Cometary nuclei consist mostly of dust and water ice(1). Previous observations have found nuclei to be low-density and highly porous bodies(2-4), but have only moderately constrained the range of allowed densities because of the measurement uncertainties. Here we report the precise mass, bulk density, porosity and internal structure of the nucleus of comet 67P/Churyumov-Gerasimenko on the basis of its gravity field. The mass and gravity field are derived from measured spacecraft velocity perturbations at fly-by distances between 10 and 100 kilometres. The gravitational point mass is GM = 666.2 +/- 0.2 cubic metres per second squared, giving a mass M = (9,982 +/- 3) x 10(9) kilograms. Together with the current estimate of the volume of the nucleus(5), the average bulk density of the nucleus is 533 +/- 6 kilograms per cubic metre. The nucleus appears to be a low-density, highly porous (72-74 per cent) dusty body, similar to that of comet 9P/Tempel 1(2,3). The most likely composition mix has approximately four times more dust than ice by mass and two times more dust than ice by volume. We conclude that the interior of the nucleus is homogeneous and constant in density on a global scale without large voids. The high porosity seems to be an inherent property of the nucleus material.

A homogeneous nucleus for comet 67P/Churyumov-Gerasimenko from its gravity field

Understanding the nature of the cometary nucleus remains one of the major problems in solar system science. Whipples (1950) icy conglomerate model has been very successful at explaining a range of cometary phenomena, including the source of cometary activity and the nongravitational orbital motion of the nuclei. However, the internal structure of the nuclei is still largely unknown. We review herein the evidence for cometary nuclei as fluffy aggregates or primordial rubble piles, as first proposed by Donn et al. (1985) and Weissman (1986). These models assume that cometary nuclei are weakly bonded aggregations of smaller, icy- onglomerate planetesimals, possibly held together only by self-gravity. Evidence for this model comes from studies of the accretion and subsequent evolution of material in the solar nebula, from observations of disrupted comets, and in particular comet Shoemaker-Levy 9, from measurements of the ensemble rotational properties of observed cometary nuclei, and from recent spacecraft missions to comets. Although the evidence for rubble pile nuclei is growing, the eventual answer to this question will likely not come until we can place a spacecraft in orbit around a cometary nucleus and study it in detail over many months to years. ESAs Rosetta mission, now en route to comet 67P/Churyumov- Gerasimenko, will provide that opportunity.

https://journals.uair.arizona.edu/index.php/maps/article/download/15601/15589

Structure and density of cometary nuclei

This chapter reviews the estimates of the dust-to-gas and refractory-to-ice mass ratios derived from Rosetta measurements in the lost materials and the nucleus of 67P/Churyumov-Gerasimenko, respectively. First, the measurements by Rosetta instruments are described, as well as relevant characteristics of 67P. The complex picture of the activity of 67P, with its extreme North-South seasonal asymmetry, is presented. Individual estimates of the dust-to-gas and refractory-to-ice mass ratios are then presented and compared, showing wide ranges of plausible values. Rosetta’s wealth of information suggests that estimates of the dust-to-gas mass ratio made in cometary comae at a single point in time may not be fully representative of the refractory-to-ice mass ratio within the cometary nuclei being observed.

/pdf/dust-to-gas-and-refractory-to-ice-mass-ratios-of-comet-67p-1wumruxhpa.pdf

Dust-to-Gas and Refractory-to-Ice Mass Ratios of Comet 67P/Churyumov-Gerasimenko from Rosetta Observations

