Venus' rotation state using Venus Express tracking data and expected outcomes for the EnVision radio-science experiment.

The main characteristic of Venus that distinguishes it from the other planets is its rotation period, which is very long at 243 Earth days. Although several solutions of this period have been determined using different methods (either from Venus’s orbit or from Earth), this parameter remains poorly constrained. In particular, there is a difference of 7 minutes between the lowest and highest estimates of the rotation period. Currently, only a 3 minute variation in the Length Of the Day (LOD) can be explained by modeling various effects such as the tidal torque exerted on Venus by the Sun and the coupling between the atmosphere and the planet. In our study, we propose a new estimate of the rotation period of Venus using Doppler tracking data from the Venus Express spacecraft. The Venus Express (VEX) mission was launched by the European Space Agency (ESA) in November 2005 and orbited Venus for almost 8 years. The main objective of the mission was to study the planet's atmosphere. To determine a new solution for the rotation period of Venus, we use the Precise Orbit Determination (POD) method, which involves a least-squares adjustment of the difference between the Doppler data collected on Earth and the Doppler data obtained by the numerical integration of forces that can affect the spacecraft's motion. We found a rotation period for Venus of 243.0200 ± 0.0007 days, within the range of values reported in the literature and obtained using different methods and databases. However, the expected periodic variations in the rotation period or the precession rate could not be detected due to the lack of sensitivity of Doppler measurements in the signature of these parameters on the VEX’s trajectory. ESA's EnVision mission, scheduled for launch around 2031, aims to study Venus from its deep core to the top of its atmosphere. We have carried out simulations to predict EnVision's performance. The predicted uncertainty in the rotation period is 0.6 seconds, compared to the uncertainty of 1 minute obtained with VEX. For the precession rate, the predicted uncertainty is 0.2%, compared to 7% obtained with ground-based radar data. The near-polar and low eccentricity of the spacecraft's orbit will provide greater sensitivity to the planet's rotational state.