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NASA NTRS1993

The structure of mass-loading shocks

A new two-fluid model which describes mass loading in the solar wind (e.g., the interaction of the solar wind with a cometary coma or the local interstellar medium) is presented. The self-consistent back-reaction of the mass-loaded ions is included through their effective scattering in low-frequency MHD turbulence and the invocation of a diffusive approximation. Such an approximation has the advantage of introducing self-consistent dissipation coefficients into the governing equations, thereby facilitating the investigation of the internal structure of shocks in mass-loading environments. To illustrate the utility of the new model, we consider the structure of cometary shocks in the hypersonic one-dimensional limit, finding that the incoming solar wind is slowed by both mass loading and the development of a large cometary ion pressure gradient. The shock is broadened and smoothed by the cometary ions with a thickness of the order of the cometary ion diffusion scale.

Zank, G. P.↗
NASA NTRS1993

The infrared light curve of Periodic Comet Halley 1986 III and its relationship to the visual light curve, C2, and water production rates

The near-IR light curve of Periodic Comet Halley 1986 III is analyzed and compared with C2 production, water production, and the visual light curve. This is the most complete IR light curve compiled to date for any comet. The scattering phase function at small sun-comet-earth angles is shown to affect the slope of near-IR light curve significantly. P/Halley's dust production, as inferred from the IR light curve showed an increased production rate near perihelion which appears to be correlated with the onset of significant jet activity. The near-IR light curve, visual light curve, C2, and water production rates displayed different heliocentric variations, suggesting that one parameter cannot be accurately estimated from another. This is particularly true of the early preperihelion visual light curve. A peak of 0.3-0.5 magnitude in the visual magnitude, representing the integrated brightness of the comet's visible coma, lagged the other parameters by about a day. The near-IR color, J-H, was less red during periods of strong dust activity.

Morris, Charles S.↗
NASA NTRS1993

Measurements of the anisotropy of the cosmic background radiation at 0.5 deg scale near the star Mu Pegasi

Results are presented from the third flight of the MAX experiment, an attitude-controlled balloon-borne millimeter-wave telescope with a 0.5 deg beam, a 1 deg chop, and a three-channel bolometric photometer. Several hours of high-quality data were obtained during a flight on 1991 June 5, including long integrations to search for CBR anisotropy, two separate measurements of dust in the Galactic plane, a brief scan of the Coma Cluster to search for the Sunyaev-Zel'dovich (SZ) effect, and a number of important systematic tests. Data from one of the long CBR integrations, carried out in a region of sky near the star Mu Pegasi, are presented. The primary structure in the data is shown to be emission from Galactic dust via its spectrum and correlation with the IRAS 100/micron map. Several approaches are used to fit this dust component and remove it from the data. An upper limit to CBR anisotropy of deltaT/T less than 2.5 x 10 exp -5 is obtained for a Gaussian autocorrelation function with coherence angle omega(c) = 25'. This limit is significantly higher than the measurement sensitivity of deltaT/T about 1 x 10 exp -5 due to the presence of residual structure in the data after removal of the dust component.

Meinhold, P.↗
NASA NTRS1993

Formaldehyde and organic molecule production in astrophysical ices at cryogenic temperatures

Thermally promoted formaldehyde (H2CO) reactions in cryogenic ices have been studied to test their importance as a source of organic molecules in comets and interstellar ices. Ices containing H2CO, H2O, CH3OH, CO, and NH3 were investigated by using infrared spectroscopy. Small traces of NH3 (NH3/H2CO equal to or greater than 0.005) are sufficient to convert significant fractions (40 percent or greater) of the H2CO into more complex organics. However, H2CO reactions do not proceed without NH3. Spectral evidence for reaction onset appeared between 40 and 80 K, depending on the ice. Five distinct products were formed. These principally consist of polyoxymethylene and related derivatives. Polyoxymethylene itself was not made in significant amounts in cometary analogs. These products differ from those produced by ultraviolet and particle irradiation. The nature and relative amounts of the products depend on the initial composition, making these materials excellent tracers of a comet's history. About 3 percent of the organics in p-Halley's coma could have been produced by thermal H2CO reactions.

