Cluster reconnaissance

Mathematicians and astrodynamicists at

are embarking on a three-year research project to test the feasibility of tethering microsatellites.

The Surrey team will create computer simulations that will demonstrate how tethers can be used to control the attitude and relative position of a cluster of about 20 tiny satellites.

They envision clusters of microsatellites that could one day orbit the Earth, studying plasma depletion in the upper atmosphere. Plasma depletion, a phenomenon also known as plasma bubbles, can disrupt communications signals.

The satellites — about the size of a mobile phone — would be tethered before pulling apart and scattering across small areas of the ionosphere. The tiny spacecraft will make individual measurements to give scientists a better understanding of plasma bubbles and the ability to forecast them.

'We're interested in launching satellites in clusters because the communication range of these satellites is going to be quite short,' said Phil Palmer, head of the astrodynamics team at Surrey Space Centre. 'When we release the tethers, the satellites will form a little cluster that will stay collocated for weeks, which is the duration of the mission, and they will be able to communicate together.'

The Surrey researchers' computer simulations will predict what can happen when 20 lightweight satellites whirl through orbit joined only by thin nylon strings similar to fishing lines.

Past experience and physics tell them they are likely to oscillate wildly. 'We're actually trying to exploit the oscillation between the satellites,' said Palmer. 'We think we can use instabilities to work to our advantage for the tethered system.'

Each satellite is connected like a pendulum, said Palmer. Disturbant forces, such as radiation, push on each of the satellites and cause vibration on the tethers. 'This will give lateral motions between the satellites, and when the tether is released, it will create differences in the satellites' velocities,' he said.

'So they'll almost be in the same orbit, but with the knowledge we gain from our research on the dynamics of tethered spacecraft, we can have some control over what the differences in their orbits are.'

The idea for the tethering system came soon after Surrey Space Centre developed its latest ultra-small PCB satellite.

'The satellites are able to fit a camera, GPS receiver, radio communication system and all the necessary subsystems, but the difficulty was there wasn't enough space to fit actuators to change their attitude,' said Palmer. 'So we had to find a different way, and the tether is just one way of doing that.'

The satellites would be grouped and launched together. Once in orbit, the tethering system would help place the satellites in the right location. After a few days, and exposure to ultraviolet light, the tether would become thinner and more plastic before snapping. 'The plasticity in the tether could generate instabilities that we could exploit,' he said.

One of the most immediate applications for microsatellites cluster missions is the study of the effects and causes of plasma bubbles in the upper atmosphere. Plasma bubbles can degrade or disrupt signals from satellites, and scientists believe the detection and characterisation of these disturbances by measuring things such as the variations in density of electrons will help them predict the disturbances.

'These small localised effects are very hard to make measurements with conventional satellites,' Palmer said. 'However, if you could put a group of satellites in a very small region together to make lots of simultaneous measurements in different places at the same time, you can build a three-dimensional picture of what's going on.'

Tethering systems are not a new idea. For about 15 years space associations have been using them for serious missions. Theoretical ideas have been floating around even longer. In one particular application, tethers can be used to electromagnetically generate power for spacecraft from the Earth's magnetic field.

'You can also attach them to pieces of space debris to de-orbit them,' Palmer said. 'You can even use them as a pumping method for changing orbit rather than using a propellant.'

With better understanding of the dynamics of tethered small spacecraft, Palmer's project could lead to many other applications besides studying plasma bubbles.

'The system could be used for anything that requires simultaneous measurements in lots of places in the same region,' he said.