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All sourcesNASA NTRSDOE OSTI
NASA NTRS2011

Interpreting Methanol v(sub 2)-Band Emission in Comets Using Empirical Fluorescence g-Factors

For many years we have been developing the ability, through high-resolution spectroscopy targeting ro-vibrational emission in the approximately 3 - 5 micrometer region, to quantify a suite of (approximately 10) parent volatiles in comets using quantum mechanical fluorescence models. Our efforts are ongoing and our latest includes methanol (CH3OH). This is unique among traditionally targeted species in having lacked sufficiently robust models for its symmetric (v(sub 3) band) and asymmetric (v(sub 2) and v(sub 9) bands) C-H3 stretching modes, required to provide accurate predicted intensities for individual spectral lines and hence rotational temperatures and production rates. This has provided the driver for undertaking a detailed empirical study of line intensities, and has led to substantial progress regarding our ability to interpret CH3OH in comets. The present study concentrates on the spectral region from approximately 2970 - 3010 per centimeter (3.367 - 3.322 micrometer), which is dominated by emission in the (v(sub 7) band of C2H6 and the v(sub 2) band of CH3OH, with minor contributions from CH3OH (v(sub 9) band), CH4 (v(sub 3)), and OH prompt emissions (v(sub 1) and v(sub 2)- v(sub 1)). Based on laboratory jet-cooled spectra (at a rotational temperature near 20 K)[1], we incorporated approximately 100 lines of the CH3OH v(sub 2) band, having known frequencies and lower state rotational energies, into our model. Line intensities were determined through comparison with several comets we observed with NIRSPEC at Keck 2, after removal of continuum and additional molecular emissions and correcting for atmospheric extinction. In addition to the above spectral region, NIRSPEC allows simultaneous sampling of the CH3OH v(sub 3) band (centered at 2844 per centimeter, or 3.516 micrometers and several hot bands of H2O in the approximately 2.85 - 2.9 micrometer region, at a nominal spectral resolving power of approximately 25,000 [2]. Empirical g-factors for v(sub 2) lines were based on the production rate as determined from the v(sub 3) Q-branch intensity; application to comets spanning a range of rotational temperatures (approximately 50 - 90 K) will be reported. This work represents an extension of that presented for comet 21P/Giacobini-Zinner at the 2010 Division for Planetary Sciences meeting [3]. Our empirical study also allows for quantifying CH3OH in comets using IR spectrometers for which the v(sub 3) and v(sub 2) bands are not sampled simultaneously, for example CSHELL/NASA IRTF or CRIRES/VLT.

DiSanti, Michael↗
NASA NTRS2012

Radio Observations of Organics in Comets

A major observational challenge in cometary science is to quantify the extent to which chemical compounds can be linked to either interstellar or nebular chemistry. Recently, there have been complimentary observations from multiple facilities to try to unravel the chemical complexity of comets and their origins. Incorporating results from various techniques can gain further insight into the abundances, production rates, distributions, and formation mechanisms of molecules in these objects [I]. Such studies have provided great detail towards molecules with a-typical chemistries, such as H2CO [2]. We report multiwavelength spectral observations of comets from two dynamical families including the JFC 103P/Hartley 2 and a long period comet C/2009 PI (Garradd) with the Arizona Radio Observatory's SMT and 12-m telescopes, as well as the NRAO Greenbank telescope, and the James Clerk Maxwell Telescope. Multiple parent volatiles (e.g. HCN, CH30H, CO) as well as daughter products (e.g, CS and 01-1) have been detected in these objects. We will present a comparison of molecular abundances in these comets to those observed in others, supporting a long-term effort of building a comet taxonomy based on composition. Previous work has revealed a range of abundances of parent species (from "organics-poor" to "organics-rich") with respect to water among comets [3,4,5], however the statistics are not well constrained.

Milam, Stefanie N.↗
NASA NTRS2012

The Rotation Temperature of Methanol in Comet 103P/Hartley 2

Considered to be relics from Solar System formation, comets may provide the vital information connecting Solar Nebula and its parent molecular cloud. Study of chemical and physical properties of comets is thus important for our better understanding of the formation of Solar System. In addition, observing organic molecules in comets may provide clues fundamental to our knowledge on the formation of prebiotically important organic molecules in interstellar space, hence, may shed light on the origin of life on the early Earth. Comet 103PIHartley 2 was fIrst discovered in 1986 and had gone through apparitions in 1991, 1997, and 2004 with an orbital period of about 6 years, before its latest return in 2010. 2010 was also a special year for Comet 103PIHartley 2 because of the NASA EPOXI comet-flyby mission.

