GETEMME—a mission to explore the Martian satellites and the fundamentals of solar system physics

GETEMME (Gravity, Einstein??s Theory, and Exploration of the Martian Moons?? Environment), a mission which is being proposed in ESA??s Cosmic Vision program, shall be launched for Mars on a Soyuz Fregat in 2020. The spacecraft will initially rendezvous with Phobos and Deimos in order to carry out a comprehensive mapping and characterization of the two satellites and to deploy passive Laser retro-reflectors on their surfaces. In the second stage of the mission, the spacecraft will be transferred into a lower 1500-km Mars orbit, to carry out routine Laser range measurements to the reflectors on Phobos and Deimos. Also, asynchronous two-way Laser ranging measurements between the spacecraft and stations of the ILRS (International Laser Ranging Service) on Earth are foreseen. An onboard accelerometer will ensure a high accuracy for the spacecraft orbit determination. The inversion of all range and accelerometer data will allow us to determine or improve dramatically on a host of dynamic parameters of the Martian satellite system. From the complex motion and rotation of Phobos and Deimos we will obtain clues on internal structures and the origins of the satellites. Also, crucial data on the time-varying gravity field of Mars related to climate variation and internal structure will be obtained. Ranging measurements will also be essential to improve on several parameters in fundamental physics, such as the Post-Newtonian parameter ?? as well as time-rate changes of the gravitational constant and the Lense-Thirring effect. Measurements by GETEMME will firmly embed Mars and its satellites into the Solar System reference frame.     

Determination of spacecraft and phobos parameters of motion in the Phobos-Grunt project

For the needs of the upcoming flight to Phobos within the framework of the project Phobos-Grunt, a dynamical numerical theory of the motion of Phobos is developed on the basis of Earth-based and spacecraft (SC) optical measurements and Earth-based radio observations of the SC motion. The theory uses the most reliable astronomical constants. Within the framework of a unified dynamical model, an algorithm is proposed for the joint correction of Phobos and the spacecraft parameters of motion, among which are the secular acceleration of Phobos and its gravitational constant. The accuracy of the obtained ephemeris is estimated for the time of the SC arrival to Phobos.     

TV system for navigation and guidance

A TV system for navigation and guidance (TVSNG) has been designed for solving the navigation and scientific tasks of the Phobos-Grunt mission. It consists of two narrow-angle and two wide-angle television cameras. Each camera has a built-in processor for the real-time processing of received images. The program of TVSNG observations includes three modes: Imaging of Stars, Sequential Imaging, and Landing. The first two modes will be used to calibrate the TVSNG against the stars, to take images of Phobos and Mars, and to detect the dust rings of Mars. In the Landing mode, the height and velocity components of the spacecraft during the landing on Phobos will be measured, and an autonomous selection of the landing site will be made.     

Diurnal Temperature Regime in the Regolith Surface Layer of the Lagado Planitia Region on Phobos: Model Predictions for Different Seasons

The paper contains the data on the thermal and physical characteristic of the surface regolith of the Martian satellite Phobos obtained from the spaceborne remote sensing (with the Mariner 9, Viking, and Mars Global Surveyor orbiters and the Phobos-2 spacecraft) and the results of the numerical modeling of the thermal regime in the surface regolith (on diurnal and seasonal scales) performed for the prospective landing site in the Lagado Planitia region located in the anti-Martian hemisphere of Phobos.     

The design philosophy of the Phobos-Grunt space vehicle

This paper covers the main features of Russia??s Phobos-Grunt space mission, whose primary goal is to return soil samples from the Martian satellite Phobos. The mission scenario, major design solutions, and evolution of the project throughout its development are described.     

Project of the mission to Phobos

This article provides the main scientific objectives and characteristics of the Phobos-Soil project, intended to fly to the Martian satellite Phobos, deliver its soil samples to the Earth, as well as explore Phobos, Mars, and the Martian environment with onboard scientific instruments. We give the basic parameters of the ballistic scenario of the mission, spacecraft, and some scientific problems to be solved with the help of the scientific instruments installed on the spacecraft.     

The selection and characterization of the <Emphasis Type="Italic">Phobos-Soil</Emphasis> landing sites

This paper shortly describes the selection technique for the landing sites of the Phobos-Soil spacecraft, the characteristics of the Phobos relief, the history of choosing the potential landing sites in the process of working on the project, and the suggestions to shift the landing sites to the region recently imaged by the Mars Express spacecraft with a high spatial resolution under favorable illumination conditions.     