Schutte, W. A.↗
NASA NTRS1991

Debris Collision Warning Sensor (DCWS) telescope design

The 60-cm diameter, f/2 DCWS telescope is a modified Ritchey Chretein configuration which collects radiation in three spectral bands, 0.4 to 0.9/micron (visible), 5.0 to 9.0/microns (MWIR), and 9.0 to 12.0 microns (LWIR). The visible and infrared radiation are separated by a dichroic beamsplitter and focused on to separate focal planes; the MWIR and LWIR radiation share a common focal plane with separate detector arrays residing side-by-side. The conic constants of the primary and secondary mirrors are that of a Ritchey Chretein telescope; to improve the image quality at the edges of the field of view, a set of zero power correcting optics have been introduced to each optical train. The Ritchey Chretein telescope is corrected to have zero third order spherical aberration and zero third order coma; the DCWS telescope is further configured such that the positive power in the primary mirror is approximately equal to the negative power in the secondary mirror, resulting in a telescope with very little field curvature.

Brown, Robert J.↗
NASA NTRS1992

Design and analysis of soft X-ray imaging microscopes

The spherical Schwarzschild microscope for soft X-ray applications in microscopy and projection lithography consists of two concentric spherical mirrors configured such that the third-order spherical aberration and coma are zero. Since multilayers are used on the mirror substrates for X-ray applications, it is desirable to have only two reflecting surfaces in a microscope. To reduce microscope aberrations and increase the field of view, generalized mirror surface profiles are here considered. Based on incoherent and sine wave modulation transfer function calculations, the object plane resolution of a microscope has been analyzed as a function of the object height and numerical aperture (NA) of the primary for several spherical Schwarzschild, conic, and aspherical Head reflecting two-mirror microscope configurations. The Head microscope with a NA of 0.4 achieves diffraction limited performance for objects with a diameter of 40 microns. Thus, it seems possible to record images with a feature size less than 100 A with a 40x microscope when using 40 A radiation.

Shealy, David L.↗
NASA NTRS1992

Crystallization of amorphous ice as the cause of Comet P/Halley's outburst at 14 AU

An explanation is provided for the postperihelion eruption of Comet P/Halley, detected in February 1991 and believed to have started three months earlier, namely, the crystallization of amorphous ice taking place in the interior of the porous nucleus, at depths of a few tens of meters, accompanied by the release of trapped gases. Numerical calculations show that for a bulk density of 0.5 g/cu cm and a pore size of 1 micron crystallization occurs on the outbound leg of Comet P/Halley's orbit, at heliocentric distances between 5 AU and 17 AU. The trapped gas is released and flows to the surface through the porous medium. It may also open wider channels, as the internal pressures obtained surpass the tensile strength of cometary ice. The outflowing gas carries with it grains of ice and dust, and thus can explain the large amounts of dust observed in the coma at 14.3 AU and beyond.

Prialnik, D.↗
NASA NTRS1992

A different view of plasma flow inside P/Halley

The Giotto spacecraft carried two different instruments - the JPA and the IMS - for the observation of hot ions in the coma of P/Halley. Although there are many similarities in the time and distance profiles of the plasma flow parameters (bulk velocity, number density, and temperature) computed from the two data sets, there are also some significant differences, especially at cometocentric distances less than 500,000 km. The principal discrepancies between the JPA results presented by Formisano et al. (1990) and the IMS observations are: (1) the IMS did not detect the levelling off of the speed and temperature profiles that Formisano et al. interpreted as flow stabilization; (2) the IMS detected differential north-south flow between the solar wind and cometary ions for only a brief interval when the magnetic field was oriented nearly southward, whereas Formisano et al. reported more extensive differential north-south flow that was independent of the direction of the field; (3) the JPA ion densities were factors of 2 to 4 higher than the IMS ion densities which, in turn, were an order of magnitude greater than theoretical values.

Neugebauer, M.↗
NASA NTRS1993

On the formation and evolution of gaseous structures in Comet P/Halley

The possibility of a dynamic connection between the CN jets and CN shells in the coma of Comet P/Halley is here investigated through numerical simulations on the spatial and temporal evolution of gaseous jets. The evolution of such a jet into a shell is found to be straightforward for several geometries. It is noted that a closed shell structure may be due to a near-equatorial observational view of material derived from the active region near the nucleus equator.

