In this review the theory and application of Smoothed particle hydrodynamics (SPH) since its inception in 1977 are discussed. Emphasis is placed on the strengths and weaknesses, the analogy with particle dynamics and the numerous areas where SPH has been successfully applied.

https://iopscience.iop.org/article/10.1088/0034-4885/68/8/R01/pdf

Smoothed particle hydrodynamics

The amplitude and frequency of laser light can be routinely measured and controlled on a femtosecond (10(-15) s) timescale. However, in pulses comprising just a few wave cycles, the amplitude envelope and carrier frequency are not sufficient to characterize and control laser radiation, because evolution of the light field is also influenced by a shift of the carrier wave with respect to the pulse peak. This so-called carrier-envelope phase has been predicted and observed to affect strong-field phenomena, but random shot-to-shot shifts have prevented the reproducible guiding of atomic processes using the electric field of light. Here we report the generation of intense, few-cycle laser pulses with a stable carrier envelope phase that permit the triggering and steering of microscopic motion with an ultimate precision limited only by quantum mechanical uncertainty. Using these reproducible light waveforms, we create light-induced atomic currents in ionized matter; the motion of the electronic wave packets can be controlled on timescales shorter than 250 attoseconds (250 x 10(-18) s). This enables us to control the attosecond temporal structure of coherent soft X-ray emission produced by the atomic currents--these X-ray photons provide a sensitive and intuitive tool for determining the carrier-envelope phase.

Attosecond control of electronic processes by intense light fields

During the interval from September through early December 2005, the Hayabusa spacecraft was in close proximity to near-Earth asteroid 25143 Itokawa, and a variety of data were taken on its shape, mass, and surface topography as well as its mineralogic and elemental abundances. The asteroid9s orthogonal axes are 535, 294, and 209 meters, the mass is 3.51 × 1010 kilograms, and the estimated bulk density is 1.9 ± 0.13 grams per cubic centimeter. The correspondence between the smooth areas on the surface (Muses Sea and Sagamihara) and the gravitationally low regions suggests mass movement and an effective resurfacing process by impact jolting. Itokawa is considered to be a rubble-pile body because of its low bulk density, high porosity, boulder-rich appearance, and shape. The existence of very large boulders and pillars suggests an early collisional breakup of a preexisting parent asteroid followed by a re-agglomeration into a rubble-pile object.

https://science.sciencemag.org/content/sci/312/5778/1330.full.pdf

The Rubble-Pile Asteroid Itokawa as Observed by Hayabusa

https://arxiv.org/pdf/astro-ph/9907117.pdf

Catastrophic Disruptions Revisited

More than three years of high-resolution (1.5-20 m/pixel) photographic observations of the surface of Mars have dramatically changed our view of that planet. Among the most important observations and interpretations derived therefrom are that much of Mars, at least to depths of several kilometers, is layered; that substantial portions of the planet have experienced burial and subsequent exhumation; that layered and massive units, many kilometers thick, appear to reflect an ancient period of large-scale erosion and deposition within what are now the ancient heavily cratered regions of Mars; and that processes previously unsuspected, including gully-forming fluid action and burial and exhumation of large tracts of land, have operated within near-contemporary times. These and many other attributes of the planet argue for a complex geology and complicated history.

/pdf/mars-global-surveyor-mars-orbiter-camera-interplanetary-s30da0bni0.pdf

Mars Global Surveyor Mars Orbiter Camera: Interplanetary cruise through primary mission

Orbits Close to Asteroid 4769 Castalia

Dynamics of Orbits Close to Asteroid 4179 Toutatis

Recent numerical studies1,2,3,4,5 suggest that ‘rubble-pile’ asteroids (gravitationally bound aggregates of collisional debris) are common in the Solar System, and that self-gravitation may equal or exceed material cohesion for planetary bodies as small as several hundred metres. Because analytical scaling relations for impact cratering and disruption6,7,8 do not extend to this size regime, where gravity and material strength are both important, detailed simulations are needed to predict how small asteroids evolve through impact, and also to ascertain whether powerful explosions offer a viable defence against bodies headed for a collision with Earth. Here we present simulations, using a smooth-particle hydrodynamics code9, of energetic impacts into small planetary bodies with internal structure ranging from solid rock to porous aggregate. We find that the outcome of a collision is very sensitive to the configuration of pre-existing fractures and voids in the target. A porous asteroid (or one with deep regolith) damps the propagation of the shock wave from the impactor, sheltering the most distant regions, while greatly enhancing the local deposition of energy. Multiple-component asteroids (such as contact binaries) are also protected, because the shock wave cannot traverse the discontinuity between the components. We conclude that the first impact to significantly fragment an asteroid may determine its subsequent collisional evolution, and that internal structure will greatly influence attempts to disrupt or deflect an asteroid or comet headed towards Earth.

