Research on High Temperature Gas Dynamics in Relation to Thermal Protection System of Solar Probe

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In order to realize the "Solar Probe" mission, in which the probe enters the solar corona to clarify the acceleration mechanism of the solar wind, research on the phenomena of high temperature gas dynamics in relation to the heat shield to protect the spacecraft from severe radiative heating from the sun has been made. The major conclusions are as follows :1)The rarefied gas dynamics analyses by the Direct Simulation Monte Carlo method show that the velocity distribution function of the solar wind is changed due to the molecular collision between the solar wind and the sublimation gas. For such disturbance to be negligible at observation of the solar wind, the total sublimation rate from the heat shield must be smaller than 2 mg/s.2)The disk type heat shield, which is made from graphite and is inclined at about 40 degrees to the solar direction, is promising because of its simple shape and small sublimation rate. Observation of the solar wind is disturbed by not only sublimation gas injection but also reflection of the incoming solar wind at the spacecraft surface. To reduce the disturbance, the sensors should be located on the top of the boom extended backward from the spacecraft base.3)The experiments on the laser ablation of graphite sheet have been conducted. It is experimentally clarified that the disk type heat shield works well from viewpoints of small sublimation rate and reduction in heat transfer to the spacecraft main body behind it.4)Feasibility of the small solar probe mission using the solar sail is studied. Trajectory analysis shows that the probe can reach the solar corona within five years. The balloon type heat shield is newly proposed to obtain higher thermal protection performance, smaller sublimation rate and smaller structural mass.