Engineering topics

Hodges, R. R.

Publications and source records attributed to Hodges, R. R..

18 records

NASA NTRS2014

On Lunar Exospheric Column Densities and Solar Wind Access Beyond the Terminator from ROSAT Soft X-Ray Observations of Solar Wind Charge Exchange

We analyze the Rontgen satellite (ROSAT) position sensitive proportional counter soft X-ray image of the Moon taken on 29 June 1990 by examining the radial profile of the surface brightness in three wedges: two 19 deg wedges (one north and one south) 13-32 deg off the terminator toward the dark side and one wedge 38 deg wide centered on the antisolar direction. The radial profiles of both the north and the south wedges show significant limb brightening that is absent in the 38 deg wide antisolar wedge. An analysis of the soft X-ray intensity increase associated with the limb brightening shows that its magnitude is consistent with that expected due to solar wind charge exchange (SWCX) with the tenuous lunar atmosphere based on lunar exospheric models and hybrid simulation results of solar wind access beyond the terminator. Soft X-ray imaging thus can independently infer the total lunar limb column density including all species, a property that before now has not been measured, and provide a large-scale picture of the solar wind-lunar interaction. Because the SWCX signal appears to be dominated by exospheric species arising from solar wind implantation, this technique can also determine how the exosphere varies with solar wind conditions. Now, along with Mars, Venus, and Earth, the Moon represents another solar system body at which SWCX has been observed.

Exospheric↗

NASA NTRS2012

ROSAT Observations of Solar Wind Charge Exchange with the Lunar Exosphere

We analyze the ROSAT PSPC soft X-ray image of the Moon taken on 29 June 1990 by examining the radial profile of the count rate in three wedges, two wedges (one north and one south) 13-32 degrees off (19 degrees wide) the terminator towards the dark side and one wedge 38 degrees wide centered on the anti-solar direction. The radial profiles of both the north and the south wedges show substantial limb brightening that is absent in the 38 degree wide antisolar wedge. An analysis of the count rate increase associated with the limb brightening shows that its magnitude is consistent with that expected due to solar wind charge exchange (SWCX) with the tenuous lunar atmosphere. Along with Mars, Venus, and Earth, the Moon represents another solar system body at which solar wind charge exchange has been observed. This technique can be used to explore the solar wind-lunar interaction.

Collier, Michael R.↗

NASA NTRS2012

Redistribution of Lunar Polar Water to Mid-latitudes and its Role in Forming an OH veneer - Revisited

We suggest that energization processes like ion sputtering and impact vaporization can eject/release polar water molecules residing within lunar cold trapped regions with sufficient velocity to allow their redistribution to mid-latitudes. We consider the possibility that these polar-ejected molecules can be an additional (but not dominant) contribution to the water/OH veneer observed as a 3 micron absorption feature at mid-latitudes by Chandrayaan-I, Cassini, and EPOXI. Taking the conservative case that polar water is ejected only from the floor of polar craters with an 0.1 % icy regolith then overall source rates are near 10(exp 18) H20s/s. This outflow amounts to approx 10(exp -7) kg/s of water to be ejected from each pole and is a water source rate that is 10(exp .5 lower than the overall exospheric source rate for all species. Hence, the out-flowing polar water is a perturbation in the overall exosphere composition & dynamics. This polar water 'fountain' model may not fully account for the relatively high concentrations in the mid-latitude water veneer observed in the IR (approx 10-1000 ppm). However, it may account for some part of the veneer. We note that the polar water fountain source rates scale linearly with ice concentration, and larger mass fractions of polar crater water should provide correspondingly larger fractions of water emission out of the poles which then 'spills' on to mid-latitude surfaces.

