B. Hausler

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

Cometary nuclei consist mostly of dust and water ice. Previous observations have found nuclei to be low-density and highly porous bodies, 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) × 109 kilograms. Together with the current estimate of the volume of the nucleus, 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. 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.

The structure of Venus’ middle atmosphere and ionosphere

The atmosphere and ionosphere of Venus have been studied in the past by spacecraft with remote sensing or in situ techniques. These early missions, however, have left us with questions about, for example, the atmospheric structure in the transition region from the upper troposphere to the lower mesosphere (50–90 km) and the remarkably variable structure of the ionosphere. Observations become increasingly difficult within and below the global cloud deck (<50 km altitude), where strong absorption greatly limits the available investigative spectrum to a few infrared windows and the radio range. Here we report radio-sounding results from the first Venus Express Radio Science (VeRa) occultation season. We determine the fine structure in temperatures at upper cloud-deck altitudes, detect a distinct day–night temperature difference in the southern middle atmosphere, and track day-to-day changes in Venus’ ionosphere.

OUTFLOW STRUCTURE OF THE QUIET SUN CORONA PROBED BY SPACECRAFT RADIO SCINTILLATIONS IN STRONG SCATTERING

Radio scintillation observations have been unable to probe flow speeds in the low corona where the scattering of radio waves is exceedingly strong. Here we estimate outflow speeds continuously from the vicinity of the Sun to the outer corona (heliocentric distances of 1.5-20.5 solar radii) by applying the strong scattering theory to radio scintillations for the first time, using the Akatsuki spacecraft as the radio source. Small, nonzero outflow speeds were observed over a wide latitudinal range in the quiet-Sun low corona, suggesting that the supply of plasma from closed loops to the solar wind occurs over an extended area. The existence of power-law density fluctuations down to the scale of 100 m was suggested, which is indicative of well-developed turbulence which can play a key role in heating the corona. At higher altitudes, a rapid acceleration typical of radial open fields is observed, and the temperatures derived from the speed profile show a distinct maximum in the outer corona. This study opened up a possibility of observing detailed flow structures near the Sun from a vast amount of existing interplanetary scintillation data.

Gravity field determination of a Comet Nucleus: Rosetta at P/Wirtanen

One of the prime objectives of the Rosetta Radio Science Investigations (RSI) experiment is the determination of the mass, the bulk density and the low degree and order gravity of the nucleus of comet P/Wirtanen, the target object of the international Rosetta mission. The RSI experiment will use the spacecrafts radio carrier frequencies at X-band (8.4 GHz) and S-band (2.3 GHz) in order to measure slight changes of the orbit velocity via the classical Doppler effect induced by the gravity attraction of the comet nucleus. Based on an estimate of the background Doppler noise, it is expected that a mass determination (assuming a representative radius of 700 m and a bulk density of 500 kg/m^3) at an accuracy of 0.1% can be achieved if the spacecrafts orbit is iteratively reduced below 7 km altitude. The gravity field of degree and order two can be detected for reasonable tracking times below 5 km altitude. The major competing forces acting on the spacecraft are the radiation pressure and the gas mass flux from cometary activity. While the radiation pressure may be predicted, it is recommended to begin a gravity mapping campaign well before the onset of outgassing activity (>3.25 AU heliocentric distance). Radial acceleration by water outgassing is larger by orders of magnitude than the accelerations from the low degree and order gravity field and will mask the contributions from the gravity field.

Mass and density determination of 140 Siwa and 4979 Otawara as expected from the Rosetta flybys.

During its interplanetary cruise to comet P/Wirtanen, the Rosetta spacecraft will encounter the asteroids 4979 Otawara and 140 Siwa on 11 July 2006 and 24 July 2008, respectively. The objective of the Rosetta Radio Science Investigations (RSI) experiment at these flybys is a determination of the asteroids mass and bulk density by analyzing the radio tracking data (Doppler and range) received from Rosetta before, during and after closest approach. The spacecrafts flyby trajectory will be gravitationally deflected by an amount proportional to the mass of the asteroid for a given flyby distance and velocity. An analysis of the Doppler noise sources indicates that the mass can be determined to an accuracy of 1% for 140 Siwa. The corresponding bulk density show be accurate to 20% . Unfortunately, a detectable trajectory perturbation seems to be hopeless for Otawara because of its small size and the large nominal flyby distance.

