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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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At least 181 records · Page 10
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

Multi-fluid model of a sun-grazing comet in the rapidly ionizing, magnetized low corona

Two Sun-grazing comets were recently imaged in the low solar corona by space telescopes in unprecedented detail, revealing a wide range of new phenomena. This sparked growing interest in the interaction of comets with the coronal plasma and magnetic field and their diagnostic potential as solar probes. However, interpretation of such rich observational data requires profound understanding of relevant physical processes in an unexplored regime. Here advanced numerical modeling can provide critical clues. To this end, we present a prototype, multi-fluid, magnetohydrodynamic model of a steady-state comet in the low solar corona. These simulation results are compared with previously modeled comets in the solar wind environment. By inspecting their projection and column densities, we find a dominance of O{sup 6+} ions in the cometary tail, which can explain the observed extreme ultraviolet emission. The tail is found to be comparable to recent EUV images of these comets. In addition, the comet tail appears wider when the observer's line of sight is perpendicular rather than parallel to the local magnetic field. This is opposite to the trend in the interplanetary space permeated in the solar wind, because the ratio between dynamic pressure and magnetic pressure is an order of magnitude smaller than at 1 AU. On the other hand, we find that iron ions in the comet head build up to a density comparable to that of oxygen ions, but are unlikely to form a visible tail because of the shorter mean free paths of the neutrals.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

IMAGING OBSERVATIONS OF THE HYDROGEN COMA OF COMET 67P/CHURYUMOV–GERASIMENKO IN 2015 SEPTEMBER BY THE PROCYON/LAICA

The water production rate of a comet is one of the fundamental parameters necessary to understand cometary activity when a comet approaches the Sun within 2.5 au, because water is the most abundant icy material in the cometary nucleus. Wide-field imaging observations of the hydrogen Lyα emission in comet 67P/Churyumov–Gerasimenko were performed by the Lyman Alpha Imaging Camera (LAICA) on board the 50 kg class micro spacecraft, the Proximate Object Close Flyby with Optical Navigation (PROCYON), on UT 2015 September 7.40, 12.37, and 13.17 (corresponding to 25.31, 30.28, and 31.08 days after the perihelion passage of the comet, respectively). We derive the water production rates, Q{sub H{sub 2O}}, of the comet from Lyα images of the comet by using a 2D axi-symmetric Direct Simulation Monte-Carlo model of the atomic hydrogen coma; (1.46 ± 0.47) × 10{sup 28}, (1.24 ± 0.40) × 10{sup 28}, and (1.30 ± 0.42) × 10{sup 28} molecules s{sup −1} on 7.40, 12.37, and 13.17 September, respectively. These values are comparable to the values from in situ measurements by the Rosetta instruments in the 2015 apparition and the ground-based and space observations during the past apparitions. The comet did not show significant secular change in average water production rates just after the perihelion passage for the apparitions from 1982 to 2015. We emphasize that the measurements of absolute Q{sub H{sub 2O}} based on the wide field of view (e.g., by the LAICA/PROCYON) are so important to judge the soundness of the coma models used to infer Q{sub H{sub 2O}} based on in situ measurements by spacecraft, like the Rosetta.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2018

Finding Long Lost Lexell's Comet: The Fate of the First Discovered Near-Earth Object

Jupiter-family Comet D/1770 L1 (Lexell) was the first discovered Near-Earth Object (NEO) and passed the Earth on 1770 July 1 at a recorded distance of 0.015 au. The comet was subsequently lost due to unfavorable observing circumstances during its next apparition followed by a close encounter with Jupiter in 1779. Since then, the fate of D/Lexell has attracted interest from the scientific community, and now we revisit this long-standing question. We investigate the dynamical evolution of D/Lexell based on a set of orbits recalculated using the observations made by Charles Messier, the comet’s discoverer, and find that there is a 98% chance that D/Lexell remains in the solar system by the year of 2000. This finding remains valid even if a moderate non-gravitational effect is imposed. Messier’s observations also suggest that the comet is one of the largest known near-Earth comets, with a nucleus of ≳10 km in diameter. This implies that the comet should have been detected by contemporary NEO surveys regardless of its activity level if it has remained in the inner solar system. We identify asteroid 2010 JL{sub 33} as a possible descendant of D/Lexell, with a 0.8% probability of chance alignment, but a direct orbital linkage of the two bodies has not been successfully accomplished. We also use the recalculated orbit to investigate the meteors potentially originating from D/Lexell. While no associated meteors have been unambiguously detected, we show that meteor observations can be used to better constrain the orbit of D/Lexell despite the comet being long lost.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2019