The Mars Express Orbiter Radio Science (MaRS) experiment will employ radio occultation to (I) sound the neutral martian atmosphere to derive vertical density, pressure and temperature profiles as functions of height to resolutions better than 100 m, (II) sound the ionosphere to derive vertical ionospheric electron density profiles and a description of the ionosphere through its diurnal and seasonal variations with solar wind conditions; MaRS will also (III) determine the dielectric and scattering properties of the martian surface in target areas by a bistatic radar experiment, (IV) determine gravity anomalies for the investigation of the structure and evolution of the martian crust and lithosphere in conjunction with observations of the High Revolution Stereo Camera as a base for 3D topography, and (V) sound the solar corona during the superior conjunction of Mars with the Sun. The radio carrier links of the spacecraft Telemetry, Tracking and Command subsystem between the Orbiter and Earth will be used for these investigations. Simultaneous and coherent dual-frequency downlinks at X-band (8.4 GHz) and S-band (2.3 GHz) via the High Gain Antenna will permit separation of contributions from the classical Doppler shift and the dispersive media effects caused by the motion of the spacecraft with respect to the Earth and the propagation of the signals through the dispersive media, respectively. The investigation relies on the observation of the phase, amplitude, polarisation and propagation times of radio signals transmitted from the spacecraft and received with antennas on Earth. The radio signals are affected by the medium through which they propagate (atmospheres, ionospheres, interplanetary medium, solar corona), by the gravitational influence of the planet on the spacecraft and, finally, by the performances of the various systems aboard the spacecraft and on Earth.

MaRS: Mars Express Orbiter Radio Science

One of the two planetary cornerstone missions of the European Space Agency is the Rosetta mission to comet 67P/Churyumov-Gerasimenko. Rosetta is a rendezvous mission with a comet nucleus, which combines an Orbiter with a Lander. It will monitor the evolution of the comet nucleus and the coma as a function of increasing and decreasing solar flux input along the comet’s pre- and post-perihelion orbit. Different instrumentations will be used in parallel, from multi-wavelength spectrometry to in-situ measurements of coma and nucleus composition and physical properties. Rosetta will go in orbit around the nucleus of its target comet 67P/Churyumov-Gerasimenko, when it is still far from the Sun and accompany the comet along its way to perihelion and beyond. In addition the Rosetta Lander Philae will land on the nucleus surface, before the comet is too active to permit such a landing (i.e. at around r = 3 AU) and examine the surface and subsurface composition of the comet nucleus as well as its physical properties.

Rosetta—one comet rendezvous and two asteroid fly-bys

The Rosetta spacecraft has been successfully launched on 2nd March 2004 to its new target comet 67 P/Churyumov-Gerasimenko. The science objectives of the Rosetta Radio Science Investigations (RSI) experiment address fundamental aspects of cometary physics such as the mass and bulk density of the nucleus, its gravity field, its interplanetary orbit perturbed by nongravitational forces, its size and shape, its internal structure, the composition and roughness of the nucleus surface, the abundance of large dust grains, the plasma content in the coma and the combined dust and gas mass flux. The masses of two asteroids, Steins and Lutetia, shall be determined during flybys in 2008 and 2010, respectively. Secondary objectives are the radio sounding of the solar corona during the superior conjunctions of the spacecraft with the Sun during the cruise phase. The radio carrier links of the spacecraft Telemetry, Tracking and Command (TT&C) subsystem between the orbiter and the Earth will be used for these investigations. An Ultrastable oscillator (USO) connected to both transponders of the radio subsystem serves as a stable frequency reference source for both radio downlinks at X-band (8.4 GHz) and S-band (2.3 GHz) in the one-way mode. The simultaneous and coherent dual-frequency downlinks via the High Gain Antenna (HGA) permit separation of contributions from the classical Doppler shift and the dispersive media effects caused by the motion of the spacecraft with respect to the Earth and the propagation of the signals through the dispersive media, respectively. The investigation relies on the observation of the phase, amplitude, polarization and propagation times of radio signals transmitted from the spacecraft and received with ground station antennas on Earth. The radio signals are affected by the medium through which the signals propagate (atmospheres, ionospheres, interplanetary medium, solar corona), by the gravitational influence of the planet on the spacecraft and finally by the performance of the various systems involved both on the spacecraft and on ground.

Rosetta Radio Science Investigations (RSI)

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Rosetta Radio Science Investigations (RSI)