Publication archive

Publication archive

The presence of dry fluvial river channels and the intense cloud activity in the south pole of Titan over the past few years suggest the presence of methane rain. The nitrogen atmosphere of Titan therefore appears to support a methane meteorological cycle that sculptures the surface and controls its properties. Titan and Earth are the only worlds in the Solar System where rain reaches the surface, although the atmospheric cycles of water and methane are expected to be very different. Here we report three-dimensional dynamical calculations showing that severe methane convective storms accompanied by intense precipitation may occur in Titan under the right environmental conditions. The strongest storms grow when the methane relative humidity in the middle troposphere is above 80 per cent, producing updrafts with maximum velocities of 20 m s-1, able to reach altitudes of 30 km before dissipating in 5-8 h. Raindrops of 1-5 mm in radius produce precipitation rainfalls on the surface as high as 110 kg m-2 and are comparable to flash flood events on Earth.
Published: 28 July 2006
Titan is viewed as a sibling of Earth, as both bodies have rainy weather systems and landscapes formed by rivers. But as we study these similarities, Titan emerges as an intriguingly foreign world.
Published: 27 July 2006
Huygens provided an unanticipated bistatic radio scattering experiment from Titan's surface. After a successful entry and descent on Titan, on 14 January 2005, the probe remarkably survived the landing and continued radioing from the surface to the overflying Cassini, until the orbiter set below Titan's local horizon. Here we report high-quality measurements of the 2098 MHz (14.3 cm) postlanding radio signal, focusing on the striking variations observed in signal strength. The mechanism that creates this fading pattern is physically interpreted as multipath interference between the direct signal and the signal reflected on Titan's surface. The geophysical aspects that could bear on the signal analysis are described.
Published: 26 July 2006
Large radio telescopes on Earth tracked the radio signal transmitted by the Huygens probe during its mission at Titan. Frequency measurements were conducted as a part of the Huygens Doppler Wind Experiment (DWE) in order to derive the velocity of the probe in the direction to Earth. The DWE instrumentation on board Huygens consisted of an ultrastable oscillator which maintained the high Doppler stability required for a determination of probe horizontal motion during the atmospheric descent. A vertical profile of the zonal wind velocity in Titan's atmosphere was constructed from the Doppler data under the plausible assumption of generally small meridional wind, as validated by tracked images from the Huygens Descent Imager/Spectral Radiometer (DISR). We report here on improved results based on data with higher temporal resolution than that presented in the preliminary analysis by Bird et al. (2005), corroborating the first in situ measurement of Titan's atmospheric superrotation and a region of strong vertical shear reversal within the lower stratosphere. We also present the first high-resolution display and interpretation of the winds near the surface and planetary boundary layer.
Published: 21 July 2006
Contents: Hubble News Update; Staff Changes; The AstroAsciiData Python Module; Creating a Legacy Archive for Hubble; The New ST-ECF Web; Scisoft VI
Published: 16 July 2006
An electronic collisional-radiative model is proposed to predict the nonequilibrium populations and the radiation of the excited electronic states CN(A, B) and N2 (A, B, C) during the entry of the Huygens probe into the atmosphere of Titan. The model is loosely coupled with flow solvers using a Lagrangian method. First, the model was tested against measurements obtained with the shock-tube of NASA Ames Research Center. Then, the model was applied to the simulation of Huygen's entry. Our simulations predict that the population of the CN(B) state is lower than the Boltzmann population by a factor 40 at trajectory time t = 165 s and by a factor 2 at t = 187 s and that the population of the CN(A) state remains close to the Boltzmann population for both trajectory points. The radiative heat fluxes, driven by the CN(A, B) states, are lower than predictions based on the Boltzmann populations by a factor 15 at t = 165 s and a factor 2 at t = 187 s.
Published: 15 July 2006
In the proceedings of the 6th International Conference on Space Optics

This paper summarizes the results of an ESA feasibility study of a Wide-Field Optical Infrared Imager (WFI) that would search for Type Ia supernovae at low redshift with the aim to measure the changing rate of expansion of the universe. WFI multi-spectral images of the deep universe could also benefit to many other research area in astrophysics. The WFI payload includes a 2 m class telescope, a 1 square degree field of view imaging camera and a low-resolution integral field spectrometer. A mission concept was identified that consists of a 2000 kg spacecraft launched by a Soyuz-Fregat into a L2 halo orbit. The WFI mission could benefit from the technology developed for the ESA Herschel and Gaia missions and for the NIRSpec ESA instrument. A fully European WFI mission would require improvement of existing European detector and on-board processor technology as well as some effort to support the utilization of the 26 GHz Ka band.

