Cometary Mass Research Articles

A race against the clock: Constraining the timing of cometary bombardment relative to Earth’s growth

Comets are considered a potential source of inner solar system volatiles, but the timing of this delivery relative to that of Earth’s accretion is still poorly understood. Measurements of xenon isotopes in comet 67P/Churyumov-Gerasimenko revealed that comets partly contributed to the Earth’s atmosphere. However, there is no conclusive evidence of a significant cometary component in the Earth’s mantle. These geochemical constraints would favor a contribution of comets mainly occurring after the last stages of Earth’s formation. Here, we evaluate whether dynamical simulations satisfy these constraints in the context of an Early Instability model. We perform N-body simulations of the solar system’s dynamic evolution considering an outer disk of 10000 comets, different initial conditions for the inner solar system and assuming that the giant planet instability happened within ≃10 million years of the start of planet formation. Out of 30 simulations, 13 meet a list of criteria for success. We calculate the probability of collision between comets and Earth analogs component embryos through time and estimate the total cometary mass accreted in Earth analogs as a function of time. We determine that the cumulative cometary mass accreted by the Earth analogs during the first 100 million years ranges between 4 ×1021 and 5 ×1024 g. While our results are in excellent agreement with geochemical constraints, we also demonstrate that the contribution of comets on Earth might have been delayed with respect to the timing of the instability, due to a stochastic component of the bombardment. More importantly, we show that it is possible that enough cometary mass has been brought to Earth after it had finished forming so that the xenon constraint is not necessarily in conflict with an Early Instability scenario. Indeed, 2 of the 13 successful simulations provide enough comet delivery after a last giant impact happening later than 30 Myr. However, it appears very likely that a few comets were delivered to Earth early in its accretion history, thus contributing to the mantle’s budget. Finally, we compare the delivery of cometary material on Earth to Venus and Mars. These results emphasize the stochastic nature of the cometary bombardment in the inner solar system.

Distant Comet C/2017 K2 and the Cohesion Bottleneck

Abstract The distant long-period comet C/2017 K2 (PANSTARRS) has been outside the planetary region of the solar system for ∼3 Myr, negating the possibility that heat retained from the previous perihelion could be responsible for its activity. This inbound comet is also too cold for water ice to sublimate and too cold for amorphous water ice, if present, to crystallize. C/2017 K2 thus presents an ideal target in which to investigate the mechanisms responsible for activity in distant comets. We have used the Hubble Space Telescope to study the comet in the pre-perihelion heliocentric distance range 13.8 ≤ r H ≤ 15.9 au. In this range, the coma maintains a logarithmic surface brightness gradient m = −1.010 ± 0.004, consistent with mass loss proceeding in steady state. The absence of a radiation pressure swept tail indicates that the effective particle size is large (radius ≳0.1 mm) and the mass-loss rate is ∼200 kg s−1, remarkable for a comet still beyond the orbit of Saturn. Extrapolation of the photometry indicates that activity began in 2012.1 ± 0.5, at r H = 25.9 ± 0.9 au, where the isothermal blackbody temperature is only T BB = 55 K. This large distance and low temperature suggest that cometary activity is driven by the sublimation of a super-volatile ice (e.g., CO), presumably preserved by K2's long-term residence in the Oort cloud. The mass-loss rate can be sustained by CO sublimation from an area ≲2 km2, if located near the hot subsolar point on the nucleus. However, while the drag force from sublimated CO is sufficient to lift millimeter-sized particles against the gravity of the cometary nucleus, it is 102–103 times too small to eject these particles against interparticle cohesion. Our observations thus require either a new understanding of the physics of interparticle cohesion or the introduction of another mechanism to drive distant cometary mass loss. We suggest thermal fracture and electrostatic supercharging in this context.

Likely Transiting Exocomets Detected By Kepler.

