New tests for rendezvous and docking interoperability

The first step towards this future of space logistic hubs like airports or train stations on Earth is developing the standards to allow for an open, interoperable and modular ecosystem for all spacecraft manufacturers to adhere to.

ESA and industry are working together to develop and test interfaces that will allow satellites to approach in space, recognise each other and dock. New hardware tests for this step took place at ESA’s testing facilities at ESTEC, the agency’s technical centre in the Netherlands, evaluating different interfaces and technologies developed by The Exploration Company with Sener, GMV, Almatech, Thales Alenia Space and CDS.

Cosmic rendezvous

⁠“When one spacecraft approaches another in space, we call this a rendezvous,” explains Lorenzo Pasqualetto-Cassinis, ESA's Guidance, Navigation and Control system engineer.

“For in-space logistics missions, a space rendezvous entails a servicing spacecraft matching its orbit to that of the client – a spacecraft in need of servicing or transportation – and performing very precise manoeuvres to get close. In our laboratory, we tested two camera systems that would enable a servicing vehicle to approach its target, bringing the two from hundreds of metres apart to as close as a few centimetres, setting them up for docking.”

The rendezvous testing took place at ESA’s Guidance, Navigation and Control Laboratory using GRALS, the lab’s duo of robotic arms mounted on 33-m long rails. One arm holds the navigation unit of an approaching spacecraft developed by The Exploration Company, the other holds a scale model of a spacecraft target or ‘client’, developed by Thales Alenia Space in France. Both are suspended in the centre of a long dark room with only one lamp illuminating the scene, mimicking the Sun.

To establish its own position in space relative to the target, the navigation system's camera locks onto a set of black-and-white markers resembling QR codes on the interface side of the target spacecraft.

The last centimetres

The final part of the rendezvous simulation – the docking – is taken care of by ESA’s Orbital Robotics Laboratory, conveniently located on the other side of the long room that houses the GRALS robotic arms.

Here, specialised platforms hover over an extremely flat and smooth floor, simulating the state of weightless free-floating in two dimensions. On the bottom of each platform, air bearings blow air out towards the floor to create a micrometre-scale air gap, making the platforms and their payloads float without any friction.

“In this part of the testing, the servicer and client spacecraft models are each attached to one floating platform,” says Jules Noirant, ESA’s automation and robotics engineer. “The servicer-simulating platform weighs around 180 kg and has an additional 20 kg weight attached on either side – that way the mass difference between the two models is similar to a real-life scenario.”

Starting at four centimetres apart, the servicer’s docking mechanism first locks onto the client’s passive capture interface and then brings the two platforms closer and closer together until two ports – one for refilling and the other for power and data transfer – connect to their counterparts.

With the two tests performed over a couple of weeks this summer, the complete rendezvous, docking and servicing process was reenacted all in one place.

“This test allowed us to demonstrate the performances of our systems,” comments Olivier Faure, Head of Future Projects and In-Orbit Services Programs at The Exploration Company. “The rendezvous must work in all conditions – in complete darkness, but also when the markers are reflecting sunlight and blinding the camera with flare.

“Similarly, the docking must work for multiple dynamic conditions in the last centimetres, with a range of relative positions and velocities with the target. The tests were instrumental for us to get a better understanding of how our system will perform before we test it in space.”

This test campaign falls under ESA’s In-Space Proof of Concept (InSPoC) venture that aims to prove interoperable interfaces needed for thriving logistical hubs in space. Grouped into four themes, the first – InSPoC-1 – tackles rendezvous, docking and transfer of fluids or gases between spacecraft.

Yann Tincelin, InSPoC Programme Manager at ESA, concludes: “Our objective is to collaboratively design and demonstrate the interfaces that could enable an open in-space infrastructure allowing European stakeholders to operate and provide services in the orbital economy.”

Sustainable use of space

Currently, satellites are launched to space with the amount of fuel they will need for the mission’s lifetime. Depending on the orbit a satellite is flying in, running out of fuel will make it either reenter the atmosphere and burn up, or continue to float, becoming another piece of space debris.

To manage the growing number of non-functional objects in space, two approaches need to be implemented side by side. One is extending the mission life of a spacecraft through repairing, refilling or reusing it. For that, the spacecraft we launch in the future will need to be equipped with interfaces that servicing spacecraft can use to rendezvous with them, developed through projects like InSPoC-1.

The second is cleaning up our orbits by removing existing space debris and preventing the generation of new debris by safely disposing of satellites at the end of their orbital lifetime. ESA is tackling this through projects like the Cat mission, which will rendezvous with a satellite equipped with a compatible Mice interface.