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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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All sourcesNASA NTRSDOE OSTI
NASA NTRS2003

Sympathetic responses to Valsalva's manoeuvre following bed rest

The purpose of this study was to examine whether 14 days of head-down tilt bed rest (HDBR) alters autonomic regulation during Valsalva's manoeuvre (VM) and if this would predict blood pressure control during a 60 degrees head-up tilt (HUT) test. To examine autonomic control of blood pressure, we measured the changes in systolic (delta SBP) and diastolic (delta DBP) blood pressure between baseline and the early straining (Phase IIE) period of VM (20 sec straining to 40 mmHg; N = 7) in conjunction with changes in muscle sympathetic nerve activity (MSNA; microneurography) burst frequency (B/min) and total activity (% delta) from baseline over the 20-sec straining period. MSNA data were successfully recorded from 6 of the 7 individuals. The averaged responses from three repeated VMs performed in the supine position were compared between the pre- and post-HDBR tests. Compared with the pre-HDBR test, a greater reduction in SBP, DBP, and MAP was observed during Phase IIE following HDBR, p < 0.05. The increase in MSNA burst frequency during straining was augmented in the post- compared with the pre-HDBR test, p < 0.0001, as was the Phase IV blood pressure overshoot, p < 0.05. Although all subjects completed the 20-min pre-HDBR tilt test without evidence of hypotension or orthostatic intolerance, the post-HDBR test was stopped early in 5 of the 7 subjects due to systolic hypotension. The responses during the VM suggest that acute autonomic adjustments to rapid blood pressure changes are preserved after bed rest. Furthermore, MSNA and blood pressure responses during VM did not predict blood pressure control during orthostasis following HDBR.

NASA Discipline Musculoskeletal↗
NASA NTRS2005

Molecular Hydrogen Fluorescence in IC 63

This grant has supported the acquisition, reduction and analysis of data targeting the structure and excitation of molecular hydrogen in the reflection nebula IC 63 and in particular the fluorescent emission seen in the UV. In addition to manpower for analyzing the FUSE data, the grant supported the (attempted) acquisition of supporting ground-based data. We proposed for and received observing time for two sets of ground based, data; narrow band imaging ([S II], [O III) at KPNO (July 2002; Observer: Burgh) and imaging spectro-photometry of several of the near-infrared rotation-vibration lines of H2 at the IRTF (October 2003; Observer: Andersson). Unfortunately, both of these runs were failures, primarily because of bad weather, and did not result in any useful data. We combined the FUSE observations with rocket borne observations of the star responsible for exciting the H2 fluorescence in IC 63: gamma Cas, and with archival HUT observations of IC 63, covering the long-wavelength part of the molecular hydrogen fluorescence.

Andersson, B-G↗
NASA NTRS2007

Cardiac and Vascular Function in Bedrested Volunteers: Effects of Artificial Gravity Training

Cardiovascular effects of an artificial gravity (AG) countermeasure on deconditioned male volunteers were studied. In two groups of men we measured cardiovascular parameters at rest and in response to 30 minutes of 80 deg. head up tilt (HUT) before, at the end of, and four days following 21 days of 6 deg. head down bed rest (HDBR). One group (N=7) underwent no countermeasure while the other (N=8) received a daily, one hour, dose (2.5 gz at the foot, decreasing to 1.0 gz at the heart) of AG training on the Johnson Space Center short radius centrifuge. Cardiovascular parameters measured included heart rate, blood pressure, stroke volume, cardiac output, peripheral vascular resistance, plasma volume shifts, and vasoactive hormones. Untrained subjects exhibited shorter tilt survival (on average 8 minutes shorter) compared to trained subjects. By the end of bed rest, mean heart rate (MHR) was elevated in both groups (both supine and during tilt). In addition, treated subjects demonstrated lower, tilt-induced, increases in MHR four days following HDBR, indicating a more rapid return to pre bed rest conditions. Results from an index of autonomic balance (percentage of MHR spectral power in the respiratory frequency range) in control of heart rate are consistent with the interpretation that parasympathetic nervous system withdrawal was responsible for both tilt- and bed rest-induced increases in MHR. Our data support our pre-study hypothesis that AG treatment would lessen cardiovascular effects of deconditioning in bed rested men and suggest that AG should be further pursued as a space flight countermeasure.

