85 research outputs found
Fate of the runner in hit-and-run collisions
In similar-sized planetary collisions, a significant part of the impactor
often misses the target and continues downrange. We follow the dynamical
evolution of "runners" from giant impacts to determine their ultimate fate.
Surprisingly, runners re-impact their target planets only about half of the
time, for realistic collisional and dynamical scenarios. Otherwise they remain
in orbit for tens of millions of years (the limit of our N-body calculations)
and longer, or sometimes collide with a different planet than the first one.
When the runner does return to collide again with the same arget planet, its
impact velocity is mainly constrained by the outcome of the prior collision.
Impact angle and orientation, however, are unconstrained by the prior
collision.Comment: 24 pages, 14 figures, 4 tables, accepted for publication in Ap
Global Scale Impacts
Global scale impacts modify the physical or thermal state of a substantial
fraction of a target asteroid. Specific effects include accretion, family
formation, reshaping, mixing and layering, shock and frictional heating,
fragmentation, material compaction, dilatation, stripping of mantle and crust,
and seismic degradation. Deciphering the complicated record of global scale
impacts, in asteroids and meteorites, will lead us to understand the original
planet-forming process and its resultant populations, and their evolution in
time as collisions became faster and fewer. We provide a brief overview of
these ideas, and an introduction to models.Comment: A chapter for Asteroids IV, a new volume in the Space Science Series,
University of Arizona Press (Patrick Michel, Francesca E. DeMeo, William F.
Bottke, Eds.
Geophysics of Small Planetary Bodies
As a SETI Institute PI from 1996-1998, Erik Asphaug studied impact and tidal physics and other geophysical processes associated with small (low-gravity) planetary bodies. This work included: a numerical impact simulation linking basaltic achondrite meteorites to asteroid 4 Vesta (Asphaug 1997), which laid the groundwork for an ongoing study of Martian meteorite ejection; cratering and catastrophic evolution of small bodies (with implications for their internal structure; Asphaug et al. 1996); genesis of grooved and degraded terrains in response to impact; maturation of regolith (Asphaug et al. 1997a); and the variation of crater outcome with impact angle, speed, and target structure. Research of impacts into porous, layered and prefractured targets (Asphaug et al. 1997b, 1998a) showed how shape, rheology and structure dramatically affects sizes and velocities of ejecta, and the survivability and impact-modification of comets and asteroids (Asphaug et al. 1998a). As an affiliate of the Galileo SSI Team, the PI studied problems related to cratering, tectonics, and regolith evolution, including an estimate of the impactor flux around Jupiter and the effect of impact on local and regional tectonics (Asphaug et al. 1998b). Other research included tidal breakup modeling (Asphaug and Benz 1996; Schenk et al. 1996), which is leading to a general understanding of the role of tides in planetesimal evolution. As a Guest Computational Investigator for NASA's BPCC/ESS supercomputer testbed, helped graft SPH3D onto an existing tree code tuned for the massively parallel Cray T3E (Olson and Asphaug, in preparation), obtaining a factor xIO00 speedup in code execution time (on 512 cpus). Runs which once took months are now completed in hours
Graze-and-Merge Collisions under External Perturbers
Graze-and-merge collisions (GMCs) are common multi-step mergers occurring in
low-velocity off-axis impacts between similar sized planetary bodies. The first
impact happens at somewhat faster than the mutual escape velocity; for typical
impact angles this does not result in immediate accretion, but the smaller body
is slowed down so that it loops back around and collides again, ultimately
accreting. The scenario changes in the presence of a third major body, i.e.
planets accreting around a star, or satellites around a planet. We find that
when the loop-back orbit remains inside roughly 1/3 of the Hill radius from the
target, then the overall process is not strongly affected. As the loop-back
orbit increases in radius, the return velocity and angle of the second
collision become increasingly random, with no record of the first collision's
orientation. When the loop-back orbit gets to about 3/4 of the Hill radius, the
path of smaller body is disturbed up to the point that it will usually escape
the target.Comment: 19 pages, 16 figures, 1 table, accepted for publication in Ap
Collisional Formation and Modeling of Asteroid Families
In the last decade, thanks to the development of sophisticated numerical
codes, major breakthroughs have been achieved in our understanding of the
formation of asteroid families by catastrophic disruption of large parent
bodies. In this review, we describe numerical simulations of asteroid
collisions that reproduced the main properties of families, accounting for both
the fragmentation of an asteroid at the time of impact and the subsequent
gravitational interactions of the generated fragments. The simulations
demonstrate that the catastrophic disruption of bodies larger than a few
hundred meters in diameter leads to the formation of large aggregates due to
gravitational reaccumulation of smaller fragments, which helps explain the
presence of large members within asteroid families. Thus, for the first time,
numerical simulations successfully reproduced the sizes and ejection velocities
of members of representative families. Moreover, the simulations provide
constraints on the family dynamical histories and on the possible internal
structure of family members and their parent bodies.Comment: Chapter to appear in the (University of Arizona Press) Space Science
Series Book: Asteroids I
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