Abstract
Launched in October 2018, the ESA/JAXA BepiColombo mission is currently
in cruise to reach Mercury in late 2025. The Mercury Orbiter
Radioscience Experiment (MORE) is one of the 16 instruments hosted on
board the spacecraft. Testing general relativity is among the primary
objectives of MORE. Superior conjunction experiments (SCE) will be
performed during the interplanetary trajectory, with the aim of
obtaining an accurate estimate of the post-Newtonian parameter γ. This
is allowed by MORE advanced radio tracking system which provides precise
range and Doppler data almost at all solar elongation angles, thus
enabling an accurate measure of the relativistic time delay and
frequency shift undergone by the signal when the spacecraft is in a
superior solar conjunction (SSC). The rst BepiColombo SCE will take
place in March 2021, and others will follow during the cruise phase. The
nal objective is to place new limits to the accuracy of the general
relativity as a theory of gravity in the weak eld limit, improving
previous result from the Cassini SCE (Bertotti et al. 2003), which was
able to determine that γ-1=(2.1±2.3)×10-5. Because of the proximity to
the Sun, the spacecraft will undergo severe solar radiation pressure
acceleration, and the effect of the random uctuations of the solar
irradiance may become a major concern. We address the problem of a
realistic estimate of the outcome of the SCE of BepiColombo, by
including the effects of solar irradiance random variations in the
dynamical model. We analyzed the experiment under different assumptions
on the ranging system performances, observation coverage and solar
activity showing their impact on the attainable result. We propose a
numerical method to mitigate the impact of the variable solar radiation
pressure on the scienti c result. Our simulations show that, exploiting
data from multiple SSCs, the accuracy obtainable in the relativistic
time delay measurement is 13×10-6 for a strong solar activity, and
6×10-6 for weak irradiance uctuations. We found that the latter result
can be obtained by the rst SSC alone if the plasma noise calibration
works until the impact parameter reaches 6 solar radii.