Meng, M.↗
NASA NTRS2007

Extravehicular Activity (EVA) 101: Constellation EVA Systems

A viewgraph presentation on Extravehicular Activity (EVA) Systems is shown. The topics include: 1) Why do we need space suits? 2) Protection From the Environment; 3) Primary Life Support System (PLSS); 4) Thermal Control; 5) Communications; 6) Helmet and Extravehicular Visor Assy; 7) Hard Upper Torso (HUT) and Arm Assy; 8) Display and Controls Module (DCM); 9) Gloves; 10) Lower Torso Assembly (LTA); 11) What Size Do You Need?; 12) Boot and Sizing Insert; 13) Boot Heel Clip and Foot Restraint; 14) Advanced and Crew Escape Suit; 15) Nominal & Off-Nominal Landing; 16) Gemini Program (mid-1960s); 17) Apollo EVA on Service Module; 18) A Bold Vision for Space Exploration, Authorized by Congress; 19) EVA System Missions; 20) Configurations; 21) Reduced Gravity Program; and 22) Other Opportunities.

Jordan, Nicole C.↗
NASA NTRS2011

Global Precipitation Measurement, Validation, and Applications Integrated Hydrologic Validation to Improve Physical Precipitation Retrievals for GPM

Land surface modeling and data assimilation can provide dynamic land surface state variables necessary to support physical precipitation retrieval algorithms over land. It is well-known that surface emission, particularly over the range of frequencies to be included in the Global Precipitation Measurement Mission (GPM), is sensitive to land surface states, including soil properties, vegetation type and greenness, soil moisture, surface temperature, and snow cover, density, and grain size. In order to investigate the robustness of both the land surface model states and the microwave emissivity and forward radiative transfer models, we have undertaken a multi-site investigation as part of the NASA Precipitation Measurement Missions (PMM) Land Surface Characterization Working Group. Specifically, we will demonstrate the performance of the Land Information System (LIS; http://lis.gsfc.nasa.gov; Peters-Lidard et aI., 2007; Kumar et al., 2006) coupled to the Joint Center for Satellite Data Assimilation (JCSDA's) Community Radiative Transfer Model (CRTM; Weng, 2007; van Deist, 2009). The land surface is characterized by complex physical/chemical constituents and creates temporally and spatially heterogeneous surface properties in response to microwave radiation scattering. The uncertainties in surface microwave emission (both surface radiative temperature and emissivity) and very low polarization ratio are linked to difficulties in rainfall detection using low-frequency passive microwave sensors (e.g.,Kummerow et al. 2001). Therefore, addressing these issues is of utmost importance for the GPM mission. There are many approaches to parameterizing land surface emission and radiative transfer, some of which have been customized for snow (e.g., the Helsinki University of Technology or HUT radiative transfer model;) and soil moisture (e.g., the Land Surface Microwave Emission Model or LSMEM).

Peters-Lidar, Christa D.↗
NASA NTRS2005

Comparison of Two Methods for Noninvasive Determination of Stroke Volume During Orthostatic Challenge

