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NASA NTRS1989

Distribution of taxonomic classes and the compositional structure of the asteroid belt

This paper reviews previous studies on the distribution of the asteroid taxonomic classes and analyzes the techniques, results, and interpretations of these studies, with special attention given to the strong and weak points of these studies and the important caveat for their interpretation. It is concluded that, in spite of the differences in class definition, the rather ordered heliocentric distribution of the composition is real and must be due to either primordial, evolutionary, or dynamical processes, or to a combination of all three. In general, the distribution of asteroid taxonomic classes is characterized by moderate-albedo asteroids dominant in the inner asteroid belt, and low-albedo asteroids prevalent in the outer belt and beyond.

Gradie, Jonathan C.↗
NASA NTRS1997

Environmental Perturbations Caused by the Impacts of Asteroids and Comets

We review the major mechanisms proposed to cause extinctions at the Cretaceous-Tertiary geological boundary following an asteroid impact. We then discuss how the proposed extinction may relate to the impact of asteroids or comets in general. We discuss the limitations of these mechanisms in terms of the spatial scale that may be affected, and the time scale over which the effects may last. Our goal is to provide relatively simple prescriptions for evaluating the importance of colliding objects having a range of energies and compositions. We also identify the many uncertainties concerning the environmental effects of impacts. We conclude that, for impact energies below about 10(exp 4) Mts (megatons of TNT equivalent) - i.e., impact frequencies less than in 6 x 10(exp 4) yr, corresponding to comets and asteroids with diameters smaller than about 400 m and 650 m, respectively - blast damage, earthquakes, and fires should be important on a scale of 10(exp 4) or 10(exp 5) km (exp 2), which corresponds to the area damaged in many natural disasters of recent history. However, tsunami could be more damaging, flooding a kilometer of coastal plane over entire ocean basins. In the energy range of 10(exp 4) to 10 (exp 5) Mts (intervals up to 3 x 10(exp 5) yr; comets and asteroids with sizes up to 800 m and 1.5 km, respectively) water vapor injections and ozone loss become significant on the global scale. In the submicrometer dust injection fraction from the pulverized target material is much higher than is presently thought to be most likely, then dust injection could be important in this energy range.

Toon, Owen B.↗
NASA NTRS2012

In Situ Analysis of Orthopyroxene in Diogenites Using Laser Ablation ICP-MS

Howardites, eucrites and diogenites (HED) form a suit of igneous achondrite meteorites that are thought to have formed on a single asteroidal body. While there have been many different models proposed for the formation of the HED parent asteroid they can be generalized into two end member models. One is the magma ocean model (e.g. [1]) in which the entire HED parent body was continuously fractionated from a planet wide magma ocean with diogenites representing the lower crust and eucrites being upper crustal rocks. The second model hypothesizes that diogenites and eucrites were formed as a series of intrusions and/or extrusions of partial melts of a primitive proto-Vesta [2]. We use in situ trace element analysis together with major and minor element analysis to try and distinguish between these different hypotheses for the evolution of the HED parent body.

Elk, Mattias↗
NASA NTRS1992

The recovery of asteroids after two observations

It is shown that a generalization of the use of 'Vaisala orbits', briefly mentioned at the Asteroids 2 Conference, can be very conveniently accomplished by means of an inversion of the 'GEM' form of the Gauss method. The procedure can also be applied to Apollo objects and to indeterminate cases of normal three-observation orbit computation, and there is also a simple extension to situations involving four or more observations.

Marsden, Brian G.↗
NASA NTRS1990

A spherical harmonics method for asteroid pole determination

A new and general method is presented for determining pole orientations of unresolved solar system bodies that exhibit brightness variation with rotation. The accuracy of the method is limited only by the number of available apparitions, which imposes the degree where the Legendre expansion must be truncated. It is shown that power spectra of lightcurves provide enough information to extract pole coordinates from the data, independent of body shape and albedo variegation. The method has been tested using several computer-generated synthetic light-curves, and the accuracy is found to be about 5 deg in both the longitude and latitude of the pole, which is considerably better than that obtained from the standard triaxial ellipsoid model. Finally, the new spherical harmonics method has been applied to six well-observed asteroids: 16 Psyche, 22 Kalliope, 39 Laetitia, 44 Nysa, 45 Eugenia, and 349 Dembowska.

