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At least 163 records · Page 9
All sourcesNASA NTRSDOE OSTI
NASA NTRS1986

Speckle interferometry of asteroids. III - 511 Davida and its photometry

Photometric interferometry data on the fifth largest minor planet, 511 Davida, are used to generate orbital elements and physical dimensions. The Seward 2.3 m telescope was employed to obtain the speckle interferometric data. The asteroid was assumed to be a uniform-surfaced triaxial ellipsoid rotating about its shortest axis. A simultaneous amplitude-magnitude aspect method is defined for determining the pole position of the object, which is calculated to be within 4 deg of (307 deg; +32). A weighted amplitude aspect relation provides similar data. The albedos are estimated to lie within the range 0.24-0.42, and an a/b axial ratio of 0.97-1.66 is projected. The variations in the photometric albedos are taken as evidence of a nonuniform surface, a conclusion similar to that reached after earlier photometric measurements of the asteroid Herculina.

Drummond, J. D.↗
NASA NTRS2015

Rapid Generation of Optimal Asteroid Powered Descent Trajectories Via Convex Optimization

Mission proposals that land on asteroids are becoming popular. However, in order to have a successful mission the spacecraft must reliably and softly land at the intended landing site. The problem under investigation is how to design a fuel-optimal powered descent trajectory that can be quickly computed on-board the spacecraft, without interaction from ground control. An optimal trajectory designed immediately prior to the descent burn has many advantages. These advantages include the ability to use the actual vehicle starting state as the initial condition in the trajectory design and the ease of updating the landing target site if the original landing site is no longer viable. For long trajectories, the trajectory can be updated periodically by a redesign of the optimal trajectory based on current vehicle conditions to improve the guidance performance. One of the key drivers for being completely autonomous is the infrequent and delayed communication between ground control and the vehicle. Challenges that arise from designing an asteroid powered descent trajectory include complicated nonlinear gravity fields, small rotating bodies and low thrust vehicles.

Pinson, Robin↗
NASA NTRS1996

Radar Observations of Asteroid 1620 Geographos

Goldstone radar observations of Geographos from August 28 through September 2, 1994 yield over 400 delay-Doppler images whose linear spatial resolutions range from approximately 75 - 151 m, and 138 pairs of dual-polarization (OC, SC) spectra with one-dimensional resolution of 103 m. Each data type provides thorough rotational coverage. The images contain an intrinsic north/south ambiguity, but the equatorial view allows accurate determination of the shape of the radar-facing part of the asteroid's pole-on silhouette at any rotation phase. Sums of co-registered images that cover nearly a full rotation have defined the extremely elongated shape of that silhouette (S. J. Ostro, K. D. Rosema, R. S. Hudson, R. F. Jurgens, J. D. Giorgini, R. Winkler, D. K. Yeomans, D. Choate, R. Rose, M. A. Slade, S. D. Howard, and D. L. Mitchell 1995, Nature 375, 474-477). Here we present individual images and co-registered sums over approximately 30 deg of rotation phase that show the silhouette's structural characteristics in finer detail and also reveal numerous contrast features "inside" the silhouette. Those features include several candidate craters as well as indications of other sorts of large-scale topographic relief, including a prominent central indentation. Protuberances at the asteroid's ends may be related to the pattern of ejecta removal and deposition caused by the asteroid's gravity field. The asteroid's surface is homogeneous and displays only modest roughness at centimeter-to-meter scales. Our estimates of radar cross section and the currently available constraints on the asteroid's dimensions are consistent with a near-surface bulk density between 2 and 3 g/cc. The delay-Doppler trajectory of Geographos' center of mass has been determined to about 200 m on August 28 and to about 100 m on August 31, an improvement of two orders of magnitude over pre-observation ephemerides.

