Entry Probe Research Articles

A smoothing technique for improving atmospheric reconstruction for planetary entry probes

Accelerometer instruments are commonly used on planetary entry probes to determine vertical profiles of atmospheric density, pressure, and temperature. A key consideration for the design and implementation of such investigations is extending the atmospheric results to the highest altitudes possible, which requires minimizing uncertainties in measured accelerations whilst maintaining adequate vertical resolution. Since atmospheric density depends exponentially on altitude, the arithmetic mean of a subset of raw acceleration data points is a biased estimate of the true acceleration at the center of the time interval in question. This diminishes the quality of derived atmospheric properties. Here we show how this problem can be alleviated by using a specialized averaging technique that takes advantage of the inherent exponential variation in acceleration with time at atmospheric entry. This technique is demonstrated successfully on Mars Phoenix data.

Radiative-Collisional Models in Non-Equilibrium Aerothermodynamics of Entry Probes

To design space vehicles aimed for returning payloads from a geostationary orbit, the Moon and other large or small planets of Solar system, a knowledge of the total (convective and radiative) heating from an environment is required. It is well known that the radiative heat load on a space vehicle moving through the atmosphere increases as the speed and the size increase, therefore, in many of these missions the large part of the trajectory will pass at high altitude, where the low atmospheric density can lead to significant thermal, chemical and physical nonequilibrium effects. Physical models and computational codes used to predict the aerothermodynamics must account for not only high temperature equilibrium thermodynamics (as a rule, within the framework of the local thermodynamic equilibrium (LTE) approach), but also for nonequilibrium one. Therefore, an accurate prediction of radiative heating as well as convective one under both equilibrium and nonequilibrium conditions becomes important to designers and space mission planners. To develop a prediction computational fluid dynamics (CFD) tool for reentry flows, where dissociation, ionization and radiation are important, some major areas are addressed. The most significant of them are following: (1) physical-chemical kinetics of high temperature dissociated and ionized gases, (2) transport properties of the gas mixtures, (3) spectral radiation properties of high temperature gases and low-temperature plasmas, (4) numerical simulation algorithms for prediction of nonequilibrium gas mixtures dynamics and radiation heat transfer in volumes of various geometry, and (5) models of physical and chemical processes accompanied by interaction of gas flows and radiation with thermoprotection systems (TPS) of space vehicles (including their thermochemical destruction, ablation, sublimation, etc.). In literatures (See Refs. (Park, C, 1990, Nonequilibrium Hypersonic Aerothermodynamics, Willey-Interscience Publication, J. Wiley & Sons, New York; Park, C., 1993, “Review of Chemical Kinetic Problems of Future NASA Missions. I: Earth Entries,” J. Thermophys. Heat Transfer, 7(3), pp. 385–398; Park, et al., 1994, “Review of Chemical-Kinetic Problems of Future NASA Missions, II: Mars Entries,” J. Thermophys. Heat Transfer, 8(1), pp. 9–23; Sarma, G., 2000, “Physico-Chemical Modelling in Hypersonic Flow Simulation,” Prog. Aerosp. Sci., 36, pp. 281–349; Huo, and Thuemmel, 1995, Electron-Air Molecule Collisions in Hypersonic Flows. Molecular Physics and Hypersonic Flows, Capitelli M., ed., Kluwer Academic Publishers, pp. 115–138.)) one can find reviews of governing equations used in the aerophysics, boundary conditions and the associated inputs using the physical-chemical models and their partially successful applications. This article presents the states of the art of models of electronic kinetics in the nonequilibrium low-temperature plasma of complex chemical compositions (air and carbon dioxide mixtures) widely met in various aerospace applications. Special attention is given to electronic kinetics of atoms and diatomic molecules within the framework of the radiative-collisional models.