C/2010 U3 (Boattini): A Bizarre Comet Active at Record Heliocentric Distance

We present a photometric and dynamical study of comet C/2010 U3 (Boattini), which was seen active in prediscovery data as early as 2005 November at a new inbound record heliocentric distance of r {sub H} = 25.8 au. Two outburst events around 2009 and 2017 were observed. The coma and tail of the comet consist of dust grains of ~10 μm in radius, ejected protractedly at speeds ≲50 m s{sup -1} near the subsolar point, and are subjected to the Lorentz force, solar gravitation, and radiation pressure force all together. The prolonged activity indicates that sublimation of supervolatiles (e.g., CO, CO{sub 2}) is at play, causing a net mass-loss rate ≳1 kg s{sup -1}. To sustain the mass loss, the nucleus radius has to be ≳0.1 km. The color of the cometary dust, similar to other long-period comets, is redder than the solar colors, but we also observed potential color variations when the comet was at 10 < r {sub H} < 15 au, concurrent with the onset of crystallization of amorphous water ice, if at all. Using publicly available and our refined astrometric measurements, we estimated the precise trajectory of the comet and propagated it backward to its previous perihelion. We found that the comet visited the planetary region 1.96 ± 0.04 Myr ago, with barycentric perihelion distance q = 8.364 ± 0.004 au. Thus, C/2010 U3 (Boattini) is almost certainly a dynamically old comet from the Oort cloud, and the observed activity cannot be caused by retained heat from the previous apparition.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2019

Orbital Deflection of Comets by Directed Energy

Cometary impacts pose a long-term hazard to life on Earth. Impact mitigation techniques have been studied extensively, but they tend to focus on asteroid diversion. Typical asteroid interdiction schemes involve spacecraft physically intercepting the target, a task feasible only for targets identified decades in advance and in a narrow range of orbits—criteria unlikely to be satisfied by a threatening comet. Comets, however, are naturally perturbed from purely gravitational trajectories via solar heating of their surfaces, which activates sublimation-driven jets. Artificial heating of a comet, such as by a laser, may supplement natural heating by the Sun to purposefully manipulate its path and thereby avoid an impact. Deflection effectiveness depends on the comet’s heating response, which varies dramatically depending on factors including nucleus size, orbit, and dynamical history. These factors are incorporated into a numerical orbital model to assess the effectiveness and feasibility of using high-powered laser arrays in Earth orbit and on the ground for comet deflection. Simulation results suggest that a diffraction-limited 500 m orbital or terrestrial laser array operating at 10 GW for 1% of each day over 1 yr is sufficient to fully avert the impact of a typical 500 m diameter comet with primary nongravitational parameter A {sub 1} = 2 × 10{sup −8} au day{sup −2}. Strategies to avoid comet fragmentation during deflection are also discussed.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2020