We present the first good evidence for exocomet transits of a host star in continuum light in data from the Kepler mission. The Kepler star in question, KIC 3542116, is of spectral type F2V and is quite bright at Kp = 10. The transits have a distinct asymmetric shape with a steeper ingress and slower egress that can be ascribed to objects with a trailing dust tail passing over the stellar disk. There are three deeper transits with depths of ≃ 0.1% that last for about a day, and three that are several times more shallow and of shorter duration. The transits were found via an exhaustive visual search of the entire Kepler photometric data set, which we describe in some detail. We review the methods we use to validate the Kepler data showing the comet transits, and rule out instrumental artefacts as sources of the signals. We fit the transits with a simple dust-tail model, and find that a transverse comet speed of ∼35-50 km s-1 and a minimum amount of dust present in the tail of ∼ 1016 g are required to explain the larger transits. For a dust replenishment time of ∼10 days, and a comet lifetime of only ∼300 days, this implies a total cometary mass of ≳ 3 × 1017 g, or about the mass of Halley's comet. We also discuss the number of comets and orbital geometry that would be necessary to explain the six transits detected over the four years of Kepler prime-field observations. Finally, we also report the discovery of a single comet-shaped transit in KIC 11084727 with very similar transit and host-star properties.

Water production activity of nine long-period comets from SOHO/SWAN observations of hydrogen Lyman-alpha: 2013–2016

Nine recently discovered long-period comets were observed by the Solar Wind Anisotropies (SWAN) Lyman-alpha all-sky camera on board the Solar and Heliosphere Observatory (SOHO) satellite during the period of 2013 to 2016. These were C/2012 K1 (PanSTARRS), C/2013 US10 (Catalina), C/2013 V5 (Oukaimeden), C/2013 R1 (Lovejoy), C/2014 E2 (Jacques), C/2014 Q2 (Lovejoy), C/2015 G2 (MASTER), C/2014 Q1 (PanSTARRS) and C/2013 XI (PanSTARRS). Of these 9 comets 6 were long-period comets and 3 were possibly dynamically new. Water production rates were calculated from each of the 885 images using our standard time-resolved model that accounts for the whole water photodissociation chain, exothermic velocities and collisional escape of H atoms. For most of these comets there were enough observations over a broad enough range of heliocentric distances to calculate power-law fits to the variation of production rate with heliocentric distances for pre- and post-perihelion portions of the orbits. Comet C/2014 Q1 (PanSTARRS), with a perihelion distance of only ∼0.3 AU, showed the most unusual variation of water production rate with heliocentric distance and the resulting active area variation, indicating that when the comet was within 0.7 AU its activity was dominated by the continuous release of icy grains and chunks, greatly increasing the active sublimation area by more than a factor of 10 beyond what it had at larger heliocentric distances. A possible interpretation suggests that a large fraction of the comet's mass was lost during the apparition.

Polluting white dwarfs with perturbed exo-comets

We present a model to account for the observed debris disks around young white dwarfs and the presence of metal-lines in their spectra. Stellar evolution models predict that the mass-loss on the AGB will be pulsed; furthermore, observations indicate that the bulk of the mass-loss occurs on the AGB. In this case, if the progenitors of the white dwarfs had remnants of planetary formation like the Sun's Oort cloud or the Kuiper Belt and a planet lying within that cloud or nearby, we find that up to 2% of the planetesimals will fall either into planet-crossing orbits or into chaotic regions after the mass-loss, depending on the location and mass of the planet (from Mars to Neptune). This yields a sufficient mass of comets that can be scattered toward the star, form a debris disk and pollute the atmosphere.

