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Kennedy, B. M.

Publications and source records attributed to Kennedy, B. M..

18 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Atmospheric 81 Kr as an Integrator of Cosmic-Ray Flux on the Hundred-Thousand-Year Time Scale

The atmospheric abundance of 81 Kr is a global integrator of cosmic rays. It is insensitive to climate shifts, geographical variations, and short-term solar cycle activity, making it an ideal standard to test models of cosmic-ray flux on the time scale of 10 5 years. In this work, we present the first calculation of absolute 81 Kr production rates in the atmosphere, and a measurement of the atmospheric 81 Kr/Kr abundance via the Atom Trap Trace Analysis method. The measurement result significantly deviates from previously reported values. The agreement between measurement and model prediction supports the current understanding of the production mechanisms. Additionally, the calculated 81 Kr atmospheric inventory over the past 1.5 Myr provides a more accurate input function for radiokrypton dating.

58 GEOSCIENCES↗
NASA NTRS2013

Dawn Maneuver Design Performance at Vesta

The Dawn spacecraft orbited the asteroid Vesta from July 16, 2011 to September 5, 2012, successfully accomplishing the four planned science orbits and two planned rotational characterization orbits. The lowest-altitude science orbit lasted four months, with 20 planned orbit maintenance maneuvers. Navigation results from Vesta demonstrate that the navigation plan was sufficient to achieve orbit delivery accuracy requirements. This paper compares the flown Dawn trajectory against the planned trajectory and expected maneuver dispersions. Understanding the effectiveness of the Vesta maneuver design plan is a key component of planning for operations at Ceres, the next destination for the Dawn mission.

Ion Propulsion↗
NASA NTRS2003

Radiometric orbit determination activities in support of navigating Deep Space 1 to Comet Borrelly

After several months of planning, development and testing, new software was uploaded that allowed Deep Space 1 to restore celestial inertia reference and begin thrusting towards an encounter with comet Borrelly. The new mission plan would have to work within the constraints of the new software as well as minimize use of the dwindling supply of hydrazine, the fuel needed to maintain the spacecraft attitude.

DS1↗
NASA NTRS1997

Navigation for the new millennium: Autonomous navigation for Deep Space 1

The autonomous optical navigation system technology for the Deep Space 1 (DS1) mission is reported on. The DS1 navigation system will be the first to use autonomous navigation in deep space. The systems tasks are to: perform interplanetary cruise orbit determination using images of distant asteroids; control and maintain the orbit of the spacecraft with an ion propulsion system and conventional thrusters, and perform late knowledge updates of target position during close flybys in order to facilitate high quality data return from asteroid MaAuliffe and comet West-Kohoutek-Ikemura. To accomplish these tasks, the following functions are required: picture planning; image processing; dynamical modeling and integration; planetary ephemeris and star catalog handling; orbit determination; data filtering and estimation; maneuver estimation, and spacecraft ephemeris updating. These systems and functions are described and preliminary performance data are presented.

Reidel, J. E.↗
NASA NTRS1989

The early solar system abundance of Pu-244 as inferred from the St. Severin chondrite

The isotopic composition of Xe released in stepwise heating of neutron-irradiated samples of the St. Severin chondrite was measured. This analysis shows that at the time of formation of most chondritic meteorites, approximately 4.56 x 10 to the 9th yr ago, the atomic ratio of Pu-244/U-238 was 0.0068 + or - 0.0010 in chondritic meteorites. This value is believed to be more reliable that inferred from earlier analyses of St. Severin and is the best estimate for the early solar system abundance of Pu-244. The comparison of Xe compositions between irradiated and unirradiated samples shows that the composition of trapped (ambient) Xe in St. Severin has a significantly lower value of Xe-136/Xe-130 than average carbonaceous chondrites.

Hudson, G. B.↗
NASA NTRS1988

I-129/I-127 variations among enstatite chondrites

The iodine isotopic compositions and corresponding I-Xe ages were obtained for eight enstatite chondrites, bringing the total number of enstatite chondrites examined by the I-Xe technique to 11. Iodine isotopic compositions of these 11 chondrites indicate a well-defined hiatus correlated with the hiatus in chemical composition defining the two distinct enstatite groups: EH(4,5) and EL(6). Judging by the I-129/I-127 ratios and assuming that both the EH and EL groups originated from a reservoir with a uniform initial iodine isotopic composition, the EH(4,5) chondrites were estimated to be about 4 mln older than the El chondrites.

Kennedy, B. M.↗
NASA NTRS1982

Shock disturbance of the I-Xe system

Three separate samples of the meteorite Bjurbole were artificially shocked at pressures of 70 kb, 200 kb, and 400 kb. Analysis of xenon released in stepwise heating shows that the I-Xe system of the 400 kb sample is substantially altered by the shock loading, and it is no longer possible to infer an age or trapped xenon composition for that sample. The 200-kb and 70-kb samples display isotopic structures progressively less altered demonstrating the gradations in shock disturbance likely to be found in natural systems. Interpretations of the I-Xe and Ar-40-Ar-39 systems for several naturally shocked meteorites are also presented. New data for Arapahoe do not confirm the previously reported age and trapped xenon composition, demonstrating instead that its I-Xe structure has been strongly disturbed by shock.

