Asteroid 1950 DA's encounter with Earth in 2880: physical limits of collision probability prediction

Integration of the orbit of asteroid (29075) 1950 DA, which is based on radar and optical measurements spanning 51 years, reveals a 20-minute interval in March 2880 when there could be a nonnegligible probability of the 1-kilometer object colliding with Earth. Trajectory knowledge remains accurate until then because of extensive astrometric data, an inclined orbit geometry that reduces in-plane perturbations, and an orbit uncertainty space modulated by gravitational resonance. The approach distance uncertainty in 2880 is determined primarily by uncertainty in the accelerations arising from thermal re-radiation of solar energy absorbed by the asteroid. Those accelerations depend on the spin axis, composition, and surface properties of the asteroid, so that refining the collision probability may require direct inspection by a spacecraft.     

PLANETARY SCIENCE: Asteroids Come of Age

Since the discovery of the first asteroid almost 200 years ago, these celestial bodies have advanced from nuisances hampering stellar studies to the focus of geological studies aimed at understanding the early solar system. Binzel describes the history of their study, which has now culminated in NASA's NEAR mission to the asteroid Eros. Four papers report first results from this first rendezvous mission with an asteroid.     

The rubble-pile asteroid Itokawa as observed by Hayabusa

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 asteroid's orthogonal axes are 535, 294, and 209 meters, the mass is 3.51 x 10(10) 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.     

The breakup of a main-belt asteroid 450 thousand years ago

Collisions in the asteroid belt frequently lead to catastrophic breakups, where more than half of the target's mass is ejected into space. Several dozen large asteroids have been disrupted by impacts over the past several billion years. These impact events have produced groups of fragments with similar orbits called asteroid families. Here we report the discovery of a very young asteroid family around the object 1270 Datura. Our work takes advantage of a method for identification of recent breakups in the asteroid belt using catalogs of osculating (i.e., instantaneous) asteroid orbits. The very young families show up in these catalogs as clusters in a five-dimensional space of osculating orbital elements.     

Discovery of a basaltic asteroid in the outer main belt

Visible and near-infrared spectroscopic observations of the asteroid 1459 Magnya indicate that it has a basaltic surface. Magnya is at 3. 15 astronomical units (AU) from the sun and has no known dynamical link to any family, to any nearby large asteroid, or to asteroid 4 Vesta at 2.36 AU, which is the only other known large basaltic asteroid. We show that the region of the belt around Magnya is densely filled by mean-motion resonances, generating slow orbital diffusion processes and providing a potential mechanism for removing other basaltic fragments that may have been created on the same parent body as Magnya. Magnya may represent a rare surviving fragment from a larger, differentiated planetesimal that was disrupted long ago.     

Mainbelt asteroids: dual-polarization radar observations

Observations of 20 asteroids in the main belt between Mars and Jupiter provide information about the nature of these objects' surfaces at centimeter-to-kilometer scales. At least one asteroid (Pallas) is extremely smooth at centimeter-to-meter scales. Each asteroid appears much rougher than the Moon at some scale between several meters and many kilometers. The range of asteroid radar albedos is very broad and implies substantial variations in porosity or metal concentration (or both). The highest albedo estimate, for the asteroid Psyche, is consistent with a surface having porosities typical of lunar soil and a composition nearly entirely metallic.     

Mass and local topography measurements of Itokawa by Hayabusa

The ranging instrument aboard the Hayabusa spacecraft measured the surface topography of asteroid 25143 Itokawa and its mass. A typical rough area is similar in roughness to debris located on the interior wall of a large crater on asteroid 433 Eros, which suggests a surface structure on Itokawa similar to crater ejecta on Eros. The mass of Itokawa was estimated as (3.58 +/- 0.18) x 10(10) kilograms, implying a bulk density of (1.95 +/- 0.14) grams per cubic centimeter for a volume of (1.84 +/- 0.09) x 10(7) cubic meters and a bulk porosity of approximately 40%, which is similar to that of angular sands, when assuming an LL (low iron chondritic) meteorite composition. Combined with surface observations, these data indicate that Itokawa is the first subkilometer-sized small asteroid showing a rubble-pile body rather than a solid monolithic asteroid.     

Interplanetary magnetic field enhancements and their association with the asteroid 2201 oljato

Comparison of the times of occurrence of a newly discovered type of disturbance in the interplanetary magnetic field at 0.72 astronomical unit with the passage of the small Venus-crossing asteroid, 2201 Oljato, reveals a possible association, but the source of these disturbances appears to be associated with outgassing material in the Oljato orbit some distance behind the asteroid and not with the asteroid itself. This suggested association can account for one quarter of the total number of events seen in eight Venus years.     

Organic matter on asteroid 130 elektra

Infrared absorption spectra of a low-albedo water-rich asteroid appear to show a weak 3.4-micrometer carbon-hydrogen stretching mode band, which suggests the presence of hydrocarbons on asteroid 130 Elektra. The organic extract from the primitive carbonaceous chondritic Murchison meteorite shows similar spectral bands.     

Oxygen isotopic compositions of asteroidal materials returned from Itokawa by the Hayabusa mission

Meteorite studies suggest that each solar system object has a unique oxygen isotopic composition. Chondrites, the most primitive of meteorites, have been believed to be derived from asteroids, but oxygen isotopic compositions of asteroids themselves have not been established. We measured, using secondary ion mass spectrometry, oxygen isotopic compositions of rock particles from asteroid 25143 Itokawa returned by the Hayabusa spacecraft. Compositions of the particles are depleted in (16)O relative to terrestrial materials and indicate that Itokawa, an S-type asteroid, is one of the sources of the LL or L group of equilibrated ordinary chondrites. This is a direct oxygen-isotope link between chondrites and their parent asteroid.     

