The MicroCarb satellite aims to monitor CO2 fluxes from Low Earth Orbit. Its objective is to improve our understanding of CO2 sources and sinks, help understand the carbon cycle and therefore better grasp climate evolution. For this purpose, MicroCarb must reach a phased Sun-synchronous orbit with a Mean Local Time of the Ascending Node (MLTAN) around 22:30, at 649 km altitude. This orbit provides global coverage with a 25-day repeat cycle and a 7-day sub-cycle thanks to the satellite's depointing capability.

MicroCarb was originally scheduled to fly as a secondary passenger at the end of 2023, on a VEGA-C launcher, alongside with S2C satellite. At that time, the mission analysis team had to account for the rising phase of Solar Cycle 25, which increases atmospheric drag and strongly affects propellant budget. The 185-kg microsatellite is designed for a nominal three-year scientific mission, with a planned two-year extension. With a tight fuel budget of only 4.7 kg, optimizing propellant usage was a significant challenge.

A previous paper [1], presented at IAC 2022, introduced innovative operational strategies designed for each mission phase to minimize propellant consumption while taking into account equipment’s and payload’s non-glare constraints. However, following the Vega-C technical issues revealed after the VV22 failure at the end of 2022, the launch schedule shifted and MicroCarb had to seek a new launcher. As a secondary payload, MicroCarb had no control over its injection orbit and had to adapt to that of the primary passenger. Several launch opportunities were studied, many involving non-optimal injection conditions (differences in initial altitude, inclination, and MLTAN). MicroCarb eventually secured a launch aboard VV27 with CO3D, on 24 July 2025, which met both propellant and mission constraints.

In this paper, we describe how the mission analysis was adapted to this new launch opportunity, along with the operational manoeuvre strategy. We detail how the launch delay and the compromises made with the launcher and CO3D regarding the injection trajectory ultimately benefited our propellant budget and made a non-optimal injection feasible. Finally, we highlight the robustness of both the mission and the platform with respect to this injection scenario. In particular, we present how the in-flight satellite data helped to resolve initial uncertainties about the thermal model, enabling us to adjust the MLTAN station keeping window and to define an appropriate station keeping strategy. We also address the adaptations made to the instrument calibration strategy to cope with the lack of repeating ground tracks during the orbit drift phase.

[1] P. Annat and E. Montagnon, "Minimum-fuel Orbit Acquisition, Station-Keeping and Deorbiting Operations for a Phased Sun-synchronous Mission," presented at the 73rd International Astronautical Congress (IAC), Paris, France, 2022