Ostro, Steven J.↗
NASA NTRS1995

Radar Observations of Asteroid 1620 Geographos

Goldstone radar observations of Geographos from Aug. 28 through Sep. 2, 1994, yield over 400 delay-Doppler images whose linear spatial resolutions range from ~75 m to ~151 m, and 138 pairs of dual-polarization (OC, SC) spectra with one-dimensional resolution of 103 m. Each data type provides thorough rotational coverage. The images contain an intrinsic north/south ambiguity, but the equatorial view allows accurate determination of the shape of the radar-facing part of the asteroid's pole-on silhouette at any rotation phase. Sums of co-registered images that cover nearly a full rotation have defined the extremely elongated shape of that silhouette (Ostro et al. 1995, Nature 375, 474-477). Here we present individual images and co-registered sums over ~30û of rotation phase that show the silhouette's structural characteristics in finer detail and also reveal numerous contrast features "inside" the silhouette. Those features include several candidate craters as well as indications of other sorts of large-scale topographic relief, including a prominent central indentation. Protuberances at the asteroid's ends may be related to the pattern of ejecta removal and deposition caused by the asteroid's gravity field. The asteroid's surface is homogeneous and displays only modest roughness at centimeter-to-meter scales. Our estimates of radar cross section and the currently available constraints on the asteroid's dimensions are consistent with a near-surface bulk density between 2 and 3 g cm-3. The delay-Doppler trajectory of Geographos' center of mass has been determined to about 200 m on Aug. 28 and to about 100 m on Aug. 31, an improvement of two orders of magnitude over pre-observation ephemerides.

Asteroid↗
NASA NTRS1996

Radar Observations of Asteroid 1620 Geographos

Goldstone radar observations of Geographos from August 28 through September 2, 1994 yield over 400 delay-Doppler images whose linear spatial resolutions range from approx. 75 to approx. 151 in, and 138 pairs of dual-polarization (OC, SC) spectra with one-dimensional resolution of 103 m. Each data type provides thorough rotational coverage. The images contain an intrinsic north/south ambiguity, but the equatorial view allows accurate determination of the shape of the radar-facing part of the asteroid's pole-on silhouette at any rotation phase. Sums of co-registered images that cover nearly a full rotation have defined the extremely elongated shape of that silhouette. Here we present individual images and co-registered sums over approx. 30 deg of rotation phase that show the silhouette's structural characteristics in finer detail and also reveal numerous contrast features "inside" the silhouette. Those features include several candidate craters as well as indications of other sorts of large-scale topographic relief, including a prominent central indentation. Protuberances at the asteroid's ends may be related to the pattern of ejecta removal and deposition caused by the asteroid's gravity field. The asteroid's surface is homogeneous and displays only modest roughness at centimeter-to-meter scales. Our estimates of radar cross section and the currently available constraints on the asteroid's dimensions are consistent with a near-surface bulk density between 2 and 3 g/cu cm. The delay-Doppler trajectory of Geographos' center of mass has been determined to about 200 m on August 28 and to about 100 m on August 31, an improvement of two orders of magnitude over pre-observation ephemerides.

Ostro, Steven J.↗
NASA NTRS1980

Hektor - The largest highly elongated asteroid

New observations of Trojan asteroid 624, Hektor, in April 1979 establish that the high amplitude of the rotational light curve of this object is caused by its elongated shape and not by patches of dark and light albedo on opposing hemispheres. These observations confirm the fact that Hektor is a very unusual object and are consistent with the hypothesis that it may be a compound asteroid formed when two Trojans of comparable size fell together - a rare fossilized example of a planetary accretion process.

Hartmann, W. K.↗
NASA NTRS2015

The Mission Accessibility of Near-Earth Asteroids

Astrodynamical Earth departure dates; mission v; mission duration; stay time; etc. Physical I NEO size(?); rotation rate; dust satellites environment; chemistry; etc. Architectural Launch vehicle(s); crew vehicle(s); habitat module(s); budget; etc. Operational Operations experience; abort options profiles; etc. Astrodynamical Accessibility is the starting point for understanding the options and opportunities available to us. Here we shall focus on. Astrodynamical Accessibility.2 Earth departure date between 2015-01-01 and 2040-12-31 Earth departure C3 60 km2s2. Total mission v 12 kms. The total v includes (1) the Earth departure maneuver from a 400 km altitude circular parking orbit, (2) the maneuver to match the NEAs velocity at arrival, (3) the maneuver to depart the NEA and, (4) if necessary, a maneuver to control the atmospheric re-entry speed during Earth return. Total round trip mission duration 450 days. Stay time at the NEA 8 days Earth atmospheric entry speed 12 kms at an altitude of 125 km. A near-Earth asteroid (NEA) that offers at least one trajectory solution meeting those criteria is classified as NHATS-compliant.