/pdf/disruption-of-kilometre-sized-asteroids-by-energetic-16e4ewzweb.pdf

Disruption of kilometre-sized asteroids by energetic collisions

Ostro et al.: Asteroid Radar Astronomy Asteroid Radar Astronomy Steven J. Ostro Jet Propulsion Laboratory, California Institute of Technology R. Scott Hudson Washington State University Lance A. M. Benner and Jon D. Giorgini Jet Propulsion Laboratory, California Institute of Technology Christopher Magri University of Maine at Farmington Jean-Luc Margot California Institute of Technology Michael C. Nolan Arecibo Observatory Radar is a uniquely powerful source of information about the physical properties and orbits of asteroids. Measurements of the distribution of echo power in time delay (range) and Dop- pler frequency (radial velocity) produce two-dimensional images that can provide spatial reso- lution as fine as a decameter if the echoes are strong enough. With adequate orientational coverage, such images can be used to construct detailed three-dimensional models, define the rotation state precisely, and constrain the object’s internal density distribution. As of May 2002, radar signatures have been measured for 75 main-belt asteroids (MBAs) and 105 near-Earth asteroids (NEAs). We summarize specific results for radar-detected asteroids, which span 4 or- ders of magnitude in diameter and rotation period. Radar has revealed both stony and metallic objects, principal-axis and complex rotators, smooth and extremely rough surfaces, objects that must be monolithic and objects that probably are not, spheroids and highly elongated shapes, contact-binary shapes, and binary systems. Radar also has expanded accurate orbit-prediction intervals for NEAs by as much as several centuries. INTRODUCTION One of the goals of this book is to outline developments since Asteroids II (Binzel et al., 1989), which was completed in 1988. The subsequent 13 years have seen critical devel- opments in technical aspects of asteroid radar astronomy, including increases in sensitivity and versatility of tele- scopes, the evolution and optimization of observational techniques, and the invention of analytical methods to opti- mize extraction of information from radar images. Observa- tions commensurate with those developments have raised the number of radar-detected asteroids from 52 (19 NEAs + 33 MBAs) in mid-1988 (Ostro, 1989) to 180 (105 NEAs + 75 MBAs) in May 2002 and have produced an enormous body of information about the physical properties of aster- oids. An equally significant development since Asteroids II is an increase in the number of persons lead-authoring aster- oid radar papers at an average rate of one person per year. The following sections outline the technical develop- ments and observational highlights of the past 13 years, summarize the major conclusions drawn from the explo- sive increase in data, and discuss current problems and chal- lenges to be faced during the coming decade. See Ostro (2002a) for a list of radar-detected asteroids, Ostro (1993, 2002b) for reviews of planetary radar principles and tech- niques, Ostro (1994) for a discussion of radar’s role in NEO hazard identification and mitigation, and Harmon et al. (1999) for a review of radar observations of comets. 2. STRATEGIES, TELESCOPES, AND TECHNICAL DEVELOPMENTS Telescopes and Observing Strategies The basic strategy of an asteroid radar experiment is to measure the distribution of echo power in time delay and Doppler frequency, usually in the opposite sense (OC) of circular polarization as transmitted as well as in the same sense (SC) as a function of the object’s orientation and plane-of-sky (POS) direction. SC echo would be absent in

/pdf/asteroid-radar-astronomy-17ep8ilzet.pdf

Asteroid Radar Astronomy

Observations of the icy Galilean satellites, performed during 1987-1991 with the Goldstone 3.5 cm system and the Arecibo 13 cm system, show significant enhancements in the knowledge of the satellite's radar properties. The most prominent radar features are tentatively identified with Galileo Regio and the Valhalla basin. Estimates of echo Doppler frequencies show Callisto to be lagging its ephemeris by 200 +/- 50 km.

R. S. Hudson

Papers

Orbits Close to Asteroid 4769 Castalia

Dynamics of Orbits Close to Asteroid 4179 Toutatis

Disruption of kilometre-sized asteroids by energetic collisions

Asteroid Radar Astronomy

Europa, Ganymede, and Callisto - New radar results from Arecibo and Goldstone