Pisa, 06/16/2010

Great influence of Paolo Farinella in studies of dynamics of Near-Earth asteroids and

collisional processes

Vth Catastrophic Disruption Workshop, Oregon, 1998

• Amors: a>1 AU 1.017<q<1.3 AU

• Apollos: a>1 AU q<1.017 AU

• Atens: a<1 AU Q>0.987 AU

• IEOs: a<1 AU Q<0.987

1000 Objects with D>1 km

Years of studies have shown that asteroids from different regions of the Main Belt (MB) are injected into resonances which transport them on Earth-crossing orbits

But once NEOs, what are the main dynamical mechanisms at work? Are there only planetary approaches, as

commonly assumed at that time?

• Eros: target of NEAR (NASA/APL)

• Nereus: initial target of Muses-C (Hayabusa,JAXA)

Hayabusa cambe back on June 13th, one day before Paolo’s meeting: Re-entry capsule in the Woomera desert (Australia)

Semimajor axis close to 1 AU or 0.72 AU Low inclination < 20° ω librates around 0° or 180°

From Michel&Thomas 1996, AA 307

ν13 and ν14 cause great inclination changes

Nature, Vol. 380, 25 April 1996

ν16

ν4

Distance To Earth

Collision

Longitude of perihelions Longitude of nodes Michel&Froechlé, 1997, Icarus 128

Proper i, versus proper a, for e=0.1

Polar diagrams eccentricity, ν3 Overlapping of ν13 and ν14 = source of chaos

Inclination, ν13

Michel, 1997, Icarus 129

« It makes Boom »!

Prof. A. Fujiwara: leader of the Science Team of the mission Hayabusa, Pioneer of impact experiments, and good colleague of Paolo!

Setoh, Nakamura, Michel et al. 2010, Icarus 205

Experiments using the two-stage light gas gun of ISAS in Japan

Comparison with impact experiments on basalt

Nakamura & Fujiwara 93

dust removed

largest fragment as a function of impact angle

→ SPH simulations using 3.5×106 particles

Benz & Asphaug 1994!High-res. Runs by M. Jutzi!

Example: a projectile hits a 100 km-size body at 5 km/s (average impact velocity in the MB)

From Jutzi, Michel, Benz, Richardson 2010.

Red zones are damaged

Gravitational phase: once fragments have been generated they reaccumulate due to their mutual attractions

Snapshots centered on the largest fragment; time t=0 to 84 minutes Increase of realism of simulations: model of rigid body allows Reproducingthe shapes of aggregates formed during a collision (Richardson, Michel et al. 2009, PSS 57):

Michel P. et al. 2001. 2002, 2003, 2004 Michel P. 2006, Lecture Notes Physics Michel P. 2009, Lecture Notes in Physics

Itokawa

Forming asteroid families: Testing the gravity regime

P. Michel, W. Benz, P. Tanga and D. Richardson, 2001

2) expanding debris are re-accumulating to form family members

1) the parent body is totally disrupted by a catastrophic impact

→ SPH simulations of impact

time

→ N-body simulations of re-accumulation

Size distribution of an asteroid family: actual and simulated members

Michel et al., Science 294 (2001)

Michel et al., Nature 421 (2003)

Our simulations of asteroid disruptions reproduced for the first time asteroid families and suggest that objects > km are gravitational agregates (rubble piles)

Impact energies and disruption outcomes greatly depend on the initial internal structure of the impacted body

Internal structure of small bodies: Characterisation and role

Impact speed: 3 km/s

Jutzi, Michel, Hiraoka, Nakamura, Benz, 2009, Icarus 201

Différent kinds of porosity

Winter 2010 with A. Nakamura, a good colleague of Paolo

T = 1.5 ms!Experiment! Simulation!

Jutzi, Michel, Hiraoka, Nakamura, Benz, 2009, Icarus 201

First validations of a model of fragmentation of porous body

T = 8 ms!Experiment! Simulation!

First application at large scale: formation of the crater on the asteroid Stein (Rosetta image)

Jutzi, Michel, Benz 2010. A&A 509, L2

Asphaug et al. 2003

Impact energy threshold for disruption of a solid target vs. Target’s radius

This parameter still needs to be better estimated for monolithic targets, and was not estimated for rubble piles and porous bodies

Threshold for "which the "largest "fragment has "50% of the "mass of the "original body"

(thick line with dots:"Benz & Asphaug "1999)"

Impact angle: 45° Impact velocity: 3 km/s

Q*D=Q0(R/1cm)a + B ρ(R/1cm)b

Impact angle: 45°

Q*D=Q0(R/1cm)a + B ρ(R/1cm)b

Impact angle: 45°

Jutzi, Michel, Benz, Richardson 2010, Icarus, in press.

Jutzi, Michel, Benz, Richardson 2010, Icarus, in press

Slope Independent On target’s diameter

Particles initially at the surface in orange, those initially inside in blank (the pole of the primary should be less weathered than that at the equator)

1999 KW4 (Radar Image Ostro et al.)

Walsh K., Richardson D.C., Michel, P., Nature 454

Spin-up due to YORP, a variant of the re-discovered Yarkowsky effect by Paolo

Final Snapshot of a simulation of spin-up of a km-size aggregate resulting in a binary with similar properties as observed ones

Thank you!!

Porous versus non-porous!!

And thank you, Paolo, for accompanying my researches every day!!