QUENCHING OF CARBON MONOXIDE AND METHANE IN THE ATMOSPHERES OF COOL BROWN DWARFS AND HOT JUPITERS

We explore CO-CH4 quench kinetics in the atmospheres of substellar objects using updated time-scale arguments, as suggested by a thermochemical kinetics and diffusion model that transitions from the thermochemical-equilibrium regime in the deep atmosphere to a quench-chemical regime at higher altitudes. More specifically, we examine CO quench chemistry on the T dwarf Gliese 229B and CH4 quench chemistry on the hot-Jupiter HD 189733b. We describe a method for correctly calculating reverse rate coefficients for chemical reactions, discuss the predominant pathways for CO-CH4 interconversion as indicated by the model, and demonstrate that a simple time-scale approach can be used to accurately describe the behavior of quenched species when updated reaction kinetics and mixing-length-scale assumptions are used. Proper treatment of quench kinetics has important implications for estimates of molecular abundances and/or vertical mixing rates in the atmospheres of substellar objects. Our model results indicate significantly higher Kzz values than previously estimated near the CO quench level on Gliese 229B, whereas current model-data comparisons using CH4 permit a wide range of Kzz values on HD 189733b. We also use updated reaction kinetics to revise previous estimates of the Jovian water abundance, based upon the observed abundance and chemical behavior of carbon monoxide. The CO chemical/observational constraint, along with Galileo entry probe data, suggests a water abundance of approximately 0.51-2.6x solar (for a solar value of H2O/H2=9.61E-04) in Jupiter's troposphere, assuming vertical mixing from the deep atmosphere is the only source of tropospheric CO.

Contrast sources for the infrared images taken by the Venus mission AKATSUKI

A feasibility study has been carried out to look for the source of contrast that will be seen in the infrared images of Venus taken by the cameras on board AKATSUKI. This procedure used a cloud model based on the measurements of previous entry probes and radiative transfer calculations. The source of the small contrast expected in the 0.90-/xm dayside image was found to be due to inhomogeneity in the cloud optical thickness, with the variations in cloud altitude and temperature having little effect. The source of the large contrast expected in the 2.26-μm nightside image was also found to be due to inhomogeneity in the cloud optical thickness. We attempted to determine the representative altitude of the cloud layers, but this could not be specified to one particular layer. The brightness expected in the 2.02-μm dayside image was found to be affected by the cloud altitude, as expected since the 2.02-μm is in a moderate CO2 absorption band; however, it is also affected by the cloud optical thickness. The brightness expected in the 10-μm image was found to be affected mostly by temperature; however, the effects due to cloud optical thickness and cloud altitude are also important. It is necessary to use all of the information obtained at various wavelengths to gain a correct understanding of the brightness distribution of the Venus images taken by AKATSUKI.

Comparison of velocity and temperature measurements with simulations in a hypersonic wake flow

A hypersonic shock-tunnel flow around an axisymmetric model of a planetary entry probe is analyzed. Planar laser-induced fluorescence is applied to measure both the velocity and the rotational temperature everywhere in the central plane of the flow field. The experimental test case is compared to simulations using the direct simulation Monte Carlo (DSMC) method. While the Mach 9.7 flow at a freestream Reynolds number based on the model diameter of 35,000 is chemically frozen, effects of thermal non-equilibrium and localized rarefaction cannot be neglected. DSMC and measurements agree well within the outer wake, but disagree close to the centerline, where in particular the measured velocity is higher than values predicted by the simulations. The experimental results indicated a shorter recirculation region and increased local fluctuations in the free shear layer upstream of the wake recompression shock when compared to the simulation. These effects are attributed to incipient transition, which is not observed in the simulations, as the simulations did not model the effects of freestream fluctuations. Furthermore, measured and simulated vorticities are compared with theoretical predictions.

Trajectory and atmospheric structure from entry probes: Demonstration of a real-time reconstruction technique using a simple direct-to-Earth radio link

The reconstruction of the trajectory and atmospheric structure associated with an entry probe has traditionally relied upon onboard accelerometer measurements. Here we outline an equivalent reconstruction technique that uses Doppler-shifted direct-to-Earth transmissions instead. A critical assumption is that the entry probe's angle of attack is zero. The technique is successfully demonstrated on the atmospheric entry of the Mars Exploration Rover Opportunity, terminating at parachute deployment. This technique can be applied in real-time, which supports mission operations and public engagement. It can also be applied to entry probes that fail during their high-risk atmospheric entry.

