Stellar line-up

8 min read

The success of the two major ESA projects will depend on getting spacecraft to fly in perfect formation with each other. Niall Firth reports.

As any Red Arrows pilot will tell you, formation flying is far from easy. This is acknowledged with a passing comment on ESA’s website as it celebrates the sixth anniversary of Cluster, the first mission to keep satellites in reasonably tight formation hundreds of thousands of miles above the Earth. Describing the ground-breaking mission with wry humour it reads: ‘Formation flying is a daily reality for the Cluster scientific mission. Some days more so than others.’

With help from a control centre on the ground the four identical satellites have flown in formation, mapping the Earth’s magnetosphere separated by up to as much as 10,000km at certain points in their mission lifetime. The fact that these satellites have remained more or less aligned for such long stretches of time has rightly been hailed by ESA as a marvellous achievement. However, the next generation of the agency’s space missions will require a degree of formation flying accuracy that is far more precise than has ever gone before, Cluster included.

Two of Europe’s biggest space projects are to rely on formation flying technology that will ensure the spacecraft can keep themselves aligned within nanometres, communicating with each other via lasers that are beamed across millions of miles of dark, empty space. One of these projects is the Laser Interferometric Space Antenna (LISA), a joint ESA/NASA mission that aims to detect gravitational waves from the far reaches of space. Scheduled for launch an estimated five years later than LISA is Darwin, a mission to look for Earth-like planets in the galaxy. Due for launch in 2009, the LISA Pathfinder mission will test much of the technology needed for when the three full-scale LISA spacecraft lift off in around 2015.

Prof Mike Cruise, director of the Centre for Space and Gravity Research at Birmingham University, is developing key technology to be used in the LISA Pathfinder mission. With researchers at Glasgow University, Cruise’s team is building the sophisticated optical bench and laser assembly that sits inside the LISA spacecraft and makes the precise laser measurements that are so crucial to the mission’s success. ‘An incredible amount of engineering is going into this. A combination of very high-precision lasers, high-precision optics and exquisitely sophisticated electronics,’ said Cruise.

When the LISA mission itself is launched it will comprise three identical spacecraft positioned five million km apart in a giant equilateral triangle that will travel 50 million km behind the Earth. The spacecraft need to be kept at a great enough distance from the Earth to minimise the disrupting effect of any gravitational forces between the Moon and the Earth.

Scientists envisage that LISA will be able to detect gravitational waves caused by some of the most violent astrophysical events in the universe including the creation of super-massive black holes, and the enormous impact caused by the collision between distant galaxies. The sensitivity required to detect the minute changes in gravitational waves caused by distant cosmic events means that the equipment must be placed far away from any Earth-based interference; positioning the detectors in space is the only option.

To obtain the best signal each spacecraft in LISA must also be as far apart as possible while their exact relative location must be known with an unprecedented degree of accuracy. This is because LISA will measure the slight inward and outward movement of a special ‘proof mass’ on-board each craft as the gravitational wave passes through.

The proof mass is an isolated 40mm cube of platinum-gold that floats freely in space, with the surrounding spacecraft acting as a shield blocking out harmful solar radiation or tiny asteroid particles. Without touching it, the spacecraft follows the movement of the proof mass inside, tracking its movement to an accuracy of around 10 picometers, or 10 millionths of a metre.

‘These proof masses will be the most stable objects ever built,’ said Cruise. ‘They will stick to the inertial fabric of space and the spacecraft will act as a cocoon to protect it from anything that happens in space that would disturb our measurements, however small.’

Imperial College London is also developing a sophisticated system for shining UV light on the proof masses to rid them of the electric charge carried by cosmic rays.

The optical assembly emits an infrared laser that is beamed towards the two other remote spacecraft. As it leaves the craft a small portion of the beam is reflected back on to the proof mass inside. When the signal is received back from one of the remote proof masses — five million km away — the phase difference between the two is measured. The accuracy of this measurement is crucial because a passing gravitational wave will slightly change the length of one laser’s optical path. This difference can then be compared to the length of the other two laser beams and the passing of the gravitational wave can be monitored and its effects measured.

Despite the fact that much of the same technology may well be used for both the missions, Cruise noted the small but crucial difference between the formation flying needed for LISA and that to be used in Darwin. ‘These spacecraft need position control to the nanometre, over millions of kilometres in space. The difference with LISA is that we will be measuring the separation between the craft rather than making sure they are always aligned as in Darwin,’ said Cruise.

One thing that both Darwin and LISA do have in common is that they are likely to use Field Emission Electric Propulsion (FEEP) ion engines — developed by EADS Astrium — for the micro-newton thrusters necessary to allow precise positional control of these spacecraft. For Darwin an important part of the formation flying technology lies in the success of this micro-propulsion system that will gently push a spacecraft back into alignment if it falls out of step with the others.

In LISA if the laser interferometry indicates that the exact distance between the proof mass and spacecraft has changed slightly, these extremely low-thrust micro-thrusters will carefully nudge the spacecraft back into place. FEEP thrusters are unique ion engines that extract ions directly from the liquid phase of the stored propellant, often caesium. FEEP engines are recognised as being able to provide extremely accurate amounts of thrust at micro-newton level, ideal for precision movement crucial for accurate formation flying.

According to Cruise, the work that LISA will do in developing precision formation flying technologies, particularly the laser metrology, will be critical to the success of the subsequent Darwin mission.

