Mastering the universe

XMM Newton, with its 170 wafer-thin cylindrical mirrors spread over three telescopes, was a huge breakthrough. Its successor, The XEUS system, above left to right, consists of two spacecraft flying in close formation no more than 50m apart, one carrying a mirror and the other fitted with the X-ray detectors

XEUS will be enormous. The planned system consists of two spacecraft flying in close formation no more than 50m apart. One spacecraft will carry a mirror while the other will be fitted with the X-ray detectors.

Nicola Rando is responsible for the mission’s preliminary studies as part of the Science Payload and Advanced Concepts Office at ESTEC, the European Space Research and Technology Centre, itself part of ESA. He described XEUS as XMM Newton’s natural successor but said the technology needed to make it work was of a completely different order. For engineers, science requirements that call for much higher resolutions translate directly into engineering challenges that must be addressed.

One way XEUS will allow astronomers to see ever-fainter signals in space is to greatly increase the telescope’s optics size and capability. The spectroscopic equipment will also be upgraded, providing better resolution. X-ray optics require a much larger collecting area than optical telescopes because of the way the rays must be collected. The trick is to focus the light rather than simply reflect it, so the mirrors on the optics spacecraft must lie at shallow angles — known as grazing incident optics — to focus the X-rays towards a single focal point.

Because of its science requirements, if XEUS was made to the same design as XMM-Newton then it would require almost 16 tonnes of nickel, making it impossible to launch. The mirrors’ maximum weight should be no more than two tonnes, according to ESA.

‘We need to be able to achieve a larger collecting area for the optics but at a low mass,’ said Rando. ‘This is the challenge, because the existing technology delivers a level of effective area and image quality but only at a given mass. If you extrapolate to the size we need, you end up with incredibly heavy optics. The challenge is to meet the collecting requirement at a lower mass while keeping optics quality.’

Luckily, a recent development known as high-precision micro-pore optics could solve the problem. Instead of the heavy conventional nickel mirrors used on the XMM-Newton, micro-pore optics uses tiny channels etched into silicon wafers. They are then stacked up in layers to form a unique reflective surface that is far lighter and thinner than any existing optics materials. ‘Without micro-pore optics, it just wouldn’t be possible to do anything on this kind of scale,’ admitted Rando.

Adding to the problem of the size of the mirror is the fact that the range of X-rays to be detected means the optimum focal length between the mirror and the detectors is about 30-50m. It would be impossible to incorporate that kind of distance between the two ends of the optics array along with all of the science equipment on a single spacecraft. The only answer is to launch the XEUS mission on two separate craft.

Formation flying, a feature of a number of future space missions including Darwin and Lisa (The Engineer, 13 February 2006), will certainly be required. Described by Rando as ‘a new territory in spacecraft engineering’, formation flying relies on a sophisticated metrology package that uses a hierarchy of technologies ranging from radio frequency through to laser-optics to remain in synch with another spacecraft. The detector spacecraft will be able to adjust its position in space automatically so the instrument in use is at the focal point of the reflected X-rays.

The final challenge for engineers is the instrumentation. Scientists want to be able to measure the energy of incoming photons down to the level of one electron volt — an unprecedented level of sensitivity.

‘It’s a factor of 1,000 more accurate in resolution than anything else available. To do this we need to develop a new generation of instrument, which not only detects the arrival of a photon but is also capable of recognising its colour and energy in a very accurate way,’ said Rando. ‘We don’t know of many systems that can support this energy resolution.’

Existing systems such as diffraction gratings that have been used in the past are considered inefficient for XEUS because they require too many photons to work — a real problem when astronomers are trying to detect the universe’s faintest objects.

At the moment there are two competing technologies to use for the main X-ray imaging spectrometer (also known as the Narrow field imager) on board XEUS. One is a micro-caloriometer, or bolometer. This technology measures the minute temperature increase that occurs when a photon is absorbed and from which the energy of the photon can be calculated. The other contender is a superconducting tunnel junction. Regarded as the next step in astronomical detection, this junction consists of two layers of a superconducting metal such as niobium or tantalum, separated by a thin strip of insulating material. By using a small voltage across the junction and a magnetic field the device can measure the charge created when the detector absorbs a photon. ESA’s researchers are carrying out extensive programmes to work out which system is the most suitable.

The Wide-Field Imager, however, is likely to be based on established charge-coupled device (CCD) technology, building on the work that went into XMM-Newton. As a leading supplier for space-rated CCD technology for many of NASA and ESA’s missions, Chelmsford-based e2V is likely to be heavily involved in the XEUS programme.

