INTEGRAL Picture Of the Month
April 2022

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INTEGRAL dancing with the Moon 'till the Earth's atmosphere

The orbit of the INTEGRAL mission belongs to the class of Highly Elliptical Orbits (HEOs). These orbits are characterised by relatively low pericentre altitudes and extremely high apocentre altitudes. For this reason, HEO missions offer a privileged position for astrophysics and astronomy operations as the spacecraft spends most of its time at an altitude above the Earth's radiation belts, offering a radiation-free environment for the scientific observations. Specifically, INTEGRAL is currently on an orbit whose lowest point is 2000 kilometres and its highest point near 150000 kilometres.

Given its peculiar shape, the orbit of INTEGRAL is not only influenced by Earth's gravity and the atmospheric drag, but also by the gravitational attraction of the Moon and by the Solar Radiation Pressure (SRP). These additional forces can drastically influence the trajectory of the spacecraft, leading to different predictions about its future. INTEGRAL is a large spacecraft, five metres tall and more than 4 tonnes in weight. Its disposal will be performed via a re-entry in Earth's atmosphere and given its size, it is important to predict its re-entry and its demise in the atmosphere to ensure the safety of people and properties on the ground.

Given INTEGRAL's orbit and the little fuel left on-board, the re-entry of the spacecraft is only possible after several years from the manoeuvre; therefore, its predicted entry is not trivial. A study funded by the European Research Council COMPASS ( combines the long-term prediction of INTEGRAL's orbit with re-entry dynamics (the concept of overshoot boundary) to help improve the re-entry predictions of HEO satellites and study their demisability. By performing a sensitivity analysis on the disposal manoeuvre of INTEGRAL, the variability of the re-entry scenarios could be studied and the difference with approaches that were neglecting the re-entry dynamics be highlighted (by directly targeting a very low pericentre radius).

The left graph shows the predicted entry trajectories, highlighting in colour the Liquid Mass Fraction (LMF) that is the portion of the spacecraft mass that is predicted to demise during re-entry. In black, instead, the entry trajectory for a fixed targeted entry interface.

The middle graph and right graph show the evolution of the mechanical and thermal loads, respectively, on the spacecraft during the different entry trajectories. The colours highlight the value of the peak acceleration and heat flux, respectively.

From the analysis, it is clear that when one predicts the re-entry, one needs to consider an array of possible solutions and understand the sensitivity to the initial conditions, as even small differences in the manoeuvre can lead to largely different trajectories and thermomechanical loads on the spacecraft, which in turn affect its demise.

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