Chuang, Yo-Ling↗
NASA NTRS2012

Development of Primary Volatile Production in COMET C/20O9 Pl (GARRADD) During its 2011-2O12 Apparition

We quantified primary volatiles in comet C/2009 Pl (Garradd) through pre- and post-perihelion observations acquired during its apparition in 2011-12 [1,2,3]. Detected volatiles include H2O, CO, CH4, C2H2, C2H6, HCN, NH3, H2CO, and CH3OH. We present production rates and chemical abundance ratios (relative to water) for all species, and I-D spatial profiles for multiple primary volatiles. We discuss these findings in the context of an emerging taxonomy based on primary volatiles in comets [4]. We used three spectrometer/telescope combinations. On UT 20ll August 7 (Rh 2.4 AU) and September 17-21 (Rh 2.0 AU), we used CRIRES at ESO's Very Large Telescope (VLT) [1]. On September 8 and 9 (Rh 2.1 AU), we used NIRSPEC at Keck-2 and CSHELL at IRTF [2]. Using NIRSPEC on October 13 and 2012 January 08 (Rh 1.83 and 1.57 AU, respectively), we detected nine primary volatiles pre-perihelion, and six post-perihelion [3]. CO was enriched in Garradd while C2H2 was strongly depleted. C2H6 and CH3OH displayed abundances close to those measured for the majority of Oort cloud comets observed to date. The high fractional abundance of CO identifies comet C12009 P1 as a CO-rich comet. Spatial profiles revealed notable differences among individual primary species. Given the relatively large heliocentric distance of C/2009 Pl, we explored the effect of water not being fully sublimated within our field of view and we identi$, the "missing" water fraction needed to reconcile the retrieved abundance ratios with the mean values found for "organics-normal" comets.

Mumma, M. J.↗
NASA NTRS2011

Navigation of the EPOXI Spacecraft to Comet Hartley 2

On November 4, 2010, the EPOXI spacecraft flew by the comet Hartley 2, marking the fourth time that a NASA spacecraft successfully captured high resolution images of a cometary nucleus. EPOXI is the extended mission of the Deep Impact mission, which delivered an impactor on comet Tempel-1 on July 4, 2005. EPOXI officially started in September 2007 and eventually took over 3 years of flight time and had 3 Earth gravity assists to achieve the proper encounter conditions. In the process, the mission was redesigned to accommodate a new comet as the target and changes in the trajectory to achieve better imaging conditions at encounter. Challenges in navigation of the spacecraft included precision targeting of several Earth flybys and the comet encounter, uncertainties in determining the ephemeris of the comet relative to the spacecraft, and the high accuracy trajectory knowledge needed to image the comet during the encounter. This paper presents an overview of the navigation process used for the mission.

Deep Impact (DI)↗
NASA NTRS2016

Comet Dust: The Diversity of "Primitive" Particles and Implications

Comet dust is primitive and shows significant diversity. Our knowledge of the properties of primitive particles has expanded significantly through microscale investigations of cosmic dust samples ( IDP's(Interplanetary Dust Particles) and AMM's (Antarctic Micrometeorites)) and of comet dust samples (Stardust and Rosetta's COSIMA), as well as through remote sensing (spectroscopy and imaging) via Spitzer and via spacecraft encounters with 103P/Hartley 2 and 67P/Churyumov-Gerasimenko. Microscale investigations show that comet dust and cosmic dust are particles of unequilibrated materials, including aggregates of materials unequilibrated at submicron scales. We call unequilibrated materials "primitive" and we deduce they were incorporated into ice-rich (H2O-, CO2-, and CO-ice) parent bodies that remained cold, i.e., into comets, because of the lack of aqueous or thermal alteration since particle aggregation; yet some Stardust olivines suggest mild thermal metamorphism. Primitive particles exhibit a diverse range of: structure and typology; size and size distribution of constituents; concentration and form of carbonaceous and organic matter; D-, N-, and O- isotopic enhancements over solar; Mg-, Fe-contents of the silicate minerals; the compositions and concentrations of sulfides, and of less abundant mineral species such as chondrules, CAIs and carbonates. The uniformity within a group of samples points to: aerodynamic sorting of particles and/or particle constituents; the inclusion of a limited range of oxygen fugacities; the inclusion or exclusion of chondrules; a selection of organics. The properties of primitive particles imply there were disk processes that resulted in different comets having particular selections of primitive materials. The diversity of primitive particles has implications for the diversity of materials in the protoplanetary disk present at the time and in the region where the comets formed.