Two-component Doppler-shift fluorescence velocimetry applied to a generic planetary entry probe model

This study discusses the development and application of planar laser-induced fluorescence of nitric oxide (NO PLIF) to measure velocities in an axisymmetric hypersonic near-wake flow field around a model planetary-entry vehicle configuration. Shapes and positions of NO spectral lines at every location in the flow are determined over several successive shock tunnel runs. The lines experience Doppler shifts proportional to the local flow velocity component in the direction of the fluorescence-generating laser. A Gaussian line shape function is then fitted to the acquired wavelength-dependent fluorescence measurements, the line center of which is correlated to the time-averaged velocity at each pixel location. The flow field is probed successively by a laser in two orthogonal directions, which yields the velocity magnitude and direction everywhere in the illuminated plane. The accuracy of the measurement technique is discussed, and various strategies to characterize systematic errors are presented. The variation of random uncertainties in different regions of the flow field provides information about the local steadiness of the flow. To the authors’ knowledge, the measurements represent the first two-component velocity map of a hypersonic near-wake flow.

Radiofrequency Ablation of Facial Nerve Branches Controlling Glabellar Frowning

Hyperdynamic activity of the corrugator supercilii and procerus muscles causes glabellar furrows. Recently, a novel radiofrequency device has become available that can effectively ablate the efferent nerves controlling corrugator and procerus contraction, producing clinical results that are similar to those of botulinum toxin. To assess the efficacy, longevity of effect, and side effects of the radiofrequency ablation device in the treatment of hyperdynamic glabellar furrows. Four probe entry points were used to access branches of the temporal and angular nerves. Seven and two ablations, respectively, were delivered to each temporal branch and angular nerve. Twenty-nine patients underwent bilateral radiofrequency ablation of temporal branches of the facial nerve and the angular nerves. Abrogation of glabellar furrowing was achieved in 90% of patients. No major adverse events were observed. All patients developed mild to moderate swelling, and nine patients (31%) developed purpura in the treated areas. Sixty-nine percent of patients had effects that lasted 4 months or longer, 41% had effects that lasted 6 months or longer, and 10% had effects lasting longer than 12 months. Radiofrequency ablation of efferent branches of the temporal and angular nerves effectively eliminates corrugator and procerus contraction and concomitant glabellar furrowing.

Saturn Satellites as Seen by Cassini Mission

In this paper we will summarize some of the most important results of the Cassini mission concerning the satellites of Saturn. The Cassini Mission was launched in October 1997 on a Titan IV-Centaur rocket from Cape Canaveral. Cassini mission was always at risk of cancelation during its development but was saved many times thanks to the great international involvement. The Cassini mission is in fact a NASA-ESA-ASI project. The main effort was made by NASA, which provided the launch facilities, the integration and several instruments; ESA provided the Huygens probe while ASI some of the key elements of the mission such as the high-gain antenna, most of the radio system and important instruments of the Orbiter, such as the Cassini Radar and the visual channel of the VIMS experiment. ASI contributed also to the development of HASI experiment on Huygens probe. The Cassini mission was the first case in which the Italian planetology community was directly involved, developing state of the art hardware for a NASA mission. Given the long duration of the mission, the complexity of the payload onboard the Cassini Orbiter and the amount of data gathered on the satellites of Saturn, it would be impossible to describe all the new discoveries made, therefore we will describe only some selected, paramount examples showing how Cassini’s data confirmed and extended ground-based observations. In particular we will describe the achievements obtained for the satellites Phoebe, Enceladus and Titan. We will also put these examples in the perspective of the overall evolution of the system, stressing out why the selected satellites are representative of the overall evolution of the Saturn system. Cassini is also an example of how powerful could be the coordination between ground-based and space observations. In fact coordinated ground-based observations of Titan were performed at the time of Huygens atmospheric probe mission at Titan on 14 January 2005, connecting the in situ observations by the probe with the general view provided by ground-based measurements. Different telescopes operating at different wavelengths were used, including radio telescopes (up to 17-tracking of the Huygens signal at 2040 MHz), eight large optical observatories studying the atmosphere and surface of Titan, and high-resolution infrared spectroscopy used to observe radiation emitted during the Huygens Probe entry (Witasse et al. J. Geophys. Res. 111:E07S01, 2006).

