ESA Clean Space tackles space junk one component at a time
VALETTA, Malta – The European Space Agency’s Clean Space Initiative is using a novel test approach to support the development of satellite components that are designed to endure. This approach to satellite development advocates the safe disposal of spacecraft through destructive re-entry into the atmosphere.
ESA’s Clean Space Initiative was launched in 2012 to study the environmental impact of the Agency’s missions over their lifecycle. One of the main focuses of Clean Space since its earliest days has been to reduce space debris through “Design for Demise”. The goal: to make design decisions that ensure that a spacecraft component has a less than 1 in 10,000 chance of surviving re-entry and posing a threat to the local people.
In considering how to design components that are more likely to burn in the atmosphere upon re-entry, the Clean Space team took a novel approach. Rather than relying on modeling software to understand the forces exerted on a particular component during a satellite reentry, the team tracked their models with real world tests.
To simulate what a satellite and its components would experience upon re-entry, the team used a plasma wind tunnel in the Cologne plant of the German space agency DLR. The wind tunnel can mimic the overheated gas or plasma that satellites encounter upon re-entry, and expose individual components and sections of satellites to several thousand degrees Celsius.
After a series of initial studies to determine which components of a satellite were most likely to survive re-entry, the ESA Clean Space team began initial plasma wind tunnel tests in 2015. These initial tests were conducted on a selection of materials used in satellite construction, including aluminum, titanium alloy and stainless steel.
The first test on a component of a satellite was carried out in 2018. During this test campaign, the team used a reaction wheel that enables three-axis position control through a simulated re-entry. In addition to individual satellite components, the 2018 test campaign also put a complete Cubesat through its paces.
The final annual Clean Space test campaign for the sinking was completed in March. This was the most ambitious test series to date, in which the team provided a wide range of components through simulated re-entries, including an on-board computer, a battery module and a magnetorquer.
According to Tiago Soares, systems engineer at Clean Space, every test managed to produce at least one surprising result. In an interview, Soares said that a particularly revealing test was on a magnetorquer from the Portuguese space company LusoSpace.
A magnetorquer is a widely used satellite actuator that interacts with the Earth’s magnetic field to create torque that can be used for attitude control and stabilization. The LusoSpace Magnetorquer consists of a carbon fiber reinforced polymer composite shell in which an iron-cobalt core is wrapped with copper coils. The Clean Space team expected the carbon fiber shell to melt first, followed by the copper coils, and then the core. But this was not the case.
Instead of just melting away, the carbon fiber shell remained largely intact before it peeled away from the internal parts of the component. The copper coils then melted as expected, leaving the core and one final surprise.
The team found that the core had higher-than-expected energy emissivity, which meant the iron-cobalt material was emitting most of the energy it received. This resulted in the melting taking much longer than expected. In fact, the ceramic mounts used to hold the magnetorquer in place began to melt while the core remained largely intact.
The lessons learned from the test and similar findings indicated that it was necessary to expose the core as early as possible in the re-entry phase to ensure that a satellite’s magnetorqueror burned safely. With this in mind, LusoSpace has made some changes to a next-generation Magnetorquer, which was developed after a preliminary Clean Space study completed in 2015.
According to LusoSpace representative André Rodrigues Santos, the new design has a revised shape that exposes part of the core. The result is a magnetorquer that is expected to burn completely if it separates from a spacecraft structure at an altitude of at least 78 kilometers. Earlier models of magnetorquerors would only have had a 60% burn probability if released at such altitudes.
The new Magnetorquer is expected to reach phase 6 of the technology readiness stage of its development in 2021, which is mature enough for LusoSpace to offer the product to satellite manufacturers. According to Santos, the company is currently in the tendering phase for a number of projects.
With the success of the first design, LusoSpace received an ESA order in early 2019 to qualify a larger magnetorquer and produce a new line of components. The contract was awarded under the agency’s Generic Support Technology Program with a small contribution from ESA’s Earth Observation Program, a future potential customer for this type of demisable hardware.
According to Soares, the Magnetorquer pioneered the Clean Space team’s design-for-demise efforts. However, thanks to the ESA initiative, it is one of many that are currently being developed. The Clean Space team has been involved in developing fuel tanks, reaction wheels, and battery modules for doom, some of which are also nearing maturity.
In addition to the satellite components already under development, the Clean Space team expects to return to the plasma wind tunnel to run a new set of re-entry simulations. This new test campaign will include drive mechanisms for solar panels and monofuel tanks. An updated test bench will also be introduced, allowing the team to rotate parts to mimic the tumbling of a satellite upon reentry.
In addition to promoting space debris reduction, ESA’s clean space initiative is also looking at the design of green satellites, in-orbit maintenance, and active debris removal solutions. The Clean Space team consists of six full-time employees and is led by ESA veteran Luisa Innocenti. The initiative receives input and support from several other ESA departments, notably the Technology Development Element program, which pursues next generation technologies in line with ESA objectives.
This article originally appeared in the November 16, 2020 issue of SpaceNews magazine.