Effects of disc asymmetries on astrometric measurements

Astrometry covers a parameter space that cannot be reached by RV or transit methods to detect terrestrial planets on wide orbits. In addition, high accuracy astrometric measurements are necessary to measure the inclination of the planet's orbits. Here we investigate the principles of an artefact of the astrometric approach. Namely, the displacement of the photo-centre due to inhomogeneities in a dust disc around the parent star. Indeed, theory and observations show that circumstellar discs can present strong asymmetries. We model the pseudo-astrometric signal caused by these inhomogeneities, asking whether a dust clump in a disc can mimic the astrometric signal of an Earth-like planet. We show that these inhomogeneities cannot be neglected when using astrometry to find terrestrial planets. We provide the parameter space for which these inhomogeneities can affect the astrometric signals but still not be detected by mid-IR observations. We find that a small cross section of dust corresponding to a cometary mass object is enough to mimic the astrometric signal of an Earth-like planet. Astrometric observations of protoplanetary discs to search for planets can also be affected by the presence of inhomogeneities. Some further tests are given to confirm whether an observation is a real planet astrometric signal or an impostor. Eventually, we also study the case where the cross section of dust is high enough to provide a detectable IR-excess and to have a measurable photometric displacement by actual instruments such as Gaia, IRAC or GRAVITY. We suggest a new method, which consists in using astrometry to quantify asymmetries (clumpiness) in inner debris discs that cannot be otherwise resolved.

Magnetotail structure of the giant magnetospheres: Implications of the viscous interaction with the solar wind

The internal sources of plasma in the giant magnetospheres of Jupiter and Saturn affect magnetospheric dynamics in terms of magnetosphere‐ionosphere coupling and auroral current systems, as well as the interaction with the solar wind. The radial transport of plasma at Jupiter is well constrained to vary between 300 kg/s and 1200 kg/s over timescales of 20–60 days. Saturn's neutral‐dominated inner magnetosphere has presented a challenge for determining the radial mass transport rates with values ranging between 20 kg/s and 280 kg/s over timescales of 20–80 days. We present an estimate of the radial mass transport rates associated with magnetospheric plasma production for Jupiter and Saturn, assuming that magnetospheric plasma loss (equal to the plasma source) can be treated as mass pickup by the solar wind. That is, the magnetospheric plasma is assumed to be at rest with respect to the solar wind, analogous to cometary mass loading of the solar wind flow. This Alfvénic coupling between the solar wind and the magnetosphere suggests mass loading rates of 500–1500 kg/s for Jupiter and 20–50 kg/s for Saturn, consistent with observations and physical chemistry models. We discuss implications for magnetotail structure by considering the contribution of the viscous interaction of the solar wind with the giant magnetospheres. We suggest that a significant portion of the magnetotail structure can be attributed to this viscous interaction.

Comets as solar probes

By rapidly evaporating as they fly through the Sun’s hot outer atmosphere, sungrazing comets reveal an otherwise invisible magnetic field that shapes the very beginnings of the solar wind.

Cosmogenic nuclide enhancement via deposition from long-period comets as a test of the Younger Dryas impact hypothesis

We explore the idea that detectable excursions in 26Al may arise from direct deposition by any bolide, and excursions in 14C and 10Be abundances in the atmosphere may result from long-period comet impacts. This is very different from the usual processes of production by cosmic rays within Earthʼs atmosphere. Long-period comets experience greatly increased cosmic ray flux beyond the protection of the sunʼs magnetic field. We report the computed amount of 14C, 10Be, and 26Al present on long-period comets as a function of comet mass. We find that the amount of nuclide mass on large long-period comets entering the Earthʼs atmosphere may be sufficient for creating anomalies in the records of 14C and 10Be from past impacts. In particular, the estimated mass of the proposed Younger Dryas comet is consistent with its having deposited sufficient isotopes to account for recorded 14C and 10Be increases at that time. The 26Al/10Be ratio is much larger in extraterrestrial objects than in the atmosphere, and so, we note that measuring this ratio in ice cores is a suitable definitive test for the Younger Dryas impact hypothesis, even if the hypothetical bolide is not a long-period comet and/or did not contribute to the 14C and 10Be increases.