Farrell, W. M.↗

NASA NTRS2012

Calibration of the Neutral Mass Spectrometer for the Lunar Atmosphere and Dust Environment Explorer

Science objectives of the LADEE Mission are to (1) determine the composition, and time variability of the tenuous lunar atmosphere and (2) to characterize the dust environment and its variability. These studies will extend the in-situ characterization of the environment that were carried out decades ago with the Apollo missions and a variety of ground based studies. The focused LADEE measurements will enable a more complete understanding of dust and gas sources and sinks. Sources of gas include UV photo-stimulated desorption, sputtering by plasma and micrometeorites, as well as thermal release of species such as argon from the cold service or venting from the lunar interior. Sinks include recondensation on the surface and escape through a variety of mechanisms. The LADEE science payload consists of an Ultraviolet Spectrometer, a Neutral Mass Spectrometer, and a Dust Detector. The LADEE orbit will include multiple passes at or below 50 km altitude and will target repeated sampling at the sunrise terminator where exospheric density will be highest for some thermally released species. The science mission will be implemented in approximately three months to allow measurements to be made over a period of one or more lunations In addition to the science mission NASA will use this mission to demonstrate optical communication technology away from low Earth orbit.

Mahaffy, P. R.↗

NASA NTRS1995

Gravity Waves Near 300 km Over the Polar Caps

Distinctive wave forms in the distributions of vertical velocity and temperature of both neutral particles and ions are frequently observed from Dynamics Explorer 2 at altitudes above 250 km over the polar caps. These are interpreted as being due to internal gravity waves propagating in the neutral atmosphere. The disturbances characterized by vertical velocity perturbations of the order of 100 m/s and horizontal wave lengths along the satellite path of about 500 km. They often extend across the entire polar cap. The associated temperature perturbations indicate that the horizontal phase progression is from the nightside to the dayside. Vertical displacements are inferred to be of the order of 10 km and the periods to be of the order of 10(exp 3) s. The waves must propagate in the neutral atmosphere, but they usually are most clearly recognizable in the observations of ion vertical velocity and ion temperature. By combining the neutral pressure calculated from the observed neutral concentration and temperature with the vertical component of the neutral velocity, an upward energy flux of the order of 0.04 erg/sq cm-s at 250 km has been calculated, which is about equal to the maximum total solar ultraviolet heat input above that altitude. Upward energy fluxes calculated from observations on orbital passes at altitudes from 250 to 560 km indicate relatively little attenuation with altitude.

Johnson, F. S.↗

NASA NTRS1994

Methanol and hydrogen sulfide in comet P/Halley

The Neutral Mass Spectrometer on the Giotto spacecraft measured the gas and ion composition in the coma of comet P/Halley. A detailed model of the ion chemistry inside the contact surface located at 4660 km is used to interpret the measured ion desnity profiles in the 32 to 35 amu/e mass range. The masses 33 and 35 amu/e are dominated by the protonated methanol and hydrogen sulfide ions CH3OH2(+) and H3S(+). Both profiles are essentially compatible with CH3OH and H2S originating from the nucleus only. The production rates relative to water are Y(CH3OH) = Q(CH3OH)/Q(H2O) = 1.7% and Y(H2S) = 0.41%. Our Y(CH3OH) agrees well with a determination from IR spectra obtained about 6 weeks after the Giotto encounter with P/Halley. In 7 other comets IR and microwave observations give Y(CH3OH) values between about 0.7 and 6%, indicating that the methanol abundance shows a strong variability from comet to comet. In three other comets Y(H2S) values between 0.2 and 0.5% have been reported. In addition to H2S(+), only ions containing minor isotopes of H, C, O and S contribute to mass 34 amu/e (e.g. (34)S(+), (13)CH3OH2(+), CH4DO(+)). These contributions can be calculated from the measured densities of the ions containing the major isotopes and the H2S(+) contribution from the measured H3S(+) density. From mass 34 amu/e we can also derive an upper limit of 1% for the abundance of deuterated methanol. This limit is at most marginally compatible with a direct interstellar origin of the CH3OH in P/Halley as the measured interstellar abundance of deuterated methanol is 1 to 6%.