Wooden, Diane H.↗
NASA NTRS2015

The CO2 Abundance in Comets C2012 K1 (PanSTARRS), C2012 K5 (LINEAR), and 290P Jager as Measured with Spitzer

Carbon dioxide is one of the most abundant ices present in comets and is therefore important for understanding cometary composition and activity. We present analysis of observations of CO2 and [O I] emission in three comets to measure the CO2 abundance and evaluate the possibility of employing observations of [O I] emission in comets as a proxy for CO2. We obtained NIR imaging sensitive to CO2 of comets C/2012 K1 (PanSTARRS), C/2012 K5 (LINEAR), and 290P/Jager with the IRAC instrument on Spitzer. We acquired observations of [O I] emission in these comets with the ARCES echelle spectrometer mounted on the 3.5-m telescope at Apache Point Observatory and observations of OH with the Swift observatory (PanSTARRS) and with Keck HIRES (Jager). The CO2/H2O ratios derived from the Spitzer images are 12.6 +/- 1.3% (PanSTARRS), 28.9 +/- 3.6% (LINEAR), and 31.3 +/- 4.2% (Jager). These abundances are derived under the assumption that contamination from CO emission is negligible. The CO2 abundance for PanSTARRS is close to the average abundance measured in comets at similar heliocentric distance to date, while the abundances measured for LINEAR and Jager are significantly larger than the average abundance. From the coma morphology observed in PanSTARRS and the assumed gas expansion velocity, we derive a rotation period for the nucleus of about 9.2 h. Comparison of H2O production rates derived from ARCES and Swift data, as well as other observations, suggest the possibility of sublimation from icy grains in the inner coma. We evaluate the possibility that the [O I] emission can be employed as a proxy for CO2 by comparing CO2/H2O ratios inferred from the [O I] lines to those measured directly by Spitzer. We find that for PanSTARRS we can reproduce the observed CO2 abundance to an accuracy of approximately 20%. For LINEAR and Jager, we were only able to obtain upper limits on the CO2 abundance inferred from the [O I] lines. These upper limits are consistent with the CO2 abundances measured by Spitzer.

McKay, Adam J.↗
NASA NTRS2016

On the Absence of EUV Emission from Comet C/2012 S1 (ISON)

When the sungrazing comet C2012 S1 (ISON) made its perihelion passage within two solar radii of the Sun's surface, it was expected to be a bright emitter at extreme ultraviolet (EUV) wavelengths. However, despite solar EUV telescopes repointing to track the orbit of the comet, no emission was detected. This null result is interesting in its own right, offering the possibility of placing limits on the size and composition of the nucleus. We explain the lack of detection by considering the properties of the comet and the solar atmosphere that determine the intensity of EUV emission from sungrazing comets. By comparing these properties with those of sungrazing comet C2011 W3 (Lovejoy), which did emit in the EUV, we conclude that the primary factor resulting in non-detectable EUV emission from C2012 S1 (ISON) was an insufficiently large nucleus. We conclude that the radius of C2012 S1 (ISON) was at least a factor of four less than that of C2011 W3 (Lovejoy). This is consistent with white-light observations in the days before perihelion that suggested the comet was dramatically reducing in size on approach.

