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Millimetric precision for flying satellites

It is a technological breakthrough. The two satellites of the Proba-3 mission are expected to maintain their respective positions with a precision of one millimetre. This system of satellites flying in formation and forming an artificial eclipse will act as a giant coronagraph, able to study the Sun’s corona with unprecedented precision.  © ESA
It is a technological breakthrough. The two satellites of the Proba-3 mission are expected to maintain their respective positions with a precision of one millimetre. This system of satellites flying in formation and forming an artificial eclipse will act as a giant coronagraph, able to study the Sun’s corona with unprecedented precision. © ESA

European scientists are preparing to notch up a world first in satellite formation flying. Two spacecraft will soon be flying side-by-side with extreme precision.

Planned for launch in 2017, the Proba-3 mission will be the European Space Agency’s – and the world’s – first precision space formation flying mission. A pair of satellites will fly together maintaining a fixed configuration as if they were a virtual large rigid structure in space. The mission, costing more than 100 million euros, will demonstrate precise formation flying as part of a large-scale science experiment.

Both spacecraft on this mission are part of ESA’s Proba series. ‘If successful, future missions could be created on a much larger scale at lower cost, using separate satellites that fly as one,’ said the Proba-3 project manager, Agnes Mestreau-Garreau.

She added, ‘In recent years, Europe’s multi-satellite missions have made significant progress. The “Automated Transfer Vehicle” supply vessel has demonstrated centimetre accuracy docking with the International Space Station, while Sweden’s Prisma mission has demonstrated formation flying for brief periods. Proba-3 is the next step in formation flying. Its two satellites will autonomously move in unison, without guidance from the ground.’ The two satellites are expected to maintain their respective position with a precision of one millimetre at distances of 150 m or more.

Proba stands for the ‘PRoject for OnBoard Autonomy’ and this is how it all works. The paired Proba-3 satellites will together form a 150-m long solar ‘coronagraph’ to study the Sun’s faint corona. It will get closer to its solar rim than has ever been achieved before.

The larger ‘coronagraph’ satellite will point the instrument sunward to observe the solar corona while the smaller ‘occulter’ satellite will obscure the glaring solar disk so the wispy corona can be continuously observed, forming an artificial eclipse.

GPS tracking and inter-satellite radio links

The Sun is a million times brighter than its surrounding corona, so eclipsing it is essential for coronal studies. This giant ‘coronagraph system’ will enrich solar science with an unprecedented study of the corona.

Previous Sun-observing missions incorporate ‘internal’ coronagraphs to study the corona. But their effectiveness is limited by a phenomenon called diffraction, where stray light overspills the edge of the occulting disk.

Progress on this front requires moving the occulter much further away while still preserving eclipse-like conditions for long periods of time – precisely the performance offered by Proba-3. The technique was previously attempted during the 1975 manned Apollo-Soyuz mission, when an Apollo command module blocked light falling on a Soyuz spacecraft. The manoeuvre only lasted for a brief time during the 44-hour mission, but allowed the crew of the Soyuz craft to take pictures of the solar corona.

Beside its scientific interest, Mestreau-Garreau said the Proba-3 experiment will be a ‘perfect instrument’ to measure the precise positioning of the two satellites.

Launched together, formation flying will be achieved using a combination of technologies, including GPS tracking and inter-satellite radio links from a distance, to determine their relative positions. Over a two year-period they will orbit the Earth every 20 hours and the satellites will test a variety of strategies and algorithms.

‘Achieving precise formation flying will open up a whole new era’, said Mestreau-Garreau. ‘For science and Earth observation, larger apertures, longer focal lengths and baselines far beyond what can be achieved with a single spacecraft can be met with precision formation flying. In-orbit servicing and deorbiting becomes feasible, as does the automated rendezvous and docking needed for the ambitious Mars Sample Return mission, retrieving a sample Martian regolith (dust) to bring back to Earth,’ she said.

‘Proba 3 will provide the technology required for missions several times more expensive and that would otherwise be unaffordable.’

Agnes Mestreau-Garreau, Proba-3 project manager, European Space Agency

A broad range of novel technologies will be essential to make Proba-3 a success. This technology could be used for a future Mars mission.

In the shorter term, Mestreau-Garreau says Proba-3 is needed because current scientific challenges call for the detection of ever fainter signals. Larger apertures that are beyond what can be achieved with a single spacecraft will be required to meet these goals. ‘As we progress in science and services,’ said Mestreau-Garreau, ‘we have to capture weaker signals and observe smaller features.’

Like its Proba predecessors, Proba-3 - effectively a ‘laboratory in space’ - will be controlled from ESA´s Redu Centre deep in Belgium’s Ardennes.

Mestreau-Garreau said Proba-3 will pave the way for future, highly demanding missions which need maintained large structures, overcoming the limitations of large deployable structures in space.

‘Proba-3 will provide the technology required for missions several times more expensive and that would otherwise be unaffordable. It will allow us to observe the Sun corona down to the rim like never before and to understand so far elusive phenomena.’

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