Schulz, Rita↗
NASA NTRS1993

An experimental study of the organic molecules produced in cometary and interstellar ice analogs by thermal formaldehyde reactions

Results of an experimental study tracing thermal formaldehyde reactions in astrophysically relevant ices in dense molecular clouds are reported. The formaldehyde chemistry during warm-up of ices containing H2CO and one or more of the molecules H2O, CH3OH, CO, O2, and NH3 were monitored using IR spectroscopy. Conversion of H2CO into residues was observed to start at about 40 K for NH3:H2CO ices and at about 80 K in H2O-rich ices. A total of five different organic products of these reactions were distinguished: POM and reaction products of H2CO and H2O, CH3OH, and NH3. Given the measured reaction paths and efficiencies, it is estimated that on the order of 1 percent of the organics found in the coma of Comet P/Halley could have been produced by thermal formaldehyde reactions taking place in the nucleus.

Schutte, W. A.↗
NASA NTRS1993

On-orbit refueling

The goal of this program is to increase operational availability of space assets by refueling with an expendable launch vehicle (not the Shuttle) in LEO or GEO. Information is drawn from the on-orbit refueling model, COSEMS model, COMA study, and information provided from the San Antonio Air Logistics Center.

POSPISIL↗
NASA NTRS1992

Giotto data analysis: Electron plasma and heavy ion composition measurements at Comet Halley

This investigation involved the analysis of electron plasma and heavy ion composition measurements made by the COPERNIC (COmplete Positive ion, Electron, and Ram Negative Ion measurements near Comet Halley) plasma experiment during the close fly-by of Halley by the European Space Agency's Giotto spacecraft. The experiment provided measurements of the full 3-dimensional distribution of 10 eV-30 keV electrons, and mass analysis of cold cometary ions from 10-210 amu. The analysis of the COPERNIC data has yielded some remarkable results, including: The discovery of negatively charged ions in the inner coma; the discovery of far heavier (mass is greater than 50 amu) ions than predicted, dominated by complex molecular ions made up of C, H, O, and N; the discovery of an adiabatic heating effect on electrons from the compression of the solar wind plasma; the identification of several organic and sulfur bearing ions; and the discovery of a new 'mystery region' where electrons are accelerated to high energies. These discoveries were in addition to the detailed analysis of 'expected' features at Comet Halley. Although this grant has expired, analysis continues on the data at a low (unfunded) level, and it is expected that more significant results will be obtained. A bibliography of the papers resulting from this research is attached, and a copy of each paper is included.

Lin, Robert P.↗
NASA NTRS1993

C2, CN and dust in Comet Wilson (1987VII)

Column density profiles in C2 and CN as well as in blue and red continuum (BC and RC) were constructed from two dimensional images of the coma of Comet Wilson (1987VII). The authors' analysis showed that the continuum profiles decrease with the nuclear distance rho as 1/rho. From the C2 and CN profiles the authors determined the parent and the daughter lifetimes as well as the production rates in terms of the vectorial model.

Schulz, R.↗
NASA NTRS1993

The velocity distribution of cometary hydrogen - Evidence for high velocities?

The Hamilton Echelle spectrograph on the 3-m Shane telescope at Lick Observatory was used to obtain high-velocity and spatial resolution 2D spectra of H-alpha 6563-A emission in Comets Austin and Levy. The presence of the components expected from water dissociation and collisional thermalization in the inner coma is confirmed by the hydrogen velocity distribution. In Comet Austin, the potential high-velocity hydrogen includes velocities of up to about 40 km/s and is spatially symmetric with respect to the nucleus. In Comet Levy, the high-velocity hydrogen reaches velocities of up to 50 km/s and is situated exclusively on the sunward side of the nucleus. The two distinct signatures of high-velocity hydrogen imply two distinct sources.