Bryans, Paul↗
NASA NTRS2017

Beyond 3 Au from the Sun: the Hypervolatiles CH4, C2H6, and CO in the Distant Comet C2006 W3 (Christensen)

Comet C/2006 W3 (Christensen) remained outside a heliocentric distance (Rh) of 3.1 au throughout its apparition, but it presented an exceptional opportunity to directly sense a suite of molecules released from its nucleus. The Cryogenic Infrared Echelle Spectrograph at ESO-VLT detected infrared emissions from the three hypervolatiles (CO, CH4, and C2H6) that have the lowest sublimation temperatures among species that are commonly studied in comets by remote sensing. Even at Rh 3.25 au, the production rate of each molecule exceeded those measured for the same species in a number of other comets, although these comets were observed much closer to the Sun. Detections of CO at Rh = 3.25, 4.03, and 4.73 au constrained its post-perihelion decrease in production rate, which most likely dominated the outgassing. At 3.25 au, our measured abundances scaled as CO/CH4/C2H6 approx. = 100/4.4/2.1. The C2H6/CH4 ratio falls within the range of previously studied comets at Rh < 2 au, while CO/CH4 is comparatively high and similar to in situ measurements from Rosetta at approx.10 km from the nucleus of 67P/Churyumov-Gerasimenko conducted at a very similar Rh (3.15 au). The independent detections of H2O (Herschel Space Observatory) and CO (this work) imply a coma abundance H2O/CO approx. = 20% in C/2006 W3 near Rh = 5 au. All these measurements are of high value for constraining models of nucleus sublimation (plausibly CO-driven) beyond Rh = 3au, where molecular detections in comets are still especially sparse.

Bonev, Boncho P.↗
NASA NTRS2007

Fragmentation hierarchy of bright sungrazing comets and the birth and orbital evolution of the Kreutz System. II. the case for cascading fragmentation

We examine the process of cascading fragmentation for the Kreutz sungrazer system to continue our exploration of its birth, orbital evolution, and temporal clumping. We modify and broaden the two-superfragment model from Paper I to include clusters of ~30 bright comets spanning four centuries and 1000 SOHO sungrazers from 1996 to 2006. The spectacular parent sungrazer X/1106 C1 is assumed to have tidally split shortly after perihelion into a train of major protofragments immersed in a cloud of particulate debris, which at larger heliocentric distances were breaking up nontidally over and over again. We describe potential evolutionary paths for the Kreutz system by linking X/1106 C1 in subgroup I-type orbit with the comet of February 423 in one scenario or with the comet of February 467 in another. The latter scenario accounts for sungrazer clusters in as early as the 16th century, suggests that the progenitor object may have been observed as the comet of 214 BCE, is quite consistent with the orbital distribution of the SOHO sungrazers that sample the central filament of the Kreutz system between the clusters of major sungrazers, and predicts future clusters until ~2120. Comet X/1106 C1 and the common parent of C/1882 R1 and C/1965 S1 were two first-generation fragments of the progenitor that split nontidally on the way to its 5th century perihelion, reminiscent of the superfragments in Paper I. We provide computational tools needed for solving the problem of the Kreutz system’s orbital evolution, but no unique scenarios are presented for the individual comets. Another cluster of bright sungrazers is expected to arrive in the coming decades, its earliest member possibly just several years from now.

Chodas, Paul W.↗
NASA NTRS2022

The breakup of a long-period comet is not a likely match to the Chicxulub impactor

Since the discovery of Ir in the clay layer at the K-Pg boundary, scientists have sought to constrain the origin of the extraterrestrial impactor that triggered the end-Cretaceous mass extinction of the non-avian dinosaurs and other species. While the first proposal was for an asteroid, for a while some theories invoked a cometary impactor to explain perceived periodicities in mass extinctions. Such models have long been disfavored by the mass of Ir in the layer, inferred to be 2.0−2.8×10^(11)cu.g. The size of the Chicxulub crater leads to an estimated asteroid impactor diameter, D≈10km. Comets typically impact at higher speeds, reducing the impactor mass for the same impact energy. Although it is increasingly recognized that a continuum exists between comets and asteroids, ’comets’ are considered to be more ice-rich (estimates for 67P/Churyumov-Gerasimenko are about 20%;), implying lower Ir contents per impactor mass. A carbonaceous chondrite-like asteroid of the appropriate size would likely deliver ≈2.3×10^(11)g of Ir, in the center of the estimated mass range of the global Ir layer; but a comet would only deliver ∼0.1×10^(11)g, because it would be less massive. Although these conclusions are long standing, Siraj and Loeb have recently argued anew in favor of a comet over an asteroid, based on dynamical and geochemical evidence. Here we demonstrate that their arguments are based on misinterpretations of the literature, and that an asteroid is in fact still highly favored over a comet.