Depletion of the Outer Asteroid Belt

During the early history of the solar system, it is likely that the outer planets changed their distance from the sun, and hence, their influence on the asteroid belt evolved with time. The gravitational influence of Jupiter and Saturn on the orbital evolution of asteroids in the outer asteroid belt was calculated. The results show that the sweeping of mean motion resonances associated with planetary migration efficiently destabilizes orbits in the outer asteroid belt on a time scale of 10 million years. This mechanism provides an explanation for the observed depletion of asteroids in that region.     

Itokawa dust particles: a direct link between S-type asteroids and ordinary chondrites

The Hayabusa spacecraft successfully recovered dust particles from the surface of near-Earth asteroid 25143 Itokawa. Synchrotron-radiation x-ray diffraction and transmission and scanning electron microscope analyses indicate that the mineralogy and mineral chemistry of the Itokawa dust particles are identical to those of thermally metamorphosed LL chondrites, consistent with spectroscopic observations made from Earth and by the Hayabusa spacecraft. Our results directly demonstrate that ordinary chondrites, the most abundant meteorites found on Earth, come from S-type asteroids. Mineral chemistry indicates that the majority of regolith surface particles suffered long-term thermal annealing and subsequent impact shock, suggesting that Itokawa is an asteroid made of reassembled pieces of the interior portions of a once larger asteroid.     

Regolith migration and sorting on asteroid Itokawa

High-resolution images of the surface of asteroid Itokawa from the Hayabusa mission reveal it to be covered with unconsolidated millimeter-sized and larger gravels. Locations and morphologic characteristics of this gravel indicate that Itokawa has experienced considerable vibrations, which have triggered global-scale granular processes in its dry, vacuum, microgravity environment. These processes likely include granular convection, landslide-like granular migrations, and particle sorting, resulting in the segregation of the fine gravels into areas of potential lows. Granular processes become major resurfacing processes because of Itokawa's small size, implying that they can occur on other small asteroids should those have regolith.     

Capture resonance of the asteroid 1685 toro by the Earth

The asteroid 1685 Toro has its perihelion inside the Earth's orbit and a period which is 8/ 5 of that of the Earth. A calculation of Toro's orbit covering 200 years shows that this asteroid at present is captured in resonance with the Earth. The capture is due to the gravitational interaction at close encounters between the bodies.     

Mass of vesta

The asteroid 197 (Arete) approaches asteroid 4 (Vesta) within 0.04 astronomical unit once every 18 years. It may therefore be possible to determine the mass of Vesta from observations of Arete. From 72 observations a value of (1.20 +/- 0.08) x 10(-10) solar mass was derived, the indicated uncertainty being a probable error.     

Asteroids: surface composition from reflection spectroscopy

Minerals partly composing the surfaces of 14 asteroids are determined by using asteroid reflectance spectra and optical properties of meteorites and other materials. Individual electronic absorption features are identified in the asteroids' spectra. The energies, relative strengths, and shapes of these features are interpreted by using laboratory and theoretical studies. Analysis of the initial 14 asteroid reflectance spectra indicates the presence of the following types of surface materials: six carbonaceous chondrite-like; two stony-iron-like (metal/silicate approximately 1); one iron meteorite-like; one basaltic achondrite-like; and four silicate-metal assemblages (metal/silicate approximately 0.25). These results support the conclusion that the asteroid belt is a source of at least some meteoritic material, and they show a relation between certain asteroids and certain classes of meteoritcs.     

The violent collisional history of asteroid 4 Vesta

Vesta is a large differentiated rocky body in the main asteroid belt that accreted within the first few million years after the formation of the earliest solar system solids. The Dawn spacecraft extensively imaged Vesta's surface, revealing a collision-dominated history. Results show that Vesta's cratering record has a strong north-south dichotomy. Vesta's northern heavily cratered terrains retain much of their earliest history. The southern hemisphere was reset, however, by two major collisions in more recent times. We estimate that the youngest of these impact structures, about 500 kilometers across, formed about 1 billion years ago, in agreement with estimates of Vesta asteroid family age based on dynamical and collisional constraints, supporting the notion that the Vesta asteroid family was formed during this event.     

Pioneer 11 meteoroid detection experiment: preliminary results

The concentration of meteoroids of mass ~ 10(-8) gram in interplanetary space, in the asteroid belt, and near Jupiter has been measured. The data confirm the Pioneer 10 observation that the asteroid belt is not highly populated with small meteoroids, suggest that the high concentration of small particles around Jupiter is the result of gravitational focusing, and provide an indication of the mass distribution of meteoroids in interplanetary space.     

PLANETARY SCIENCE: Most-Common Meteorites Find a Home Among the Asteroids

Meteoriticists have long been puzzled by the fact that the most common meteorites, so-called ordinary chondrites, don't appear to have come from the most common asteroids, the S-types. Last week, however, a group of researchers attending the spring meeting of the American Geophysical Union here announced that the 31-kilometer-long S-type asteroid Eros now being orbited by the NEAR Shoemaker spacecraft is made of the same stuff as ordinary chondrites. That conclusion comes from NEAR Shoemaker's first-ever analysis of the elemental composition of an asteroid.     

New evidence on the size and possible effects of a late pliocene oceanic asteroid impact

Debris from a late Pliocene asteroid impact is spread across at least 600 kilometers of the ocean floor in the southeast Pacific. On the basis of iridium concentrations in sediments from six deep-sea cores, the asteroid diameter was at least 0.5 kilometer; the impacting projectile may have been one of the largest in the last few million years. The stratigraphic age of this impact is the same as that inferred for the onset of Northern Hemisphere glaciation.