Project

GRAINS

Goal: Study of rubble pile asteroids as granular systems through N-body simulations of gravitational aggregation.
GRAINS project receives funding from EU Horizon 2020 Research & Innovation Programme (Marie Skłodowska-Curie Global Fellowship).
Partner institutions: Politecnico di Milano, NASA JPL, Observatoire de la Cote d'Azur.

Date: 1 June 2018 - 31 May 2020

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Project log

Fabio Ferrari
added 3 research items
In the last decades, in-situ and remote observations of asteroids have brought evidence to support the idea that celestial bodies up to a few hundreds meters in size may be piles of loosely consolidated material, namely "rubble piles" [6]. A popular method to study these bodies is through numerical N-body simulations of gravitational aggregation. We discuss here the reshaping dynamics of rubble pile aggregates and how these change depending on initial conditions given to particles. We show the main features of the numerical code used, including its capability of handling contacts and collisions between a large number of non-spherical particles. A comparative study between spherical and angular bodies is shown for the scenario under study.
A recently popular way to study the internal properties and structure of "rubble pile" asteroids is to study them as gravitational aggregates, through numerical N-body simulations. These methods are suitable to reproduce aggregation scenarios after disruption and allow the study of the dynamical and collisional evolution of fragments up to the formation of a stable aggregate or to their dispersion. This work presents a numerical implementation of the gravitational N-body problem with contact dynamics between non-spherically shaped rigid bodies. The work builds up on a previous implementation of the code and extends its capabilities. The number of bodies handled is significantly increased through the use of a CUDA/GPU-parallel octree structure. The main features of the code are described and its performance are compared against CPU-parallel ar-chitectures and classical direct N 2 integration. Preliminary results and examples of scenarios that could benefit from such implementation are presented, with application to asteroid gravitational aggregation problems .
The study of small bodies addresses a number of questions relevant for space engineers, planetary scientists and physicists. Near Earth Asteroids (NEA) are a great opportunity for technological and human exploration missions. NEAs can also represent a threat for the planet. The scientific and technological exploitation and the implementation of mitigation actions for hazardous asteroids rely upon our knowledge of their properties. Remote surveys play a fundamental role to estimate asteroid properties, but they can provide very limited information about their internal structure. GRAINS addresses such problem through N-body numerical simulations of gravitational aggregation. This method is suitable to estimate the internal properties of rubble-pile asteroids: gravitational aggregates with very low tensile strength and high level of porosity. The study of aggregation phenomena currently relies on codes optimized for a large number of mutually interacting particles, regardless of their individual shape and rigid body motion. Although not relevant for many applications, this limitation could be relevant for the case of asteroids, as suggested by results of granular dynamics in terrestrial engineering applications. The latter are commonly studied using multi-body codes, able to simulate contact interactions between a large number of complex-shaped bodies, but not suitable for gravitational dynamics. GRAINS is able to joint the advantages of both classes of codes into a single implementation, to reproduce N-body gravitational dynamics between a large number of complex-shaped rigid bodies. In this work, aggregation simulations are presented: favorable conditions leading to the formation of an asteroid are investigated, when starting from a cloud of boulders. The analysis is performed under many degrees of freedom, to include different geometries, physical and dynamical properties of the boulders and final aggregate. The problem to be investigated is twofold: (a) the study of gravitational aggregation dynamics, and (b) the study of the physical and dynamical properties of the final aggregate. The first aspect includes the analysis and numerical simulation of typical scenarios, for small and medium sized (hundreds of meters) asteroid aggregation, to identify the conditions that lead to the formation of a single or multiple aggregate, or to the dispersion of the particle cloud. The second aspect analyzes the properties of the final aggregate, as a result of the dynamical simulation, which are then studied and classified depending on the initial parameters and simulation scenario. Preliminary results show good agreement between theory and observation, and confirm the capability of the numerical code to predict natural aggregation phenomena.
Fabio Ferrari
added a research item