Lyα Observations of Comet C/2013 A1 (Siding Spring) Using MAVEN IUVS Echelle

The close approach of comet C/2013 A1 (Siding Spring) to Mars in 2014 October provided a unique opportunity to observe a dynamically new Oort cloud comet with potential for interaction with a planet’s atmosphere. The water-originating hydrogen coma of the comet extended to over 20 million km from the nucleus. Determining the properties of this coma contributes to characterizing the comet’s water content and production rate. The present study analyzes a unique data set of high spectral resolution UV observations of comet C/2013 A1 Siding Spring measured by the Mars Atmosphere and Volatile Evolution spacecraft. The Siding Spring observations capture Lyα emissions from the Martian corona, the interplanetary medium, as well as the cometary H and D reservoirs. The isolated cometary spectra are analyzed to reveal a velocity distribution of H atoms that are consistent with model estimates of H{sub 2}O photodissociated H emissions and of OH photodissociated H emissions, Doppler shifted from the main comet H emission line center by 18 km s{sup −1} and 8 km s{sup −1}, respectively. The variations in comet H brightness with distance from the nucleus are used to constrain cometary water production to a rate of 0.5 × 10{sup 28} molecules s{sup −1} at a time when Siding Spring was at 1.5 au, pre-perihelion.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS2012

Autonomous Onboard Science Data Analysis for Comet Missions

Coming years will bring several comet rendezvous missions. The Rosetta spacecraft arrives at Comet 67P/Churyumov-Gerasimenko in 2014. Subsequent rendezvous might include a mission such as the proposed Comet Hopper with multiple surface landings, as well as Comet Nucleus Sample Return (CNSR) and Coma Rendezvous and Sample Return (CRSR). These encounters will begin to shed light on a population that, despite several previous flybys, remains mysterious and poorly understood. Scientists still have little direct knowledge of interactions between the nucleus and coma, their variation across different comets or their evolution over time. Activity may change on short timescales so it is challenging to characterize with scripted data acquisition. Here we investigate automatic onboard image analysis that could act faster than round-trip light time to capture unexpected outbursts and plume activity. We describe one edge-based method for detect comet nuclei and plumes, and test the approach on an existing catalog of comet images. Finally, we quantify benefits to specific measurement objectives by simulating a basic plume monitoring campaign.

comets↗
NASA NTRS2012

Autonomous Navigation Performance During The Hartley 2 Comet Flyby

On November 4, 2010, the EPOXI spacecraft performed a 700-km flyby of the comet Hartley 2 as follow-on to the successful 2005 Deep Impact prime mission. EPOXI, an extended mission for the Deep Impact Flyby spacecraft, returned a wealth of visual and infrared data from Hartley 2, marking the fifth time that high-resolution images of a cometary nucleus have been captured by a spacecraft. The highest resolution science return, captured at closest approach to the comet nucleus, was enabled by use of an onboard autonomous navigation system called AutoNav. AutoNav estimates the comet-relative spacecraft trajectory using optical measurements from the Medium Resolution Imager (MRI) and provides this relative position information to the Attitude Determination and Control System (ADCS) for maintaining instrument pointing on the comet. For the EPOXI mission, AutoNav was tasked to enable continuous tracking of a smaller, more active Hartley 2, as compared to Tempel 1, through the full encounter while traveling at a higher velocity. To meet the mission goal of capturing the comet in all MRI science images, position knowledge accuracies of +/- 3.5 km (3-?) cross track and +/- 0.3 seconds (3-?) time of flight were required. A flight-code-in-the-loop Monte Carlo simulation assessed AutoNav's statistical performance under the Hartley 2 flyby dynamics and determined optimal configuration. The AutoNav performance at Hartley 2 was successful, capturing the comet in all of the MRI images. The maximum residual between observed and predicted comet locations was 20 MRI pixels, primarily influenced by the center of brightness offset from the center of mass in the observations and attitude knowledge errors. This paper discusses the Monte Carlo-based analysis that led to the final AutoNav configuration and a comparison of the predicted performance with the flyby performance.

flyby↗
NASA NTRS1973

Solar wind interaction with Comet Bennett /1969i/.

