The Quest for Interstellar Exploration
NASA strategic planning includes the objective for a set of missions that would begin exploration beyond our solar system.
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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.
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The interstellar wake of the solar wind
The present work examines theoretically the cooling of the subsonic solar wind by the interstellar hydrogen gas entering the solar system. It is assumed that the interstellar hydrogen gas is distributed uniformly in the space where the solar wind is subsonic, and the solar-wind wake is represented by an idealized column flow of plasma in which the flow velocity is constant and the temperature varies along, not across, the wake. The results are therefore to be taken only in their order of magnitude. In the hot region near the shock sphere the electrons are cooled mainly through electron ionization while the protons cool through charge exchange. In the cooler regions far away from the shock, the protons are still cooled by charge exchange and the electron is cooled by collisions with protons. As the temperature of the solar wind decreases, the magnetic field becomes relatively more important. When the spin axis of the sun is parallel to the direction of the incident interstellar gas flow, the lines of force are in spiral form and the tension causes the plasma to be concentrated along the center of the wake.
Yu, G.↗
Laboratory for Extraterrestrial Physics
The NASA Goddard Space Flight Center (GSFC) Laboratory for Extraterrestrial Physics (LEP) performs experimental and theoretical research on the heliosphere, the interstellar medium, and the magnetospheres and upper atmospheres of the planets, including Earth. LEP space scientists investigate the structure and dynamics of the magnetospheres of the planets including Earth. Their research programs encompass the magnetic fields intrinsic to many planetary bodies as well as their charged-particle environments and plasma-wave emissions. The LEP also conducts research into the nature of planetary ionospheres and their coupling to both the upper atmospheres and their magnetospheres. Finally, the LEP carries out a broad-based research program in heliospheric physics covering the origins of the solar wind, its propagation outward through the solar system all the way to its termination where it encounters the local interstellar medium. Special emphasis is placed on the study of solar coronal mass ejections (CME's), shock waves, and the structure and properties of the fast and slow solar wind. LEP planetary scientists study the chemistry and physics of planetary stratospheres and tropospheres and of solar system bodies including meteorites, asteroids, comets, and planets. The LEP conducts a focused program in astronomy, particularly in the infrared and in short as well as very long radio wavelengths. We also perform an extensive program of laboratory research, including spectroscopy and physical chemistry related to astronomical objects. The Laboratory proposes, develops, fabricates, and integrates experiments on Earth-orbiting, planetary, and heliospheric spacecraft to measure the characteristics of planetary atmospheres and magnetic fields, and electromagnetic fields and plasmas in space. We design and develop spectrometric instrumentation for continuum and spectral line observations in the x-ray, gamma-ray, infrared, and radio regimes; these are flown on spacecraft to study the interplanetary medium, asteroids, comets, and planets. Suborbital sounding rockets and groundbased observing platforms form an integral part of these research activities. This report covers the period from approximately October 1999 through September 2000.
Vondrak, Richard R.↗
The Laboratory Production of Complex Organic Molecules in Simulated Interstellar Ices
Much of the volatiles in interstellar dense clouds exist in ices surrounding dust grains. Their low temperatures preclude most chemical reactions, but ionizing radiation can drive reactions that produce a suite of new species, many of which are complex organics. The Astrochemistry Lab at NASA Ames studies the UV radiation processing of interstellar ice analogs to better identify the resulting products and establish links between interstellar chemistry, the organics in meteorites, and the origin of life on Earth. Once identified, the spectral properties of the products can be quantified to assist with the search for these species in space. Of particular interest are findings that UV irradiation of interstellar ice analogs produces molecules of importance in current living organisms, including quinones, amphiphiles, and amino acids.