Eberhardt, P.↗

NASA NTRS1994

Ammonia in comet P/Halley

In comet P/Halley the abundances of ammonia relative to water reported in the literature differ by about one order of magnitude from roughly 0.1% up to 2%. Different observational techniques seem to have inherent systematic errors. Using the ion mass channels m/q = 19 amu/e, 18 amu/e and 17 amu/e of the Neutral Mass Spectrometer experiment aboard the spacecraft Giotto, we derive a production rate of ammonia of (1.5(sub -0.7)(sup +0.5))% relative to water. Inside the contact surface we can explain our data by a nuclear source only. The uncertainty in our abundance of ammonia is primarily a result of uncertainties in some key reaction coefficients. We discuss in detail these reactions and the range of error indicated results from extreme assumptions in the rate coefficients. From our data, even in the worst case, we can exclude the ammonia abundance to be only of the order of a few per mill.

Meier, R.↗

NASA NTRS1993

Theory and operation of a three-gate time-of-flight velocity analyzer

Theoretical considerations and test results are presented for a new-type velocity analyzer for incident fast neutral particles, positive ions, and negative ions. Velocity analysis is carried out by means of a pulsed, three-gate time-of-flight (TOF) technique capable of eliminating alias velocities (harmonics) to sixth order. In addition the design and operation are presented of a four-element ion lens system, with small spherical and chromatic aberrations, suitable for interfacing a large-diameter ion beam from the TOF section with a subsequent mass analyzer.

Martus, K. E.↗

NASA NTRS1987

Comment on the papers 'On the influx of small comets into the earth's upper atmosphere. I - Observations and II - Interpretation'

The Generic Screening Object hypothesis of Frank et al. (1986) is refuted, and it is indicated that the DE-1 dark pixel events do not serve as evidence for a large cometary influx. It is shown that the average scan line count profiles of the dark pixel events show no significant reductions in the two neighboring pixel positions, although at least a 9 percent reduction would be expected if the dark pixels were the result of shielding of the photometric field of view. In a reply, Frank et al. uphold their previous hypothesis, pointing out that the suggestion that the atmospheric holes are not a geophysical phenomenon is incorrect due to: (1) the use of an overly simplified model for the instrument responses to small, opaque objects as seen against a statistically fluctuating screen of dayglow intensities; and (2) an insufficient treatment of the observations.

Cragin, B. L.↗

NASA NTRS1986

Angular and energy distribution of low energy cometary ions measured in the outer coma of Comet Halley

During the early phase of the Giotto encounter with comet Halley, at distances from the nucleus greater than 350,000 km, the neutral mass spectrometer was operated in a mode allowing the measurement of low energy ions. Data reveal two important features of the outer coma: the presence of a sharp discontinuity in the plasma flow at 550,000 km from the nucleus which results in a significant decrease of the plasma flow accompanied by an increase in temperature; and the detection of newly born ions identified as O(+) and CO(+), at distances from the comet greater than 800,000 km.

Berthelier, J. J.↗

NASA NTRS1986

Expansion velocity and temperatures of gas and ions measured in the coma of Comet Halley

In situ measurements of flow velocity and temperature in the inner coma were obtained from ram energy spectra of molecules and ions observed by the Giotto neutral mass spectrometer. Radial flow speed is 800 km/sec; near 2,500 km from the nucleus increasing to more than 1000 m/sec at 20,000 km. A near zero volt spacecraft potential was inferred. Ion temperatures are approx. 200 K inside the contact surface (approx. 4500 km) where the plasma is collisionally coupled to the gas; at this boundary ion temperature rises by approx. 1000 K. The observations indicate a heat source which is effective to large distances.