Background: The real time, beat-by-beat, non-invasive determination of stroke volume (SV) is an important parameter in many aerospace related physiologic protocols. In this study, we compared simultaneous estimates of SV calculated from peripheral pulse waveforms with a more conventional non-invasive technique. Methods: Using a prospective, randomized blinded protocol, ten males and nine females completed 12-mm tilt table protocols. The relative change (%(Delta)) in beat-to-beat SV was estimated non-invasively from changes in pulse waveforms measured by application of infrared finger photoplethysmography (IFP) with a Portapres(Registered TradeMark) blood pressure monitoring device and by thoracic impedance cardiography (TIC). The %(Delta) SV values were calculated from continuous SV measurements in the supine posture and over the first 10 s (T1), second 10 s (T2), and 3.5 minutes (T3) of 80deg head-up tilt (HUT). Results: The average %(Delta) SV measured by IFP at T1 (-11.7 +/- 3.7 %) was statistically less (P <0.02) than the average %(Delta) SV measured by TIC at T1 (-21.7 +/- 3.1 %), while the average %(Delta) SV measured by 1FF at T2 (-16.2 +/- 3.9 %) and T3 (-19.1 +/- 3.8 %) were not statistically distinguishable (P > or = 0.322) than the average %(Delta) SV measured by TIC at T2 (-21.8 +/- 2.5 %), and T3 (-22.6 +/- 2.9 %). Correlation coefficients (r(sup 2)) between IFP and TIC were 0.117 (T1), 0.387 (T2), and 0.7 18 (T3). Conclusion: IFP provides beat-to-beat (real time) assessment of %(Delta) SV after 20 sec of transition to an orthostatic challenge that is comparable to the commonly accepted TIC. Our data support the notion that IFP technology which has flown during space missions can be used to accurately assess physiological status and countermeasure effectiveness for orth static problems that may arise in astronauts after space flight. While the peripherally measured IFP response is slightly delayed, the ease of implementing this monitor in the field is advantageous.

Doerr, Donald F.↗
NASA NTRS2016

Z-2 Architecture Description and Requirements Verification Results

The Z-2 Prototype Planetary Extravehicular Space Suit Assembly is a continuation of NASA's Z series of spacesuits. The Z-2 is another step in NASA's technology development roadmap leading to human exploration of the Martian surface. The suit was designed for maximum mobility at 8.3 psid, reduced mass, and to have high fidelity life support interfaces. As Z-2 will be man-tested at full vacuum in NASA JSC's Chamber B, it was manufactured as Class II, making it the most flight-like planetary walking suit produced to date. The Z-2 suit architecture is an evolution of previous EVA suits, namely the ISS EMU, Mark III, Rear Entry I-Suit and Z-1 spacesuits. The suit is a hybrid hard and soft multi-bearing, rear entry spacesuit. The hard upper torso (HUT) is an all-composite structure and includes a 2-bearing rolling convolute shoulder with Vernier sizing mechanism, removable suit port interface plate (SIP), elliptical hemispherical helmet and self-don/doff shoulder harness. The hatch is a hybrid aluminum and composite construction with Apollo style gas connectors, custom water pass-thru, removable hatch cage and interfaces to primary and auxiliary life support feed water bags. The suit includes Z-1 style lower arms with cam brackets for Vernier sizing and government furnished equipment (GFE) Phase VI gloves. The lower torso includes a telescopic waist sizing system, waist bearing, rolling convolute waist joint, hard brief, 2 bearing soft hip thigh, Z-1 style legs with ISS EMU style cam brackets for sizing, and conformal walking boots with ankle bearings. The Z-2 Requirements Verification Plan includes the verification of more than 200 individual requirements. The verification methods include test, analysis, inspection, demonstration or a combination of methods. Examples of unmanned requirements include suit leakage, proof pressure testing, operational life, mass, isometric man-loads, sizing adjustment ranges, internal and external interfaces such as in-suit drink bag, partial pressure relief valve, purge valve, donning stand and ISS Body Restraint Tether (BRT). Examples of manned requirements include verification of anthropometric range, suit self-don/doff, secondary suit exit method, donning stand self-ingress/egress and manned mobility covering eight functional tasks. The eight functional tasks include kneeling with object pick-up, standing toe touch, cross-body reach, walking, reach to the SIP and helmet visor. This paper will provide an overview of the Z-2 design. Z-2 requirements verification testing was performed with NASA at the ILC Houston test facility. This paper will also discuss pre-delivery manned and unmanned test results as well as analysis performed in support of requirements verification.