To seek out planets in distant space that are similar to our own, and could therefore harbour life, is both an ambitious and exciting proposal. But Darwin is also likely to be extremely technically challenging. The mission, to be managed by EADS Astrium and Alcatel, will comprise a flotilla of four spacecraft, stationed around 150 million miles from Earth at the so-called Lagrange Point to detect the light from planets circling other stars in the galaxy.

Three of the spacecraft will carry a 3m space telescope that will monitor distant areas of space in the infrared spectrum. The images from all three will then be combined and will act as if they were a single large telescope with a diameter of up to several 100m. The fourth spacecraft acts as a central hub, recombining the images.

Darwin will use a technique called ‘nulling interferometry’ to detect the indistinct light from a distant planet. Darwin is essentially a gigantic version of a Michelson interferometer. Beam-splitting optics are used to slightly delay and invert the light from one of the telescopes to leave a central ‘destructive interference’ patch in the direction you are pointing the interferometer. This blocks the light from the star but still allows the light from the planet to be detected.

If it were not for this ‘nulling’, the starlight would overwhelm the planet’s feeble glow. ESA and the European Southern Observatory (ESO) are developing a nulling interferometer to be used at the Very Large Telescope (VLT) ground-based observatory facility to practise the technique before Darwin is launched.

However, recombining the light collected by the telescopes has to be achieved with extremely high precision; a deviation of more than just 100 thousandth of a millimetre would blur and ruin the image. Because of this Darwin requires the telescopes and the central hub spacecraft to stay in formation with millimetric precision.

To develop the precision systems needed for such a mission, ESA turned to researchers at Cranfield University. Formation flying in space is a discipline that its researchers have been studying for the past five years. The university’s Space Research Centre, where the bulk of the work has taken place, received funding from both EADS Astrium and ESA to research some of the technical aspects of Darwin.

One researcher at the centre, Jenny Roberts, has spent three years studying the ways in which the level of accuracy that Darwin requires can be achieved through the use of innovative formation flying technologies. Roberts believes that the considerable precision demands imposed by the nulling interferometer create the biggest challenges for Darwin.

The most testing aspects of the Darwin mission will be when it is manoeuvring to focus on a new target or immediately after it is deployed from the launch vehicle, be that in one or two launches, said Roberts. For the spacecraft to find their positions relative to one another a number of metrology technologies are used in sequence to gradually refine the precision control for the formation.

Initially each spacecraft will use radar or a local GPS system to confirm its relative position, allowing the four spacecraft to move into a rough formation within an accuracy of around one centimetre. In this instance radar is useful as it can quickly and easily bring spacecraft together from across long distances.

Darwin will then switch to using laser-ranging, as in LISA, to increase the accuracy down to a few micrometers. The fine-laser metrology system is able to ‘slew’ the constellation in formation and point it towards the target star. Finally a system known as optical path delay uses a sophisticated set of high-frequency precision mirrors inside the hub craft which can then calibrate the way light enters each craft. Used alongside the laser metrology measurements this system can stabilise Darwin’s imaging down to the nanometre level.

But formation flying relies on more than just metrology to keep spacecraft fixed in position. Manoeuvring technologies are just as crucial in maintaining high-precision formations and to allow the telescope to change its focus while remaining precisely aligned. ‘Darwin would need to be able to slowly rotate or for the entire formation to slowly expand or retract to perfect the resolution, as well as slewing from target to target,’ said Roberts.

There are two main competing propulsion systems, one of which is FEEP, as used in LISA. However, some researchers are not convinced of FEEP’s appropriateness for Darwin, particularly as it has never been tested in space, or over a long period of time.

The use of cold gas mini-thrusters, which utilise expelled gas such as helium or nitrogen to achieve a small degree of thrust, are the most likely alternative. Cold gas thrusters have the advantage that the technology is well proven and they are reliable and cheap — but they carry with them another potential headache for engineers. The fuel in gas thrusters is much heavier than the caesium in FEEP thrusters and after every manoeuvre it is possible that one craft may end up using more gas than the others.

If one spacecraft uses a disproportionate amount of fuel during a manoeuvre then its mass properties can change and it will begin to drift differently. As the mission continues the situation is liable to become more complicated as the biggest force on the craft is surface pressure from solar radiation. If one spacecraft is marginally lighter because it is carrying less fuel it will react differently to this pressure, making it accelerate slightly more quickly than the other spacecraft. This in turn will require a burst of more fuel to bring it back into formation and so the imbalance will become a spiralling problem that is not easily solved. Researchers at Cranfield are about to begin a study in a bid to solve this problem of fuel usage which threatens to add yet another level of complication to a mission that already plans to push the boundaries of precision flying.

According to Dr Peter Roberts, a lecturer at the Space Research Centre, despite the challenges imposed by the optics, controlling the separation between the spacecraft to a high degree of accuracy is the most complicated aspect of the entire Darwin mission. ‘Formation flying has only really been done in a very limited way in the past. It is a very new technology and it has needed a big important project like Darwin to make it worth spending the money on researching it,’ he said.

It is in this way that the requirements of science are helping to further refine technologies used in formation flying. It would seem that what began with Cluster in the late 1990s will soon reach a new apogee of sophistication, and that it will not be long before European spacecraft, like well-drilled soldiers stationed many millions of miles from Earth, are accurately — and with great care — arranging themselves across the quietness of space.