Engineering topics

Verbiscer, Anne J.

Publications and source records attributed to Verbiscer, Anne J..

7 records

DOE OSTI2020

Size and Shape Constraints of (486958) Arrokoth from Stellar Occultations

We present the results from four stellar occultations by (486958) Arrokoth, the flyby target of the New Horizons extended mission. Three of the four efforts led to positive detections of the body, and all constrained the presence of rings and other debris, finding none. Twenty-five mobile stations were deployed for 2017 June 3 and augmented by fixed telescopes. There were no positive detections from this effort. The event on 2017 July 10 was observed by the Stratospheric Observatory for Infrared Astronomy with one very short chord. Twenty-four deployed stations on 2017 July 17 resulted in five chords that clearly showed a complicated shape consistent with a contact binary with rough dimensions of 20 by 30 km for the overall outline. A visible albedo of 10% was derived from these data. Twenty-two systems were deployed for the fourth event on 2018 August 4 and resulted in two chords. The combination of the occultation data and the flyby results provides a significant refinement of the rotation period, now estimated to be 15.9380 ± 0.0005 hr. The occultation data also provided high-precision astrometric constraints on the position of the object that were crucial for supporting the navigation for the New Horizons flyby. This work demonstrates an effective method for obtaining detailed size and shape information and probing for rings and dust on distant Kuiper Belt objects as well as being an important source of positional data that can aid in spacecraft navigation that is particularly useful for small and distant bodies.

79 ASTRONOMY AND ASTROPHYSICS↗

NASA NTRS2016

Observing Planetary Rings and Small Satellites with the James Webb Space Telescope: Science Justification and Observation Requirements

The James Webb Space Telescope (JWST) will provide unprecedented opportunities to observe the rings and small satellites in our Solar System, accomplishing three primary objectives: (1) discovering new rings and moons, (2) unprecedented spectroscopy, and (3) time-domain observations. We give details on these science objectives and describe requirements that JWST must fulfill in order to accomplish the science objectives.

observational - planets and satellites↗

NASA NTRS1994

A photometric study of Enceladus

We have supplemented Voyager imaging data from Enceladus (limited to phase angles of 13 deg-43 deg) with recent Earth-based CCD observations to obtain an improved determination of the Bond albedo, to construct an albedo map of the satellite, and to constrain parameters in Hapke's (1986) photometric equation. A major result is evidence of regional variations in the physical properties of Enceladus' surface. The average global photometric properties are described by single scattering albedo omega(sub 0) average = 0.998 +/- 0.001, macroscopic roughness parameter theta average = 6 +/- 1 deg, and Henyey-Greenstein asymmetry parameter g = -0.399 +/- 0.005. The value of theta average is smaller than the 14 deg found by fitting whole-disk data, which include all terrains on Enceladus. The opposition surge amplitude B(sub 0) = 0.21 +/- 0.07 and regolith compaction parameter h = 0.014 +/- 0.02 are loosely constrained by the scarcity of and uncertainty in near-opposition observations. From the solar phase curve we determine the geometric albedo of Enceladus p(sub v) = 0.99 +/- 0.06 and phase integral q = 0.92 +/- 0.05, which corresponds to a spherical albedo A = p(sub v)q = 0.91 +/- 0.1. Since the spectrum of Enceladus is fairly flat, we can approximate the Bond albedo A(sub B) with the spherical albedo. Our photometric analysis is summarized in terms of an albedo map which generally reproduces the satellite's observed lightcurve and indicates that normal reflectances range from 0.9 on the leading hemisphere to 1.4 on the trailing one. The albedo map also revels an albedo variation of 15% from longitudes 170 deg to 200 deg, corresponding to the boundary between the leading and trailing hemispheres.

Verbiscer, Anne J.↗

NASA NTRS1992

Mimas - Photometric roughness and albedo map

The backscattering phase function asssociated with bright icy satellites may render Hapke's (1986) isotropic approximation to multiple scattering inadequate. A reanalysis is accordingly conducted here of the Voyager observations of Mimas, using a modification to Hapke's equation that accommodates anisotropic multiple scattering, in order to characterize the physical and photometric properties of the heavily cratered icy surface. The roughness parameter of Mimas is in this way redefined, and an accurate albedo map is obtained which deminstrates small latitudinal and longitudinal albedo variations.

Verbiscer, Anne J.↗

NASA NTRS1990

Scattering properties of natural snow and frost - Comparison with icy satellite photometry

The Hapke (1986) equation is presently fit to ascertain the single-scattering albedo of the icy satellites of Uranus and Neptune and the one-term Henyey-Greenstein particle-phase function g for each of the Middleton and Mungall (1952) goniophotometric data samples. There emerge both very high single-scattering albedos and strongly forward-scattering particle phase functions; while these are in keeping with Mie theory-based theoretical considerations, they contrast with the observed backscattering behavior of icy satellites. It is suggested the icy satellite frost grains are aggregated into particles of complex texture, which produce the unusual backscattering behavior.

Verbiscer, Anne J.↗

NASA NTRS1989

Albedo dichotomy of Rhea - Hapke analysis of Voyager photometry

The Hapke (1986) model has been well fitted to both full-disk and disk-resolved Voyager observations. The low phase angle data indicate a substantial opposition effect, and the Hapke analysis results show that while the regolith compaction parameter for Rhea is definitely larger than for Titania, it is comparable to that of the moon. Photometric differences other than albedo are noted between the leading and trailing hemispheres of the satellite. The albedo map of Rhea presented reproduces the observed lightcurve and demonstrates that no terrain or feature in the trailing hemisphere is as bright as any in the leading hemisphere. A quasi-circular low albedo region near the antiapex of motion is discovered.

Verbiscer, Anne J.↗

NASA NTRS1988

Estimating phase integrals - A generalization of Russell's law

An attempt is made to demonstrate, by means of Hapke's (1981) photometric function, that a simple and reliable method exists for the estimation of phase integrals from limited higher phase angle measurements. This method is a generalization of the approximation first proposed by Russell (1916) and more recently treated by Veverka (1971). It is shown that this generalization of Russell's law can employ observations anywhere in the range of phase angles from 40 to 90 deg; optimum estimates of q will be obtained if the data near 70 deg are obtainable.

Verbiscer, Anne J.↗