10%, 20% and 50%.
These results suggest that the crater diameters predicted by the numerical simulations are consistent with the existing scaling laws for analogous materials, from lab-scale impact experiments, when the lower cohesion of the targets are taken into account. [ but wrong crater size estimate proves that all these are just guesses & not accurate, as claimed ]
The crater diameters were found to vary from about 9 m to 36 m.
Consistent with previous work, as the porosity of the target material is increased the cratering efficiency decreases slightly.
The presence of porosity leads to more energy being used in pore compaction, which leaves less energy available to displace material. On the other hand, with increasing porosity the material becomes less dense and easier to excavate. Our results suggest that the former effect dominates slightly over the latter, as an increase in target porosity from 10% to 50% leads to a decrease in crater diameter and crater volume of only 20%, while the crater depth remains almost the same.
For porous targets, more of the energy imparted from the impactor is used to compact the pores, making less available to set the target material in motion and eject material. In addition, as the target is made more porous, the ejected material becomes less dense and so less total mass is ejected from the same radial distance, compared to the denser lower porosity targets.
The behaviour of the ejecta in our numerical simulations is consistent with impact experiments. Highly porous materials had much lower ejecta velocity compared to the less porous sands.
The weaker the target, the more deflection we should expect.
DART could hit anything from a smooth, very weak terrain (which would provide a very large deflection of the asteroid), or it could hit a strong boulder (which could result in almost no momentum enhancement).
The porosity of the surface material can also influence the deflection, but not as considerably as the cohesive strength.
For example, Bruck Syal et al. (2016) found beta= 4, for a 20% porous target with a cohesion of 1 kPa [ now proven right with beta=3.7 ]
On the other hand, the Holsapple and Housen (2012), Jutzi and Michel (2014), Stickle et al. (2015) and Cheng et al. (2016), found beta values between about 1 and 2 for experiments and simulations, in which much stronger targets (cohesions of a few MPa) were used. [ all proven wrong ]
These results reinforce our conclusion that for impacts on small asteroid surfaces, beta is most sensitive to the cohesion of the target.
Conclusion : We found that an increase in porosity or in internal friction leads to lower ejection velocities, while an increase in target cohesion only slows down the last ejecta to leave the crater. This is consistent with the results of recent numerical simulations of ejecta behaviour in gravity-dominated impacts.
An accurate measurement of the DART impact crater diameter would help constrain the cohesion of the target surface.
Numerical impact simulations that replicated conditions in 3 lab experiments demonstrate that our approach to quantify ejecta properties is consistent with impact experiments in analog materials. [ bogus claim ]
Furthermore, the final crater sizes predicted by our numerical simulations are consistent with strength-regime crater scaling relationships, based on lab impact experiments in 2 asteroid analogous materials : weakly cemented basalt (WCB) and the Sand Fly/ash (SFA). [ proven wrong already ]
SD Raducan et al, 2020 (No.3)
Our results also show that some of our simulated impacts into different targets produced very similar β-1 values, yet very different crater sizes and morphologies. Resulting crater diameters for same deflection amplification (β-1 = 1.9), as cohesion and porosity varied :
- 10 m (100 kPa cohesion & few% porosity)
- 30 m (10 kPa cohesion & 20% porosity)
- 46 m (1 kPa cohesion & 30% porosity)
- 74 m (0.1 kPa cohesion & 50% porosity)
Sabina Raducan & Martin Jutzi, June 2022 (No. 8)
It is possible that NASA's DART impact on Dimorphos will take place in a low-gravity, low-strength regime, which has been mostly unexplored so far. In this study, we use the Bern SPH code to numerically simulate DART-like impacts on weak asteroids that use realistic material models. We used a novel approach to model the entire cratering process, which uses shock physics code calculations directly and tracks the evolution of the target for up to 2 hr after the impact.
In the strength-dominated impact scenarios, the cohesion is the dominant force that stops the crater cavity from growing.
We find that a gravity-dominated impact into a small cohesionless target can eject up to 3% of the target mass.
At the same time, up to 20% of the target material is displaced, causing excavation of material from the asteroid interior, global deformation, and resurfacing.
In such impact scenarios, the momentum enhancement, β, can be as high as 6.
For surface cohesion stronger than 50 Pa : about 98% of the ejecta momentum leaves the Didymos system, while for cohesionless targets, this percentage is reduced to 80%.
This means that if there is a measurable difference between the β derived from Dimorphos's orbital dynamics and the β derived from the heliocentric orbit of the system, it is likely that Dimorphos is weaker than 50 Pa.
For surface cohesion less than 10 Pa : DART impact is likely to produce morphologies that are dissimilar to cratering and change the global morphology of the asteroid.
In such cases, Hera will not find a classical crater morphology. Our results also show that the impact angle will likely not influence the target morphology or the direction of the target momentum significantly.
We find that for the current energy of the DART impact, the catastrophic disruption of the target is not possible. However, the DART impact may probe not only the surface but also the interior of the asteroid.
SD Raducan & Martin Jutzi, 2021 (No. 4)
Asteroid-scale consequences of the DART impact: the role of target strength and heterogeneity
Recent results of the SCI impact on Ryugu [6] inferred that at least the near-surface of the asteroid may not be dominated by strength and impact events are controlled to a large extent by gravity, despite its very low value. These findings might also be applicable to Dimorphos.
[6] Arakawa, M. et al. (2020) Science, 368:67–71
Studies by [7] suggest that for a low cohesion target, an impact on the same magnitude as DART might have a large enough specific impact energy to cause global deformation of the target.
[7] Jutzi, M. et al. (2017) A&A, 597:A61
Simulation software : Bern’s parallel Smooth Particle Hydrodynamics (SPH) impact code
We found that for homogeneous asteroids, impacts into targets stronger than Y0=10 Pa create well defined bowl-shaped craters, while impacts into weaker targets create morphologies that do not resemble an impact crater anymore.
Our results suggest that for a low friction, cohesionless target, the momentum enhancement can be as high as 6.
SD Raducan & Martin Jutzi, 2021 (No. 5)
Previous numerical models focused only on impacts on asteroid targets dominated by strength (from 100 Pa up to few MPa) and had relatively low cratering efficiencies. In these scenarios, the crater evolution spanned over only a few seconds after the impact.
Large shape deformations of Dimorphos are expected to affect the mutual orbit period, which is a critical parameter for calculating the kinetic impact deflection.
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