Article - Issue 16, July/August 2003
Space technology in the UK
Professor Richard Holdaway FREng
Many people will not be aware of the breadth of the UK’s contribution to the global space programme. With research and development in critical space technology taking place in a large number of British universities, the UK has a role in many current projects (including forthcoming missions to Mars, to the Moon and to a comet), and will be heavily involved in future missions such as the replacement of the Hubble Space Telescope. This wide ranging review of British-developed technologies provides an insight into the deeper realms of the UK’s space technology programme.
The UK has a major programme of world-class civil space projects which it undertakes in collaboration with NASA, ESA (the European Space Agency) and other agencies around the world. The UK Minister for Science and Innovation (Lord Sainsbury, whose brief includes Space) has defined the Government’s vision for civil space technology: ‘The UK will be the most developed user of space-based systems in Europe for science, enterprise and environment. UK citizens will provide and exploit the advanced space-based systems and services which will stimulate innovation in the knowledge-driven society’.
Many of the early pioneers of the space programme were British. In the 1920s the E-region of the ionosphere was discovered by the Nobel prize winner Edward (later Sir Edward) Appleton. In 1945 Arthur C. Clarke published his now famous paper on geostationary satellites, a technology that has the most profound effect on the lives of all of us. British development of the Black Arrow rocket led to the ELDO launch vehicle and subsequently the Ariane launch vehicles now so widely used throughout the world. There have been three British astronauts to date, and in the coming months the UK will be a major participant in missions to the Moon, to Mars, to a comet, and in various other space science and Earth observation missions.
On the academic side there are currently 45 UK universities offering courses in space science and/or space technology. There are over 20 university hardware groups developing instruments for space, including Birmingham University, Leicester University, and UCL’s Mullard Space Science Laboratory. The UK has major space companies such as Astrium-UK, Logica and Sky TV, and major space research institutes including the Rutherford Appleton Laboratory (RAL) near Oxford, and Surrey Satellite Technology Ltd (SSTL), which is the world leader in microsatellites.
The major funders of the UK civil space programme are the Particle Physics and Astronomy Research Council (PPARC), the Natural Environment Research Council (NERC) and the Meteorological Office. The Ministry of Defence also funds a significant programme of defence satellites for communications and for surveillance. Much of the strategy for UK space is coordinated by the British National Space Centre (BNSC) based within the Department of Trade & Industry.
In many cases it is the development of UK space technology that enables world-class space science. In this respect, the ‘faster, better, cheaper’ paradigm coined by former NASA administrator Dan Goldin in the early 1990s has had significant impact on the development of space technology, and that is particularly true in the UK. Microtechnology and nanotechnology are now playing a revolutionary role in the way we ‘do’ space. The ‘space-crafton- a-chip’ is approaching reality. So too is fast-track technology development, as evidenced by the ESA Star Tiger programme pioneered at RAL with the development of the world’s first terahertz imager. There are numerous examples of UK excellence in space technology (often leading to world-class science). Some examples of these are described here.
Space science programmes developed through advanced technology
The UK is one of the leading partners in the Planck Surveyor Telescope (Figure 1) that will be launched on the Ariane 5 rocket in 2007. Astronomers at Cardiff University, Manchester University’s Jodrell Bank Observatory and the Rutherford Appleton Laboratory are developing parts of the two instruments on the telescope which are designed to unravel the mysteries of the early universe through making observations of the 3 K Cosmic Microwave Background (CMB) radiation anisotropy to unprecedented precision. Both the instruments utilise novel solutions to obtain sensitivities which allow the small 3 K blackbody signal to be measured to an accuracy of ~1 mK over the whole sky. The Low Frequency Instrument, LFI, which has radiometer channels at 30, 44, 70 and 100 GHz, uses high electron mobility transistors cooled by a hydrogen absorption cooler to 20 K. The High Frequency Instrument, HFI, which has photometric channels at 100, 143, 217, 353, 545 and 853 GHz, uses an additional 4 K cryocooler to precool an open cycle 100 mK He3/He4 dilution refrigerator to cool a suite of bolometric detectors.
To accommodate all these detection channels in the focal plane requires that the LFI be wrapped around the HFI to minimise off-axis aberrations in the telescope focal plane and to optimise the use of common cryogenic cooling stages.
The waveguide horn feeds on the right-hand side collect the faint signals from the CMB in the telescope focal plane and channel them to the sensitive detectors. The outer circles of LFI horns are at 20 K, while the inner array of back-to-back horns are all at 4 K, feeding an array of tertiary horns on the 100 mK dilution stage which is needed to get the required bolometric sensitivity.
ESA’s four tonne International Gamma-Ray Astrophysics Laboratory (INTEGRAL) (Figure 2), the heaviest ESA payload to date, was launched in October 2002 from the Baikonur Cosmodrome in the Republic of Kazakhstan on a Russian Proton rocket.