Limits on the size and orbit distribution of main belt comets

The first of a new class of objects now known as main belt comets (MBCs) or “activated asteroids” was identified in 1996. The seven known members of this class have orbital characteristics of main belt asteroids yet exhibit dust ejection like comets. In order to constrain their physical and orbital properties we searched the Thousand Asteroid Light Curve Survey (TALCS; Masiero, J.R., Jedicke, R., Durech, J., Gwyn, S., Denneau, L., Larsen, J. [2009]. Icarus 204, 145–171) for additional candidates using two diagnostics: tail and coma detection. This was the most sensitive MBC survey effort to date, extending the search from MBCs with H ∼ 18 ( D ∼ 1 km) to MBCs as small as H ∼ 21 ( D ∼ 150 m). We fit each of the 924 objects detected by TALCS to a PSF model incorporating both a coma and nuclear component to measure the fractional contribution of the coma to the total surface brightness. We determined the significance of the coma detection using the same algorithm on a sample of null detections of comparable magnitude and rate of motion. We did not identify any MBC candidates with this technique to a sensitivity limit on the order of cometary mass loss rate of about 0.1 kg/s. Our tail detection algorithm relied on identifying statistically significant flux in a segmented annulus around the candidate object. We show that the technique can detect tail activity throughout the asteroid belt to the level of the currently known MBCs. Although we did not identify any MBC candidates with this technique, we find a statistically significant detection of faint activity in the entire ensemble of TALCS asteroids. This suggests that many main belt asteroids are active at very low levels. Our null detection of MBCs allows us to set 90% upper confidence limits on the number distribution of MBCs as a function of absolute magnitude, semi-major axis, eccentricity, and inclination. There are ≲400,000 MBCs in the main belt brighter than H V = 21 (∼150-m in diameter) and the MBC:MBA ratio is ≲1:400. We further comment on the ability of observations to meaningfully constrain the snow line’s location. Under some reasonable and simple assumptions we claim 85% confidence that the contemporary snow line lies beyond 2.5 AU.

Masses of long-period comets derived from non-gravitational effects - analysis of the computed results and the consistency and reliability of the non-gravitational parameters

We estimate masses for a selected sample of long-period comets (LPCs) (with orbital periods P > 1000 yr and perihelion distances q < 2 au), with good photometric visual light curves and known non-gravitational parameters. We follow a procedure similar to that developed by Szutowicz et al. to estimate the masses of comets C/1995 O1 (Hale–Bopp) and C/1996 B2 (Hyakutake). The method also requires the knowledge of the water production rates Q ,f or which we find a new correlation between Q and the visual total heliocentric magnitude mh of LPCs, which can be expressed as log10Q = 30.53 − 0.234mh. The computed masses are in the range [0.5, 10] × 10 12 kg. From these masses, and by assuming a cometary bulk density ρ of 0.4 g cm −3 , we estimate the effective nuclear radii RN for the studied LPCs to be in the range [0.7, 1.8] km. From the computed values of RN ,w e find that the active surface areas are (in most cases) greater than the geometric ones (fractions f ranging from 0.8 to 2.4). We argue that this could be understood as a state of hyperactivity, which can be explained if a significant fraction of the sublimated water molecules come from the sublimation of icy grains in the coma. Hyperactivity would make LPC nuclei smaller (i.e. less massive) than expected from their coma brightness. Other effects, like extremely low densities or overestimated non-gravitational parameters, might also contribute to the rather high computed value of f . For comet C/1995 O1 (Hale–Bopp), we find a seeming inconsistency between some size estimates and some computed non-gravitational parameters. We analyse some alternatives to this apparent paradox. We also compute absolute total magnitudes and photometric indices for the studied sample of LPCs and find a new correlation between masses and absolute total visual magnitudes H. By assuming ρ = 0.4 g cm −3 , we can convert masses to diameters D, leading to a relation log10D(km) = 1.5 − 0.13H.