Thermal protection system development, testing, and qualification for atmospheric probes and sample return missions: Examples for Saturn, Titan and Stardust-type sample return

The science community has continued to be interested in planetary entry probes, aerocapture, and sample return missions to improve our understanding of the Solar System. As in the case of the Galileo entry probe, such missions are critical to the understanding not only of the individual planets, but also to further knowledge regarding the formation of the Solar System. It is believed that Saturn probes to depths corresponding to 10 bars will be sufficient to provide the desired data on its atmospheric composition. An aerocapture mission would enable delivery of a satellite to provide insight into how gravitational forces cause dynamic changes in Saturn’s ring structure that are akin to the evolution of protoplanetary accretion disks. Heating rates for the “shallow” Saturn probes, Saturn aerocapture, and sample Earth return missions with higher re-entry speeds (13–15 km/s) from Mars, Venus, comets, and asteroids are in the range of 1–6 KW/cm 2. New, mid-density thermal protection system (TPS) materials for such probes can be mission enabling for mass efficiency and also for use on smaller vehicles enabled by advancements in scientific instrumentation. Past consideration of new Jovian multiprobe missions has been considered problematic without the Giant Planet arcjet facility that was used to qualify carbon phenolic for the Galileo probe. This paper describes emerging TPS technologies and the proposed use of an affordable, small 5 MW arcjet that can be used for TPS development, in test gases appropriate for future planetary probe and aerocapture applications. Emerging TPS technologies of interest include new versions of the Apollo Avcoat material and a densified variant of Phenolic Impregnated Carbon Ablator (PICA). Application of these and other TPS materials and the use of other facilities for development and qualification of TPS for Saturn, Titan, and Sample Return missions of the Stardust class with entry speeds from 6.0 to 28.6 km/s are discussed.

High-Fidelity Aerothermal Engineering Analysis for Planetary Probes Using DOTNET Framework and OLAP Cubes Database

This publication presents the architecture integration and implementation of various modules inSpartaframework.Spartais a trajectory engine that is hooked to an Online Analytical Processing (OLAP) database for Multi-dimensional analysis capability. OLAP is an Online Analytical Processing database that has a comprehensive list of atmospheric entry probes and their vehicle dimensions, trajectory data, aero-thermal data and material properties like Carbon, Silicon and Carbon-Phenolic based Ablators. An approach is presented for dynamic TPS design. OLAP has the capability to run in one simulation several different trajectory conditions and the output is stored back into the database and can be queried for appropriate trajectory type. An OLAP simulation can be setup by spawning individual threads to run for three types of trajectory:Nominal,Undershoot and Overshoot trajectory. Sparta graphical user interface provides capabilities to choose from a list of flight vehicles or enter trajectory and geometry information of a vehicle in design. DOTNET framework acts as a middleware layer between the trajectory engine and the user interface and also between the web user interface and the OLAP database. Trajectory output can be obtained in TecPlot format, Excel output or in a KML (Keyhole Markup Language) format. Framework employs an API (application programming interface) to convert trajectory data into a formatted KML file that is used by Google Earth for simulating Earth-entry fly-by visualizations.

Metastable oxygen atom velocity and temperature in supersonic CO2 plasma expansions

Radial and axial velocity distribution functions of metastable oxygen atoms were obtained by way of resonant laser induced fluorescence spectroscopy at 777.19 nm in rarefied supersonic plasma jets produced from pure CO2 and CO2–N2 gas mixtures. The N2 seeded fraction is 3% in volume in order to mimic the Martian atmosphere composition. The measured lineshapes allow computation of the mean radial velocity and the perpendicular temperature, with respect to the jet axis, as well as the mean axial velocity and the parallel temperature. With an arcjet plasma source input power of 6.5 kW, the metastable O(5S) atom axial velocity is around 4500 m s−1 and the atom perpendicular temperature reaches 4500 K at the nozzle outlet. Under identical voltage and current conditions, a small addition of N2 does not change the measured values, i.e. it does not modify the overall energy balance. Distribution of flow parameters along both the radial and the axial directions reveals the existence of a barrel shock wave and a stationary shock front, which originate in shock wave regular reflexion. Flow properties, shock wave structure, departure from thermal equilibrium and impact of the applied power are debated for the two gas mixtures. Finally, the capacity of supersonic plasma expansions to simulate interplanetary probe entry into the Martian atmosphere is discussed in the light of experimental outcomes.