Laemmerzahl, P.↗

NASA NTRS1986

Evidence for HCS(+) and CH2SH(+) in the inner coma of Comet Halley

Number densities for ion species with masses 44, 45, and 47 amu/q were derived from ram energy spectra of the neutral gas mass spectrometer experiment on Giotto in the inner coma of comet Halley. The relative abundances of the masses suggest the presence of the ions CS(+), HCS(+), and CH2SH(+).

Krankowsky, D.↗

NASA NTRS1986

On the CO and N2 abundance in Comet Halley

The mass 28 amu/e signal observed in the neutral mode of the Giotto neutral gas mass spectrometer (NMS) is evaluated. At 1000 km from the nucleus number density n(CO)/ n(H2O) is less than or = 0.07. The production rate of CO as a parent molecule directly from the nucleus is thus less than 7% of the H2O production rate. However, CO is also produced from an extended source in the inner coma (r is less than 20,000 km) and at 20,000 km from the nucleus, for the total equivalent CO production rate 0.05 is less than or = Q(CO)/Q(H2O) is less than or = 0.15. For N2 an upper limit Q(N2)/Q(H2O) is less than or = 0.1 is derived. No parent molecule for the CO is identified in agreement with the NMS measurements. It is proposed that CO or a very short-lived parent is released in the coma from cometary dust grains, such as the CHON particles.

Eberhardt, P.↗

NASA NTRS1986

The D/H ratio in water from Halley

The neutral gas mass spectrometer on Giotto made neutral and ion composition measurements with a high mass resolution. Evaluation of the ion data within the contact surface gives a D/H ratio in water from Halley between 0.00006 and 0.00048. While this ratio is definitely not compatible with the D/H in molecular hydrogen of the protosolar nebula or the Jovian and Saturnian atmospheres, it is in the range observed for hydrogen in solar system objects which acquired their hydrogen as part of volatile molecules, e.g., as ices.

Eberhardt, P.↗

NASA NTRS1986

Measurements of thermal ion energy spectra from the Giotto encounter with Comet Halley

Ram ion measurements made by the Giotto neutral mass spectrometer show the spatial evolution of the composition and energy spectrum of coma ions. Near the nucleus the plasma is cold and has many constituents, but at distances of the order of 200,000 km the main ions are the terminal products of water dissociation, H(+) and O(+). The ion velocity distribution at the time of the Giotto encounter is highly anisotropic. Ion flow is generally away from the comet with energies increasing with distance to the order of 100 eV in the comet frame of reference. Similarities in the proton and oxygen ion energy spectra in the comet frame suggest an acceleration mechanism other than v x B pickup.

Hodges, R. R.↗

NASA NTRS1979

Pioneer Venus large probe neutral mass spectrometer

The Pioneer Venus large probe neutral mass spectrometer (LNMS) uses a single focusing magnetic sector field mass analyzer with mass range of 1-208 amu, resolution sufficient to separate the mercury isotopes, and sensitivity sufficient to detect minor constituents in the 1 ppm range relative to the CO2 in the Venus atmosphere. A combination of ion and chemical pumping is used to maintain a vacuum in the mass analyzer and to remove the atmosphere gases which enter the ion source chamber through a special leak. A microprocessor controls the operation of the instrument through a highly efficient peak stepping and data compression program, permitting acquisition of a complete mass spectrum roughly once each minute.

Hoffman, J. H.↗

NASA NTRS1972

Lunar orbital mass spectrometer experiment

The experiment to measure the composition and distribution of the ambient lunar atmosphere by mass spectrometer is reported. The lunar orbital mass spectrometer is described along with the calibration procedures for the instrument. Preliminary results indicate that a large number of gas molecules were observed in the vicinity of the spacecraft in lunar orbit. Many of these molecules are considered to be of spacecraft origin.

Hoffman, J. H.↗

NASA NTRS1972

Lunar orbital mass spectrometer experiment

A lunar orbital mass spectrometer carried by the Apollo 16 command and service module was used to detect the lunar atmosphere and to search for active lunar volcanism. The experimental procedure and results are described.

Hodges, R. R.↗