Dworkin, J. P.↗
Gas Phase Formation of the Interstellar Molecule Methyltriacetylene
Methyltriacetylene – the largest methylated polyacetylene detected in deep space – has been synthesized in the gas phase via the bimolecular reaction of the 1-propynyl radical with diacetylene under single-collision conditions. In this work, the barrier-less route to methyltriacetylene represents a prototype of a polyyne chain extension through a radical substitution mechanism and provides a novel low temperature route, in which the propynyl radical piggybacks a methyl group to be incorporated into methylated polyynes. This mechanism overcomes a key obstacle in previously postulated reactions of methyl radicals with unsaturated hydrocarbon, which fail the inclusion of methyl groups into hydrocarbons due to insurmountable entrance barriers thus providing a fundamental understanding on the electronic structure, chemical bonding, and formation of methyl-capped polyacetylenes. These species are key reactive intermediates leading to carbonaceous nanostructures in molecular clouds like TMC-1.
74 ATOMIC AND MOLECULAR PHYSICS↗
DISCOVERY OF A DISK GAP CANDIDATE AT 20 AU IN TW HYDRAE
We present a new Subaru/HiCIAO high-contrast H-band polarized intensity (PI) image of a nearby transitional disk associated with TW Hydrae. The scattered light from the disk was detected from 0.″ 2 to 1.″ 5 (11–81 AU) and the PI image shows a clear axisymmetric depression in PI at ∼0.″ 4 (∼20 AU) from the central star, similar to the ∼80 AU gap previously reported from Hubble Space Telescope images. The azimuthal PI profile also shows that the disk beyond 0.″ 2 is almost axisymmetric. We discuss two possible scenarios explaining the origin of the PI depression: (1) a gap structure may exist at ∼20 AU from the central star because of a shallow slope seen in the PI profile, and (2) grain growth may be occurring in the inner region of the disk. Multi-band observations at near-infrared and millimeter/submillimeter wavelengths play a complementary role in investigating dust opacity and may help reveal the origin of the gap more precisely.
79 ASTRONOMY AND ASTROPHYSICS↗
Graphite revisited
Laboratory measurements are used to constrain the dielectric tensor for graphite, from microwave to X-ray frequencies. The dielectric tensor is strongly anisotropic even at X-ray energies. The discrete dipole approximation is employed for accurate calculations of absorption and scattering by single-crystal graphite spheres and spheroids. For randomly oriented single-crystal grains, the so-called 1/3--2/3 approximation for calculating absorption and scattering cross sections is exact in the limit a/λ→0 and provides better than ∼10% accuracy in the optical and UV even when a/λ is not small, but becomes increasingly inaccurate at infrared wavelengths, with errors as large as ∼40% at λ=10 μm. For turbostratic graphite grains, the Bruggeman and Maxwell Garnett treatments yield similar cross sections in the optical and ultraviolet, but diverge in the infrared, with predicted cross sections differing by over an order of magnitude in the far-infrared. It is argued that the Maxwell Garnett estimate is likely to be more realistic, and is recommended. The out-of-plane lattice resonance of graphite near 11.5 μm may be observable in absorption with the MIRI spectrograph on James Webb Space Telescope. Aligned graphite grains, if present in the interstellar medium, could produce polarized X-ray absorption and polarized X-ray scattering near the carbon K edge.
79 ASTRONOMY AND ASTROPHYSICS↗
Space Congress, 29th, Cocoa Beach, FL, Apr. 21-24, 1992, Proceedings
The present volume on the quest for new frontiers in space discusses weather impacts on space operations, planning for the performance of future space bases, a new guidance and control unit for the Titan IV vehicle, and nondestructive evaluation of Shuttle Columbia tiles. Attention is given to Space Shuttle payload accommodations and trends in customer demands, a generic propellants transfer unit, making space part of general education, space station on-orbit solar array loads during assembly, and dimensional stability of the attitude reference assembly on SSF. Topics addressed include National Launch System payload accommodations and launch operations, the integrated factory/launch site processing concept, Pioneer 10 interstellar studies, and the role of advanced nuclear propulsion systems in precursor interstellar missions. Also discussed are legal challenges in realizing interstellar initiatives, Mars transportation system synthesis, and NASA's commercial space program.