Trajectory Optimization↗
NASA NTRS1989

The microwave spectra of the asteroids Pallas, Vesta, and Hygiea

Microwave observations of Pallas, Vesta, and Hygiea at 2 and 6 cm wavelength yield brightness temperatures that are much lower than would be expected for a rapidly rotating blackbody. An analysis of the wavelength dependence of the observed brightness temperatures shows that, as was found for Ceres, these asteroids may be covered by a layer of material with the physical properties of finely divided dust. Models with layer depths of greater than 6 cm (Pallas), 6 cm (Vesta), and 8 cm (Hygiea) were found to represent well the variation in emissivity at radio wavelengths. The properties of the underlying layer are not well constrained by the microwave observations. It does appear that the real part of the dielectric vector for the substrate is similar to that of basaltic rock. Major compositional changes, if any, must take place at depths greater than about 10 cm. No evidence for water ice was found. Disk-resolved observations of the 2-cm emission of Vesta yield physical dimensions consistent with the recently published speckle-interferometry results.

Johnston, K. J.↗
NASA NTRS2002

Hydrated Minerals on Asteroids: The Astronomical Record

Knowledge of the hydrated mineral inventory on the asteroids is important for deducing the origin of Earth's water, interpreting the meteorite record, and unraveling the processes occurring during the earliest times in solar system history. Reflectance spectroscopy shows absorption features in both the 0.6-0.8 and 2.5-3.5 micrometers regions, which are diagnostic of or associated with hydrated minerals. Observations in those regions show that hydrated minerals are common in the mid-asteroid belt, and can be found in unexpected spectral groupings, as well. Asteroid groups formerly associated with mineralogies assumed to have high temperature formation, such as M- and E-class asteroids, have been observed to have hydration features in their reflectance spectra. Some asteroids have apparently been heated to several hundred degrees Celsius, enough to destroy some fraction of their phyllosilicates. Others have rotational variation suggesting that heating was uneven. We summarize this work, and present the astronomical evidence for water- and hydroxyl-bearing minerals on asteroids.

Rivkin, A. S.↗
NASA NTRS2003

Hydrated Minerals on Asteroids: The Astronomical Record

Knowledge of the hydrated mineral inventory on the asteroids is important for deducing the origin of Earth's water, interpreting the meteorite record, and unraveling the processes occurring during the earliest times in solar system history. Reflectance spectroscopy shows absorption features in both the 0.6-0.8 and 2.5-3.5-micron regions, which are diagnostic of or associated with hydrated minerals. Observations in those regions show that hydrated minerals are common in the mid-asteroid belt, and can be found in unexpected spectral groupings as well. Asteroid groups formerly associated with mineralogies assumed to have high-temperature formation, such as M- and E-class steroids, have been observed to have hydration features in their reflectance spectra. Some asteroids have apparently been heated to several hundred degrees Celsius, enough to destroy some fraction of their phyllosilicates. Others have rotational variation suggesting that heating was uneven. We summarize this work, and present the astronomical evidence for water- and Hydroxl-bearing minerals on asteroids.

Rivkin, A. S.↗
NASA NTRS1990

The collisional evolution of the spin of a nonspherical body

The nonspherical shapes of asteroids are considered through a generalization of the Harris (1979) model of asteroid spin rates' collisional evolution. Since a body in spin equilibrium rotates about its axis of maximum moment of inertia, the computation of the mean-squared impact parameter through averaging over all directions of flux is not strictly valid; nevertheless, these complications have been found not to alter the conclusions reachable by means of the simpler model.

Harris, A. W.↗
NASA NTRS2004

Multi-Wavelength Observations of 2100 Ra-Shalom: Radar and Lightcurves

We conducted a multi-wavelength campaign to study the near-Earth asteroid (NEA) 2100 Ra-Shalom during its August 2003 encounter. Rotationally resolved observations were acquired at Arecibo (12.6 cm radar), the IRTF (0.8-2.5 micron and 3 micron), McDonald Observatory (0.48-0.92 micron), Palomar Observatory (8-15 micron), and Ondrejov Observatory (optical lightcurves). Our objectives were to determine Ra-Shalom's size and shape, and the composition and physical state of its near-surface material. Preliminary results from radar and lightcurve measurements will be presented here.