Simulations of a comet impact on the Moon and associated ice deposition in polar cold traps

Modeling results of the water vapor plume produced by a comet impact on the Moon and of the resulting water ice deposits in the lunar cold traps are presented. The water vapor plume is simulated near the point of impact by the SOVA hydrocode and in the far field by the Direct Simulation Monte Carlo (DSMC) method using as input the SOVA hydrocode solution at a fixed hemispherical interface. The SOVA hydrocode models the physics of the impact event such as the surface deformation and material phase changes during the impact. The further transport and retention processes, including gravity, photodestruction processes, and variable surface temperature with local polar cold traps, are modeled by the DSMC method for months after impact. In order to follow the water from the near field of the impact to the full planetary induced atmosphere, the 3D parallel DSMC code used a collision limiting scheme and an unsteady multi-domain approach. 3D results for the 45° oblique impact of a 2 km in diameter comet on the surface of the Moon at 30 km/s are presented. Most of the cometary water is lost due to escape just after impact and only ∼3% of the cometary water is initially retained on the Moon. Early downrange focusing of the water vapor plume is observed but the later material that is moving more slowly takes on a more symmetric shape with time. Several locations for the point of impact were investigated and final retention rates of ∼0.1% of the comet mass were observed. Based on the surface area of the cold traps used in the present simulations, ∼1 mm of ice would have accumulated in the cold traps after such an impact. Estimates for the total mass of water accumulated in the polar cold traps over 1 byr are consistent with recent observations.

An analysis of outgassing pressure forces on the Rosetta orbiter using realistic 3D+t coma simulations

A model for the interaction between a multicomponent Maxwellian atmosphere and a spacecraft is described. Multidimensional, time-dependent gasdynamical simulations of the gas coma around the recently reconstructed aspherical rotating nucleus of comet 67P/C-G is used to analyze the outgassing pressure forces on the ESA spacecraft Rosetta. The forces were in general found to be directed significantly away from the cometocentric position vector of the spacecraft. It was also found that in a maximum outgassing scenario at comet rendezvous, the outgassing pressure force exceeds the gravitational attraction from the nucleus in the cometocentric direction of the Sun. Furthermore, the highly non-spherical pressure field was found to undergo very large changes as the nucleus rotated. Still, it was possible to represent the mean pressure field experienced by Rosetta by a fairly simple model, which can be used for the determination of the comet mass and the so-called oblateness coefficient c 20 from the spacecraft Doppler signal. The oblateness coefficient represents a type of asphericity of the gravity field. The determination of the so-called triaxiality coefficient of the gravity field c 22 may require using the true pressure field instead of the mean pressure field.

Non-gravitational forces and masses of some long-period comets. The cases of Hale-Bopp and Hyakutake

AbstractBy means of a simple non-gravitational force model of the cometary nucleus, which relies on the observed light curves assumed to be a good representation of the water sublimation rate, we estimate the masses of a sample of long-period comets (LPCs).A critical issue of our method is the assumption of a correlation between visual heliocentric magnitudes and water production rates. This is a necessary assumption because of the sparse observational data of gas production rates (with the exception of very few comets like Hale-Bopp or Hyakutake). In this regard we present here a new correlation for LPCs. We also present the preliminary results for the masses of comets Hale-Bopp and Hyakutake.

A ballistics analysis of the Deep Impact ejecta plume: Determining Comet Tempel 1's gravity, mass, and density

In July of 2005, the Deep Impact mission collided a 366 kg impactor with the nucleus of Comet 9P/Tempel 1, at a closing speed of 10.2 km s −1. In this work, we develop a first-order, three-dimensional, forward model of the ejecta plume behavior resulting from this cratering event, and then adjust the model parameters to match the flyby-spacecraft observations of the actual ejecta plume, image by image. This modeling exercise indicates Deep Impact to have been a reasonably “well-behaved” oblique impact, in which the impactor–spacecraft apparently struck a small, westward-facing slope of roughly 1 / 3 – 1 / 2 the size of the final crater produced (determined from initial ejecta plume geometry), and possessing an effective strength of not more than Y ¯ = 1 – 10 kPa . The resulting ejecta plume followed well-established scaling relationships for cratering in a medium-to-high porosity target, consistent with a transient crater of not more than 85–140 m diameter, formed in not more than 250–550 s, for the case of Y ¯ = 0 Pa (gravity-dominated cratering); and not less than 22–26 m diameter, formed in not less than 1–3 s, for the case of Y ¯ = 10 kPa (strength-dominated cratering). At Y ¯ = 0 Pa , an upper limit to the total ejected mass of 1.8 × 10 7 kg ( 1.5 – 2.2 × 10 7 kg ) is consistent with measurements made via long-range remote sensing, after taking into account that 90% of this mass would have stayed close to the surface and then landed within 45 min of the impact. However, at Y ¯ = 10 kPa , a lower limit to the total ejected mass of 2.3 × 10 5 kg ( 1.5 – 2.9 × 10 5 kg ) is also consistent with these measurements. The expansion rate of the ejecta plume imaged during the look-back phase of observations leads to an estimate of the comet's mean surface gravity of g ¯ = 0.34 mm s −2 (0.17–0.90 mm s −2), which corresponds to a comet mass of m t = 4.5 × 10 13 kg ( 2.3 – 12.0 × 10 13 kg ) and a bulk density of ρ t = 400 kg m −3 (200–1000 kg m −3), where the large high-end error is due to uncertainties in the magnitude of coma gas pressure effects on the ejecta particles in flight.