Graziosi, Dave↗
NASA NTRS2016

Human Health Countermeasures - Partial-Gravity Analogs Workshop

The experimental conditions that were deemed the most interesting by the HHC Element lead scientists are those permitting studies of the long-term effects of exposure to (a) chronic rotation when supine or in head down tilt (ground-based); and (b) long-radius centrifugation (space based). It is interesting to note that chronic ground based slow rotation room studies have not been performed since the 1960's, when the USA and USSR were investigating the potential use of AG for long-duration space missions. On the other hand, the other partial gravity analogs, i.e., parabolic flight, HUT, suspension, and short-radius centrifugation, have been regularly used in the last three decades (see review in Clément et al. 2015). Based on the workshop evaluations and the scores by the HHC scientific disciplines indicated in tables 3 and 4, simulation of partial G between 0 and 1 should be prioritized as follows: Priority 1. Chronic space-based partial-G analogs: a. Chronic space-based long-radius centrifugation. The ideal scenario would be chronic long-radius centrifugation of cells, animals and humans in a translational research approach - ideally beyond low earth orbit under deep space environmental effects and at various rotations - to obtain different G-effects. In this scenario, all physiological systems could be evaluated and the relationship between physiological response and G level established. This would be the most integrative way of defining, for the first time ever, G-thresholds for each physiological system. b. Chronic space-based centrifugation of animals. Chronic centrifugation of rodents at various G levels in space would allow for determination of AG thresholds of protection for each physiological system. In this case, all physiological systems will be of interest. Intermittent centrifugation will be of secondary interest. c. Chronic space-based centrifugation of cell cultures (RWV). Bioreactor studies of cells and cell cultures of various tissues at various G levels would allow for intracellular investigations of the effects of partial-G. Priority 2. Acute, intermittent space based partial-G analogs: a. Acute, intermittent space-based short radius human centrifugation. Intermittent centrifugation of humans would allow determination of thresholds of AG for protection of astronaut health in space. Priority 3. Chronic ground-based partial-G analogs: a. Chronic centrifugation of supine or head-down tilted humans. b. Chronic head-up tilt in humans. c. Chronic head-out graded dry immersion in humans. d. Chronic partial suspension of rodents e. Chronic rotating bioreactor cell culture studies (RWV) Priority 4. Acute ground based partial-G analogs. a. Parabolic flights. Very acute and short term effects of G levels between 0 and 1 in humans for fast responding systems like cardiovascular and sensorimotor as well as for acute responses in cell cultures and animals. b. Other acute models as indicated in table 3.

Barr, Yael↗
NASA NTRS2019

Evaluation of Brightness Temperature Sensitivity to Snowpack Physical Properties Using Coupled Snow Physics and Microwave Radiative Transfer Models

There are multiple existing microwave radiative transfer models (RTMs) to simulate the brightness temperature (Tb) of snowpacks. It is still challenging to have consistent Tb responses from RTMs due to individual physical formulations of the snowpack scattering process. This article examines three of the widely-used multi-layer RTMs: 1) the microwave emission model of layered snowpacks (MEMLS); 2) the dense media radiative transfer based on the quasi-crystalline approximation (QCA) of Mie scattering of densely packed sticky spheres (DMRTQMS); and 3) the Helsinki University of Technology (HUT) model. Interestingly, these models yield slightly different Tb responses when driven by the same physical snowpack properties. Tb variations, dependent on the choice of RTMs, are then evaluated to improve the understanding of model differences in microwave emission from a snowpack. We first perform a sensitivity study of the Tb predictions from the three RTMs as a function of snow grain sizes, densities, and depths. While Tb from all three RTMs decreases with increasing snow grain sizes, it is found that a scaling factor is required to have the same amount of Tb attenuation for small grain sizes within the Rayleigh scattering regime. For larger grain sizes, however, a scaling coefficient is not enough to match the model outputs due to the different scattering assumptions of the RTMs. For a single snow layer with increasing snow depths and densities, all three RTMs exhibit Tb attenuations arising from the increase in path lengths and optical depths. Further evaluations are conducted by feeding the three RTMs with the output of a snow physics model driven by in situ weather forcing in a coupled simulation. Outputs of this coupled model include snowpack physical properties and Tbs. By using snow stratigraphy observations, another set of Tb simulation is also conducted with RTMs driven by in situ snowpit observations. The snow physics outputs from the coupled case are compared against in situ snow stratigraphy observations.