Secular acceleration of Phobos and Q of Mars

Reductions of Mariner 9 TV data of Phobos and Deimos tend to corroborate the existence of a secular acceleration of Phobos commensurate with two recently reported values based on a reprocessing of Earth-based data. These values of secular acceleration have been used together with Mariner 9 data on the physical size of Phobos and Earth-based photoelectric observations which infer a carbonaceous composition for Phobos to place bounds of 50 < Q < 150 on the tidal dissipation function of Mars. The corresponding bounds on the tidal lag angle are 0.19° < Φ < 0.57°.     

Ballistic capture into distant retrograde orbits around Phobos: an approach to entering orbit around Phobos without a critical maneuver near the moon

This paper discusses an approach for designing missions to Phobos that do not require a critical maneuver in proximity of the moon. A low-energy transfer is designed that utilizes the aspherical mass distribution of Phobos to capture a spacecraft into a distant retrograde orbit (DRO) for the mission duration. The process for generating stable DROs in the Mars–Phobos system is discussed along with the method for generating and surveying a set of ballistic capture trajectories (BCTs) for DROs with altitudes between 0.5 and 14 km above Phobos. Statistical analysis of the BCT set reveals options for designing a mission to the desired DRO. This approach can be used in any three-body system when a significant perturbation is present, such as Phobos’ aspherical co-rotating gravity field.     

Mariner 9 polarimetry of Phobos and Deimos

We have used the Mariner 9 A-camera system to measure the polarization (P) of Phobos and Deimos at large phase angles (α). For Deimos, P = +22 ± 4% at α = 74°; for Phobos P = +20.5 ± 4% at α = 77°, and P = +24.5 ± 4% at α = 81°. These measurements refer to orange light at about 0.57 μm. A comparison with laboratory measurements of powdered rock samples indicates that the observations are consistent with the presence of regoliths on the satellites.     

Tidal rigidity of phobos

The Martian satellites Phobos and Deimos move along nearly circular coplanar, stable orbits and have created surfaces older than ～ 10⁹ years. The accretion hypothesis suggests that their primordial orbits were also very regular. However, tides raised on Mars and Phobos can substantially alter the semimajor axis a of Phobos' orbit over time. The effect of the Martian tidal torque alone on Phobos' orbit implies that the primordial e was ～0.1 to 0.2 about 4.6 × 10⁹ years ago if the present observed e = 0.015 is naively interpreted as a tidally damped remnant. Significant tidal friction in Phobos reduces the time scale for Phobos to achieve a crossing orbit with Deimos to less than 10⁹ years and permits the primodial e to approach unity. The consequences of orbital intersections cannot easily be resolved by assuming either a catastrophic origin for both satellites (namely, that both are fragments of a common parent body fractured by an impact) or that they were captured sequentially by Mars. Either hypothesis is difficult to accept, given that Deimos' orbit, which is only slightly affected by tides, is now so regular. An alternative scenario is proposed in this paper in which the observed e of Phobos results from several gravitational resonance excitations within the last 10⁹ years, assuming tidal friction in Phobos has had only a small effect on its orbit. In facr, both the primordial e and the inclination i may have been much smaller than presently observed. The constraints imposed on tidal friction in Phobos by both the apparent age of Phobos' surface (> 10⁹yrs) and the above scenario can be satisfied only of μQ > 10¹²dynes/cm². Since the Q factor is ～10², the rigidity μ > 10¹⁰dynes/cm². Thus Phobos should have substantial internal strength.     

The Temperature Regime of the Surface Layer of the Phobos Regolith in the Region of the Potential Fobos–Grunt Space Station Landing Site

Because of the absence of the atmosphere, the short duration of the Phobos day (7.7 hours), and the presence of a highly porous and fine-grained soil on the Phobos surface, all components of the future Russian Fobos–Grunt lander will operate under frequent and sharp temperature changes: from positive to extremely low negative temperatures. As a consequence, information about the temperature regime directly on the surface of the Martian satellite and in the near-surface layer appears to be extremely important. The proposed publication contains both the information about the thermophysical properties of the surface regolith of Phobos, derived from observations made with the Mariner 9 orbiter, the Viking orbiter, the Fobos-2 spacecraft, and the Mars Global Surveyor orbiter, and the results of the numerical modeling of the thermal regime of the surface regolith layer (on diurnal and seasonal time scales) in the area of the potential Fobos–Grunt landing site. We performed this modeling by taking into account the seasons on Mars and the effects due to the eclipse of Phobos by Mars.     