Peter Pool, the company’s chief imaging engineer, said e2V will probably provide the CCDs that cover a wide area around the central focal plane where much of the other instrumentation is based. Although the technology is likely to be derived from work that went into XMM Newton, on XEUS it will be faster, bigger and more accurate, said Pool.

PPARC has already commissioned preparatory work at e2V’s centre of electronic imaging research group, based at Brunel University. The detector spacecraft will be fitted with an area of thick, heavily-doped silicon compound into which large CCDs will be embedded.

The main difference for the XEUS design will be that the pixels will be much larger than those used in past space missions such as XMM. While research is ongoing, it is likely that these new pixels could be as large as 100 microns across. Larger pixels make it less likely that so-called ‘split-events’ will occur. When a photon is absorbed it creates a charge cloud, which can spread over a number of pixels making it harder to accurately determine its exact position and energy.
‘In imaging spectroscopy, capturing all the energy in a single pixel makes it much easier to work out its origin and what elements were involved in its formation millions of years earlier,’ said Pool.

His team is also looking into ways of collecting the charge from a photon event deeper into the silicon layer, which will make the instrument more efficient at transferring charge. However, even the world’s most advanced imaging instruments owe as much to luck as new technology, according to Pool.
‘The exciting thing about these space missions is you are never 100 per cent sure what you are going to find,’ he said. ‘We expect to get a lot of what we call ‘serendipitous science’ out of this.’

Much of the key design work on XEUS’ detector spacecraft is being undertaken by Stevenage-based Astrium UK. One of the key challenges in its work is how to fit all of the different instrumentation on to the spacecraft while maintaining the key temperature requirements of each system. This tricky balancing act, the cryogenics, is key to the mission’s success. ‘This stuff is difficult in a laboratory but to reproduce it in space adds further complexity,’ said Rando.

Marie-Claire Perkinson is one of Astrium’s systems engineers on the project. Working in the space firm’s Mission System Department, Perkinson and her team have been looking at ways of tackling the engineering problems of satisfying the complex cryogenic requirements. At the moment the design seems to be heading towards that of a cryostat, or cryocooler, a system that incorporates all of the necessary cooling technologies in one compact device. ‘The main thing is providing the right temperature for each instrument. What we are proposing is that the Narrow Field instruments will be housed inside a cryostat, much like inside a Russian doll, with a series of shells,’ said Perkinson.

One of the main problems is that each instrument operates at different temperatures. The Wide Field Imager typically operates at 210K, while for the Narrow Field Imager there are two options: one that will operate at 50mili-Kelvins (mK) or one that works at 300mK.

With the payload craft operating at an ambient space temperature, normally around 300K, a series of sophisticated and power-hungry coolers will be needed to get down to these very low temperatures.

With the outside shell at 300K the next shell of the cryostat will be cooled using a Stirling cycle cooler to reduce the temperature even further. This first shield is then used as a pre-cooling stage for a Joule-Thompson cooler, which takes the temperature down again to no more than a few Kelvin.
A Joule-Thompson cooler uses the expansion of gases to decrease temperature and is used in the petrochemical industry to liquefy gases. The last stage in this incremental process of refrigeration is a final cooler that surrounds the instruments, with the Narrow Field Imager sitting snugly inside, operating at close to absolute zero.

Around the same time that XMM-Newton was providing its crucial images of distant space, NASA was using its own X-ray telescope, Chandra, to produce equally enlightening images. Like ESA, NASA also has plans to launch a new generation of X-ray observatories but, in contrast to its European counterparts , things are not progressing quite as smoothly.

The Constellation-X (Con-X) telescope forms part of NASA’s Beyond Einstein series of future programmes which also includes the LISA mission. However, with recent changes in priorities at NASA the scale of the planned telescope has shrunk in scale considerably. Nick Price, a project scientist at NASA’s Goddard Space Flight Centre near Washington, admits that Con-X is far less ambitious than XEUS, a project NASA will be following with interest.

‘It is likely now that Con-X will consist of one spacecraft divided into four small telescopes rather than one big one,’ said Price. ‘XEUS is much more ambitious and is in fact closer to something that would be the next generation from Con-X — possibly called Generation X — which will also use formation flying.’

As things stand, it could be quite a while before even XEUS is launched. Listed on ESA’s website with a launch date of 2015, all involved in the project say that X-ray astronomers may have to show a little patience.

A decision is due early this year about which projects should be fully funded and it is expected that XEUS will be a shoo-in to be selected. However, systems level work is unlikely to take place until at least 2008, according to Perkinson, so those cosmic X-rays beaming across the universe from the beginning of time might just have to wait a few more years before they can pass on their secrets.