A ballistics analysis of the Deep Impact ejecta plume: Determining Comet Tempel 1's gravity, mass, and density

In July of 2005, the Deep Impact mission collided a 366 kg impactor with the nucleus of Comet 9P/Tempel 1, at a closing speed of 10.2 km s −1. In this work, we develop a first-order, three-dimensional, forward model of the ejecta plume behavior resulting from this cratering event, and then adjust the model parameters to match the flyby-spacecraft observations of the actual ejecta plume, image by image. This modeling exercise indicates Deep Impact to have been a reasonably “well-behaved” oblique impact, in which the impactor–spacecraft apparently struck a small, westward-facing slope of roughly 1 / 3 – 1 / 2 the size of the final crater produced (determined from initial ejecta plume geometry), and possessing an effective strength of not more than Y ¯ = 1 – 10 kPa . The resulting ejecta plume followed well-established scaling relationships for cratering in a medium-to-high porosity target, consistent with a transient crater of not more than 85–140 m diameter, formed in not more than 250–550 s, for the case of Y ¯ = 0 Pa (gravity-dominated cratering); and not less than 22–26 m diameter, formed in not less than 1–3 s, for the case of Y ¯ = 10 kPa (strength-dominated cratering). At Y ¯ = 0 Pa , an upper limit to the total ejected mass of 1.8 × 10 7 kg ( 1.5 – 2.2 × 10 7 kg ) is consistent with measurements made via long-range remote sensing, after taking into account that 90% of this mass would have stayed close to the surface and then landed within 45 min of the impact. However, at Y ¯ = 10 kPa , a lower limit to the total ejected mass of 2.3 × 10 5 kg ( 1.5 – 2.9 × 10 5 kg ) is also consistent with these measurements. The expansion rate of the ejecta plume imaged during the look-back phase of observations leads to an estimate of the comet's mean surface gravity of g ¯ = 0.34 mm s −2 (0.17–0.90 mm s −2), which corresponds to a comet mass of m t = 4.5 × 10 13 kg ( 2.3 – 12.0 × 10 13 kg ) and a bulk density of ρ t = 400 kg m −3 (200–1000 kg m −3), where the large high-end error is due to uncertainties in the magnitude of coma gas pressure effects on the ejecta particles in flight.

Ptolemy – an Instrument to Measure Stable Isotopic Ratios of Key Volatiles on a Cometary Nucleus