A Massive Core in Jupiter Predicted from First-Principles Simulations

Hydrogen-helium mixtures at conditions of Jupiter's interior are studied with first-principles computer simulations. The resulting equation of state (EOS) implies that Jupiter possesses a central core of 14-18 Earth masses of heavier elements, a result that supports core accretion as standard model for the formation of hydrogen-rich giant planets. Our nominal model has about 2 Earth masses of planetary ices in the H-He-rich mantle, a result that is, within modeling errors, consistent with abundances measured by the 1995 Galileo Entry Probe mission (equivalent to about 5 Earth masses of planetary ices when extrapolated to the mantle), suggesting that the composition found by the probe may be representative of the entire planet. Interior models derived from this first-principles EOS do not give a match to Jupiter's gravity moment J4 unless one invokes interior differential rotation, implying that jovian interior dynamics has an observable effect on the measured gravity field.

Enceladus Mission Architecture Using Titan Aerogravity Assist for Orbital Capture About Saturn

R ECENTobservations of the Saturn system have generated great interest in the moon Enceladus. Water geysers have been identified in the southern hemisphere and have been found to contain possible organic compounds [1]. These observations strongly support a return to the Saturn system specifically designed to evaluate Enceladus in greater detail and perhaps to return a sample from the geyser’s plume. We previously described an aerogravity assist (AGA) maneuver, in which Titan’s atmosphere and gravitational field are used to decelerate a spacecraft and deflect its trajectory, resulting in a closed orbit about Saturn [2]. A similar concept using wave-rider configurations was described by Randolph and McRonald [3,4] and evaluated using an analytical approach. In contrast, we have numerically simulated AGA trajectories for low and medium lift-todrag (L=D) vehicles. In this paper, we describe Titan AGA maneuvers in greater depth, and evaluate their potential for orbital capture of a Cassini-class vehicle. The targeted Saturn orbit has a periapsis near the radius of Enceladus’s orbit, allowing repeated close passes. This orbit can be achieved by directing the outbound hyperbolic excess speed (V1) with respect to Titan nearly opposite in direction to Titan’s orbital velocity, resulting in a final apoapsis near Titan’s orbital radius. Depending on the interplanetary trajectory and the exact Titan encounter location, the proposed maneuver would provide a velocity increment ( V) in excess of 6 km=s. Titan has a near-circular, equatorial orbit about Saturn at a radius from the planetary center of 1:22 10 km. Its orbital velocity (VT) and period are 5:58 km=s and 15.95 Earth days, respectively. Titan’s orbit lies well outside the ring system, which extends in the equatorial plane to a radius of approximately 480,000 km. Enceladus orbits in nearly the same plane, at a radius of 238,000 km andwith an orbital period of 32.9 h. The mission concept is illustrated in Fig. 1, where the upper diagram shows Titan’s velocity vector and that of the probe before and after the AGA maneuver, and the lower panel illustrates the range of potential directions for the outbound V1, any of which will produce a periapsis at Enceladus’s orbital radius. To achieve this desired periapsis, the probe’s velocity immediately after completion of the Titan AGA should have a magnitude 3:18 km=s with respect to (WRT) Saturn and a flight path angle of 0 deg. The various possible directions for this velocity vector (Fig. 1b) determine the inclination of the final orbit about Saturn. Some variation in the velocity after the AGA from this target value may be acceptable (or even desirable) and would yield a periapsis slightly inside or outside Enceladus’s orbital radius. In addition, the vehicle may have a radial velocity component WRT Saturn after the maneuver, resulting in an apoapsis outside Titan’s orbital radius. For simplicity, the latter scenario is not considered in this preliminary study. Titan is particularly well suited for an AGA maneuver because of its relatively weak gravitational field, coupled with its unique atmospheric structure. Although Titan’s radius is only 2575 km, atmospheric pressure at ground level is approximately 1.6 times that on Earth, and the atmosphere extends to an altitude of approximately 1000 km (compared with approximately 125 km for Earth). These factors combine to yield relatively low atmospheric flight velocities at a given density level for a planetary entry probe, especially when compared with a vehicle entering Saturn’s atmosphere. This makes a Titan AGAmaneuver preferable to direct aerocapture using Saturn’s atmosphere because of the much lower entry speed and the lower aerodynamic heating. The less-severe heating translates directly into weight savings in the vehicle’s thermal protection system (TPS).

