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The PHOBOS experiment ran at the RHIC collider from 2000 to 2005, under the leadership of Wit Busza. These proceedings summarize selected PHOBOS results, highlighting their continuing relevance amidst the wealth of new results from the lead–lead program at the Large Hadron Collider (LHC)
One of the deep-space human exploration activities proposed for the post-Shuttle era is a mission to one of the moons of Mars, Phobos or Deimos. There are several options available to the mission architect for operations around these bodies. These options include distant retrograde orbits (DROs), Lagrange-point orbits such as halos and Lyapunov orbits, and fixed-point stationkeeping or "hovering." These three orbit options are discussed in the context of the idealized circular restricted three body problem, full-dynamics propagations, and a concept of operations. The discussion is focused on Phobos, but all results hold for Deimos
The origin of the martian moons Phobos and Deimos is obscure and enigmatic. Hypotheses include the capture of small bodies originally from the outer main belt or beyond, residual material left over from Mars' formation, and accreted ejecta from a large impact on Mars, among others. Measurements of reflectance spectra indicate a similarity to low-albedo, red D-type asteroids, but could indicate a highly space-weathered veneer. Here we suggest a way of constraining the near-surface composition of the two moons, for comparison with known meteoritic compositions. Neutron spectroscopy, particularly the thermal and epithermal neutron flux, distinguishes clearly between various classes of meteorites and varying hydrogen (water) abundances. Perhaps most surprising of all, a rendezvous with Phobos or Deimos is not necessary to achieve this. Multiple flybys suffice.
The presented trajectory design and analysis was performed for the Phobos and Deimos & Mars Environment (PADME) mission concept as part of a NASA proposal submission managed by NASA Ames Research Center in the 2014-2015 timeframe. The PADME spacecraft would be a derivative of the successfully flown Lunar Atmosphere & Dust Environment Explorer (LADEE) spacecraft. While LADEE was designed to enter low-lunar orbit, the PADME spacecraft would instead enter an elliptical Mars orbit of 2-week period. This Mars orbit would pass by Phobos near periapsis on successive orbits and then raise periapsis to yield close approaches of Deimos every orbit thereafter.
The origins of Phobos and Deimos are uncertain; both are so space weathered that their surface compositions are difficult to determine using spectral reflectance measurements. We show how the winter temperatures and associated conditions in the polar regions of Phobos could make the regolith susceptible to space weathering from dielectric breakdown caused by solar energetic particles (SEPs). During SEP events, charged particles accumulate throughout the top ~ 1 mm of the regolith, which has low condukctivity, and create subsurface electric fields that act to dissipate any net buildup of charge. The faster the net charge accumulates, the larger the electric field needed to dissipate it. If the magnitude of the subsurface electric field exceeds ~10 6 V m -1, then dielectric breadkown is likely. This process rapidly dissipates the buildup of charge by vaporizing electrically conducting channels through the regolith. Dielectric breakdown
Phobos and Deimos are the two small moons of Mars. Recent works have shown that they can accrete within an impact-generated disk. However, the detailed structure and initial thermodynamic properties of the disk are poorly understood. In this paper, we perform high-resolution SPH simulations of the Martian moon-forming giant impact that can also form the Borealis basin. This giant impact heats up the disk material (around ∼2000 K in temperature) with an entropy increase of ∼1500 J K{sup −1} kg{sup −1}. Thus, the disk material should be mostly molten, though a tiny fraction of disk material (<5%) would even experience vaporization. Typically, a piece of molten disk material is estimated to be meter sized owing to the fragmentation regulated by their shear velocity and surface tension during the impact process. The disk materials initially have highly eccentric orbits (e ∼ 0.6–0.9), and successive collisions between meter-sized fragments at high impact velocity (∼1–5 km s{sup −1}) can grind them down to ∼100 μm sized particles. On the other hand, a tiny amount of vaporized disk material condenses into ∼0.1 μm sized grains. Thus, the building blocks of the Martian moons are expected to be a mixture of these different sized particles from meter-sized down to ∼100 μm sized particles and ∼0.1 μm sized grains. Our simulations also suggest that the building blocks of Phobos and Deimos contain both impactor and Martian materials (at least 35%), most of which come from the Martian mantle (50–150 km in depth; at least 50%). Our results will give useful information for planning a future sample return mission to Martian moons, such as JAXA’s MMX (Martian Moons eXploration) mission.
Analyses of the Mariner 9 pictures of the Martian satellites have yielded much new information: improved ephemerides, estimates of their principal axes, information about the texture of their surfaces, and estimates of the structural strength of their interiors. Both satellites are found to be in synchronous rotation, as was expected from tidal theory. A close examination of the preorbital satellite pictures has failed to show any unknown satellite. The photometric behavior of Phobos and Deimos indicates that they have intricate surface layers consistent with the presence of a regolith. Morphological features on Phobos and the expected collisional history of both satellites imply that both are made of well-consolidated material.
Radiometric measurements of Phobos at 10 and 20 microns show that the surface is covered by a material whose conductivity is extremely low, around .000001 cal per cm per sec per deg K. It is concluded that Phobos is covered with a layer of dust at least 1 mm thick.