Published: 14 July 2006
In the context of the Cassini/Huygens mission, we performed supporting ground-based observations to complement the results from the NASA/ESA/ASI space mission to the Saturnian system with particular focus on Titan. On the nights of 18 and 19 December 2004, we conducted adaptive optics observations with VLT/NACO to search for and map the distribution of CO2 ice deposits on the spatially resolved surface of Titan (65 mas resolution). This experiment became possible because (1) solid CO2 possesses two narrow and strong absorption lines at 2012 nm and 2070 nm that fall into the 2.05 micron window of Titan's atmosphere and (2) the width of these bands matches the band pass of the Fabry-Perot instrument installed in NACO. We do not detect this chemical compound, but we can derive firm limits on the abundance of CO2 ice on the surface of Titan at sub-Earth longitudes 284° W and 307° W. With a spatial sampling of 65 mas, we conclude that a partial surface coverage of segregated CO2 ice does not exceed 7% or 14% for bright or dark surface regions, respectively.
Published: 07 July 2006
The profile of intense high-altitude electric fields on auroral field lines has been studied using Cluster data. A total of 41 events with mapped electric field magnitudes in the range between 0.5-1 V/m were examined, 27 of which were co-located with a plasma boundary, defined by gradients in particle flux, plasma density and plasma temperature. Monopolar electric field profiles were observed in 11 and bipolar electric field profiles in 16 of these boundary-associated electric field events. Of the monopolar fields, all but one occurred at the polar cap boundary in the late evening and midnight sectors, and the electric fields were typically directed equatorward, whereas the bipolar fields all occurred at plasma boundaries clearly within the plasma sheet. These results support the prediction by Marklund et al. (2004), that the electric field profile depends on whether plasma populations, able to support intense field-aligned currents and closure by Pedersen currents, exist on both sides, or one side only, of the boundary.
Published: 04 July 2006
Plasmaspheric plumes have been routinely observed by CLUSTER and IMAGE. The CLUSTER mission provides high time resolution four-point measurements of the plasmasphere near perigee. Total electron density profiles have been derived from the electron plasma frequency identified by the WHISPER sounder supplemented, in-between soundings, by relative variations of the spacecraft potential measured by the electric field instrument EFW; ion velocity is also measured onboard these satellites. The EUV imager onboard the IMAGE spacecraft provides global images of the plasmasphere with a spatial resolution of 0.1 RE every 10 min; such images acquired near apogee from high above the pole show the geometry of plasmaspheric plumes, their evolution and motion. We present coordinated observations of three plume events and compare CLUSTER in-situ data with global images of the plasmasphere obtained by IMAGE. In particular, we study the geometry and the orientation of plasmaspheric plumes by using four-point analysis methods. We compare several aspects of plume motion as determined by different methods: (i) inner and outer plume boundary velocity calculated from time delays of this boundary as observed by the wave experiment WHISPER on the four spacecraft, (ii) drift velocity measured by the electron drift instrument EDI onboard CLUSTER and (iii) global velocity determined from successive EUV images. These different techniques consistently indicate that plasmaspheric plumes rotate around the Earth, with their foot fully co-rotating, but with their tip rotating slower and moving farther out.
Published: 04 July 2006
Another approach (Multiple Triangulation Analysis, MTA) is presented to determine the orientation of magnetic flux rope, based on 4-point measurements. A 2-D flux rope model is used to examine the accuracy of the MTA technique in a theoretical way. It is found that the precision of the estimated orientation is dependent on both the spacecraft separation and the constellation path relative to the flux rope structure. However, the MTA error range can be shown to be smaller than that of the traditional MVA technique. As an application to real Cluster data, several flux rope events on 26 January 2001 are analyzed using MTA, to obtain their orientations. The results are compared with the ones obtained by several other methods which also yield flux rope orientation. The estimated axis orientations are shown to be fairly close, suggesting the reliability of the MTA method.
Published: 04 July 2006
BepiColombo is the name of an ESA cornerstone mission to Mercury with a launch scheduled for 2013. After a journey of 6 years, two probes, the Magnetospheric Orbiter (JAXA) and the Planetary Orbiter (ESA), will reach their target orbits. The interplanetary trajectory includes multiple planetary flybys as well as several low-thrust arcs provided by the solar electric propulsion module. The navigation analysis of such a trajectory requires the implementation of complex guidance laws for the low thrust arcs, and the inclusion of trim manoeuvres near each planet encounter. Further challenges are imposed by the occurrence of solar conjunctions, noise introduced by the thrust, limited range and Doppler availability, stringent solar aspect angle constraints, and the definition of recovery solutions in case of thrust outages. A baseline trajectory taking into account all these operational constraints is here presented. The software tools LOTNAV and INTNAV were used to perfom a navigation analysis for the interplanetary trajectory.
Published: 03 July 2006