Examination of the relations between the solar-wind and Comet Bennett during the period from Mar. 23 to Apr. 5, 1970. A large kink was observed in the ion tail of the comet on April 4, but no solar-wind stream was observed in the ecliptic plane which could have caused the kink. Thus, either there was no correlation between the solar wind at the earth and that at Comet Bennett (which was 40 deg above the ecliptic) or the kink was caused by something other than a high-speed stream. The fine structure visible in photographs of the kink favors the second of these alternatives. It is shown that a shock probably passed through Comet Bennett on March 31, but no effect was seen in photographs of the comet. A stream preceded by another shock and a large abrupt change in momentum flux might have intercepted the comet between March 24 and March 28, but again no effect was seen in photographs of the Comet.

Burlaga, L. F.↗
NASA NTRS1974

Comets: Data, problems and objectives

Relevant results from the observations of Comet Kohoutek are presented with an outline summary of basic knowledge concerning comets, both subjects being confined to data related to the nature and origin of comets rather than the phenomena (for example, plasma phenomena are omitted). Places of cometary origin in the developing solar system are discussed, along with the proto-Uranus-Neptune region versus the much more distant fragmented interstellar cloud region, now frequented by comets of the Opik-Oort cloud. The Comet Kohoutek results add new insights, particularly with regard to the parent molecules and the nature of meteoric solids in comets, to restrict the range of the physical circumstances of comet formation.

Whipple, F. L.↗
NASA NTRS1974

Multiband photometry of Comets Kohoutek, Bennett, Bradfield, and Encke

Observations of Comets Kohoutek (1973f), Bradfield (1974b), and P/Encke have been made at a number of wavelengths between 0.55 and 18 microns. The silicate feature first observed in Comet Bennett (1969i) seems to be a common characteristic of cometary material. The comas of these comets radiate infrared with an effective temperature higher than the black-body temperature at the given distance from the sun. The albedo of the dust particles is between 0.10 and 0.20. The particles in the coma and tail are small, but the particles in the anti-tail of Comet Kohoutek must be larger than about 10 microns diameter. The observations give an absolute upper limit to the diameter of Comet Kohoutek of 30 km. A consistent interpretation would indicate that Comets Kohoutek and Bradfield have nuclear diameters of 5 to 10 km, that Bennett was several times larger, and that P/Encke is 10 times smaller. The peculiar behavior of Bradfield showed that the coma of a single comet can abruptly change its dust composition.

Ney, E. P.↗
NASA NTRS1979

Gas and dust in some recent periodic comets

Results are presented for narrow-band photometry of four recent periodic comets. The observed fluxes are used to estimate carbon radical production rates, gas-to-dust ratios, and the colors of the dust grains. It is found that in most respects the periodic comets exhibit the same range of properties typical of parabolic comets. Comet P/Chernykh is one of the dustiest comets on record, and its dust is physically different from that of the other comets considered. A correlation between the gas-to-dust ratio and the color of the grains is tentatively proposed. The need for homogeneous data on a large number of comets is emphasized.

Ahearn, M. F.↗
NASA NTRS1979

Empirical data on the origin of 'new' comets

The mass distribution of a group of 62 comets from the sample of Marsden et al (1978) was determined by assuming that the size of a comet is in proportion to the surface area of its vaporizing nucleus. A bimodal distribution was found, with a gap between magnitudes 8.1 and 9. The 50 brighter comets (magnitudes 1 to 8) have been tentatively identified as true 'new' comets; their size distribution cannot be fit to a constant slope on a log-log graph. These comets were apparently formed by a mechanism which differs from the fragmentation process. The 12 fainter may be fragments of comets which split or dimmed during their first passages. Results indicate that new comets could be identified with pristine planetesimals; they decay rapidly, probably by splitting when they enter the inner solar system.