The problem was investigated of estimating the masses of Phobos and Deimos from Doppler and onboard optical measurements during the Viking extended mission. A Kalman filter was used to analyze the effects of gravitational uncertainties and nongravitational accelerations. These accelerations destroy the dynamical integrity of the orbit, and multibatch or limited memory filtering is preferred to single batch processing. Optical tracking is essential to improve the relative orbit geometry. The masses can be determined to about 10% and 25% respectively for Phobos and Deimos, assuming satellite densities of about 3 gr/cu cm.
This paper addresses the problem of estimating the masses of Phobos and Deimos from Doppler and onboard optical measurements during the Viking extended mission. A Kalman filter is used to analyze the effects of gravitational uncertainties and nongravitational accelerations. These accelerations destroy the dynamical integrity of the orbit and multi-batch or limited memory filtering is preferred to single batch processing. Optical tracking is essential to improve the relative orbit geometry. The masses can be determined to about 10% and 25% respectively for Phobos and Deimos, assuming satellite densities of about 3 g per cu cm.
Computations of figures of hydrostatic equilibrium for isolated rotating bodies are discussed. It is shown that the theoretical figure of equilibrium for Phobos is a triaxial ellipsoid whose a and b axes differ by about 25% and that this difference is less than 2% for Deimos. A density of less than 3.9 g/cu cm is estimated for Phobos. Equilibrium figures are also examined for the inner satellites of Jupiter and Saturn as well as for small asteroids.
Revised crater counts for Phobos are presented which are based on uniform Mariner 9 imagery and Duxbury's (1974) map of the satellite. The contiguous portion of the satellite's surface on which all craters down to the limiting resolution of 0.2 to 0.3 km in diameter would be expected to be identified is delineated and found to contain 87 identifiable craters larger than 0.2 km in diameter. Analysis of the crater size distribution shows that the surface appears to be saturated for craters exceeding 1 km in diameter but the crater counts definitely fall below the saturation curve for smaller craters. Reasons for this fall-off are considered, and it is noted that too few craters are visible in Mariner 9 images of Deimos to permit meaningful crater counts on that satellite's surface. It is concluded that, contrary to a previous assertion, the surfaces of Phobos and Deimos are not known to be saturated with craters larger than 0.2 km in diameter.
Ground-based and spacecraft observations of Phobos and Deimos are reviewed and the satellites' origin is discussed. The crater densities of both bodies are close to the saturation level. The largest impact events may have caused extensive fracturing of their surfaces. The surfaces are at least 1.5 billion years old and may date back to the early history of the solar system. The Martian satellites display large deviations from sphericity. As a result of tidal processes, they are in synchronous rotation. Several independent lines of evidence show that they have regoliths. Despite some provocative arguments, their internal strengths and the nature of their interior are poorly known at present. Photometric measurements suggest that they are made of either carbonaceous chondritic material or a basalt. Sinclair (1972), Born and Duxbury (1975) and Shor (1975) apparently have successfully determined Phobos' secular acceleration. Their value of approximately .001 deg/year/year implies that the interior of Mars has a low specific dissipation factor (about 100), may indicate that a portion of the Martian interior is experiencing partial melting. The low inclination of the satellites' orbits indicates that they were formed as part of the same process that resulted in Mars.
Results of geodesy studies of Phobos and Deimos based on Mariner 9 imaging data are presented. This analysis includes a review of the surface coverage and high resolution pictures obtained and the determined sizes, shapes, topographies, and librations of the two Martian satellites. Also, exploration of Phobos and Deimos by missions such as Viking is discussed.
A series of striations or grooves over a great part of the surface of Phobos, the inner satellite of Mars, can be observed on Viking orbiter photographs. It is suggested that some of these features are related to a readjustment of the satellite's figure with increasing tidal stress as the orbit evolves inwards under the action of tidal friction. If the width of the grooves is in fact due to tidal readjustments of the figure of Phobos, then the older craters should be systematically deformed from their initial nearly circular shapes.
The improved resolution of the Viking images has led to the discovery of a number of unusual surface features on Phobos. The features include elongated rill-like depressions associated with the crater Stickney, chains and clusters of irregular elongated craters, and linear striations or grooves. Crater counts are also considered. A total of 27 craters ranging in size from 0.3 to 1.4 km are visible on the best Deimos frame available. Craters ranging from 50 m to 4.9 km in diameter can be observed on the best of the Phobos images.
The age, surface density, and association with the 10-km crater Stickney of the grooves (long linear depression) on Phobos are investigated, and the results support the cratering hypothesis of groove formation and seem to rule out a tidal mechanism. The appearance of the grooves is described. The age of the grooves is estimated from the density of superimposed impact craters; the density of impact craters within the grooves is compared with the average value for all of Phobos. The old age of the grooves as well as their evident association with the crater Stickney are considered to be inconsistent with the tidal hypothesis.
A series of lower- and higher-resolution pictures of Phobos and Deimos taken from Viking spacecraft is presented. The imaging data imply that the two asteroid-size bodies may indeed have originated in the asteroid belt and may be composed of primordial material from the solar nebula. There is now sufficient knowledge on Phobos and Deimos to consider them as candidate targets for future sample/return missions or for locating bases to monitor Mars.