Do Hyuk Kang↗
NASA NTRS2019

A new environment to simulate the dynamics in the close proximity of rubble-pile asteroids

This paper presents a new environment to simulate close-proximity dynamics around rubble-pile asteroids. The code provides methods for modeling the asteroid’s gravity field and surface through granular dynamics. It implements stateof-the-art techniques to model both gravity and contact interaction between particles: 1) mutual gravity as either direct N2 or Barnes-Hut GPU-parallel octree and 2) contact dynamics with a soft-body (force-based, smooth dynamics), hard-body (constraint-based, non-smooth dynamics), or hybrid (constraint-based with compliance and damping) approach. A very relevant feature of the code is its ability to handle complex-shaped rigid bodies and their full 6D motion. Examples of spacecraft close-proximity scenarios and their numerical simulations are shown.

Ferrari, Fabio↗
NASA NTRSDate not supplied

xEMU Padding Optimized for Body Mobility in Lunar EVAs

We developed a new design framework for padding in the xEMU hard upper torso (HUT) that will optimize the crewmembers’ mobility and comfort. The new padding was geometrically optimized for anthropometric body curvatures and motion dependent body deformations.

Yaritza Bernal↗
NASA NTRSDate not supplied

NASA Advanced Space Suit xEMU Development Report – Components

For the past several years, the Exploration Extra-Vehicular Mobility Unit (xEMU) team at NASA’s Johnson Space Center has focused on the development and detailed design of the xEMU to support missions to the International Space Station (ISS) and a moon landing in 2024. In that context, this paper examines the development and baseline detailed design of the xEMU helmet, extra-vehicular visor assembly (EVVA), hard upper torso (HUT), shoulders, liquid cooling and ventilation garment (LCVG), boots, waist brief hip (WBH), ancillary hardware, and environmental protection garment (EPG) . This paper will outline the challenging technical requirements, significant architectural trades, technical solutions required to overcome these challenges, and a current status of the detailed design. The preliminary results of Design Verification Testing (DVT) as it relates specifically to these components will also be provided, along with a forward strategy for final maturation into a flight-ready design.

Shane McFarland↗
NASA NTRS2023

Modeling and Simulation of the Angel Upper Limb Offload Device: Branching Into New Methods