Source record↗
Oblique and rippled heliosphere structures from the Interstellar Boundary Explorer
Abstract Past analysis has shown that the heliosphere structure can be deduced from correlations between long-scale solar wind pressure evolution and energetic neutral atom emissions. However, this required spatial and temporal averaging that smoothed out small or dynamic features of the heliosphere. In late 2014, the solar wind dynamic pressure increased by roughly 50% over a period of 6 months, causing a time and directional-dependent rise in around 2–6 keV energetic neutral atom fluxes from the heliosphere observed by the Interstellar Boundary Explorer. Here, we use the 2014 pressure enhancement to provide a simultaneous derivation of the three-dimensional heliospheric termination shock (HTS) and heliopause (HP) distances at high resolution from Interstellar Boundary Explorer measurements. The analysis reveals rippled HTS and HP surfaces that are oblique with respect to the local interstellar medium upwind direction, with significant asymmetries in the heliosphere structure compared to steady-state heliosphere models. We estimate that the heliosphere boundaries contain roughly ten astronomical unit-sized spatial variations, with slightly larger variations on the HTS surface than the HP and a large-scale, southwards-directed obliquity of the surfaces in the meridional plane. Comparisons of the derived HTS and HP distances with Voyager observations indicate substantial differences in the heliosphere boundaries in the northern versus southern hemispheres and their motion over time.
79 ASTRONOMY AND ASTROPHYSICS↗
The Formation of Formaldehyde on Interstellar Carbonaceous Grain Analogs by O/H Atom Addition
An understanding of possible scenarios for the formation of astrophysically relevant molecules, particularly complex organic molecules, will bring us one step closer to the understanding of our astrochemical heritage. In this context, formaldehyde is an important molecule as a precursor of methanol, which in turn is a starting point for the formation of more complex organic species. In the present experiments, for the first time, following the synthesis of CO, formaldehyde has been produced on the surface of interstellar grain analogs, hydrogenated fullerene-like carbon grains, by O and H atom bombardment. The formation of H{sub 2}CO is an indication for a possible methanol formation route in such systems.
79 ASTRONOMY AND ASTROPHYSICS↗
Bi-Kappa Proton Mirror and Cyclotron Instabilities in the Solar Wind
Abstract The charged particles in the solar wind are often observed to possess a nonthermal tail in the velocity distribution function, a feature that can be fitted with the Kappa model. The anisotropic, or bi-Kappa, model of protons, electrons, and other charged particles is thus adopted in the literature for interpreting the data as well as in the context of the analysis of wave–particle interactions. The present paper develops an approximate but efficient theory of the mirror and cyclotron instabilities excited by the bi-Kappa protons in the solar wind. A velocity moment-based quasi-linear theory of these instabilities is also formulated in order to investigate the saturation behavior. Applications of the formalism are made for instabilities close to the marginally unstable state, which is typical of the solar wind near 1 au.
79 ASTRONOMY AND ASTROPHYSICS↗
Observations of cosmic-ray electrons between 2.7 and 21.5 Mev.
IMP 4 satellite measurements of cosmic ray electrons in interplanetary space compared to predictions of electron intensity from interstellar proton-electron collisions
Mcdonald, F. B.↗
Coronal gas in the Galaxy. II - A statistical analysis of O VI absorptions
This paper deals with general inferences about the low-density phase of interstellar gas having temperatures well above 200,000 K (the 'coronal gas') which can be drawn from O VI absorption data for 72 stars. Attention is given to the behavior of radial velocities, possible evidence for circumstellar O VI, the space distribution of the O VI gas, crowding of normal interstellar gas, and temperature distributions for the coronal gas. A model is adopted in which the coronal gas is contained within randomly distributed and nonoverlapping parcels, each with a size, pressure, and internal temperature distribution that do not vary markedly from one unit to the next. It is shown that the one-dimensional velocity dispersion for O VI regions (26 km/s) is substantially higher than the value for ordinary interstellar clouds (6.4 km/s).
