Collisional history of asteroids: Evidence from Vesta and the Hirayama families

Collisional evolution studies of asteroids indicate that the initial asteroid population at the time mean collisional velocities were pumped up to ～5 km/sec was only modestly larger than it is today; i.e., the asteroid belt was already depleted relative to the mean surface density elsewhere in the planetary region. Numerical simulations of the collisional evolution of hypothetical initial asteroid populations have been run, subject to three constraints: they must (a) evolve to the present observed asteroid size distribution, (b) preserve Vesta's basaltic crust, and (c) produce at least the observed number of major Hirayama families. A “runaway growth” initial asteroid population distribution is found to best satisfy these constraints. A new model is presented for calculating the fragmental size distribution for the disruption of large, gravitationally bound bodies in which the material strength is increased by hydrostatic self-compression. This model predicts that large asteroid behave as intrinsically strong bodies, even if they have had a history of being collisionally fractured. This model, when applied to the breakup of the Themis and Eos family parent bodies, gives size distributions in reasonably good agreement with those observed.     

The Mount Wilson magnetograph

Alterations to the Mount Wilson Observatory solar magnetograph were made during 1981. The present state of the instrument, including the spectrograph, is described. The magnetic and Doppler velocity signals and the setup procedure for the magnetogram observation are discussed. The advantages of the new system are described.     

Asteroids and meteorites: Parent bodies and delivered samples

Meteorites may be pieces of main-belt asteroids, derived by cratering collisions. The physical strength of an asteroid critically affects the quantity of ejecta that can be placed in orbits (probably resonant) that evolve to cross the Earth's. Asteroid strengths very widely due to initial composition and size (e.g., weak carbonaceous material or strong rock), subsequent geophysical evolution (e.g., formation of a strong iron core), and subsequent collisional evolution (e.g., conversion of a strong rocky body into a weak rubble pile). The meteorite yield on Earth further depends on meteorite strength, which affects longevity in space and survival through the atmosphere. We show that meteorites may be derived mainly by cratering rather than by disruptive fragmentation and from large main-belt asteroids rather than from small, Earth-approaching bodies. The model combines a wide variety of evidence from various disciplines to yield results consistent with meteorite statistics. However, no claim is made for uniqueness of this model, and many elements still admit considerable uncertainty.     

FEDERAL INITIATIVES PROMOTING THE DISPERSAL OF LOW-INCOME HOUSING IN SUBURBS

Because low-income housing is predominantly concentrated in central cities and the older suburbs of metropolitan areas, the poor have little opportunity to disperse into higher-status suburbs. The federal initiative to ease the barriers to low-income suburbanization commenced with the Fair Housing Act of 1968. Subsequent legislation—particularly the Housing and Community Development Act of 1974—and associated litigation have significantly extended the reach of this effort to disperse low-income housing opportunities. The same forces that promote exclusionary practices in suburbs, however, also tend to moderate the federal initiative.     

The response of palaeomagnetic data to Earth expansion

Summary. Attempts to estimate palaeo-radii of the Earth, using palaeomagnetic data have necessarily been based on simplistic Earth models. It has been asserted that real geological processes are too complex to enable us to approach the problem quantitatively, and such attempts yield invalid results. We examine this and argue that, to the contrary, it appears that errors introduced by allowing for more realistic behaviour of the continents, e.g. 'orange-peel effect' and crustal extension, are smaller by an order of magnitude than the response of palaeomagnetic data to simplified expansion models.
From a qualitative argument, it is shown that the observed Late Palaeozoic and Early Mesozoic palaeomagnetic data are not what should be expected from an expanded Earth. We conclude that it appears unlikely that the Earth has expanded significantly since the Early Mesozoic.     

Spectrophotometry (0.33 to 1.07 μm) of 433 Eros and compositional implications

The spectral reflectance (0.33–1.07 μm) for the asteroid 433 Eros was determined as a function of rotational phase during January 28–30, and February 15, 1975. Interpretation of absorption features suggests Eros is composed of an undifferentiated assemblange of moderate to high temperature minerals (iron, pyroxene, and olivine, but no carbon). H-type ordinary chondrites are such assemblages, but it would be premature to conclude that Eros is like an H chondrite meteorite in composition until a better understanding is reached of possible physical differences between laboratory powders and asteroid regoliths for metal-bearing assemblages. There are no large-scale major compositional variations on the different sides of Eros.     

Trapped gravity waves and the five-minute oscillations of the solar atmosphere

The various modes of hydrodynamic waves are considered for a model of the solar atmosphere which is based on the Bilderberg model and includes the effects of ionization. The atmosphere forms a potential well for internal gravity waves, since the stability is low at the base (near the convection) and low again in the region of partial ionization in the chromosphere. Calculations show that there are two resonant (trapped) modes of internal gravity waves for horizontal wavelengths based on the scale of the granulation. The properties of these modes are in close agreement with the two modes of oscillation observed by Frazier (1968). Trapped acoustic modes are found to have periods too short to account for the observations.Presently Visiting Fellow, Joint Institute for Laboratory Astrophysics, University of Colorado, Boulder, Colo.