BepiColombo is an interdisciplinary mission to the planet Mercury that has been selected as the 5th cornerstone in the Cosmic Vision programme of the European Space Agency (ESA). Owing to the high scientific potential related to the planet and its environment the mission will open a new frontier in the study of our solar system. Planned for launch in 2013, BepiColombo is a collaboration between ESA and the Japan Aerospace Exploration Agency (JAXA). It consists of two scientific orbiters, the Mercury Planetary Orbiter (MPO) and the Mercury Magnetospheric Orbiter (MMO), which will study the origin and evolution of the planet, Mercury's interior dynamics and the origin of the magnetic field. The interplanetary trajectory includes flybys at the Moon, Earth, Venus (twice) and Mercury (twice), as well as several long thrust arcs provided by solar electric propulsion. The Mercury orbit capture and lowering to the operational orbits will be done with chemical propulsion. The launch configuration consists of a stack of the two spacecraft and the chemical and electrical propulsion modules.
Published: 03 July 2006
A conventional Mercury sample return mission requires significant outbound and return trips, and the large mass of a planetary lander and ascent vehicle. In this paper, it is shown that solar sailing can be used to reduce lander mass allocation by delivering the lander to a low, thermally safe orbit close to the planetary terminator. In addition, the ascending node of the solar sail parking orbit plane can be artificially forced to avoid out-of-plane manoeuvres during ascent from the planetary surface. Propellant mass is not an issue for solar sails, so a sample can be returned relatively easily, without resorting to lengthy, multiple gravity assists. A 275 m square solar sail with a sail assembly loading of 5.9 g m-2 is used to deliver a lander, cruise stage and science payload to a forced Sun-synchronous orbit at Mercury in 2.85 years. The lander acquires samples, and conducts limited surface exploration. An ascent vehicle delivers a small cold gas rendezvous vehicle containing the samples for transfer to the solar sail. The solar sail then spirals back to Earth in one year. The total mission launch mass is 2353 kg, on an H2A202-4S class launch vehicle (C3=0). Extensive launch date scans have revealed an optimal launch date in April 2014 with sample return to Earth 4.4 years later. Solar sailing reduces launch mass by 60% and trip time by 40%, relative to conventional mission concepts. In comparison, mission analysis has demonstrated that solar sail Mars and Venus sample return appears to have only modest benefit in terms of reduced launch mass, at the expense of longer mission durations than conventional propulsion systems.
Published: 02 June 2006
Magnetic reconnection is one of the most important processes in astrophysical, space and laboratory plasmas. Identifying the structure around the point at which the magnetic field lines break and subsequently reform, known as the magnetic null point, is crucial to improving our understanding of reconnection. But owing to the inherently three-dimensional nature of this process, magnetic nulls are only detectable through measurements obtained simultaneously from at least four points in space. Using data collected by the four spacecraft of the Cluster constellation as they traversed a diffusion region in the Earth's magnetotail on 15 September 2001, we report here the first in situ evidence for the structure of an isolated magnetic null. The results indicate that it has a positive-spiral structure whose spatial extent is of the same order as the local ion inertial length scale, suggesting that the Hall effect could play an important role in 3D reconnection dynamics.
Published: 02 June 2006
This article is based on a talk given by Dr. Oliver Botta to the PRO ISSI Association.
Published: 02 June 2006
We present the results of a five-day monitoring campaign with xmm of six X-ray bright Young Stellar Objects in the star-forming complex L1551, in Taurus. All stars present significant variability on the five-day time scale. Modulation of the light curve on time scales comparable with the stars rotational period appear to be present in the case of one WTTS. The CTTS XZ~Tau is the only star showing significant spectral variations between the 2000 and the 2004 observations: a hot plasma component which was present in the X-ray spectrum in 2000 has disappeared. As XZ~Tau was undergoing a strong (EXor-like) optical outburst in 2000, which has since then terminated, we speculate on the possible relationship between episodic, burst accretion and X-ray heating. The transition object HL~Tau undergoes a strong flare with a complex temperature evolution, indicative of an event confined within a very large magnetic structure (few stellar radii), similar to the ones found in YSOs in the Orion Nebula Cluster.
Published: 02 June 2006
Magnetic reconnection is one of the most important processes in astrophysical, space and laboratory plasmas. Identifying the structure around the point at which the magnetic field lines break and subsequently reform, known as the magnetic null point, is crucial to improving our understanding of reconnection. But owing to the inherently three-dimensional nature of this process, magnetic nulls are only detectable through measurements obtained simultaneously from at least four points in space. Using data collected by the four spacecraft of the Cluster constellation as they traversed a diffusion region in the Earth's magnetotail on 15 September 2001, we report here the first in situ evidence for the structure of an isolated magnetic null. The results indicate that it has a positive-spiral structure whose spatial extent is of the same order as the local ion inertial length scale, suggesting that the Hall effect could play an important role in 3D reconnection dynamics.
Published: 02 June 2006
Determining the origin and composition of asteroids is a key step in understanding the nature of the solar system. Believed to be a captured asteroid, Deimos, Mars's moon, is therefore of dual scientific interest. The upper regolith of the moon contains Martian material accreted during the late heavy bombardment period. Retrieving a sample from Deimos would contain both asteroidal and Martian material. The perceived scientific interest in Deimos, and for small body sample return missions, are the key reasons that Deimos Sample Return (DSR) was chosen as one of ESA's Technology Reference Studies.