Predictions of the electrical conductivity and charging of the cloud particles in Jupiter's atmosphere

The electrical conductivity and electrical charge on cloud particles (composed of ammonia, ammonium hydrosulfide, and water) in the atmosphere of Jupiter are computed for pressures between 5.5 and 0.1 bars. The source of ionization is galactic cosmic rays (GCR). The distribution of charge among the various reservoirs is a function of altitude and the total area of the aerosol particles. For pressures below 4 bars, the electrons are scavenged efficiently by the cloud particles, decreasing the electron‐ion recombination rate and resulting in increased positive ion abundance over that in the absence of the particles. For the upper regions of each cloud layer, the area of the aerosols and the large diffusion rate of the electrons cause most aerosol particles to be negatively charged. Near the bases of the cloud layers, the larger total area of the aerosols causes most of the charge, positive and negative, to reside on particles. Where clouds are present, the reduction of the electron conductivity ranges from a factor of 30 at 0.1 bar to 104 at 4 bars. At pressures near 1 bar and 4 bars, the positive ion conductivity increases by a factor of 10 over that expected for the clear atmosphere. A parametric study of negative ions shows that they are likely to be insignificant. For altitudes below the 0.3‐bar level the predicted positive and negative conductivities are well below the detection limit of the relaxation and mutual impedance instruments such as those employed on the Huygens entry probe.

Numerical Computation of Radiative Heating Environment for Huygens Probe Entry Flight

A trajectory-based analysis for obtaining aerodynamic heating environment for the Huygens probe is carried out using the thermochemical nonequilibrium computational fluid dynamics code. Radiative heat transfer is accounted for in computational fluid dynamics calculations where a modern multiband radiation model is employed. In this study, we first compare the radiative heat flux obtained by the ray-tracing approach in three-dimensional space with that given by the tangent-slab approximation, to determine how the difference in obtaining radiative heat flux can alter the overall aerodynamic heating environment. We then explore the radiative cooling effect on surface heat flux through radiation coupled computational fluid dynamics calculations. It is shown that the radiative heat flux value at the stagnation point obtained by the ray-tracing approach becomes about 17-19% smaller than that given by the tangent-slab approximation due to body curvature at all the chosen trajectory points. Furthermore, it is also shown that the radiative cooling effect can reduce the surface radiative heat flux by almost the same amount at the stagnation point when computational fluid dynamics calculation coupled with radiation is conducted. It is therefore confirmed that, even for the relatively lower radiative heating rate such as for the Huygens entry flight, we need to employ the ray-tracing approach instead of the tangent-slab approximation, and also need to account for radiative cooling effect through radiation coupled computational fluid dynamics calculation, to evaluate the surface heating condition accurately.