BACKGROUND: The Active Response Gravity Offload System (ARGOS) provides an analog environment for extravehicular activity (EVA) testing and training. Discomfort has been observed during longer suited test sessions. While the subject’s core is offloaded during surface EVA evaluations, his/her arms experience full Earth gravity and can become overly fatigued, especially during suited tests which involve reaching and prolonged arm extensions. A device (ARGOS Negation of Gravitational Effects on the Limbs: ANGEL) to offload the weight of the arms and suit sleeves is being developed by JSC’s Flight Systems Branch of the Software, Robotics, and Simulation Division. Previously we have shared preliminary modeling of that device and kinematics based on motion capture data. Here we present an alternative approach to determine device kinematics by calculating ANGEL component angles with an OpenSim plugin. We compare calculated angles to inverse kinematics (IK) derived ones with the goal of validating the model. This new method can be further informative for device design and analytically testing different configurations to achieve desired reduced gravity conditions (e.g., lunar gravity (Lg) or Martian gravity (Mg)). We have compared calculated angles with IK-derived angles in tests with a shirt-sleeve subject positioned in a test stand with a Mark-III Hard Upper Torso (HUT) and Portable Life Support System (PLSS) mockup and arm weights to emulate the weight of the suit sleeve as well as a suited subject in ARGOS with a Mark-III suit. A variety of upper body tasks were completed in the former and full-body tasks in the latter. METHODS AND RESULTS: To model the offload device, we augment the OpenSim human model topology with the offload mechanism components and joints, using CAD models to represent the mechanism graphically. The joint angles of the device are calculated in the OpenSim plugin by modeling how the components configure themselves under the offloading spring tension given a particular IK-derived arm position. There are four ANGEL components with a total of 5 degrees of freedom (DOFs), each component has a single DOF except for the cuff which is modeled as 2 DOFs. The sickle/yaw bracket and cuff rotation angles are determined statically based on the assumptions that the sickle will track the attachment point of the cuff and that the cuff will rotate such that the attachment point is at its highest point. The cuff tilt, linker and V-bracket angles are then determined by optimizing their positions to approach a mechanical equilibrium. The calculated linker angle is compared to three different methods of determining the linker line-of-force kinematically (from V-bracket to center-cuff, cuff highest point or marker-derived position). Given the joint angles of the device, the spring force and resulting force on the arm is computed by the plugin and applied as an external load in inverse dynamics (ID) to enable study of overall shoulder joint torques as well as offload achieved. We verify the calculated joint angles by using the inverse kinematic data. The average difference in angles is the smallest for the V-bracket and linker, around 1 to 5 degrees for most trials. The resulting offload and shoulder torque are comparable between calculated and IK-derived angles. In summary, we have developed a method to calculate the joint angles of an exoskeleton-like upper limb offloading device currently in development. We have also developed a custom plugin which will be a valuable tool to optimize device configurations for a desired gravitational environment, probe the offload achieved for motions recorded outside of our test suite, and inform future design improvements.

L B Nilsson↗
NASA NTRS2023

Modeling and Simulation of The Angel Upper Limb Offload Device: Branching into New Methods

BACKGROUND: The Active Response Gravity Offload System (ARGOS) provides an analog environment for extravehicular activity (EVA) testing and training. Discomfort has been observed during longer suited test sessions. While the subject’s core is offloaded during surface EVA evaluations, his/her arms experience full Earth gravity and can become overly fatigued, especially during suited tests which involve reaching and prolonged arm extensions. A device (ARGOS Negation of Gravitational Effects on the Limbs: ANGEL) to offload the weight of the arms and suit sleeves is being developed by JSC’s Flight Systems Branch of the Software, Robotics, and Simulation Division. Previously we have shared preliminary modeling of that device and kinematics based on motion capture data. Here we present an alternative approach to determine device kinematics by calculating ANGEL component angles with an OpenSim plugin. We compare calculated angles to inverse kinematics (IK) derived ones with the goal of validating the model. This new method can be further informative for device design and analytically testing different configurations to achieve desired reduced gravity conditions (e.g., lunar gravity (Lg) or Martian gravity (Mg)). We have compared calculated angles with IK-derived angles in tests with a shirt-sleeve subject positioned in a test stand with a Mark-III Hard Upper Torso (HUT) and Portable Life Support System (PLSS) mockup and arm weights to emulate the weight of the suit sleeve as well as a suited subject in ARGOS with a Mark-III suit. A variety of upper body tasks were completed in the former and full-body tasks in the latter. METHODS AND RESULTS: To model the offload device, we augment the OpenSim human model topology with the offload mechanism components and joints, using CAD models to represent the mechanism graphically. The joint angles of the device are calculated in the OpenSim plugin by modeling how the components configure themselves under the offloading spring tension given a particular IK-derived arm position. There are four ANGEL components with a total of 5 degrees of freedom (DOFs), each component has a single DOF except for the cuff which is modeled as 2 DOFs. The sickle/yaw bracket and cuff rotation angles are determined statically based on the assumptions that the sickle will track the attachment point of the cuff and that the cuff will rotate such that the attachment point is at its highest point. The cuff tilt, linker and V-bracket angles are then determined by optimizing their positions to approach a mechanical equilibrium. The calculated linker angle is compared to three different methods of determining the linker line-of-force kinematically (from V-bracket to center-cuff, cuff highest point or marker-derived position). Given the joint angles of the device, the spring force and resulting force on the arm is computed by the plugin and applied as an external load in inverse dynamics (ID) to enable study of overall shoulder joint torques as well as offload achieved. We verify the calculated joint angles by using the inverse kinematic data. The average difference in angles is the smallest for the V-bracket and linker, around 1 to 5 degrees for most trials. The resulting offload and shoulder torque are comparable between calculated and IK-derived angles. In summary, we have developed a method to calculate the joint angles of an exoskeleton-like upper limb offloading device currently in development. We have also developed a custom plugin which will be a valuable tool to optimize device configurations for a desired gravitational environment, probe the offload achieved for motions recorded outside of our test suite, and inform future design improvements.