Caffee, M. W.↗
NASA NTRS1981

Xenon spallation systematics in Angra dos Reis

Literature Xe data for the Angra dos Reis meteorite have been resolved into constituent spallation, fission and trapped components. The spallation Xe compositions vary over a range wider than observed in any other samples, including lunar samples. These variations are due to the mixing of spallation Xe from Ba and rare earth element targets. It is possible to infer the Ba and rare earth spallation Xe compositions. Angra dos Reis spallation Xe compositions are systematically different from those observed in lunar samples, possibly because of differences in the irradiation conditions (geometry and shielding). Thus the Angra dos Reis data appear to be superior to lunar data for predicting spallation Xe compositions in other meteorites.

Hohenberg, C. M.↗
NASA NTRS1981

I-Xe dating - Intercomparisons of neutron irradiations and reproducibility of the Bjurbole standard

Previously unpublished data from the stepwise heating analysis of the Valecitos 1 (V1) Bjurbole standard are presented together with the results of an isotopic dilution measurement of a KI monitor included in the V1 irradiation package. The results of these analyses are compared with the values observed in other irradiations. Apparent I-Xe 'formation' ages are reproducible for three different samples of Bjurbole, suggesting isotopic homogeneity for initial iodine in the bulk material. The systematics of neutron capture in Xe-135 (produced from U-235 neutron fission) are examined and verified in irradiated BCR-1.

Hohenberg, C. M.↗
NASA NTRS1980

Fission xenon in troctolite 76535

Xenon released in stepwise heating of troctolite 76535 shows remarkable similarity to that from the gas-rich breccias. The low temperature fractions are highly enriched in fissiogenic xenon from Pu-244. Although the quantity of fission xenon is not inconsistent with reported age data, release patterns demonstrate that this component is not due to in situ decay. However, in contrast with the gas-rich breccias, the parentless radionuclear component in 76535 is not accompanied by solar wind xenon. Mechanisms for the emplacement of this component in 76535 cannot, therefore, involve exposure on the lunar surface but most likely involve adsorption along intergranular boundaries or thermally driven redistribution. The fact that xenon released from a gas-poor igneous cumulate can mimic that from gas-rich breccias, except for the presence of solar wind xenon in the latter, suggests that direct exposure to space is not required for the incorporation of these weakly bound, surface-correlated, parentless radionuclear components in lunar samples.

Hohenberg, C. M.↗
NASA NTRS1980

Noble gas component organization in 14301

A study is conducted of the organization of noble gases in the Apollo 14 breccia 14301, giving particular attention to xenon from the extinct radionuclides I-129 and Pu-244. An essential ingredient of this work is the intercomparison between noble gases released in single-stage melts of grain size fractions and those released in stepwise extractions. This approach resolves 'functional' noble gas components, which are organized by siting, into the specific genetic contributions on the basis of the 'activation energies' for diffusive loss. The procedure makes it possible to resolve various genetic components on the basis of the chemical and structural differences in their specific locations. Consequently, distinctions can be made among components acquired by different processes and among components which have undergone different regolith histories.

Bernatowicz, T. J.↗
NASA NTRS1978

Excess fission xenon at Apollo 16

Results are reported of stepwise heating analyses of Xe and Kr in four Apollo 16 low-grade breccias: 60019, 60225, 60275, and 67455. All four samples contain significant amounts of solar wind gases and at least three contain excess fission Xe. This observation indicates that the excess fission Xe effect is global rather than local, i.e., results from moon-wide redistribution of fission Xe rather than local redistribution in KREEP-rich Apollo 14 material. It is suggested that the excess fission Xe phenomenon represents a temporal variation in the amount of fission Xe available for redistribution, that it is prominent in samples from both highland sites simply because they are older than mare sites, and that samples from these sites had experienced at least some of their surface exposure at an early epoch.

Bernatowicz, T. J.↗
NASA NTRS1976

The excess fission xenon problem in lunar samples

This paper presents noble-gas (neon, krypton, and xenon) data obtained in stepwise heating of five lunar breccias: 14063, 14082, 15205, 15405, and 15445. These data are discussed in terms of spallation gas content, cosmic-ray exposure ages, and content of fission xenon. None of these samples is rich in solar-wind gases, and none contains more fission xenon than would be expected for in situ fission of U-238 and Pu-244. The essential features of the excess-fission-xenon phenomenon seen in several Apollo 14 samples are reviewed, as are models which might account for this phenomenon and criteria by which these models may be evaluated. The lack of excess fission xenon in 14063 and 14082 eliminates models relying on microscale xenon redistribution. A model is proposed in which the excess fission xenon originates in local material and percolates through and is adsorbed on regolith material.

Drozd, R. J.↗