Heat balance in Titan's atmosphere

The recent measurements of the vertical distribution and optical properties of haze aerosols as well as of the absorption coefficients for methane at long paths and cold temperatures by the Huygens entry probe of Titan permit the computation of the solar heating rate on Titan with greater certainty than heretofore. We use the haze model derived from the Descent Imager/Spectral Radiometer (DISR) instrument on the Huygens probe [Tomasko, M.G., Doose, L., Engel, S., Dafoe, L.E., West, R., Lemmon, M., Karkoschka, E., See, C., 2008a. A model of Titan's aerosols based on measurements made inside the atmosphere. Planet. Space Sci., this issue, doi:10.1016/j.pss.2007.11.019] to evaluate the variation in solar heating rate with altitude and solar zenith angle in Titan's atmosphere. We find the disk-averaged solar energy deposition profile to be in remarkably good agreement with earlier estimates using very different aerosol distributions and optical properties. We also evaluated the radiative cooling rate using measurements of the thermal emission spectrum by the Cassini Composite Infrared Spectrometer (CIRS) around the latitude of the Huygens site. The thermal flux was calculated as a function of altitude using temperature, gas, and haze profiles derived from Huygens and Cassini/CIRS data. We find that the cooling rate profile is in good agreement with the solar heating profile averaged over the planet if the haze structure is assumed the same at all latitudes. We also computed the solar energy deposition profile at the 10°S latitude of the probe-landing site averaged over one Titan day. We find that some 80% of the sunlight that strikes the top of the atmosphere at this latitude is absorbed in all, with 60% of the incident solar energy absorbed below 150 km, 40% below 80 km, and 11% at the surface at the time of the Huygens landing near the beginning of summer in the southern hemisphere. We compare the radiative cooling rate with the solar heating rate near the Huygens landing site averaging over all longitudes. At this location, we find that the solar heating rate exceeds the radiative cooling rate by a maximum of 0.5 K/Titan day near 120 km altitude and decreases strongly above and below this altitude. Since there is no evidence that the temperature structure at this latitude is changing, the general circulation must redistribute this heat to higher latitudes.

Huygens probe entry dynamic model and accelerometer data analysis

During the first phase of Huygens arrival into Titan's atmosphere the probe is subjected to gravitational and aerodynamic forces in aerodynamic hypersonic regime. Atmospheric drag exerts a strong deceleration on the capsule measured by Huygens atmospheric structure instrument (HASI) servo accelerometer. A 6 DOF (Degree of Freedom) model of the Huygens probe entry dynamics has been developed and used for data analysis. The accelerometer data are analysed and the model allows the retrieval of dynamics information of Huygens probe from 1545 km altitude down to end of the entry phase. Probe's initial conditions (velocity and position) were refined to match the measured deceleration profile resulting in a different altitude at interface epoch with respect to those of the Cassini Navigation Team. Velocity and position of probe at interface epoch are compatible with those used by Descent Trajectory Working Group (DTWG). Measurements acquired before atmosphere detection are used to estimate probe's angular rate, bound attitude and characterise the angle of attack profile which results to be lower than 4 ∘ during the whole entry. Probe's spin calculated (6.98 RPM) is slightly different with respect to DTWG of 7.28 RPM but considering a 2% error in the Inertia matrix these results are inside the 1 - σ error band.

Huygens probe entry trajectory and attitude estimated simultaneously with Titan atmospheric structure by Kalman filtering

Space probes entering planetary atmospheres are used for in situ study of their physical structures. During the entry phase aerodynamic forces exerted on the probe depend on atmospheric density. As a consequence accelerations measured by on-board sensors can be used to derive probe trajectory as well as atmospheric density, pressure and temperature profiles. In this work acceleration data acquired by the Huygens Atmospheric Structure Instrument (HASI) have been used to reconstruct the probe trajectory and the Titan's atmospheric structure from ≈ 1545 km down to ≈ 155 km of altitude. An accurate six degree of freedom model of Huygens during the entry phase has been developed and a new reconstruction technique based on Kalman filtering is presented. This technique estimates simultaneously the probe trajectory, the attitude profile consistent with measured data and the atmospheric density, pressure and temperature.

Huygens’ entry and descent through Titan's atmosphere—Methodology and results of the trajectory reconstruction

The European Space Agency's Huygens probe separated from the NASA Cassini spacecraft on 25 December 2004, after having been attached for a 7-year interplanetary journey and three orbits around Saturn. The probe reached the predefined NASA/ESA interface point on 14 January 2005 at 09:05:52.523 (UTC) and performed a successful entry and descent sequence. The probe softly impacted on Titan's surface on the same day at 11:38:10.77 (UTC) with a speed of about 4.54 m/s. The probe entry and descent trajectory was reconstructed from the estimated initial state vector provided by the Cassini Navigation team, the probe housekeeping data, and measurements from the scientific payload. This paper presents the methodology and discuss the results of the reconstruction effort. Furthermore the probe roll rate was reconstructed prior to the main entry phase deceleration pulse and throughout the entire descent phase under the main and drogue parachute.