Project aims to dock cubesats for modular space structures

A project at the Surrey Space Centre (SSC) is aiming to dock two cubesats in orbit, with a view to eventually enabling mission upgrades and construction of modular devices.

STRaND-2 — a collaboration between Surrey University and Surrey Satellite Technology — will use spatial awareness technology from Microsoft’s Kinect console peripheral for docking.

‘Cubesats are very handy and they have opened up the field of demonstrating technologies for a very low cost,’ SSTL project lead Shaun Kenyon told The Engineer. ‘But at the same time they are really, really small, so if we can dock two or three, or however many together then you can build a more capable system in space.’

The project will build on experience gained by STRaND-1, a cubesat that is powered by an Android Google Nexus 1 smartphone. Indeed, it continues a tradition at SSTL of using off-the shelf-components, where possible, and adapting them for a space environment.

‘The wonderful thing about the Kinect API [application programming interface] is that it looks after that very low-level calculation of distances for you, so you can get a live stream of the depth field as a video — a lot of the very complex processing has already taken place and you can concentrate on the higher-level analysis such as edge detection or object detection,’ Kenyon said.

‘There are some things obviously we’ll have to do, such as take off the plastic casing, and we’re going to do some vacuum tests too see if we have to replace any of the structural components because some materials outgas when they’re in a vacuum.’

Docking systems have never been employed on such small and low-cost missions and are usually reserved for big-budget space missions to the International Space Station (ISS) or historically, the Mir space station and the Apollo program.

The two satellites will measure 30cm in length and consist of three cubesats (10 x 10 x 10cm) joined together. The STRaND-2 twins will be separated after launch. After the initial phase of system checks, the two satellites will be commanded to perform the docking procedure and, when in close proximity, the Kinect system will provide the satellites with 3D spatial awareness to align and dock with the help of micro-thrusters.

The team hopes to have the system ready in around 24–36 months according to Kenyon. They will then use an air-bearing table facility at Surrey University to test out the docking procedure.

‘It’s not a full three-dimensional simulation, but it’s good enough to do the dynamic tests we need to do to test the control software,’ said Kenyon.

The aim is for STRaND-2 to be a technology demonstrator to then enable the docking and modular principle to be used for more complex configurations and applications.

For example, space missions could be reconfigured as mission objectives change or upgraded in-orbit with the latest available technologies and added computing power.

The concept could also provide a solution to the problem of hazardous space debris, by bolting-on thruster satellites to de-orbit end-of-life components.

In addition, SSC is currently working with the California Institute of Technology (CalTech) on a project dubbed ARREST to build a modular telescope.

‘This particular concept looks at changing the shape of the mirror and changing the aspect ratio, depending on what they want,’ said Kenyon. ‘If they want a different signal-to-noise ratio they might have the mirror in particular shape, if they want a better spatial resolution they might undock some of these satellites each with a small section of the mirror and re-dock them in a different configuration that changes the shape of the mirror and gives you a different set of properties in the telescope,’ said Kenyon.