point on the assembly, reference frame, the ISA Instrument Line of Sight (ISA ILS) is introduced. ISA ILS is defined as the closer orthogonal tern to the non-orthogonal reference formed by the three sensing axes, and it is placed in the Y sensing element CoM. ISA ILS relation between ISA URF and MPO RF is then a very simple rotation, since, by design, the two reference frames are 90 anticlockwise Y-rotated, so the XILS ∼= −ZMPO, YILS ∼= YMPO and ZILS ∼= XMPO.

1.3. The Cruise Phase The Mercury position in the Solar System is so close to the Sun that ∆V needed for a simple Hohmann transfer from the Earth is comparable to one a probe would use to reach Jupiter. For that reason the BepiColombo cruise trajectory towards Mercury implies a complex set of operations to minimise the ∆V and so the propellant stored on board. During the cruise, periods of thrusted arcs using the MTM SEPS (Solar Electric Propulsion System) alternate with periods of coasting arcs, in which the spacecraft is in pure free fall around the Sun. Moreover, the cruise foresees several planetary flybys, summarised in Figure 4. The BepiColombo cruise evolution was considered so crucial for the entire mission success that, at launch, the only scientific activity foreseen during the cruise was the MORE Superior Solar Conjunction Experiment (SCE) with the ISA support. Therefore, when approaching the Earth Flyby in 2020, the Mission Manager and the System Operation Manager relaxed those specifications considering the flybys as an important opportunity for payloads able to be switched-on despite the MCS configuration, to collect data for characterise other planet environments or calibrate instruments for the nominal Hermean mission. Mangano et al. (2021)
(a)
(b) Figure 3: ISA - IDA assembly. Figure a) IDA Flight Model assembly. Figure b) ISA ILS internal reference frames as placed on the assembly.
is also called ISA vertex, as it represents the MPO on-board vertex with respect to what the MORE experiment will perform the Precise Orbit Determination. The operation of converting an acceleration measured by ISA sensing elements in a ISA ILS acceleration is not a simple base rotation. Indeed, positional accelerations differences as Gravity Gradient effect and Inertial rotations accelerations between the sensing elements shall be kept into account. The calculation is done onground by ISA pipelines and is called ”vertex reduction”. The attitude of ISA with respect to the spacecraft is depicted in figure(3b). The figure summarise how MPO reference frame (MPO RF) is oriented with respect to the ISA URF and ISA ILS. The
1.4. The Venus swing-by #2 (VSB2) For its characteristic, clearly shown in Figure 5 and figure 6, Venus Swing-by#2 (VSB2) was highlighted as a relevant event for many instruments to monitoring the Venus environment, the Venusian exosphere ions abundance and the crossing of magnetosphere boundaries.
An accurate analysis of spacecraft attitude is the first step to understand the forces acting on board. The Venus Solar Orbital frame (VSO) is a (a)

XY VSO frame

[Figure: Figure 4: BepiColombo flybys history in terms of event times and distance from Sun.]

(b) XZ VSO frame Figure 6: BepiColombo 2nd Venus flyby geometry in VSO reference frame. Red dot references the BepiColombo closest approach. Epochs are expressed in UTC.
2018; Acton, 1996; SPICE, 2024). To describe the orientation of MCS in space and the forces acting on it, a body fixed reference frame was used. The reference is defined as follows: ˆZMPO it is along the axis of the MCS, directed as the the MTM four SEP thrusters, the ˆY MPO axis is the opposite to the normal to the MPO radiator and, completing the right-handed frame, ˆXMPO axis points along the MTM Solar Arrays axis. Figure 7, produced with a tool developed by ISA team to visualise the MCS attitude and appendices position during the cruise, shows the described body fixed MPO frame (coincident with MCS reference) and the direction of Venus during the VSB2. The spacecraft was commanded to be

[Figure: Figure 5: BepiColombo ground track during the 2nd Venus flyby. Terminator and illuminated regions are referred to the flyby starting epoch. Epochs are expressed in UTC.]