Shepard, M. K.↗
NASA NTRS2005

Asteroid Shape Reconstruction From Radar Observations

I estimate near-Earth asteroid 1992 SK's physical properties from radar delay-Doppler images, Doppler-only echo spectra and optical lightcurves. The images are not very strong, but place up to 20 (40 m by 160 m) pixels on the asteroid. The radar tracks are confined to subradar latitudes between 20 and 40 degrees but have complete rotational phase coverage. The echo spectra and optical lightcurves span approx.80 degrees of sky motion, providing geometric leverage to constrain the pole direction. The optical lightcurves are essential to accurate determination of the asteroid's shape and spin state. The asteroid is approx.1.4 km in maximum extent and mildly asymmetric, with an elongation of approx.1.5 and relatively subdued topography. The radar albedo is about 0.13 and the optical albedo about 0.3. The circular polarization ratio for the object is about 0.34, implying typical cm-scale surface roughness. I estimate the asteroid's period to be 7.3182+/-0.0003 hours and its pole direction as (99deg+/-5deg,-3deg+/-5deg) in ecliptic coordinates. The radar-refined orbital solution accurately predicts planetary close approaches between the years 826 and 2690. I have used my model to predict salient characteristics of radar images and optical lightcurves obtainable during the asteroid's March 2006 approach.

Busch, Michael J.↗
NASA NTRS1984

Lightcurves and phase relations of the asteroids 82 Alkmene and 444 Gyptis

The asteroids 82 Alkmene and 444 Gyptis were observed photoelectrically at Table Mountain Observatory and at Torino Observatory during their 1979 opposition in order to determine their rotation periods and phase relations. The observations of 82 Alkmene yielded a rotation period of 12.999 h + or 0.002 h and an amplitude of variation of 0.55. The measured and corrected absolute magnitudes, when plotted against phase angle, is the best fit Lumme and Bowell (1981) theoretical phase function. The rotation period and amplitude of variation for 444 Gyptis are 6.214 h and 0.15, respectively. The phase relation of this asteroid is poorly fit by the Lumme and Bowell theory when only the multiple scattering parameter and the zero phase angle are treated as variables. A good fit can be obtained by adjusting some of the other parameters of their theory, but the physical interpretation is ambiguous.

Harris, A. W.↗
NASA NTRS1988

Collisional evolution in the Eos and Koronis asteroid families - Observational and numerical results

The origin and evolution of the Eos and Koronis families are addressed by an analysis of Binzel's (1987) observational results. The Maxwellian distribution of the Eos family's rotation rates implies a collisionally-evolved population; these rates are also faster than those of the Koronis family and nonfamily asteroids. While the age of the Eos family may be comparable to the solar system's, that of the Koronis family could be considerably younger. Greater shape irregularity may account for the Koronis family's higher mean lightcurve amplitude.

Binzel, Richard P.↗
NASA NTRS2011

Surface Contact Model for Comets and Asteroids

A contact force model was developed for use in touch and go (TAG) surface sampling simulations on small celestial bodies such as comets and asteroids. In TAG scenarios, a spacecraft descending toward the surface of a small body comes into contact with the surface for a short duration of time, collects material samples with a sampler device, and then ascends to leave the surface. The surface contact required 6-DOF (degrees of freedom) dynamics models due to coupling of the attitude and translation dynamics during the contact. The model described here is for contact scenarios that utilize a rotating brush wheel sampler (BWS) to collect surface material. The model includes stiffness and damping of the surface material during BWS vertical motion, lateral friction from the BWS dragging across the surface, and lateral shear from the rotating BWS scooping the surface material. This model is useful for any mission to asteroids or comets that incorporates surface sampling operations.

Blackmore, Lars James C.↗
NASA NTRS1980

Hektor - Nature and origin of a binary asteroid

It is argued that the elongated single body model (Dunlap and Gehrels, 1969) which reproduces the rotational light curve of 624 Hektor is unstable with respect to binary fission. It is shown that a contact binary model with ellipsoidal components of equilibrium shape matches the observed amplitude. Hektor's bulk density is inferred to be about 2.5 g/cu cm.

Weidenschilling, S. J.↗
NASA NTRS1972

Shape and Orientation of Nucleus

The shape and rate of rotation, as determined from brightness-time variations of reflected sunlight, for various asteroid nuclei are presented. The possible application of such a technique to the nucleus of comets is discussed.

Gehrels, T.↗