Steven J. Desch↗
NASA NTRS2022

The breakup of a long-period comet is not a likely match to the Chicxulub impactor

Since the discovery of Ir in the clay layer at the K-Pg boundary [1], scientists have sought to constrain the origin of the extraterrestrial impactor that triggered the end-Cretaceous mass extinction of the non-avian dinosaurs and other species. While the first proposal was for an asteroid, for a while some theories invoked a cometary impactor to explain perceived periodicities in mass extinctions. Such models have long been disfavored by the mass of Ir in the layer, inferred to be2.0−2.8×1011g. The size of Chicxulub crater leads to an estimated asteroid impactor diameter,D≈10 km. Comets typically impact at higher speeds, reducing the impactor mass for the same impact energy. Although it is increasingly recognized that a continuum exists between comets and asteroids, ’comets’ are considered to be more ice-rich (estimates for67P/ Churyumov-Gerasimenko are about 20%;), implying lower Ir contents per impactor mass. A carbonaceous chondrite-like asteroid of the appropriate size would likely deliver≈2.3×1011gof Ir, in the center of the estimated mass range of the global Ir layer; but a comet would only deliver∼0.1×1011g, because it would be less massive. Although these conclusions are long standing, Siraj & Loeb have recently argued anew in favor of a comet over an asteroid, based on dynamical and geochemical evidence. Here we demonstrate that their arguments are based on misinterpretations of the literature, and that an asteroid is in fact still highly favored over a comet.

Steven J Desch↗
DOE OSTI2021

Investigation of the Origins of Comets as Revealed through Infrared High-resolution Spectroscopy I. Molecular Abundances

We report and analyze updated molecular abundances in 20 comets obtained by employing modern data reduction procedures and molecular models. Using box and scatter plots, we examine how the different molecular species are distributed among the comet population, while by means of pie charts, we investigate the relative proportions of these molecular species in each comet. We compare these results with the orbital parameters of the selected comets to identify trends related to their dynamical history. With these analyses, we tentatively identify at least three chemical classes based mainly on relative abundances of CO, CH{sub 3}OH, CH{sub 4}, C{sub 2}H{sub 6}, HCN, and NH{sub 3}. The combination of relative abundances and orbital parameters is then compared with recent chemical models of planetary system formation. This approach may help in investigating the origins and evolution of the material in cometary nuclei. Among other aspects, we underline the need to increase our sample size, especially for hypervolatiles (i.e., CH{sub 4} and CO) in Jupiter family comets.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Disintegration of Long-period Comet C/2019 Y4 (ATLAS). I. Hubble Space Telescope Observations

The near-Sun comet C/2019 Y4 (ATLAS) is the first member of a long-period comet group observed to disintegrate well before perihelion. Here we present our investigation into this disintegration event using images obtained in a three-day Hubble Space Telescope campaign. We identify two fragment clusters produced by the initial disintegration event, corresponding to fragments C/2019 Y4-A and C/2019 Y4-B identified in ground-based data. These two clusters started with similar integrated brightness but exhibit different evolutionary behavior. C/2019 Y4-A was much shorter-lived compared to C/2019 Y4-B and showed signs of significant mass loss and changes in size distribution throughout the three-day campaign. The cause of the initial fragmentation is undetermined by the limited evidence but crudely compatible with either the spin-up disruption of the nucleus or runaway sublimation of subsurface supervolatile ices, either of which would lead to the release of a large amount of gas as inferred from the significant bluing of the comet observed shortly before its disintegration. Gas can only be produced by the sublimation of volatile ices, which must have survived at least one perihelion passage at a perihelion distance of q = 0.25 au. We speculate that Comet ATLAS is derived from the ice-rich interior of a nonuniform, kilometer-wide progenitor that split during its previous perihelion. This suggests that comets down to a few kilometers in diameter can still possess complex, nonuniform interiors that can protect ices against intense solar heating.

