G.G. Swinerd

A FREE MOLECULE AERODYNAMIC INVESTIGATION USING MULTIPLE SATELLITE ANALYSIS

Abstract The inaccuracies of numerical satellite positioning in Low Earth Orbit are predominantly due to the errors in calculating the aerodynamic forces. This is due to the unknown interactions of the rarefied atmosphere on the satellite surface, and the errors of the atmospheric density models. To improve knowledge of the gas-surface interactions, the effects of aerodynamic forces upon the orbits of satellites can be isolated, using precise orbital analysis (POA). When examining the orbit of a single satellite, the solution is restricted by the limited observational data, atmospheric model errors and the inevitable correlations between the atmospheric and aerodynamic models. To address these, three satellite orbits are examined simultaneously. The satellite orbits analysed are those of ERS1, a remote sensing satellite, and the geodetic satellites STARLETTE and STELLA. They orbit at 800 to 1000 km where helium and atomic oxygen are dominant. The collisional energies of helium and atomic oxygen upon the spacecraft surface are 1.25 and 5 eV, respectively. This preliminary analysis suggests that the re-emission of these particles are at lower speeds and lower emitted angles than those measured in laboratory molecular beam experiments. The analysis suggests that the bulk speed of the emitted particles is about 2 km s−1, and in a direction approximately half way between the specular direction and the surface normal. This emission produces a drag coefficient on an aluminium sphere of 2.52.

The space debris environment: future evolution

Space debris represents a significant risk to satellite operations, due to the possibility of damaging or catastrophic collisions. Consequently, many satellite operators screen the orbiting population for close approaches with their on-orbit assets and a public conjunction assessment service, Satellite Orbital Conjunction Reports Assessing Threatening Encounters in Space (SOCRATES), generates close approach predictions on a daily basis for all satellite payloads in the catalogue. These screening capabilities are used to inform operational decisions relating to risk mitigation but it is anticipated that the demands placed on these services will increase as debris becomes more prolific. This hypothesis is explored in a preliminary analysis of conjunction data for the years 2004 to 2009 and a new ‘Business As Usual’ study using the Debris Analysis and Monitoring Architecture for the Geosynchronous Environment (DAMAGE) model. The results suggest a 50% increase in the number of close approaches reported by SOCRATES (or its equivalent) within the next ten years. By 2059, daily conjunction reports could contain over 50,000 close approaches below 5 km, affecting the demands placed on tracking facilities and satellite resources.

Analysis of satellite laser ranging data to investigate satellite aerodynamics

Abstract In Low Earth Orbit (LEO) (under 1500 km) the effects of aerodynamic forces upon the trajectory of a satellite are far from negligible when compared to the accuracy of instrumentation flown on remote sensing satellites. To accurately calculate the net aerodynamic force, the atmospheric conditions and the interactions between the surface and the atmosphere need to be known continuously. There have been few ground-based facilities which have accurately reproduced the space environment in LEO and so there are limited direct experimental results upon which to base interaction models. Using Precise Orbital Analysis (POA) techniques and Satellite Laser Ranging (SLR) data, the aerodynamic orbital perturbations can be isolated and related to the gas/surface interactions. The objectives of this paper are to assess the performance of a number of spacecraft aerodynamic models, and to attempt to improve the knowledge of the gas/surface interactions. Lift to drag ratios greater than 0.1 are found at 800 km, with the “best fit” gas/surface emission being strongly quasi-specular in character.

Deriving Accurate Satellite Ballistic Coefficients from Two-Line Element Data

Using a specially developed orbital propagator, it is shown that only two-line element data are required to accurately predict a satellite’s ballistic coefficient. By analyzing orbit degradation due to atmospheric drag, a ballistic coefficient can be estimated using an inexpensive way to determine an important satellite characteristic, without the need for additional satellite information or a design model. As atmospheric drag is a significant perturbing force for satellites in low Earth orbit, any improvements in its estimation are vital for many applications, such as reentry predictions, satellite lifetimes, and orbital evolution. The motivation for this work originates from the study of longterm thermospheric density changes. The publicly available two-line element catalog of currently orbiting and decayed objects is a vast resource of atmospheric density information. However, two-line element data modestly provide orbital ephemerides with an accuracy of hundreds of meters up to a few kilometers. Therefore, due to these inaccuracies, deriving an accurate estimate of ballistic coefficient requires an averaging technique using multiple estimates from multiple two-line element sets.

Short-Term Debris Risk to Large Satellite Constellations

A debris collision hazard analysis is carried out on a large 800-satellite constellation, representative of the initial Teledesic cone guration, using the Space Debris Simulation (SDS) software. Usage of the software focuses on the short- to medium-term effects of explosive and collision-induced breakups. Two potentially constellationthreatening scenarios are considered: 1 ) constellation member fragmentation and 2 ) constellation launch-vehicle breakup. The constellation collision probability and the debris impact-energy to target-mass ratio are used as indicatorsoftheseverityoftherisk posed totheconstellation.Itisfound that,inthecaseinvestigated,thecollisional risk to the constellation is low in the short term. Of the scenarios examined, it is the collision-induced breakup of a constellation satellite that poses the greatest danger, in terms of the possibility of a secondary fragmentation. The SDS software is proven to be a useful tool in the analysis of the short-term, relative threat posed by debris impact to large constellation systems.