L B Nilsson↗
NASA NTRS1985

Maximum mass of objects that constitute unseen disk material

It is suggested that the mass of the individual unseen 'disk things' which furnish unseen mass satisfies an inequality that is based on the demonstrated existence of a large number of wide binary stars in the Galaxy whose projected separations are as large as 0.1 pc. Since these binaries are weakly bound, with orbital velocities of 0.3 km/sec and periods of the order of 1 million years, they can be disrupted by passing stars or other pointlike objects. This implies that the mass of the unseen disk objects, which provide about half the local mass density, must be less than 2 solar masses.

Bahcall, J. N.↗
NASA NTRS1987

Comet showers as a cause of mass extinction

Three independent pieces of evidence supporting a connection between comet showers and clustering in terrestrial cratering and mass extinctions are presented. The temporal profile of a comet shower triggered by a star passing through the Oort cloud is calculated. Four weak peaks are found in the age of distribution of impact craters over the past 100 Myr, as well as two compact clusters of ages of impact glass broadly coincident with crater-age peaks. Recent paleontological observations are reviewed that indicate a stepwise character for some well-documented mass extinctions in the past 100 Myr which roughly coincide with three of the four peaks in crater ages and which have a duration compatible with comet shower predictions.

Hut, Piet↗
NASA NTRS1988

On the evolution of globular cluster systems. I - Present characteristics and rate of destruction in our Galaxy

The current dynamic state and rate of destruction for the system of globular clusters in the Galaxy are modeled. It is found that gravitational shocks due to the central bulge of the Galaxy are very efficient in destroying clusters on highly radial orbits. This effect, extrapolated at the present unweighted destruction rate over 10 to the 10th yr, can account for the observed difference between the velocity distribution of globular clusters and other tracers of the halo population.

Aguilar, Luis↗
NASA NTRS1991

A post-core-collapse model for the nucleus of M33

A model for the post-collapse core of M33 is proposed. By analogy with globular clusters, the formation of tight binary systems powers the reexpansion of the core, and it is expected that low-mass X-ray binaries form at a rate about equal to that of all galactic globular clusters combined. About a dozen such binaries should be present, and their combined emission may explain the large and enigmatic unresolved X-ray emission from the nucleus of M33. In addition, tidal formation of cataclysmic binaries may lead to a nova rate of more than about one per century. Numerous blue stragglers have probably formed through mergers caused by stellar collisions, but their density is probably too low to explain the blue color of the nucleus. Young stars are the likely cause for the color, and their presence may complicate the simple dynamical picture presented here.

Hernquist, Lars↗