Engineering topics

Pokorny, Petr

Publications and source records attributed to Pokorny, Petr.

7 records

DOE OSTI2022

EarthShine: Observing our world as an exoplanet from the surface of the Moon

NASA’s return to the Moon coincides with explosive growth in exoplanet discovery. Missions are being formulated to search for habitable planets orbiting other stars, making this the ideal time to deploy an instrument suite to the lunar surface to help us recognize a habitable exoplanet when we see it. We present EarthShine, a technically mature, three-instrument suite to observe the whole Earth from the Moon as an exoplanet proxy. EarthShine data will validate and improve models critical for designing missions to image and characterize exoplanets, thus informing observing strategies for flagship missions to directly image exoplanets. EarthShine will answer interconnected questions in Earth and lunar science, exoplanets, and astrobiology, related to the credo “follow the water.” EarthShine can take advantage of current NASA programs to conduct science from the Moon with low-cost, mature space hardware to reduce risk and assure success. Like the 1968 Apollo Earthrise image of our home planet, lonely in the black sky, the appeal of EarthShine to a multidisciplinary array of researchers in Earth Science, Planetary Science, and astrophysics will maximize both its scientific impact and its impact on the general public.

79 ASTRONOMY AND ASTROPHYSICS↗

NASA NTRS2019

Micrometeoroid Events in LISA Pathfinder

The zodiacal dust complex, a population of dust and small particles that pervades the solar system, provides important insight into the formation and dynamics of planets, comets, asteroids, and other bodies. We present a new set of data obtained from direct measurements of momentum transfer to a spacecraft from individual particle impacts. This technique is made possible by the extreme precision of the instruments flown on the LISA Pathfinder spacecraft, a technology demonstrator for a future space-based gravitational wave observatory. Pathfinder employed a technique known as drag-free control that achieved rejection of external disturbances, including particle impacts, using a micropropulsion system. Using a simple model of the impacts and knowledge of the control system, we show that it is possible to detect impacts and measure properties such as the transferred momentum, direction of travel, and location of impact on the spacecraft. In this paper, we present the results of a systematic search for impacts during 4348 hr of Pathfinder data. We report a total of 54 candidates with transferred momenta ranging from 0.2 to 230 μNs. We furthermore make a comparison of these candidates with models of micrometeoroid populations in the inner solar system, including those resulting from Jupiter-family comets (JFCs), Oort Cloud comets, Halley-type comets, and asteroids. We find that our measured population is consistent with a population dominated by JFCs, with some evidence for a smaller contribution from Halley-type comets, in agreement with consensus models of the zodiacal dust complex in the momentum range sampled by LISA Pathfinder.

Thorpe, J. I.↗

NASA NTRS2018

Constraining the Ratio of Micrometeoroids from Short- and Long-Period Comets at 1 AU from LADEE Observations of the Lunar Dust Cloud

We interpret recent observations of the secondary dust ejecta cloud around the Moon from the Lunar Dust Experiment (LDEX) on board the NASA Lunar Atmosphere and Dust Environment Explorer (LADEE) spacecraft with help from dynamical models of meteoroids. Results suggest that in order to match the spatial structure of observed ejecta profiles, the flux of meteoroids on the Moon must be primarily provided by short-period comets with an excess ratio of at least 1.3:1 compared to long-period comets. This ratio increases significantly if the dependence of the ejecta yield on impactor velocity is stronger than generally believed. The model accounts for the orbital geometry of LADEE and shows no indication of a large asymmetry in the meteoroid flux impacting from the Helion and Anti-Helion directions.

Janches, Diego↗

NASA NTRS2017

Formation and Past Evolution of the Showers of 96P/Machholz Complex

In this work we model the dynamical evolution of meteoroid streams of comet 96P/Machholz, and the largest member of the Marsden sunskirters, comet P/1999 J6. We simultaneously fit the characteristics of eight meteor showers which have been proposed to be linked to the complex, using observations from a range of techniques - visual, video, TV and radar. The aim is to obtain a self-consistent scenario of past capture of a large comet into a short-period orbit, and its subsequent fragmentation history. Moreover, we also aim to constrain the dominant parent of these showers. The fit of our simulated shower characteristics to observations is consistent with the scenario of a capture of a proto-comet 96P/Machholz by Jupiter circa 20000 BCE (Before Christian Era), and a subsequent major breakup around 100-950 CE (Christian Era) which resulted in the formation of the Marsden group of comets. We find that the Marsden group of comets are not the immediate parents of the daytime Arietids and Northern and Southern delta-Aquariids, as previously suggested. In fact, the hypothesis that the Northern delta-Aquariids are related to the Marsden group of comets is not supported by this study. The bulk of the observational characteristics of all eight showers can be explained by meteoroid ejection primarily from comet 96P/Machholz between 10000 BCE and 20000 BCE. Assuming the Marsden group of comets originated between 100 CE-950 CE, we conclude that sunskirting comets contribute mainly to the meteoroid stream near the time of the peak of the daytime Arietids, Southern delta-Aquariids, kappa-Velids. Finally, we find that the meteor showers identified by Babadzhanov and Obrubov (1992) as the alpha-Cetids, the Ursids and Carinids correspond to the daytime lambda-Taurids, the November iota-Draconids or December alpha-Draconids and the theta-Carinids.