Brown, Michael E.↗
NASA NTRS1993

CCD photometry of 2060 Chiron in 1985 and 1991

CCD photometry of 2060 Chiron from two apparitions, in January 1985 and January 1991, is reported. No evidence is found for a change in the amplitude of Chiron's lightcurve with ecliptic longitude, as would be expected of an object having a high-obliquity rotational axis. An improved value for Chiron's synodic period is reported: 5.917813 +/- 0.000007 hr. No outbursts larger than one-half of one percent were found during the 1991 observations. Secular (yearly) change is due to the gradual dissipation and resupply of material (mainly dust) throughout the coma, while the impulsive (daily) brightenings are probably due to the sudden release of pockets of entrapped volatiles, such as CO2, CN, or possibly H2O.

Marcialis, Robert L.↗
NASA NTRS1994

Techniques for in situ collection and measurement of volatiles released during hypervelocity impact

The capability of the gas capture cell to collect particles and semivolatile species which were released during impact is examined. The proposed Comet Coma Sample Return (CCSR) payload contains a variety of instrumentation for characterizing and collecting cometary dust. In this suite of instruments the Gas Capture Cell (GCC) is unique in that it not only collects the vaporization products resulting from the dust particle impacts, but also provides chemical characterization information prior to return of the dust particles for analysis on Earth. The GCC provides near real-time characterization of the volatile species, such as low- and medium-molecular-weight organic compounds that evolve from dust particles on impact with metal targets. Instrument sensitivity is sufficient for analyzing the volatile impact products resulting from single, individual dust particles. This capability will enable characterization of near-pristine dust particles, including the CHON particles, to be performed at a level not previously possible. Its design concept, operation and performance are detailed.

Heppner, R. A.↗
NASA NTRS1993

The Deep Space Network as an instrument for radio science research

Radio science experiments use radio links between spacecraft and sensor instrumentation that is implemented in the Deep Space Network. The deep space communication complexes along with the telecommunications subsystem on board the spacecraft constitute the major elements of the radio science instrumentation. Investigators examine small changes in the phase and/or amplitude of the radio signal propagating from a spacecraft to study the atmospheric and ionospheric structure of planets and satellites, planetary gravitational fields, shapes, masses, planetary rings, ephemerides of planets, solar corona, magnetic fields, cometary comae, and such aspects of the theory of general relativity as gravitational waves and gravitational redshift.

Asmar, S. W.↗
NASA NTRS1994

Exobiology opportunities from Discovery-class missions

Discovery-class missions that are now planned, and those in the concept stage, have the potential to expand our knowledge of the origins and evolution of biogenic compounds, and ultimately, of the origins of life in the solar system. This class of missions, recently developed within NASA's Solar System Exploration Program, is designed to meet important scientific objectives within stringent guidelines--$150 million cap on development cost and a 3-year cap on the development schedule. The Discovery Program will effectively enable "faster, cheaper" missions to explore the inner solar system. The first two missions are Mars Environmental Survey (MESUR) Pathfinder and Near Earth Asteroid Rendezvous (NEAR). MESUR Pathfinder will be the first Discovery mission, with launch planned for November/December 1996. It will be primarily a technical demonstration and validation of the MESUR Program--a network of automated landers to study the internal structure, meteorology, and surface properties of Mars. Besides providing engineering data, Pathfinder will carry atmospheric instrumentation and imaging capabilities, and may deploy a microrover equipped with an alpha proton X-ray spectrometer to determine elemental composition, particularly the lighter elements of exobiological interest. NEAR is expected to be launched in 1998 and to rendezvous with a near-Earth asteroid for up to 1 year. During this time, the spacecraft will assess the asteroid's mass, size, density, map its surface topography and composition, determine its internal properties, and study its interaction with the interplanetary environment. A gamma ray or X-ray spectrometer will be used to determine elemental composition. An imaging spectrograph, with 0.35 to 2.5 micron spectral range, will be used to determine the asteroid's compositional disbribution. Of the 11 Discovery mission concepts that have been designated as warranting further study, several are promising in terms of determining the composition and chemical evolution of organic matter on small planetary bodies. The following mission concepts are of particular interest to the Exobiology Program: Cometary coma chemical composition, comet nucleus tour, near earth asteroid returned sample, small missions to asteroids and comets, and solar wind sample return. The following three Discovery mission concepts that have been targeted for further consideration are relevant to the study of the evolution of biogenic compounds: Comet nucleus penetrator, mainbelt asteroid rendezvous explorer, and the Mars polar Pathfinder.

Meyer, Michael A.↗