INTEGRAL was first proposed in 1989 by UK and US principal investigators. The launch was the culmination of 12 years of effort by scientists in 18 countries, and is providing a new window on the gamma-ray sky, following on from NASA’s Compton Gamma-Ray Observatory (CGRO) which re-entered the Earth’s atmosphere in June 2000 after 10 years in orbit.
Gamma rays are the highest energy form of electromagnetic radiation known to man; those studied by INTEGRAL have about a million times more energy than a photon of visible light. These energetic photons bring us information from some of the most violent exotic objects in the universe, such as the explosions of stars or the infall of hot matter onto a black hole or neutron star. The combination of their great penetrating power and their intimate relationship with the nuclear particle processes ensures that g-rays are able to reach us from otherwise obscured regions to convey vital information directly to the observer regarding the physical processes that power these infernal machines. The gas and dust that causes 30 magnitudes of obscuration in the direction of the galactic centre at optical wavelengths barely affects gamma rays.
By comparison to astronomy in other wavebands, gamma-ray astronomy is still in its infancy. One of the main reasons for this is the opacity of the Earth’s atmosphere at gamma-ray energies, forcing gamma-ray astronomers to deploy their instruments on stratospheric balloons or satellites.
Within the UK, activity on INTEGRAL is centred on the astronomy group at Southampton University. The group is directly involved in the IBIS gamma-ray imaging telescope and the Integral Science Data Centre (ISDC) located near to Geneva. In addition, the Southampton team is responsible for The Integral Mass Model (TIMM), a comprehensive computer model of the entire science payload and spacecraft. This model can be exposed to the ever-changing particle and radiation environment encountered in orbit, as well as for earthly calibrations. This new and powerful modelling system is applicable to all high-energy astrophysics space missions, and has had many applications during the course of the INTEGRAL programme. Mass modelling is now being further developed as a data analysis tool, which will enable observers to ‘flat field’, the incoming data.
The INTEGRAL payload consists of four independent and co-aligned instruments carefully designed as a complementary set. The two primary telescopes both operate in the hard X-gamma ray region of the spectrum (~20 keV to ~10 MeV). One (SPI) is optimised for fine spectroscopy and the other (IBIS) for high-resolution imaging. These two instruments are supported by an X-ray monitor (JEMX), which extends the spectral range down into the classical X-ray region of the spectrum, and an optical monitor camera (OMC), which provides V-band photometric images of the same region of sky. The imager, spectrometer and the X-ray monitor share a common principle of operation: they all use coded masks, which are basically thick sheets of tungsten with a large (~50% of the area) number of holes to form a multiple pinhole camera. The main detectors used in the INTEGRAL payload are pixellated arrays of cadmium telluride, germanium and caesium iodide, totalling some 20 000 independent detector channels.
As a prime contractor of complete spacecraft for space science and Earth observation, the tri-national Astrium company (based in the UK at Stevenage and Portsmouth) is constantly involved in the extremes of space technologies. Astrium has designed the spacecraft platform for the giant three-tonne Rosetta mission that will undertake an 11-year odyssey of the solar system to rendezvous with a comet and place a lander onto its surface. This will occur far away from the Sun where the solar illumination is just 4% of that which is seen at the Earth. In order to provide the mere 670 W of power (including 270 W for the 11 scientific instruments) needed to operate the spacecraft at the comet, 10 fold-out panels of special photovoltaic cells which can operate at very low temperatures are fitted. These have an area of 65 m2 and span 32 m tip to tip. Although Rosetta uses a series of gravity slingshots at Mars and the Earth to help it accelerate to the orbital rate of the target comet, Rosetta also has 1.6 tonnes of rocket fuel and oxidiser which is fed under the pressure of helium gas to a network of 24 10 N thrusters via 56 control and isolation valves. This propulsion system is very unusual in having to provide large manoeuvres late in life. Thus particular attention has been given to avoiding any vapours of the fuel and oxidiser mixing, thus causing an explosion, as may have occurred in a previous NASA Mars mission.
At the opposite end of the size scale is the Beagle 2 Mars lander project, for which the Open University leads on science and Astrium leads the industrial team of many UK and overseas companies. This tiny craft (Figure 4), weighing just 68 kg, must separate itself from the Mars Express orbiter, plunge into the Martian atmosphere at Mach 20 before decelerating with the aid of a drogue and main parachute system and releasing the aerodynamic covers. A few seconds before impact a radar altimeter will deploy a set of airbags, and the parachutes will separate. After bouncing on the surface, the vehicle will come to rest, the airbags will be released and the actual lander will fall 2 m to the ground (the structural design case!). Like a pocket-watch – and irrespective of its orientation on the ground – it will open up five solar panels to provide electrical power. Beagle 2 is equipped with a miniaturised set of scientific instruments, including a mass spectrometer, grinding tool, stereo cameras and a tiny self-propelled burrowing tool called the mole. To make Beagle 2 feasible for a cost and mass an order of magnitude less than previous Mars landers, many individual technologies are combined in a highly integrated way. For example, the outer body of Kevlar and foam serves shock absorbing, thermal control and structural roles; the bi-directional motors which deploy the solar panels can also vibrate them to shake off surface dust; a single powerful ERC-32 computer runs software for entry, landing and surface operations as well servicing all the scientific instruments.