Abedin, Abedin↗

NASA NTRS2017

Reconciling the Dawn-Dusk Asymmetry in Mercury's Exosphere with the Micrometeoroid Impact Directionality

Combining dynamical models of dust from Jupiter-family comets and Halley-type comets, we demonstrate that the seasonal variation of the dust/meteoroid environment at Mercury is responsible for producing the dawn-dusk asymmetry in Mercury's exosphere observed by the MESSENGER spacecraft. Our latest models, calibrated recently from ground-based and space-borne measurements, provide unprecedented statistics that enable us to study the longitudinal and latitudinal distribution of meteoroids impacting Mercury's surface. We predict that the micrometeoroid impact vaporization source is expected to undergo significant motion on Mercury's surface toward the nightside during Mercury's approach to aphelion and toward the dayside when the planet is approaching the Sun.

Mercury's exosphere - meteoroids-comets-dynamical ↗

NASA NTRS2012

Dynamics of Dust Particles Released from Oort Cloud Comets and Their Contribution to Radar Meteors

The Oort Cloud Comets (OCCs), exemplified by the Great Comet of 1997 (Hale-Bopp), are occasional visitors from the heatless periphery of the solar system. Previous works hypothesized that a great majority of OCCs must physically disrupt after one or two passages through the inner solar system, where strong thermal gradients can cause phase transitions or volatile pressure buildup. Here we study the fate of small debris particles produced by OCC disruptions to determine whether the imprints of a hypothetical population of OCC meteoroids can be found in the existing meteor radar data. We find that OCC particles with diameters D < or approx. 10 microns are blown out from the solar system by radiation pressure, while those with D > or approx. 1 mm have a very low Earth-impact probability. The intermediate particle sizes, D approx. 100 microns represent a sweet spot. About 1% of these particles orbitally evolve by Poynting-Robertson drag to reach orbits with semimajor axis a approx. 1 AU. They are expected to produce meteors with radiants near the apex of the Earth s orbital motion. We find that the model distributions of their impact speeds and orbits provide a good match to radar observations of apex meteors, except for the eccentricity distribution, which is more skewed toward e approx. 1 in our model. Finally, we propose an explanation for the long-standing problem in meteor science related to the relative strength of apex and helion/antihelion sources. As we show in detail, the observed trend, with the apex meteors being more prominent in observations of highly sensitive radars, can be related to orbital dynamics of particles released on the long-period orbits.

Nesvorny, David↗

NASA NTRS2011

Dynamical Model for the Zodiacal Cloud and Sporadic Meteors

The solar system is dusty, and would become dustier over time as asteroids collide and comets disintegrate, except that small debris particles in interplanetary space do not last long. They can be ejected from the solar system by Jupiter, thermally destroyed near the Sun, or physically disrupted by collisions. Also, some are swept by the Earth (and other planets), producing meteors. Here we develop a dynamical model for the solar system meteoroids and use it to explain meteor radar observations. We find that the Jupiter Family Comets (JFCs) are the main source of the prominent concentrations of meteors arriving to the Earth from the helion and antihelion directions. To match the radiant and orbit distributions, as measured by the Canadian Meteor Orbit Radar (CMOR) and Advanced Meteor Orbit Radar (AMOR), our model implies that comets, and JFCs in particular, must frequently disintegrate when reaching orbits with low perihelion distance. Also, the collisional lifetimes of millimeter particles may be longer (approx. > 10(exp 5) yr at 1 AU) than postulated in the standard collisional models (approx 10(exp 4) yr at 1 AU), perhaps because these chondrule-sized meteoroids are stronger than thought before. Using observations of the Infrared Astronomical Satellite (IRAS) to calibrate the model, we find that the total cross section and mass of small meteoroids in the inner solar system are (1.7-3.5) 10(exp 11) sq km and approx. 4 10(exp 19) g, respectively, in a good agreement with previous studies. The mass input required to keep the Zodiacal Cloud (ZC) in a steady state is estimated to be approx. 10(exp 4)-10(exp 5) kg/s. The input is up to approx 10 times larger than found previously, mainly because particles released closer to the Sun have shorter collisional lifetimes, and need to be supplied at a faster rate. The total mass accreted by the Earth in particles between diameters D = 5 micron and 1 cm is found to be approx 15,000 tons/yr (factor of 2 uncertainty), which is a large share of the accretion flux measured by the Long Term Duration Facility (LDEF). Majority of JFC particles plunge into the upper atmosphere at <15 km/s speeds, should survive the atmospheric entry, and can produce micrometeorite falls. This could explain the compositional similarity of samples collected in the Antarctic ice and stratosphere, and those brought from comet Wild 2 by the Stardust spacecraft. Meteor radars such as CMOR and AMOR see only a fraction of the accretion flux (approx 1- 10% and approx 10-50%, respectively), because small particles impacting at low speeds produce ionization levels that are below these radars detection capabilities.

Nesvorny, David↗