5 ECTS Credits 

29.10.2022
05.11.2022
19.11.2022
26.11.2022
03.12.2022
17.12.2022
14.01.2022
21.01.2022
28.01.2022
04.02.2022
11.02.2022

Content of the Lecture

Orbital Mechanics:

Two-body Problem, Shape of the Orbit, Velocities (vis viva integral), Time-dependency of the motion, Orbit Transfers, Representing an orbital state, Time and Earth rotation, Perturbed Orbits.

Measurements: Radar, Optical Telescopes, Laser Systems, In-situ Detectors.

The on-orbit population of man-made objects: Sink Terms, Spatial Density, Source Terms.

Legal Aspects: UN Space Law, National Space Law.

Mitigation: Projection of Environment Trends, Governing and Consulting Bodies, Mitigation Measures, Mitigation Technology.

Space Surveillance: Scope of Surveillance, Orbit Determination, Observation Approaches, Correlation, Scheduling Follow-Ups.

Collision Avoidance: Collision Probability, Collision Avoidance Manoeuvres, Accepted Collision Probability Levels, Operational Practise.

Re-entry Risks: Re-entry Break-Up Analysis, Uncontrolled Re-entries, Controlled Re-entries.

Protection: Hypervelocity Impact Tests, Ballistic Limit Equations.

Remediation: Scope, Active Removal, Laser-based Momentum Transfer.

 Overview

Space Debris is a direct result of human space launch activities. Typically, a rocket launch will not only lead to the orbital injection of a functional spacecraft, but very often additional elements reach orbital velocity and can remain in space for a longer time. If the latest rocket stage is not designed to remove itself after orbit injection, it will remain on a similar orbit than the satellite. Some launches make use of adapters that separate several satellites from each other in a multi-object launch stack. Such mission related objects do not have propulsion and remain in the injection orbit.

Other mission related objects can also be released by the satellite, such as sensor covers or clamp bands. Finally, when a satellite has reached its end of life and is decommissioned, or if it cannot anymore be contacted from ground, it becomes a man-made object that does not fulfil any purpose any longer and therefore fulfils the definition of space debris.

The majority of space debris objects is, however, a result of fragmentation events. The majority of such events are break-ups due to remnant onboard energy. Space objects are not designed to survive the harsh environment in space for much longer than their mission duration. Without orbital or thermal control, temperatures on board can change from the usual range. This can lead onboard energy sources such as chemical (e.g. fuel), electrical (batteries), or even mechanical (e.g. reactions wheels) to enter into exothermic reactions, typically fuel dissociation or the generation of pressure gasses in battery cells. The aggressive environment (extreme ultraviolet radiation, atomic oxygen, impacting micro-particles) can further stimulate such destructive processes.

Figure 1.1: Objects injected with orbital velocity during a launch (example: Ariane 5)

The resulting break-ups can range from pressure bursts to high-energy explosions. The fragments receive a velocity increment induced by the explosion. In combination with orbital perturbations the fragment cloud typically gets dispersed globally in the altitude shell of the progenitor object. Another type of fragmentation are catastrophic collisions, which are still rare for the time being. Several additional, non-fragmentation sources exist. Like fragmentations they can contribute to all debris size regimes.

Figure 1.2: Distribution of a fragment cloud (released on a polar LEO orbit)

As a result of human launch activities and debris generating events, the overall current on-orbit population of man-made objects is estimated to consist of:

• Ca 150,000,000 objects with sizes > 1 mm
• Ca 900,000 objects with sizes > 1 cm
• Ca 30,000 objects with sizes > 10 cm

The majority of the objects >10 cm (ca. 22,000) can be regularly monitored with the help of powerful surveillance systems from ground. Their orbits can be determined, and the objects are registered in a catalogue that is constantly maintained. The data of such surveillance systems is essential input to spacecraft operations.

The major issue of space debris is the inherit risk of impact of these objects onto the surface of functional orbital objects. These impacts are particularly critical because of the high velocity at which they occur. A typical orbital velocity of objects orbiting in LEO (Low Earth Orbit) is 7.5 km/s. Relative impact velocities can therefore even be higher. The impact energy associated with these high relative velocities can render an impact of a 1mm object mission-critical for a functioning spacecraft. Objects of 1cm size can terminate a mission and objects larger than 10cm carry enough energy to cause a catastrophic collision (meaning a full fragmentation of both objects).

Figure 1.3: Impact of an Al-sphere of d = 1.2 cm (m ≈ 1.7 g) at v = 6.8 km/s on an Al-block of diameter 18cm and height 8.2 cm, Crater depths: 5.3 cm

The number of artificial objects scales with human spaceflight activity. While the increase of spacecraft, rocket bodies and mission related objects has been growing linearly with time (due to roughly constant launch rates), fragmentations and collisions are more connected to accumulation effects in orbit. Smaller objects (fragments) are thus produced at increasing rates.

Figure 1.4: History of US Space Surveillance Catalogue

Orbital satellites are constantly exposed to the environment of man-made particulate objects. Impact features on surfaces that returned from space, or that were inspected in space have been collected and systematically studied. They are striking evidence for the amount and effect of such events.

Figure 1.5: Onboard imagery of an impact into the backside of the solar array of Sentinel-1A on August 23^rd 2016 (left ‑ solar arrays directly after deployment in 2014, right ‑ after impact of an object with 3-4 mm at 10 km/s, impact feature has a diameter of ca. 40 cm)

Figure 1.6: Impact features on solar cells of the Hubble Space Telescope (HST) retrieved with STS‑109 in 2002 (each cell has a size of 2 cm x 4 cm)

Starting from the current risks associated with space debris, future risks might become even more severe pending on the future launch traffic and behaviour in spaceflight. The sensitivity of the dynamics of the environment is subject of current research. Models are used to predict the evolution of the number of objects in space. Such models are based on semi-analytical and stochastic methods. They are used to analyse the effectiveness of different prevention measures and provide the backbone for the definition of debris mitigation guidelines.

Figure 1.7: Simulated trends of object numbers in a so-called “business as usual” scenario (continuation of current launch rates and behaviour). Results of different Monte Carlo runs with different seed values are shown.

Mitigation measures aim at the prevention of explosions by “passivating” space systems at the end of mission. They also aim at preventing collisions. The latter is a twofold problem. During operation, large known objects (contained in the catalogue) can be avoided by initiating a manoeuvre. After operations, only the chance for collisions can be reduced by limiting the presence of the space system in orbit. In the future, active intervention into the on-orbit population by active removal (so-called remediation) might be applied.

During flight, the vulnerability of a space system can be reduced by passive shielding. Such technology is applied in human spaceflight (manned modules of the International Space Station ISS are designed to withstand impactors of up to 1cm size). In unmanned space systems, shielding effects of the structure can be used to protect more sensitive parts (tanks, electronics, etc…).

Finally, many space objects terminate their orbital presence by a re-entry into the upper atmosphere (which is a desired disposal method). Under the influence of the aerodynamic deceleration and aerothermal heating, such re-entry is a destructive process which only typically 20-40 % of the original mass survives. Mostly, heat resistant material such as titanium or stainless steel (often used for tanks and thrusters) survives. During this process, the orbital velocity is completely removed, the energy converted into torques and heating and the surviving fragment falls vertically (the last 30-40 km). Still it carries enough impact energy to cause casualties on-ground. Limiting the casualty risks is one of the major elements in spacecraft design for mitigation.

Figure 1.8: Tank of a Delta-II upper-stage retrieved 2001 in Saudi Arabia (Maley, 2003)