Vertical Wavenumber Spectra of Gravity Waves in the Venus Atmosphere Obtained from Venus Express Radio Occultation Data: Evidence for Saturation

AbstractBy using the vertical temperature profiles obtained by the radio occultation measurements on the European Space Agency (ESA)’s Venus Express, the vertical wavenumber spectra of small-scale temperature fluctuations that are thought to be manifestations of gravity waves are studied. Wavenumber spectra covering wavelengths of 1.4–7.5 km were obtained for two altitude regions (65–80 and 75–90 km) and seven latitude bands. The spectra show a power-law dependence on the high-wavenumber side with the logarithmic spectral slope ranging from −3 to −4, which is similar to the features seen in Earth’s and Martian atmospheres. The power-law portion of the spectrum tends to follow the semiempirical spectrum of saturated gravity waves, suggesting that the gravity waves are dissipated by saturation as well as radiative damping. The spectral power is larger at 75–90 km than at 65–80 km at low wavenumbers, suggesting amplitude growth with height of unsaturated waves. It was also found that the wave amplitude is la...

Pre-flyby estimates of the precision of the mass determination of asteroid (21) Lutetia from Rosetta radio tracking

The Rosetta spacecraft will fly by its second target asteroid (21) Lutetia on 10 July 2010. Simulations based on the currently known size of Lutetia and assumptions on the bulk density show that tracking of two radio-carrier frequencies at X-band (8.4 GHz) and S-band (2.3 GHz) during the flyby will determine the mass at less than 1% accuracy. Derivation of the asteroid volume by camera observation will drive the uncertainty in derivation of the bulk density. Mass and bulk density provide valuable clues that might help resolve the difficulties in determining the taxonomic class of the asteroid.

Internal structure of a coronal mass ejection revealed by Akatsuki radio occultation observations

A coronal mass ejection (CME) was observed at the heliocentric distance of 12.7 Rυ by radio occultation measurements using the Akatsuki spacecraft. The temporal developments of the bulk velocity and the electron column density along the raypath traversing the CME were obtained, and under the assumption that the irregularities are transported across the raypath, the internal structure of the CME covering the region from the core to the tail was retrieved. The suggested internal structure was compared with Large Angle Spectroscopic Coronagraph images, a numerical study and previous radio occultation observations of CMEs to propose a CME model; the bulk velocity and the electron density have relatively large values in the core, decrease behind the core, and increase again in the tail region where the fast plasma flow associated with the magnetic reconnection converges. This implies that the magnetic reconnection behind the CMEs might continue up to at least the heliocentric distance of ∼13 Rυ.

The Challenges and Opportunities for International Cooperative Radio Science; Experience with Mars Express and Venus Express Missions

Radio Science is an opportunistic discipline in the sense that the communication link between a spacecraft and its supporting ground station can be used to probe the intervening media remotely. Radio science has recently expanded to greater, cooperative use of international assets. Mars Express and Venus Express are two such cooperative missions managed by the European Space Agency with broad international science participation supported by NASAs Deep Space Network (DSN) and ESAs tracking network for deep space missions (ESTRAK). This paper provides an overview of the constraints, opportunities, and lessons learned from international cross support of radio science, and it explores techniques for potentially optimizing the resultant data sets.

Local Time Dependence of the Thermal Structure in the Venusian Equatorial Upper Atmosphere: Comparison of Akatsuki Radio Occultation Measurements and GCM Results

Plain Language Summary Temperature profiles of the Venus atmosphere obtained by the Akatsuki radio occultation measurements showed a prominent local time dependence above 65-km altitude at low latitudes equatorward of 35 degrees. A zonal wavenumber 2 component is predominant in the temperature field, and its phase (i.e., isothermal) surfaces descend with local time, suggesting its downward phase propagation. A general circulation model (GCM) for the Venus atmosphere, AFES-Venus, reproduced the local time-dependent thermal structure qualitatively consistent with the radio occultation measurements. Based on a comparison between the radio occultation measurements and the GCM results, the observed zonal wavenumber 2 structure is attributed to the semidiurnal tide. Applying the dispersion relationship for internal gravity waves to the observed wave structure, the zonally averaged zonal wind speed at 75- to 85-km altitudes was found to be significantly smaller than that at the cloud top. The decrease of the zonal wind speed with altitude is attributed to the momentum deposition by the upwardly propagating semidiurnal tide excited in the cloud layer. Akatsuki radio occultation measurements showed the local time dependence of the Venus atmosphere in the equatorial region. By comparing the measurements with a general circulation model, it is attributed to the upward propagating semidiurnal tide generated in the cloud layer. And then, we proposed a new method to estimate the zonal wind speed above the cloud layer, where any optical instruments cannot be measured, for the first time.