Lumme, K.↗
NASA NTRS1975

Asteroids - Spectral reflectance and color characteristics

We present visible and near-infrared spectral reflectance curves for 35 asteroids, making a total sample of 67 including our previous work. Several previously unknown spectral types have been discovered. Absorption bands and other features, from which information about the surface composition of the asteroids can be derived, continue to appear. Sampling statistics suggest we have observed about half of the significantly different mineralogical assemblages present in the asteroid belt. There is a general, but imperfect, correlation between spectral type and semimajor axis, which is consistent with the hypothesis that meteorites are derived from the asteroid belt. The color-frequency histogram for the 67 objects is bimodal, probably associated with 'stony' objects and 'carbonaceous' objects. Hirayama family pairs usually show disparate spectral types. 19 Fortuna may show strikingly different photometric properties on opposite sides.

Mccord, T. B.↗
NASA NTRS1987

The 1986 DA and 1986 EB: M-class asteroids in near-Earth orbits

The Earth-approaching asteroid population is composed of asteroids in orbits with short lifetimes compared with the age of the solar system. These objects which are comprised of Aten, Apollo, and Amor asteroids must be replenished from either cometary or mainbelt asteroid sources since lifetimes against collision with or ejection by a planet are on the order of 10 to 100 million years. The physical study of Earth-approaching asteroids is constrained by the generally long period between favorable apparitions and poorly known orbits. Broadband spectrophotometry on the Johnson UBVR system and the Eight-Color Asteroid Survey system were obtained at Kitt Peak National Observatory and on the Johnson JHK system and at 10 and 20 microns at the NASA Infrared Telescope Facility at Mauna Kea Observatory. These observations were used to determine the absolute visual magnitudes and to derive the visual geometric albedos and diameters on the IRAS system. The spectral reflectance properties and geometric albedos of the M-class asteroids are consistent compositions analogous to the iron nickel meteorites or the enstatite-metal assemblages of the enstatite chondrites. The issue of the source(s) of the near-Earth asteroids population was examined by comparing the classifications on the scheme employed by Gradie and Tedesco of 38 such asteroids. Most of the near-Earth objects is indeed the asteroid belt as the observations suggest, then a method for removing extinct nuclei of short period comets must be found since the rate of production of short period comets from the long period comets is relatively large.

Gradie, Jonathan↗
NASA NTRS1978

Asteroid surface materials - Mineralogical characterizations from reflectance spectra

The use of general and specific diagnostic spectral features and parameters to interpret most of the published high-quality reflectance spectra of asteroids is discussed. Such diagnostics are shown to provide the most complete and sophisticated mineralogically and petrologically based interpretation of the asteroid surface materials. Previous investigations of asteroid surface materials are exhaustively reviewed, emphasizing the general approaches employed, sources of information, previous characterizations of asteroid surface materials, and asteroid spectral groups. The interpretive methodology is then applied to spectral reflectance data for many individual members of the various spectral groups. A range of mineral assemblages similar to certain meteorite classes is identified, and evidence is presented for a selective high-temperature event that melted certain asteroid parent bodies and permitted their differentiation as some function of distance from the sun or the protosun and of the size of the protoasteroid. Possible candidates are proposed for the source bodies of different meteorite groups.

Gaffey, M. J.↗
NASA NTRS1987

Asteroid lightcurve inversion

One of the most fundamental physical properties of any asteroid is its shape. Lightcurves provide the only source of shape information for most asteroids. Unfortunately, the functional form of a lightcurve is determined by the viewing/illumination geometry and the asteroid's light scattering characteristics as well as its shape, and in general it is impossible to determine an asteroid's shape from lightcurves. A technique called convex-profile inversion (CPI) that obtains a convex profile, P, from any lightcurve is introduced. If certain ideal conditions are satisfied, then P is an estimator for the asteroid's mean cross section, C, a convex set defined as the average of all cross sections C(z) cut by planes a distance z above the asteroids's equatorial plane. C is therefore a 2-D average of the asteroid's 3-D shape.

