phase with the intent to acquire as much information possible on the behaviour of the instrument, so the team was one of the first to identify the VSB2 as an important occasion to get precious data to be used for internal characterisation scopes by comparing its measurements with the expected MCS NGPs and tidal effects. Hence, to avoid any spacecraft-generated disturbances, ISA team requested to ESOC operations to maintain the spacecraft in quasi-inertial pointing, to not perform reaction wheels desaturation manoeuvres and to avoid appendages movements, including HGA pointing mechanism to be triggered in hold status for the period t = CA ± 1hrs.

2.1.2. Expected non-gravitational perturbations Solar radiation pressure variation. As previous reported, the SAA of the spacecraft do not change during the flyby, neither Sun occultations are encountered in the trajectory close to Venus. As a consequence, the direct Sun irradiance do not change significantly in the period under analysis. Furthermore, the MCS appendices, including the MTM Solar Arrays, are commanded to be fixed for the CA ± 1hrs. The conclusion is that the Solar Radiation Pressure variation (SRP) could be considered negligible during the whole flyby.
and considering the spacecraft maximum possible surface exposed to Sun about SSC = 37m2, the spacecraft mass mSC = 3991kg, hence the NGP due to albedo can be evaluated as:
Planetary albedo. From figure (6) can be observed that the spacecraft approached Venus from its night side. Indeed, from some minutes before the closest approach, the spacecraft crossed the Venus terminator and, gradually, the entire illuminated disk of the planet was exposed to the spacecraft. An upper order of magnitude of such an effect can be calculated considering the fact that, at closest approach, almost all the full disk of Venus is illuminated. A raw formula can be used here as the full Venus disk is 550 km far from the spacecraft:
NGPalbedo = W alb
,
c ∗SSC
mSC
where, c is the speed of light, SSC is the spacecraft active surface, mSC is the spacecraft mass and W alb is the Venus albedo irradiance, calculated taking into account the θ angle, i.e. the half angle under which Venus is observed by the spacecraft at CA altitude 550 km. Assuming that, from the BepiColombo ESA internal documentation, the MCS assorbance αSC = 0.78 and the albedo optical coefficient of the planet Venus alb = 0.75 from Moroz (1983), the W alb can be arranged as:
W alb = 2π (1 −cos (θ)) ∗(1 + αSC) ∗alb
4π ∗W g (1) Since the Solar constant at Venus during the BepiColombo flyby (0.72AU) can be approximated as:
�2 = 2670[W/m2]
�Rg RL
W g = W L
NGPalbedo = 3.6 ∗10−8[m/s2] Hence, the upper approximation of the albedo radiation acceleration expected on the spacecraft is about the order of magnitude of the nominal ISA intrinsic noise (1 ∗10−8[m/s2/sqrt(Hz)]).
Planetary Infrared radiation. Venus atmosphere can be considered as a black body emitting at about 240 K. The corresponding average body irradiance can be estimated in 154 W/m2. Applying similar considerations used for the Planetary Albedo in equation 1, we can estimate the maximum infrared acceleration due to the radiation emitted by the planet in wave number between 230 cm−1 and 2300 cm−1, Taguchi et al. (2012), as:
NGPIR = 2.5 ∗10−8[m/s2] Thermal recoil acceleration. A Thermal Recoil Acceleration (TRA) is due to unbalanced thermal emissions of spacecraft surfaces, generating a net force pushing the spacecraft body in the opposite direction. A good estimate of that force can be carried out by collecting the temperature of the body surfaces and calculate the emission for each surface. In our study it is important to calculate an order of magnitude of the effect, to be sure that the acceleration source shall be neglected when cleaning the data. As already mentioned, the spacecraft points its ˆY MPO towards the Sun, so, MCS surfaces oriented along the +Y axis can be considered at a temperature much greater that the surfaces pointing -Y. During the cruise, at a Sun-spacecraft distance comparable with the Venus orbit, the delta average temperature between the two spacecraft sides can be kept around 120 K. Looking at thermistors onboard the spacecraft, we knew that the Venus albedo heated the -Y side of the MCS, increasing the temperature of of about 30 K. In terms of acceleration, considering that ISA is a relative 8 accelerometer, the changes of acceleration in its frequency band can be only addressed to the 30 K variation, since the +Y side of the MCS did not change significantly its temperature.