Recent remote measurements and in-situ observations confirm the idea that asteroids up to few hundreds of meters in size might be aggregates of loosely consolidated material, or ‘rubble piles’. The dynamics of these objects can be studied using N-body simulations of gravitational aggregation. We investigate the role of particle shape in N-body simulations of gravitational aggregation. We explore contact interaction mechanisms and study the effects of parameters such as surface friction, particle size distribution and number of particles in the aggregate. As a case study, we discuss the case of rubble pile reshaping under its own self-gravity, with no spin and no external force imposed. We implement the N-body gravitational aggregation problem with contact and collisions between particles of irregular, non-spherical shape. Contact interactions are modeled using a soft-contact method, considering the visco-elastic behavior of particles' surface. We perform numerical simulations to compare the behavior of spherical bodies with that of irregular randomly-generated angular bodies. The simulations are performed starting from an initial aggregate in a non-equilibrium state. The dynamics are propagated forward allowing particles to settle through reshaping until they reach an equilibrium state. Preliminary tests are studied to investigate the quantitative and qualitative behavior of the granular media. The shape of particles is found to play a relevant role in the settling process of the rubble pile aggregate, affecting both transient dynamics and global properties of the aggregate at equilibrium. In the long term, particle shape dominates over simulation parameters such as surface friction, particle size distribution and number of particles in the aggregate. Spherical particles are not suitable to model accurately the physics of contact interactions between particles of N-body aggregation simulations. Irregular particles are required for a more realistic and accurate representation of the contact interaction mechanisms.
Fabio Ferrari
added 2 research items
The paper presents a numerical implementation of the gravitational N-body problem with contact interactions between non-spherically shaped bodies. The work builds up on a previous implementation of the code and extends its capabilities. The number of bodies handled is significantly increased through the use of a CUDA/GPU-parallel octree structure. The implementation of the code is discussed and its performance are compared against direct N2 integration. The code features both smooth (force-based) and non-smooth (impulse-based) methods, as well as a visco-elastic non-smooth method, to handle contact interaction between bodies. The numerical problem of simulating rubble-pile asteroid gravitational aggregation processes is addressed. We discuss the features of the problem and derive criteria to set up the numerical simulation from the dynamical constraints of the combined gravitational-collisional problem. Examples of asteroid aggregation scenarios that could benefit from such implementation are finally presented.
The paper presents a numerical implementation of the gravitational N-body problem with contact interactions between non-spherically shaped bodies. The work builds up on a previous implementation of the code and extends its capabilities. The number of bodies handled is significantly increased through the use of a CUDA/GPU-parallel octree structure. The implementation of the code is discussed and its performance are compared against direct N$^2$ integration. The code features both smooth (force-based) and non-smooth (impulse-based) methods, as well as a visco-elastic non-smooth method, to handle contact interaction between bodies. The numerical problem of simulating "rubble-pile" asteroid gravitational aggregation processes is addressed. We discuss the features of the problem and derive criteria to set up the numerical simulation from the dynamical constraints of the combined gravitational-collisional problem. Examples of asteroid aggregation scenarios that could benefit from such implementation are finally presented.
Fabio Ferrari
added a research item
This paper presents a new environment to simulate close-proximity dynamics around rubble-pile asteroids. The code provides methods for modeling the as-teroid's gravity field and surface through granular dynamics. It implements state-of-the-art techniques to model both gravity and contact interaction between particles: 1) mutual gravity as either direct N2 or Barnes-Hut GPU-parallel octree and 2) contact dynamics with a soft-body (force-based, smooth dynamics), hard-body (constraint-based, non-smooth dynamics), or hybrid (constraint-based with compliance and damping) approach. A very relevant feature of the code is its ability to handle complex-shaped rigid bodies and their full 6D motion. Examples of spacecraft close-proximity scenarios and their numerical simulations are shown.
Fabio Ferrari
added a project goal
Study of rubble pile asteroids as granular systems through N-body simulations of gravitational aggregation.
GRAINS project receives funding from EU Horizon 2020 Research & Innovation Programme (Marie Skłodowska-Curie Global Fellowship).
Partner institutions: Politecnico di Milano, NASA JPL, Observatoire de la Cote d'Azur.