Ostro, Steven J.↗
NASA NTRS1996

NASA Space Missions to Asteroids: Protecting the Earth from NEO Impacts

There is now a general recognition of the hazard of impacts on Earth by comets and asteroids, but there is yet no consensus concerning international actions that should be taken to protect the planet from such impacts. An essential step in the analysis of the situation involves estimating the relative hazard posed by comets and asteroids of different sizes and orbits. All recent studies agree that the larger impacts pose the greater danger, and that our primary concern from the perspective of total risk should be on impacts that are large enough to cause global ecological catastrophe. These global catastrophes are also of special interest, since they (alone among natural disasters) have the potential to destroy civilization. Studies of the sensitivity of the Earth's environment suggest that the energy threshold energy for causing a global catastrophe is at about 1 million megatons, corresponding to impactor diameters of 1.5 to 2 km. This information leads naturally to a strategy of concentrating on the larger NEOs, say those 1 km or more in diameter. This is the rationale for the Spaceguard Survey, which must be the highest priority in mitigation efforts. The second question concerns the value of developing standing defensive systems that could deflect or destroy an incoming NEO. In the case of the asteroids larger than 1 km in diameter, no such system is needed, since there will be ample time (at least several decades) between the discovery of the threatening object by Spaceguard and the requirement to take action against it. In the case of objects smaller than 1 km diameter, development of defensive systems is not cost-effective; there are many greater dangers to persons and property that are much more urgent. Only in the case of large long-period comets is there a rationale for standing defense systems. The question is also raised whether the risks inherent in developing and maintaining a defense system might be greater than the impact risks it is intended to guard against. These and related issues are the focus of much current international debate on defense of the planet against NEO impacts. Meanwhile, the most critical issue remains the expansion of the telescopic search for NEOs.

Morrison, David↗
NASA NTRS2003

Surface Material Analysis of the S-type Asteroids: Removing the Space Weathering Effect from Reflectance Spectrum

Recent years, many researchers have been observing a lot of asteroid reflectance spectra in the UV, visible to NIR at wavelength region. Reflectance spectroscopy of asteroid at this range should bring us a lot of information about its surface materials. Pyroxene and olivine have characteristic absorption bands in this wavelength range. Low-Ca pyroxene has two absorption bands around 0.9 microns and 1.9 microns. The more Ca and Fe content, the longer both absorption band centers. On the other hand, reflectance spectrum of olivine has three complicated absorption bands around 1 m, and no absorption feature around 2 microns. In general, reflectance spectra of many asteroids that are considered to be silicate rich (i.e., S- and A type asteroids) show redder slope and more subdued absorption bands than those of terrestrial minerals and meteorites. These features are now believed to be caused by the space weathering effect, which is probably caused by micrometeorite bombardment and/or solar wind. This process causes nanophase reduced iron (npFe(sup 0)) particles near the surface of mineral grains, which leads the optical change. Therefore, the space weathering effect should be removed from asteroid reflectance spectra to compare with those of meteorite and terrestrial minerals. In this report, we will apply the expanded modified Gaussian model (MGM) to the reflectance spectra of S-type asteroids 7 Iris and 532 Herculina and compare them with those of meteorites.

Ueda, Y.↗
NASA NTRS2003

Preservation and Enhancement of the Spacewatch Data Archives

In March of 1998, the asteroid 1997 XF11 was announced to be potentially hazardous after being tracked over 90 days. A potential two year wait for confirming observations was shortened to under 24 hours because of the existence of archived photographic prediscovery images. Spacewatch was a pioneer in using CCD scanning and possesses a valuable digital archive of its scans. Unfortunately these data are aging on magnetic tape and will soon be lost. Since 1990, the Spacewatch project gathered some 1.5 Terabytes of scan data covering roughly 75,000 degrees of sky to a limiting magnitude of V = 21.5. The data have not yet been mined for all of their asteroids for scientific studies and orbit determination. Spacewatch's real-time motion detection program MODP was constrained by the computers of the era to use simplified image processing algorithms at a reduced efficiency. Jedicke and Herron estimated MODP's efficiency at finding asteroids to be approximately 60 percent to V=18 and improving somewhat thereafter. This lead to a substantial bias correction in their analyses. Larsen has developed a MODP replacement capable in excess of 90 percent efficiency in the same range and able to push a magnitude fainter in completeness. We propose a program of post-processing and re-archiving Spacewatch data. Our scans would be transferred from tape to CD-ROMs and converted to FITS images -- establishing a consistent data format and media for both past and future Spacewatch observations. Larsen's MODP replacement would mine these data for previously undetected motions, which would be made available to the Minor Planet Center and our ongoing asteroid population studies. A searchable observation record would be made generally available for prediscovery work. We estimate the net asteroid yield of this proposal is equivalent to three full years of Spacewatch operations.