Enhancement and Validation of the IDES Orbital Debris Environment Model

Orbital debris environment models are essential in predicting the characteristics of the entire debris environment, especially for altitude and size regimes where measurement data is sparse. Most models are also used to assess mission collision risk. The IDES (Integrated Debris Evolution Suite) simulation model has recently been upgraded by including a new sodium–potassium liquid coolant droplet source model and a new historical launch database. These and other features of IDES are described in detail. The accuracy of the IDES model is evaluated over a wide range of debris sizes by comparing model predictions to three major types of debris measurement data in low Earth orbit. For the large-size debris population, the model is compared with the spatial density distribution of the United States (US) Space Command Catalog. A radar simulation model is employed to predict the detection rates of mid-size debris in the field of view of the US Haystack radar. Finally, the small-size impact flux relative to a surface of the retrieved Long Duration Exposure Facility (LDEF) spacecraft is predicted. At sub-millimetre sizes, the model currently under-predicts the debris environment encountered at low altitudes by approximately an order of magnitude. This is because other small-size debris sources, such as paint flakes have not yet been characterised. Due to the model enhancements, IDES exhibits good accuracy when predicting the debris environment at decimetre and centimetre sizes. Therefore, the validated initial conditions and the high fidelity future traffic model enables IDES to make long-term debris environment projections with more confidence.

The cascade fragmentation of a satellite constellation

Abstract The possibility of a cascade fragmentation occurring within a constellation of satellites is examined. The study sets out to quantify the additional risk of a collision with debris encountered by the constellation satellites as a direct result of the breakup of a system member or a constellation launch vehicle. A new method of determining the collision probability for an object passing through a debris cloud is used which is both versatile and completely general in its application. The new method does not explicitly assume a cloud shape or require an overly simplistic breakup model and as such avoids many of the pitfalls and sources of error found in many earlier studies. The nature of the fragmentation event is found to significantly affect the additional collision risk to the constellation above the ‘background’ level and thus govern the possible occurrence of a catastrophic breakup chain reaction.

Self-induced collision hazard in high and moderate inclination satellite constellations☆

Abstract The assessment of the hazard posed by space debris to constellations of satellites in low Earth orbit is of growing importance, with the proliferation of proposed and implemented constellation systems with a variety of mission objectives. This applies to current constellation-based commercial communication systems, in particular, since these are typically deployed at altitudes where there is a peak in the space debris environment. An impact risk analysis is performed over a period of up to 1 month after a breakup event using two examples of constellation configurations. The first is similar to the IRIDIUM system, containing around 70 satellites in near-polar orbits at approximately 800 km altitude, and the second is a Globalstar-like configuration with 56 satellites at around 1400 km altitude, distributed in orbit planes inclined at 52°. The analysis is performed using the SDS software, which applies the probabilistic continuum dynamics technique. This has the benefit of being a self-contained and rigorous method. However, it is found to be not well-suited to ‘long-term’ analysis, due to the computational effort required. The risk analysis for the chosen examples is presented, as well as an investigation of the robustness of the method when applied to complex and ‘long-period’ simulations.

Long-term collision risk prediction for low Earth orbit satellite constellations

Abstract In the light of recent changes to planned Low Earth Orbit (LEO) satellite constellation designs and enhancements made to the DERA IDES model, we have conducted a new study on long-term debris environment evolution. This includes the collision interactions of constellation systems with the orbital debris environment over the next 50 years. In this new study, we use the IDES model to simulate long-term evolution in four ‘business as usual’ future traffic scenarios, which differ by the presence and absence of foreseen satellite constellation traffic and debris mitigation measures. The IDES model is capable of taking high spatial resolution snapshots of the debris flux environment at regular time intervals. By accessing these snapshots, the IDES model is able to predict the long-term variation of debris flux incident on a specific target orbit. This technique is harnessed to predict the average debris flux trends for a typical LEO constellation satellite. Furthermore, we estimate the average debris-induced satellite failure rates for a whole constellation system. Finally, we discuss our new findings on the long-term effects of constellations on the debris environment and vice versa.

The long-term implications of operating satellite constellations in the low earth orbit debris environment

Abstract DRAs Integrated Debris Evolution Suite (IDES) model is used in this study to predict the future evolution of the orbital debris environment for two distinct scenarios. For the first case, a pre-generated background debris population for 1995 and ‘business as usual’ future launch/explosion rates are used as input to the model. IDES then employs its collision event prediction algorithm to simulate evolution from 1996 to 2020 as a baseline. The second scenario uses the same initial conditions and future trends, but in addition, a large constellation is introduced into the simulation process from year 1998 onwards. The additional contribution of the constellation to the temporal variation of key environment/population parameters is presented; including enhancement from any long-term collision coupling effects.