Download Granular material dynamics and space missions to celestial bodies

Granular material dynamics and space missions to celestial bodies: a
transdisciplinary approach
Project co-funded by Academy 2 and 3 of UCA-JEDI
Logos of the space missions AIDA (ESA-NASA), Hayabusa 2 (JAXA), launch of OSIRIS-REx (NASA, Credit: United Launch Alliance)
Collaborators at the Observatoire de la Côte d’Azur have high-level involvement in three separate
spacecraft missions developed by three main space agencies (JAXA, NASA, ESA) set to rendezvous with
distinct near-Earth asteroids within the next few years. This involvement will offer great visibility to UCA given
the great public interest for asteroid-related topics. The Japanese Hayabusa 2 mission was launched on
December 3, 2014. It will visit the primitive near-Earth asteroid Ryugu (900 meters in diameter) in 2018-2019,
and will return a sample to Earth in 2020. The NASA OSIRIS-REx mission was launched on September 8th,
2016. It will visit the primitive near-Earth asteroid Bennu (450 meters in diameter) in 2018-2020, and will return
a sample to Earth in 2023. Both missions will be at their respective asteroid during the same years, which
promises amazing new images and a large media coverage. They will allow us to make a big step in our
understanding of the early phases of our Solar System, in particular the role of asteroids in planet formation and
emergence of life on Earth. Finally, the AIDA mission (ESA-NASA) consists of two separate spacecraft that
will be launched to a binary asteroid system, the near-Earth asteroid Didymos, to test the kinetic impactor
technique to deflect an asteroid. The European Asteroid Impact Mission (AIM) is set to rendezvous in late spring
2022 with the asteroid to fully characterize the smaller of the two binary components a few months prior to the
impact by the US Double Asteroid Redirection Test (DART) spacecraft in October 2022 . AIDA will be the first
test ever to use a kinetic impactor to deflect an asteroid, which will allow us to check or revise our understanding
of the impact process at asteroid scale.
The project funded by Academy 3 will contribute to different important activities related to these space
missions, in which members of OCA are co-Investigators (Hayabusa2, OSIRIS-REx) or Science lead (AIDA). In
effect, this project intends to join the independent expertise of a group from OCA and another group from Ecole
des Mines ParisTech and to develop a single numerical model to study granular material dynamics in conditions
adapted to those existing at the surface of asteroids, comets, planets and their satellites. This will allow us to
address various important issues related to regolith motion and interaction on small bodies, to model the
interaction of sampling devices or other tools with an asteroid surface and to be best prepared to interpret the
space mission data. Our highly transdisciplinary project will allow UCA to contribute greatly to these missions
and future ones, adding to the visibility of our institutions.
Asteroids range in size from small (~10 m) boulders to bodies 1000 km across, implying a wide range
in mass from a few thousand tons to 10 21 kg; however, this is still only a fraction of a percent of the mass of the
Earth. As a result, from planets to asteroids, surface gravities vary by many orders of magnitude. In addition,
surfaces of Solar System planets and small bodies are often covered by a layer of granular material, called
regolith, which can range from fine grains to gravel-like structures of varying depths. Understanding how
different types of granular materials behave on a small body’s surface, and how they react to different kinds of
stresses, is of great interest for the interpretation of surface images sent by spacecraft. It is also crucial for the
proper design of landers and sampling devices of space missions.
Regarding the applications to the design of space missions, we began modeling the interaction of the
sampling devices of the space mission OSIRIS-REx (NASA) with the surface of the asteroid target (called
Bennu) under the corresponding level of gravity and assuming various regolith properties. OSIRIS-REx will
collect a sample and return it to Earth in 2023, for analysis in laboratory. The objective is to determine the
efficiency of the sampling tool to collect the required amount of material (at least 60 g) as a function of the
assumed regolith properties, so that we are well prepared to choose the best sampling site during the visit of the
spacecraft to the asteroid in 2018-2020. The tool consists of a cylinder at the bottom of a boom that will touch
the surface and with 5 seconds, will release a gas (nitrogen) to move the material and capture it. So far, we have
studied the effect of various regolith properties (characterized by various rolling, twisting and sliding frictions of
the particles, as well as their size distribution; see Fig. 2) on the compliance of the OSIRIS-REx device with the
surface.
1/2
Figure 1: A model of the OSIRIS-REx sampling device visualized with some slight transparency penetrates a bed of about 100,000 particles
(seen in cross-section) with an initial downward vertical speed of 10 cm/s in the microgravity ennment of Bennu (ag, Bennu = 20 μm s-2). The
cylindrical container has a height and radius of 60 cm. The colored layers aid the viewer to discern the penetration depth (each layer is about
6 cm in height). The particles were assigned friction parameters similar to those of gravel (Yu et al. 2014).
The Japanese space mission Hayabusa2 is also on its way to the asteroid Ryugu. In addition to the
sampling activity, it will deploy the lander MASCOT (DLR/CNES cooperation) on the asteroid surface. One
important uncertainty, as we have seen with the lander of Philae (Rosetta mission), is the outcome of the first
interaction of the lander with the surface (how much and how long it will bounce), which depends on the
unknown surface properties and their response to the lander geometry and speed at impact. We began a study of
MASCOT’s interaction with Ryugu’s surface, assuming various impact geometries, velocities, and surface
properties. As a demonstration of principle, we performed a first study, assuming that the surface consists of
spheres of unique size (centimeter), and varying the orientation of the lander at contact (see Fig. 2).
Figure 2: From left to rithg: 3 snapshots from a simulation of MASCOT landing and bouncing on a surface composed of mono-size
spherical particles (1 cm in diameter) in the gravity conditions of the asteroid Ryugu; image of MASCOT bouncing by DLR.
Partners of our project include specialists of the numerical modeling and laboratory experiments of
granular material dynamics. Our objective is to enhance our existing numerical models based on our respective
approaches and to compare them with experiments performed both at micro-g and under Earth-gravity. This will
allow us to keep validating them, better define their limits and the pros and cons of the two approaches,
depending on the considered case, and to possibly merge them in a single numerical tool. Our aim is to be able to
both contribute to space mission activities (design, image interpretation) but also more generally to improve our
knowledge of small body’s and planetary surface evolutions and histories based on our understanding of the
dynamics of regolith on their surface.
OSIRIS-REx (NASA) and its sampling tool at the end of a long arm (credit: NASA).
2/2