47 OTHER INSTRUMENTATION↗
NASA NTRS1963

Comet Intercept Study

STL (Space Technology Laboratories, Inc.) has been studying, during the past year, the problems of carrying out a comet intercept mission. During the course of this study, the properties of 31 short-term comets have been examined to determine the feasibility of a mission to any of them during the next 15 years. In the process of selecting these comets, injection energies for each of these comets at a suitable launch period were determined. In addition, the distance of the earth at intercept, the transit and flight times, and the guidance requirements were evaluated. Also, to determine the effectiveness of such a mission, possible scientific instruments which could be used to measure the various characteristics of the comets have been studied. Finally, to determine the present feasibility of such a mission, the payload capability of available boosters was examined, and a spacecraft configuration with appropriate subsystems was also studied.

INTERCEPTION↗
NASA NTRS2007

Encounter of the Ulysses Spacecraft with the Ion Tail of Comet McNaught

Comet McNaught was the brightest comet observed from Earth in the last 40 years. For a period of five days in early 2007 February, four instruments on the Ulysses spacecraft directly measured cometary ions and key properties of the interaction of the comet's ion tail with the high-speed solar wind from the polar regions of the Sun. Because of the record-breaking duration of the encounter, the data are unusually comprehensive. O3(+) ions were detected for the first time in a comet tail, coexisting with singly charged molecular ions with masses in the range 28-35 amu. The presence of magnetic turbulence and of ions with energies up to approximately 200 keV indicate that at a distance of approximately 1.6 AU from the comet nucleus, the ion tail McNaught had not yet reached equilibrium with the surrounding solar wind.

plasmas↗
NASA NTRS2007

Earth's 2006 Encounter with Comet 73P/Schwassmann-Wachmann: Products of Nucleus Fragmentation Seen in Closeup

The large numbers of nucleus fragments observed are a spectacular illustration of the process of cascading fragmentation in progress, a concept introduced to interpret the properties of the Kreutz system of sungrazers and comet D/1993 F2. The objective is to describe the fragmentation sequence and hierarchy of comet 73P, the nature of the fragmentation process and observed events, and the expected future evolution of this comet. The orbital arc populated by the fragments refers to an interval of 3.74 days in the perihelion time. This result suggests that they all could be products (but not necessarily first-generation fragments) of two 1995 events, in early September (involving an enormous outburst) and at the beginning of November. The interval of perihelion times is equivalent to a range of about 2.5 m/s in separation velocity or 0.00012 the Sun's attraction in nongravitational deceleration. Their combined effect suggests minor orbital momentum changes acquired during fragmentation and decelerations compatible with survival over two revolutions about the Sun. Fragment B is a likely first-generation product of one of the 1995 events. From the behavior of the primary fragment C, 73P is not a dying comet, even though fragment B and others were episodically breaking up into many pieces. Each episode began with the sudden appearance of a starlike nucleus condensation and a rapidly expanding outburst, followed by a development of jets, and a gradual tailward extension of the fading condensation, until the discrete masses embedded in it could be resolved. In April-May, this debris traveled first to the southwest, but models show their eventual motion toward the projected orbit. Fainter fragments were imaged over limited time, apparently because of their erratic activity (interspersed with periods of dormancy) rather than improptu disintegration. A dust trail joining the fragments and reminiscent of comet 141P/Machholz suggests that cascading fragmentation exerts itself profoundly over an extremely broad mass range of particulate debris.

daya analysis↗
NASA NTRS2011

Attitude Determination and Control Subsystem (ADCS) Preparations for the EPOXI Flyby of Comet Haley 2

On November 4, 2010 the already "in-flight" Deep Impact spacecraft flew within 700km of comet 103P/Hartley 2 as part of its extended mission EPOXI, the 5th time to date any spacecraft visited a comet. In 2005, the spacecraft had previously imaged a probe impact comet Tempel 1. The EPOXI flyby marked the first time in history that two comets were explored with the same instruments on a re-used spacecraft-with hardware and software originally designed and optimized for a different mission. This made the function of the attitude determination and control subsystem (ADCS) critical to the successful execution of the EPOXI flyby. As part of the spacecraft team preparations, the ADCS team had to perform thorough sequence reviews, key spacecraft activities and onboard calibrations. These activities included: review of background sequences for the initial conditions vector, sun sensor coefficients, and reaction wheel assembly (RWA) de-saturations; design and execution of 10 trajectory correction maneuvers; science calibration of the two telescope instruments; a flight demonstration of the fastest turns conducted by the spacecraft between Earth and comet point; and assessment of RWA health (given RWA problems on other spacecraft).

Harley↗