Larsen, Jeffrey A.↗
NASA NTRS1987

Evolution of the inner asteroid belt: Paradigms and paradoxes from spectral studies

Recent years have witnessed a significant increase in the sophistication of asteroidal surface material characterizations derived from spectral data. An extensive data base of moderate to high spectral resolution, visible and near-infrared asteroid spectra is now available. Interpretive methodologies and calibrations were developed to determine phase abundance and composition in olivine-pyroxene assemblages and to estimate NiFe metal abundance from such spectra. A modified version of the asteroid classifications system more closely parallels the mineralogic variations of the major inner belt asteroid types. These improvements permit several general conclusions to be drawn concerning the nature of inner belt objects; their history, and that of the inner solar system; and the relationship between the asteroids and meteorites. Essentially all large belt asteroids have or are fragments of parent bodies which have undergone strong post-accretionary heating, varying degrees of melting and magmatic differentiation, and subsequent collisional disruption. These asteroids show a systematic, but not yet well characterized, mineralogic variation with semi-major axis. This suggests that the S-type asteroid families represent relatively recent collisions onto the cores of previously disrupted parent bodies.

Gaffey, Michael J.↗
NASA NTRS1977

Discovering Jupiter. II

Data derived from Pioneer 10 and Pioneer 11 and other sources on the Jovian magnetosphere, the circum-Jovian radiation belts, and Jupiter's radio emission are presented at some length, descriptions are given of the principal Jovian satellites (Io, Europa, Ganymede, Callisto), and inferences are drawn on the origin of the planet and its place in the solar system. The inner, middle, and outer regions of the magnetosphere, the bow shock wave, and the particularly heavy intensity of the inner radiation belt region (within 1.44 million km of the planet) are discussed. All of the major satellites except Callisto lie immersed in the intense radiation belts. Jupiter's failure to become a stellar companion to the sun, Io's action in 'switching on' Jovian radio emission, and other Pioneer discoveries relating to asteroids, the solar system in general, and trans-Jovian space, are discussed

Source record↗
NASA NTRS1977

Comets: Data, problems, and objectives

A highly abridged review of new relevant results from the observations of Comet Kohoutek is followed by an outline summary of our basic knowledge concerning comets, both subjects being confined to data related to the nature and origin of comets rather than the phenomena (for example, plasma phenomena are omitted). The discussion then centers on two likely places of cometary origin in the developing solar system, the proto-Uranus-Neptune region versus the much more distant fragmented interstellar cloud region, now frequented by comets of the Opik-Oort cloud. The Comet Kohoutek results add new insights, particularly with regard to the parent molecules and the nature of meteoric solids in comets, to restrict the range of the physical circumstances of comet formation. A few fundamental and outstanding questions are asked, and a plea made for unmanned missions to comets and asteroids in order to provide definitive answers as to the nature and origin of comets, asteroids, and the solar system generally.

Whipple, F. L.↗
NASA NTRS1980

A bibliography of planetary geology principal investigators and their associates, 1979 - 1980

This bibliography cites 698 reports and articles published from May 1979 through May 1980 by principal investigators and associates who received support from NASA's Office of Space Science, as part of the Planetary Geology program. Entries are arranged in the following categories: (1) general interest; (2) solar system, asteroids, comets, and satellites; (3) structure, tectonics, and stratigraphy; (4) regolith and volatiles; (5) volcanism; (6) impact craters; (7) Eolian glacial An author index is provided. The bibliography serves as a companion document to NASA TM 81776, "Reports of Planetary Geology Programs, 1979-1980".

Lettvin, E.↗
NASA NTRS1986

Pole orientation, sidereal period, and sense of rotation of asteroids

Pole orientations of asteroids were determined. The method, called photometric astrometry, takes precise epochs of lightcurves into account. Pole determination research on asteroids 532 Herculina, 45 Eugenia, and 3 Juno continues. Discrepancies between various pole determination techniques presently being used are analyzed. The study of asteroid shapes and creating a generalized master pole determination technique also continues which will incorporate the best features of several current methods.

Taylor, R. C.↗
NASA NTRS1989

Phase relations of high albedo asteroids - The unusual opposition brightening of 44 Nysa and 64 Angelina

The present photoelectric lightcurve observations of 44 Nysa at one aspect, and 64 Angelina at both polar aspects, indicate the presence of an opposition spike or brightening of approximately 0.25 mag within several deg of zero-phase angle. Since the three curves are identical within the bounds of observational scatter, the opposition spike cannot be judged anomalous within the asteroid taxonomic class E. The general similarity of the observed phase curves to those of the Uranian satellites and the rings of Saturn supports the status of the spike as an ordinary property of moderate-to-high albedo atmosphereless surfaces.

Harris, A. W.↗