The photometric functions of Phobos and Deimos. III. Surface photometry of Phobos

At least three large areas on the surface of Phobos are covered by a dark material of complex texture which scatters light according to the Hapke-Irvine Law. The average 20° to 80° intrinsic and disc-integrated phase coefficients of this material are β_i = 0.020 ± 0.001 mag/deg and β = 0.033 mag/deg, respectively. These values are slightly greater than the values found for Deimos in Paper II (preceding article). On the largest scale the surface of Phobos is rougher than the surface of Deimos, perhaps accounting for the slightly greater phase coefficients. Contrary to the situation on Deimos, no definite regions of intrinsically brighter material are apparent on Phobos. This difference could account for the slightly lower average reflectance of Phobos relative to Deimos. No evidence for large exposures of solid rock has been found in the three areas studied.     

Near-Infrared Spectrophotometry of Phobos and Deimos

We have observed the leading and trailing hemispheres of Phobos from 1.65 to 3.5 μm and Deimos from 1.65 to 3.12 μm near opposition. We find the trailing hemisphere of Phobos to be brighter than its leading hemisphere by 0.24±0.06 magnitude at 1.65 μm and brighter than Deimos by 0.98±0.07 magnitude at 1.65 μm. We see no difference larger than observational uncertainties in spectral slope between the leading and trailing hemispheres when the spectra are normalized to 1.65 μm. We find no 3-μm absorption feature due to hydrated minerals on either hemisphere to a level of ∼5-10% on Phobos and ∼20% on Deimos. When the infrared data are joined to visible and near-IR data obtained by previous workers, our data suggest the leading (Stickney-dominated) side of Phobos is best matched by T-class asteroids. The spectral slope of the trailing side of Phobos and leading side of Deimos are bracketed by the D-class asteroids. The best laboratory spectral matches to these parts of Phobos are mature lunar soils and heated carbonaceous chondrites. The lack of 3-μm absorption features on either side of Phobos argues against the presence of a large interior reservoir of water ice according to current models of Phobos' interior (F. P. Fanale and J. R. Salvail 1989, Geophys. Res. Lett.16, 287-290; Icarus88, 380-395).     

The photometric functions of Phobos and Deimos I. Disc-integrated photometry

We have used the integrated brightnesses from Mariner 9 high-resolution images to determine the large phase angle (20° to 80°) phase curves of Phobos and Deimos. The derived phase coefficients are β = 0.032 ± 0.001 mag/deg for Phobos and β = 0.030 ± 0.001 mag/deg for Deimos, while the corresponding phase integrals are q_Phobos = 0.52 and q_Deimos = 0.57. The predicted intrinsic phase coefficients of the surface material are β_i = 0.019 mag/deg and β_i = 0.017 mag/deg for Phobos and Deimos, respectively. The phase curves, phase coefficients and phase integrals are typical of objects whose surface layers are dark and intricate in texture, and are consistent with the presence of a regolith on both satellites. The relative reflectance of Deimos to Phobos is 1.15±0.10. The presence of several bright patches on Deimos could account for this slight difference in average reflectance.     

Heliospheric Observations of STEREO-Directed Coronal Mass Ejections in 2008?–?2010: Lessons for Future Observations of Earth-Directed CMEs

We present a study of coronal mass ejections (CMEs) which impacted one of the STEREO spacecraft between January 2008 and early 2010. We focus our study on 20 CMEs which were observed remotely by the Heliospheric Imagers (HIs) onboard the other STEREO spacecraft up to large heliocentric distances. We compare the predictions of the Fixed-?? and Harmonic Mean (HM) fitting methods, which only differ by the assumed geometry of the CME. It is possible to use these techniques to determine from remote-sensing observations the CME direction of propagation, arrival time and final speed which are compared to in-situ measurements. We find evidence that for large viewing angles, the HM fitting method predicts the CME direction better. However, this may be due to the fact that only wide CMEs can be successfully observed when the CME propagates more than 100^° from the observing spacecraft. Overall eight CMEs, originating from behind the limb as seen by one of the STEREO spacecraft can be tracked and their arrival time at the other STEREO spacecraft can be successfully predicted. This includes CMEs, such as the events on 4 December 2009 and 9 April 2010, which were viewed 130^° away from their direction of propagation. Therefore, we predict that some Earth-directed CMEs will be observed by the HIs until early 2013, when the separation between Earth and one of the STEREO spacecraft will be similar to the separation of the two STEREO spacecraft in 2009??C?2010.     