A fundamental goal of cometary studies is to determine the exact relationship between these bodies and the Solar System – the question(s) can be summarised as follows: did comets originate during the same events that spawned the Sun and planets, are they more primitive bodies that record a pre-solar history, or are they interstellar materials collected in relatively more recent times? Now, whatever the origin of comets, it is entirely possible that they could, in part, contain interstellar or pre-solar components – indeed, it seems rather likely in light of the fact that primitive meteorites contain such entities. These particular components are likely to be refractory (dust, macromolecular organic complexes, etc.). Of more relevance to the issues above are the volatile constituents, which make up the bulk of a comet's mass. Since these materials, by their very nature, volatilise during perihelion passage of a comet they can, in some instances, be detected and measured spectroscopically. Perhaps the most useful species for isotopic investigations are C2, HCN and CN. Unfortunately, spectroscopic measurements can only currently be made with accuracies of ±10 to ±20%. As such it is very often not practical to conclude anything further than the fact that isotopic measurements are compatible with ‘`solar’' values, which tends to imply an origin from the margins of the solar accretion disk. But there is another problem with the spectroscopic measurements – since these are made on gaseous species in the coma (and relatively minor species at that) it is impossible to be certain that these represent the true nuclear values. In other words, if the processes of sublimation, active jetting, and photochemistry in the coma impart isotopic fractionation, the spectroscopic measurements could give a false impression of the true isotope ratios. What is required is an experiment capable of measuring isotopic ratios at the very surface of a comet. Herein we describe the Ptolemy instrument, which is included on the Philae lander as part of the Rosetta mission to 67P/Churyumov-Gerasimenko. The major objective of Ptolemy is a detailed appraisal of the nature and isotopic compositions of all materials present at the surface of a comet.

Non-gravitational force modeling of Comet 81P/Wild 2: I. A nucleus bulk density estimate

The nucleus of Comet 81P/Wild 2 is modeled by assuming various smooth triaxial ellipsoidal or irregular body shapes, having different rotational periods, spin axis orientations, and thermophysical properties. For these model nuclei, a large number of surface activity patterns (e.g., maps of active and inactive areas) are studied, and in each case the resulting water production rate and non-gravitational force vector versus time are calculated. By requiring that the model nuclei simultaneously reproduce certain properties of the empirical water production curve and non-gravitational changes of the orbit (focusing on changes of the orbital period and in the longitude of perihelion), constraints are placed on several properties of the nucleus. The simulations suggest that the mass of Comet 81P/Wild 2 is M ≲ 2.3 × 10 13 kg , resulting in a rather low bulk density, ρ bulk ≲ 600 – 800 kg m −3 (depending on the assumed nucleus volume), and that the nucleus rotation is prograde rather than retrograde. The active area fraction is difficult to constrain, but at most 60% of the nucleus is likely to have near-surface ice.

A cometary neutral gas simulator for gas dynamic sensor and mass spectrometer calibration

The goal of the European Space Agency's Rosetta mission is a rendezvous with the Edgewood‐Kuiper Belt comet 67P/Churyumov‐Gerasimenko. After the initial encounter the spacecraft will accompany the comet for one to two years toward the Sun and throughout the perihelion passage. For the calibration of the onboard comet mass spectrometers a calibration system was built to simulate the neutral gas environment around a cometary nucleus. This facility consists of a fully automated three‐chamber ultra‐high vacuum system with a separate gas‐mixing unit and a docking section with a five‐axis table for instrument positioning and support. The gas‐mixing unit produces mixtures of gases and also gas water vapor mixtures. The gas mixtures are fed into the vacuum system by a leak valve (static mode) or through a nozzle to form a molecular beam (dynamic mode). All flight and flight spare sensors of the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA) have been calibrated in the static mode of the Calibration System for the Mass Spectrometer Instrument ROSINA (CASYMIR). Preliminary calibration measurements in a pressure range from 10−9 to 10−6 mbar for carbon dioxide and for noble gases from helium to xenon show very promising results.

The Pioneer 10 anomalous acceleration and Oort cloud comets

Anderson et al. [Phys. Rev. D 65 (2002) 082004] recently reported new evidence that both Pioneer 10 and 11 are experiencing nearly the same unmodeled anomalous acceleration directed toward the Sun. Numerous mechanisms, both internal and external to the spacecraft, have been proposed to explain this unmodeled acceleration. If we assume that the cause of the anomalous acceleration is (1) external to the spacecraft, (2) isotropic, and (3) acts on bodies of cometary mass, then it would imply that new comets are more tightly bound to the Sun than previously believed. Here we show that the implied higher binding energy is incompatible with the established evidence that the galactic tide is dominant in making Oort cloud comets observable. We conclude that one or more of these assumptions must be invalid.