ESA/MEO
Disposal Strategies Analysis for MEO Orbits
Contacts
Abstract
The Medium Earth Orbit (MEO) region, home of the Global Navigation Satellite Systems (GNSS), is becoming more and more exploited with the advent of the European Galileo, the Chinese Compass constellation, new generations of American GPS satellites and the restoration of full operational capability of the Russian GLONASS. The sensitive applications of the navigation constellations and the absence of any natural sink mechanism, such as atmospheric drag, call for a careful debris prevention policy able to preserve the MEO environment, avoiding the future problems now already faced in the Low Earth Orbit (LEO) and the Geostationary Orbit (GEO) environments. In particular there is the need to define a disposal practice at end-of-life that could guarantee the safety of the operational zone where active satellites are orbiting.
Deorbiting to re-entry of decommissioned satellites is not a practical option due to the high delta-v that would be required. The same holds true for upper stages which inject the satellites in a near-operational orbit. This has already lead to a significant population of space debris in the MEO region since the constellations consist of tens of satellites which have to be replaced regularly often leaving also upper stages in this orbital region. Furthermore it is transited by spacecraft, upper stages and other space debris on highly eccentric orbits such as geostationary transfer orbits, Molniya and Tundra orbits.
Since direct re-entry is not a realistic option potential graveyard orbits have been studied since the end of the 1990s and by ESOC’s Mission Analysis section in support of the GIOVE and IOV missions. A common conclusion of these studies is that the GNSS orbits and potential graveyard orbits are affected by resonances due to coupling of Earth oblateness and third body (Sun and Moon) gravity perturbations. This resonance leads to a significant long-term increase of orbital eccentricity. The eccentricity reached within a few decades (i.e. within the expected lifetime of the current and currently built GNSSs) depends strongly on the initial eccentricity at disposal and on the combination of the initial right ascension of the ascending node and the argument of perigee.
This orbital evolution gives rise to two potential disposal strategies resp. target graveyard orbits:
• Raise or lower decommissioned spacecraft and upper stages and target initial disposal orbit eccentricity and argument of perigee such that the altitudes of these orbits don’t overlap with those of the orbits of the operational spacecraft for several decades. This strategy aims at minimising the eccentricity growth since the smaller the eccentricity within the decades of interest the smaller the initial separation in altitude needs to be (and therefore the delta-v needed to reach the disposal orbit).
• Put decommissioned spacecraft and upper stages in initial disposal orbits that lead to a fast eccentricity growth such that the perigee reaches the upper atmosphere as early as possible leading to re-entry. This disposal strategy doesn’t clean the GNSS operational altitude immediately (and leads to crossing of the LEO and potentially GEO region), however it provides a definitive solution in the long-term.
Currently the first strategy is followed since it provides a “short-term safe” solution in the coming decades and is comparatively simple to implement (following a nominal disposal no collision avoidance manoeuvres are needed by the active constellation). However the second strategy is potentially attractive because of the final removal of the debris from Earth orbit.
In order to better understand the risk associated with the two strategies it should be analysed how they affect the evolution of the space debris environment and how this in turn affects the risk for the operational GNSSs and other spacecraft and the need to perform collision avoidance manoeuvres. These tasks form the objectives of this study described hereafter, together with a “benchmark” comparison case considering a scenario where no disposal is done at all.
The activity shall encompass two closely related studies on GNSS end-of-life disposal strategies:
1) Object population evolution analysis: study on the long-term evolution of the object population in the vicinity of the GNSSs’ operating altitude for different disposal scenarios (minimum eccentricity growth, maximum eccentricity growth, no disposal as background scenario). This analysis shall take into account constellation maintenance rules (constellation lifetime, replenishment schedule, disposal success rate) and provide object numbers and collision statistics between various object classes (disposed and operational Galileo objects, other operational GNSSs, LEO, GEO objects).