Delsemme, A. H.↗
NASA NTRS1980

Origin and chemistry of comets

Long-period and short-period comets are all derived from a steady flux of 'new' comets coming straight from the margin of a sphere whose radius is 50,000 AU (the Oort's cloud). No empirical evidence requires that the Oort's cloud be older than a few million years, but all satisfactory theories have so far linked its formation to the origin of the solar system. 'New' comets have a size distribution consistent with the accretion of planetesimals, in contrast with older comets that are consistent with a fragmentation distribution. The H, C, N, O elemental ratios to Si suggest the comets are more primitive than the most primitive meteorites, namely the C I chondrites. Their chemistry is poorly known, but observational data suggest a protosolar ratio of C/O at least as large as 0.66, and a drastic depletion of hydrogen in the solar nebula. This could for instance imply that the T Tauri phase of the sun happened prior to the condensation of comets. An alternate explanation is possible: the interstellar grains were relatively unprocessed during their contraction in the solar nebula, and their mantles, barely modified, would have followed suit in the accretion of comets.

Delsemme, A. H.↗
NASA NTRS1981

Comets

The nature and origin of comets is discussed. Observations of the principal parts of comets, the dust and plasma tails, the hydrogen cloud, the coma and the cometary nucleus, are presented, and the icy conglomerate model of Whipple as extended by Delsemme accounting for the observed properties and their variation with heliocentric distance is examined. The origin of comets is considered in relation to the orbital statistics of the long-period comets and the existence of the Oort cloud on the edge of the solar system, and possible roles for comets in the solar nebula and the evolution of the solar system are indicated. Particular attention is then given to the discovery and properties of Halley's Comet, which is expected to reach perihelion in 1986, and to possible flyby and rendezvous missions to Halley's Comet and others.

Brandt, J. C.↗
NASA NTRS1981

Observations of faint comets at McDonald Observatory: 1978-1980

Modern observational techniques, developed for spectroscopy and photometry of faint galaxies and quasars, successfully applied to faint comets on the 2.7 m telescope. The periodic comets Van Biesbrock, Ashbrook-Jackson, Schwassmann-Wachmann 1, Tempel 2, Encke, Forbes, Brooks 2, Stephan-Oterma and the new comets Bradfield (19791), Bowell (1980b), Chernis-Petrauskas (1980k) were observed. The comets ranged in magnitude from 10th to 20th magnitude. For comets fainter than 19th magnitude, reflectance spectra at 100A resolution and area photometry were obtained. On comets of 17th or 18th magnitude, spectrometric scans (6A resolution) of the nucleus or inner coma region. On those comets which are brighter than 16th magnitude spatial spectrophotometric (6A resolution) studies of the inner and extended comae were done. An extensive spatial study of the comae of P/Encke and P/Stephen-Oterma, correlated with heliocentric distance is taking place. The observing process used is described and examples of the results obtained to date are discussed.

Barker, E. S.↗
NASA NTRS1984

Disappearance and disintegration of comets

The present investigation has the objective to provide a summary of the existing evidence on the disappearance of comets and to draw conclusions regarding the physical processes involved in the disappearance. Information concerning the classification of evidence and the causes of apparent disappearance of comets is presented in a table. Attention is given to the dissipating comets, the headless sungrazing comet 1887 I, and the physical behavior of the dissipating comets and the related phenomena. It is found that all comets confined to the planetary region of the solar system decay on astronomically short time scales. However, only some of them appear to perish catastrophically. Some of the observed phenomena could be successfully interpreted. But little insight has been obtained into the character of the processes which the dissipating comets experience.

Sekanina, Z.↗
NASA NTRS1985

A preliminary analysis of a comet coma sample-return mission

It is pointed out that the exploration and characterization of comets can play a fundamental role in solar system science. Comets are probably primitive products of processes which occurred in the preplanetary solar nebula. The impending return of Halley's comet in 1986 provides an opportunity for a study of comets, and five spacecraft will be launched to intercept Halley. These spacecraft include two sent by the USSR, two by Japan, and one by the European Space Agency (ESA). NASA has recommended two programs for the exploration and characterization of comets. The first mission considered would be a rendezvous with a short-period comet, while the second mission would provide for the return to earth of a sample of the volatile and nonvolatile constituents of the coma of a comet. Developments related to these plans are discussed.

Swenson, B. L.↗