Technology Reference Studies (TRS) are a technology development tool introduced by ESA's Science Payload and Advanced Concepts Office, whose purpose is to provide a focus for the development of strategically important technologies that are of likely future relevance for scientific missions. This is accomplished through the study of several technologically demanding and scientifically interesting missions, which are currently not part of the ESA science programme.

The goal of the DSR TRS is to study the feasibility and the technologies required to collect a scientifically significant sample of regolith from Deimos' surface and return it to Earth. The DSR mission profile consists of a small spacecraft, launched on a Soyuz-Fregat 2B. After transferring to the Martian system, the spacecraft will enter into a co-orbit with Deimos where it will perform remote sensing observations and ultimately perform a series of sampling manoeuvres. Upon completion of sampling the spacecraft will return to Earth, where the sample canister will perform a direct Earth entry.

Published: 08 February 2006
Using the IBIS Compton mode, the INTEGRAL satellite is able to detect and localize bright and hard GRBs, which happen outside of the nominal INTEGRAL field of view. We have developed a method of analyzing such INTEGRAL data to obtain the burst location and spectra. We present the results for the case of GRB 030406. The burst is localized with the Compton events, and the location is consistent with the previous Interplanetary Network position. A spectral analysis is possible by detailed modeling of the detector response for such a far off-axis source with an offset of 36.9°. The average spectrum of the burst is extremely hard: the photon index above 400 keV is -1.7, with no evidence of a break up to 1.1 MeV at a 90% confidence level.
Published: 16 June 2006
23-Sep-2020 06:55 UT

ShortUrl Portlet

Shortcut URL

https://sci.esa.int/p/dAGeRrW