A Revisit of the Phobos Events

The magnetic field disturbances detected by the Phobos-2 spacecraft in 1989 have been suggested to be caused by a ring of dust and/or gas emitted from the Martian moon,Phobos.The physical nature of these"Phobos events"is examined using results from related investigations over the last twenty years.It is concluded that there is no clear evidence at present to support the association of magnetic field disturbances in the solar wind with Phobos.The situation will be further clarified taking advantage of the multi-spacecraft observations of the Yinghuo-1(YH-1),Mars Express and MAVEN missions beginning in 2012.It is expected that many novel features of solar wind interaction with Phobos(and possibly also Deimos) itself will also be revealed.

Groove formation on Phobos from reimpacting orbital ejecta of the Stickney crater

Numerous linear grooves have long been recognized as covering the surface of Phobos, but the mechanisms of their formation are still unclear. One possible mechanism is related to the largest crater on Phobos, the Stickney crater, whose impact ejecta may slide,roll, bounce, and engrave groove-like features on Phobos. When the launch velocity is higher than the escape velocity, the impact ejecta can escape Phobos. A portion of these high-velocity ejecta are dragged by the gravitational force of Mars, fall back, and reimpact Phobos.In this research, we numerically test the hypothesis that the orbital ejecta of the Stickney crater that reimpact Phobos could be responsible for a particular subset of the observed grooves on Phobos. We adopt impact hydrocode i SALE-2 D(impact-Simplified Arbitrary Lagrangian Eulerian, two-dimensional) to simulate the formation of the Stickney crater and track its impact ejecta, with a focus on orbital ejecta with launch velocities greater than the escape velocity of Phobos. The launch velocity distribution of the ejecta particles is then used to calculate their trajectories in space and determine their fates. For orbital ejecta reimpacting Phobos, we then apply the sliding boulder model to calculate the ejecta paths, which are compared with the observed groove distribution and length to search for causal relationships. Our ejecta trajectory calculations suggest that only ~1% of the orbital ejecta from the Stickney crater can reimpact Phobos. Applying the sliding boulder model, we predict ejecta sliding paths of 9-20 km in a westward direction to the east of the zone of avoidance, closely matching the observed grooves in that region. The best-fit model assumes an ejecta radius of ~150 m and a speed restitution coefficient of 0.3, consistent with the expected ejecta and regolith properties. Our calculations thus suggest the groove class located to the east of the zone of avoidance may have been caused by reimpact orbital ejecta from the Stickney crater.

火星探测与富布斯(Phobos)计划

一、火星探测自60年代起,美、苏便开始向火星发射火箭,对其进行探测。1965年美国Mariner(水手)4号拍摄了第一批关于火星的近距离图象,并发现火星上充满了陨坑,其表面大气的主要成份是二氧化碳、压力仅为地球的百分之几。从而表明,火星表面对生命来说,并非理想的环境。1976年两只美国的Viking(海盗)飞船在火星的两个不同地区着陆,并连续多年将测量数据发回地面。通过对这些数据的分析,使专家们大为吃惊:火星表面不仅没有生命信息,甚致连有机物质也未发现。此后。

Relativistic Warning to Space Missions Aimed to Reach Phobos

Disagreement in estimations of the observed acceleration of Phobos yields several theories empirically modifying classical description of motion of the satellite, but its orbital positions detected by Mars-aimed spacecraft differ from predictions. It is shown that the satellite’s orbital perturbations can be explained as manifestations of the relativistic time-delay effect ignored in classical models. So computed limits of Phobos’ acceleration essentially exceed the experimental values. The satellite’s expected orbital shift is calculated for the moment of contact with a landing module of the Phobos-Grunt project;the shift assessed in kilometers may prevent the mission success. Limits of the apparent relativistic accelerations are predicted for fast satellites of Jupiter.

Generalized Potentials Describing Orbits in the Solar System： Derivation of a ＇Close-Force＇ Acting on the Inner Moon Phobos

The increased velocity for the inner moon Phobos at Mars is calculated, assuming a modified gravity law with distributed mass. The motion of the moon is assumed to be balanced by the ＂close-force＂. The results from Stroberg, for planets in noncircular orbits, in conjunction with assuming a density, admit a balance equation for the rotation in a continuum flow. From this, explicit expressions for a velocity field and a so called Le-density are given. These are exploited to model distributed mass and formations, exemplified with a large asteroid, 2 Pallas, in the asteroid belt, bounds for the L-frequency, the formation of Mercury and distances between planets.

Covariance analysis of periodic and quasi-periodic orbits around Phobos with applications to the Martian Moons eXploration mission

Understanding the internal composition of a celestial body is fundamental for formulating theories regarding its origin.Deep knowledge of the distribution of mass under the body’s crust can be achieved by analyzing its moments of inertia and gravity field.In this regard,the two moons of the Martian system have not yet been closely studied and continue to pose questions regarding their origin to the space community;thus,they deserve further characterization.The Martian Moons eXploration mission will be the first of its kind to sample and study Phobos over a prolonged period.This study aims to demonstrate that the adoption of periodic and quasi-periodic retrograde trajectories would be beneficial for the scientific value of the mission.Here,a covariance analysis was implemented to compare the estimation of high-order gravitational field coefficients from different orbital geometries and for different sets of processed observables.It was shown that the adoption of low-altitude non-planar quasi-satellite orbits would help to refine the knowledge of the moon’s libration angle and gravitational field.

Working models for the gravitational field of Phobos

Models for the gravitational field of Mars moon Phobos were developed using the latest shape model and assuming homogeneous density distribution.Three methods were applied in our study.Comparisons were made between these methods and all were shown to yield consistent results.Notably,the most accurate shape model of Phobos to date,complete up to degree and order 17 was used for the first time in our analysis.A set of spherical harmonic coefficients up to degree and order 17 were derived for the gravitational field of Phobos.Also considered was the gravitational field on the surface of Phobos.Typical characteristics as well as some pronounced surface features of this irregular-shaped small body could be conveniently identified.The results are readily applicable for such purposes as spacecraft orbit analysis and assessing the dynamical environment of Phobos.

中俄联合火星电离层星-星掩星探测

中俄联合火星星-星掩星探测是人类首次在火星空间环境进行此类的联合试验。用于探测火星电离层的星-星掩星技术较以前星地间的探测技术相比,有可接收高信噪比信号,反演精度高,可探测火星上太阳天顶角大于43°,或者小于138°的区域电离层等优点。本文介绍了中俄联合火星星-星掩星探测方案、基本原理,给出了主要技术指标、地面模拟测试结果。