Solar Stereoscopy with STEREO/EUVI A and B Spacecraft from Small (6°) to Large (170°) Spacecraft Separation Angles

We performed for the first time stereoscopic triangulation of coronal loops in active regions over the entire range of spacecraft separation angles (?? _sep??6^°,43^°,89^°,127^°,and 170^°). The accuracy of stereoscopic correlation depends mostly on the viewing angle with respect to the solar surface for each spacecraft, which affects the stereoscopic correspondence identification of loops in image pairs. From a simple theoretical model we predict an optimum range of ?? _sep??22^°??C?125^°, which is also experimentally confirmed. The best accuracy is generally obtained when an active region passes the central meridian (viewed from Earth), which yields a symmetric view for both STEREO spacecraft and causes minimum horizontal foreshortening. For the extended angular range of ?? _sep??6^°??C?127^° we find a mean 3D misalignment angle of ?? _PF??21^°??C?39^° of stereoscopically triangulated loops with magnetic potential-field models, and ?? _FFF??15^°??C?21^° for a force-free field model, which is partly caused by stereoscopic uncertainties ?? _SE??9^°. We predict optimum conditions for solar stereoscopy during the time intervals of 2012??C?2014, 2016??C?2017, and 2021??C?2023.     

Microscope spectrometer for the <Emphasis Type="Italic">Phobos-Grunt</Emphasis> mission

Microscopy and spectroscopy are important methods of studies. The use of a microscope onboard a spacecraft is connected with the fact that the closer approach to the objects and the switch to the in situ measurement methods have become possible. The combination of taking an image and performing a spectral analysis forms a new type of instrument, so-called videospectrometers. The scientific payload of the Phobos-Grunt spacecraft includes the microscope spectrometer designed to analyze the composition of the surface regolith of Phobos in detail.     

Seismology of Phobos: from geophysics to cosmogony

Several of the most fundamental and feasible geophysical problems partially related to the Phobos-Grunt mission have been analyzed based on the available works. The assumed results will form the informational basis for the development of the cosmogony of planets’ small satellites and asteroids. Correspondingly, the aims of the experiment are to study the internal structure and energy state of Phobos; to analyze the manifestation of pulsed effects and fields, including the registration of seismic signals and wave fields of Phobos; and to measure the long-period oscillations on the surface of Phobos in the range of 10⁻⁵–10 Hz. Studying Phobos gives an example of specific problems peculiar to small bodies of the Solar System: specific features of cratering, grooves, and morphological structures. The registration of gas-dust streams extends the knowledge of the space-time structure of the Solar System and its objects and processes and will confirm that stellar systems can constantly interact. The physical principles of the registration of seismic fields and signals are briefly described, and the instrumental basis for cosmogonic seismology is comparatively presented. It has been indicated that the piezoelectric and electrodynamic systems of the desired signal registration complete each other, and it is desirable to use both systems if 2- and 3-D registration systems are applied. The seismometric instrumentation of the Phobos spacecraft has been considered. The device’s physical characteristics, block diagrams, energy consumption, and information content are presented. The seismoacoustic (HF) device unit and its advantages during the registration of very weak signals owing to the use of the mechanical transformer effect are described in more detail. The seismic system created can ensure the solution of the scientific problems of the mission to Phobos, including the study of the internal structure, origin, depth structures, and external impacts of the field, corpuscular, and micrometeorite types.     

Martian shock and magnetic pile-up boundary positions and shapes determined from the Phobos 2 and Mars Global Surveyor data sets

A great many Martian bow shock and magnetic pile-up boundary crossings have been identified in the Phobos 2 and Mars Global Surveyor, MGS, data. From these observations the positions and shapes of the bow shock and magnetic pile-up boundary, MPB, have been derived and modelled, using curve-fitting techniques. The models thus derived separately from the Phobos 2 and MGS data sets do not differ drastically, despite the different time and space data coverages. The purpose of the paper is therefore to show the results obtained from the mixing of the Phobos 2 and MGS data bases and to compare the derived bow shock and MPB models with the ones obtained previously. The underlying objective was to see whether it was possible to determine improved bow shock and MPB models or not. The answer is definitely yes, and particularly for the MPB, thanks to the complementary nature of the observations. The boundaries crossed close to the subsolar direction or mostly far downstream by Phobos 2 indeed allow a better coverage of the Martian space environment to be considered. Nevertheless, in order to reduce the domination of the overabundant MGS data set and/or the crossings that are close to Mars (x>−4 R_M, i.e. x>−13 562 km) weighting factors have been introduced.