Speaker
Ms
Francesca Letizia
(University of Southampton)
Description
Environmental indices for spacecraft are meant to rank objects in orbit depending on their effect on the space environment in case of fragmentation. These indices are usually designed to identify potential candidates for active debris removal and several authors have proposed formulations of environmental indices, which take into account different aspect of the debris environment (e.g. background density, spacecraft mass, orbital altitude). The index proposed in this work focusses on the assessment of the severity of the breakup of a spacecraft estimating the resulting collision probability for operational spacecraft.
First, a grid in semi-major axis and inclination is defined to map the possible initial conditions of the breakup. For each point in the grid, the NASA breakup model is applied to generate the fragment cloud (considering all objects larger than 1 cm). The case of catastrophic collision is considered to model the effect of the spacecraft mass. The spatial density of the cloud is analytically propagated for 25 years by applying the continuity equation to model the effect of atmospheric drag. An analytical approach is also used to compute the collision probability between the fragments in the cloud and target spacecraft crossing it. In this application, the target spacecraft represent the fleet of operational satellites on which the consequences of the breakup are evaluated. For computational reasons, not all operational satellites are individually propagated, but their population is sampled, considering the distribution of cross-sectional area, to define a small number (e.g. 10) of representative objects. The value of the environmental index for a specific initial condition is obtained by summing the resulting cumulative collision probability of all representative objects.
In this way, the variation of the environmental index with semi-major axis and inclination is obtained and stored in the so-called “reference layer”, which refers to a fixed value of fragmenting mass. The dependence on the mass is introduced by rescaling the “reference layer” following the power law that, in the NASA breakup model, relates the number of produced fragments and the fragmenting mass. One interesting aspect of the proposed index is that the computational effort is required only for the generation of the “reference layer”, which needs to be recomputed only in case of a significant variation in the distribution of active satellites. Given the “reference layer” and the dependence on the mass, the value of the index for any spacecraft can be obtained by a simple interpolation.
Only a few key parameters (i.e. mass, semi-major axis, inclination) are required to characterise the criticality of a mission. The index can be applied to spacecraft already in orbit (as it was done using the data available in DISCOS) and to future missions, to support their mission design and their licensing process. For example, the environmental index can be employed to assess possible waivers to mitigation requirements for spacecraft, considering the estimated severity of their breakup.
| Applicant type | First author |
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Primary author
Ms
Francesca Letizia
(University of Southampton)
Co-authors
Dr
Camilla Colombo
(University of Southampton)
Dr
Holger Krag
(European Space Agency)
Dr
Hugh Lewis
(University of Southampton)