Jenkins, E. B.↗
A survey of interstellar H I from L-alpha absorption measurements. II
This paper presents interstellar H I column densities, mean H I space densities, total neutral hydrogen column densities, and total mean hydrogen space densities toward 100 stars surveyed in the L-alpha region with the U2 detector of the Copernicus satellite. The data for 47 stars are compared with OAO 2 results, and ratios of gas to E(B-V) color excess are computed for 30 'intercloud' stars, 45 'cloud' stars, and the combined sample of 75 stars. A definitive value of 5.8 x 10 to the 21st power atoms/sq cm per mag is obtained for the mean ratio of total neutral hydrogen to color excess, indicating that this ratio seems to be a constant, independent of location in the sky, except possibly toward stars with anomalous extinction. The anomalous ratio determined for Rho Oph is examined, and the galactic distribution of neutral hydrogen is evaluated. The following values are estimated for matter in the galactic plane within 500 pc of the sun: a total mean hydrogen space density of 1.15 atoms/cu cm, a mean H2 space density of 0.143 molecule/cu cm (possibly a lower limit), and a mean H I space density of 0.86 atom/cu cm (a possible overestimate).
Bohlin, R. C.↗
Modeling the Stability of Volatile Deposits in Lunar Cold Traps
There are several mechanisms acting at the cold traps that can alter the inventory of volatiles there. Primarily, the lunar surface is bombarded by meteoroids which impact, melt, process, and redistribute the regolith. Further, solar wind and magnetospheric ion fluxes are allowed limited access onto the regions in permanent shadow. Also, although cold traps are in the permanent shadow of the Sun, there is a small flux of radiation incident on the regions from interstellar sources. We investigate the effects of these space weathering processes on a deposit of volatiles in a lunar cold trap through simulations. We simulate the development of a column of material near the surface of the Moon resulting from space weathering. This simulation treats a column of material at a lunar cold trap and focuses on the hydrogen content of the column. We model space weathering processes on several time and spatial scales to simulate the constant rain of micrometeoroids as well as sporadic larger impactors occurring near the cold traps to determine the retention efficiency of the cold traps. We perform the Monte Carlo simulation over many columns of material to determine the expectation value for hydrogen content of the top few meters of soil for comparison with Lunar Prospector neutron data.
Crider, D. H.↗
The Magnetic Field in the Outer Heliosphere
One of the great achievements of Parker was the prediction that the solar magnetic field would be drawn into Archimedian spirals as it is carried away from the Sun by the solar wind. This prediction has been amply confirmed by many in situ measurements in the intervening four decades. But, Parker made his prediction for a solar wind that expands into infinite space while we now know that the local interstellar medium (LISM) is far from empty and, in fact, confines the solar wind to a finite volume, known as the heliosphere, that extends to approximately 100 AU in the upstream direction (the solar system is moving through the LISM). Voyagers 1/2, presently at -80 AU, are approaching the upstream boundaries of the heliosphere and returning data on the properties of the magnetic field. This is important for understanding how galactic cosmic rays (GCRs) reach the Earth. Voyagers show that the IMF at 10-80 AU behaves much as Parker predicted - with two important exceptions. This is not surprising since the field is essentially passively advected by the solar wind out to 80 AU. But, new models say that nearer the heliosphere boundaries the field plays a major role in the solar wind-LISM interaction. However, of the many physical ingredients that constitute the outer heliosphere, the magnetic field poses some of the most interesting and difficult numerical modeling problems. Presently, only a few results have been published and much remains to be done. Here I will summarize the expected and measured behavior of the magnetic field at 80 AU. Then I will describe modeling predictions beyond 80 AU: magnetic "tornadoes", polarity envelopes, the Axford-Cranfill effect, inner and outer magnetic walls and more. I will also list what I believe to be important new modeling objectives. Finally, I will speculate on what is happening with the magnetic field near the nose of the heliosphere. My conclusion is that models of GCR modulation rarely incorporate even crudely realistic magnetic fields so it is a wonder that they are as successful as they are and no surprise that there are still important discrepancies between GCR modulation observations and the models.
Suess, S. T.↗
High resolution electron-impact emission of H-2. I. cross sections and emission yields 900-1200 angstrom
High-resolution (Delta lambda = 115 m Angstrom) emission spectra of molecular hydrogen produced by electron-impact excitation at 100 eV have been obtained in the wavelength range 900-1200 A.
Diffuse interstellar bands hubble-space-telescope ↗