2) Collision risk and avoidance manoeuvre analysis: study on the overall collision risk and expected avoidance manoeuvre rate per satellite and year for the three disposal scenarios taking into account orbit determination accuracies, object cross sections and the object population evolutions studied in the first part of the study.
The activity is carried out by a consortium constituted by IFAC/CNR (prime contractor), ISTI/CNR, University of Southampton (UK), IMCCE (F) and University of Strathclyde (UK).
Deorbiting to re-entry of decommissioned satellites is not a practical option due to the high delta-v that would be required. The same holds true for upper stages which inject the satellites in a near-operational orbit. This has already lead to a significant population of space debris in the MEO region since the constellations consist of tens of satellites which have to be replaced regularly often leaving also upper stages in this orbital region. Furthermore it is transited by spacecraft, upper stages and other space debris on highly eccentric orbits such as geostationary transfer orbits, Molniya and Tundra orbits.
Since direct re-entry is not a realistic option potential graveyard orbits have been studied since the end of the 1990s and by ESOC’s Mission Analysis section in support of the GIOVE and IOV missions. A common conclusion of these studies is that the GNSS orbits and potential graveyard orbits are affected by resonances due to coupling of Earth oblateness and third body (Sun and Moon) gravity perturbations. This resonance leads to a significant long-term increase of orbital eccentricity. The eccentricity reached within a few decades (i.e. within the expected lifetime of the current and currently built GNSSs) depends strongly on the initial eccentricity at disposal and on the combination of the initial right ascension of the ascending node and the argument of perigee.
This orbital evolution gives rise to two potential disposal strategies resp. target graveyard orbits:
• Raise or lower decommissioned spacecraft and upper stages and target initial disposal orbit eccentricity and argument of perigee such that the altitudes of these orbits don’t overlap with those of the orbits of the operational spacecraft for several decades. This strategy aims at minimising the eccentricity growth since the smaller the eccentricity within the decades of interest the smaller the initial separation in altitude needs to be (and therefore the delta-v needed to reach the disposal orbit).
• Put decommissioned spacecraft and upper stages in initial disposal orbits that lead to a fast eccentricity growth such that the perigee reaches the upper atmosphere as early as possible leading to re-entry. This disposal strategy doesn’t clean the GNSS operational altitude immediately (and leads to crossing of the LEO and potentially GEO region), however it provides a definitive solution in the long-term.
Currently the first strategy is followed since it provides a “short-term safe” solution in the coming decades and is comparatively simple to implement (following a nominal disposal no collision avoidance manoeuvres are needed by the active constellation). However the second strategy is potentially attractive because of the final removal of the debris from Earth orbit.
In order to better understand the risk associated with the two strategies it should be analysed how they affect the evolution of the space debris environment and how this in turn affects the risk for the operational GNSSs and other spacecraft and the need to perform collision avoidance manoeuvres. These tasks form the objectives of this study described hereafter, together with a “benchmark” comparison case considering a scenario where no disposal is done at all.
The activity shall encompass two closely related studies on GNSS end-of-life disposal strategies:
1) Object population evolution analysis: study on the long-term evolution of the object population in the vicinity of the GNSSs’ operating altitude for different disposal scenarios (minimum eccentricity growth, maximum eccentricity growth, no disposal as background scenario). This analysis shall take into account constellation maintenance rules (constellation lifetime, replenishment schedule, disposal success rate) and provide object numbers and collision statistics between various object classes (disposed and operational Galileo objects, other operational GNSSs, LEO, GEO objects).
2) Collision risk and avoidance manoeuvre analysis: study on the overall collision risk and expected avoidance manoeuvre rate per satellite and year for the three disposal scenarios taking into account orbit determination accuracies, object cross sections and the object population evolutions studied in the first part of the study.
The activity is carried out by a consortium constituted by IFAC/CNR (prime contractor), ISTI/CNR, University of Southampton (UK), IMCCE (F) and University of Strathclyde (UK).
Duration
37 Months
Financial Institution
Internazionali