Other space programmes enabled through UK technology
Disaster Monitoring Constellation
The Disaster Monitoring Constellation (DMC) is another remarkable example of international collaboration in space led by SSTL. Seven organisations from Africa, Asia and Europe have agreed to contribute microsatellites into the first satellite constellation dedicated to the monitoring and mitigating of man-made and natural disasters.
The first microsatellite, AlSAT-1 for Algeria (Figure 5), was launched on 28 November 2002 and is now fully operational, providing 32 m resolution multi-spectral Earth observation images with four-day revisit capability. Two further launches in mid-2003 and early 2004 will see the constellation complete and producing daily imaging revisit capability worldwide to benefit disaster relief agencies. Each satellite will also serve its country’s national needs and provide commercial exploitation opportunities. AlSAT-1 was built at the Surrey Space Centre in Guildford by a joint UK–Algerian team.
A further three microsatellites, for Nigeria, Turkey and the UK, are under construction at Surrey and will join AlSAT-1 in orbit in mid-2003.
Working with a team led by QinetiQ for the UK Ministry of Defence, SSTL is building the 125 kg TopSat enhanced microsatellite platform (Figure 6). TopSat is scheduled for launch on the 3rd DMC mission onboard Kosmos in mid-2004. The agile microsatellite platform will provide 2.5 m high-resolution panchromatic and 6.5 m multispectral imaging under the direct control of users in the field. The complex optical assembly (camera) is being developed by RAL.
TopSat will enable users to gain practical experience of operating a small imaging satellite, as well as exploring its applications in operational situations.
Led by SSTL, the GEMINI geostationary communications minisatellite mission is being developed as a low-cost, small geostationary communications minisatellite to support a diverse range of data, telephone, television and radio services.
SSTL’s geostationary 400 kg minisatellite, employing cost-effective commercial off-the-shelf technologies (COTS) will enable customers to own a dedicated communications satellite to provide real-time services at a fraction of the conventional cost.
It is hoped that this demonstration mission will open up a brand new and commercially attractive export market for this system and for its UK supply chain.
Generic areas of technology
Miniature plasma analysers
In situ plasma measurements in space provide important diagnostics of the solar wind and its interaction with objects in the solar system, including the Earth. Such measurements are required for several future space projects at national and international level, and add to the understanding of the solar wind itself and of planetary magnetospheres. They are also vital in space weather monitoring. Instruments must be sensitive enough, with good enough energy and angular resolution, to achieve the science objectives of the relevant mission. They must also survive the launch and radiation environments, be easily testable and have low mass.
University College London’s Mullard Space Science Laboratory (MSSL) has a current programme to produce a prototype miniaturised plasma analyser for use in future space missions. The target of the programme is to produce a working prototype for less than onethird the mass, but with comparable or improved performance, compared with earlier generation sensors from missions such as Cluster or Cassini. A prototype has now been produced which has several advantages over earlier designs:
a modular design for ease of assembly and testing
a novel readout scheme, eliminating the need for heavy decoupling capacitors
a miniaturised version of Cluster and Cassini analyser heads
coarse and fine angular resolutions available, depending on science goals
integrated front end electronics (underway)
miniaturised high voltage supplies (underway)
a mass target of 600 g.
A prototype of the analyser (Figure 7) has been tested in MSSL’s electron calibration system with excellent results – its performance is close to the simulated response. The new analyser is now being proposed as an instrument for several future missions.
Adiabatic demagnetisation refrigerators
Adiabatic demagnetisation refrigerators (ADRs) are also being developed for space at MSSL (Figure 8), with industrial, PPARC and ESA support. The ADR is intended to cool cryogenic photon detectors, which function in the region of 30–100 mK. This technology is being developed for future space astrophysics missions, for example ESA’s XEUS mission. These refrigerators can be coupled to space qualified cryocoolers offering a completely cryogen-free solution to ultra low temperature cooling in space, eliminating the need to carry consumable superfluid liquid helium. ADRs cool by the magnetic reduction of entropy, via the magnetic interaction with electron spins, in a paramagnetic material. Subsequent removal of the magnetic field on the material in adiabatic conditions results in cooling from the liquid helium region (~4 K) to ultra low temperatures (<100 mK). The magnetic fields are generated using ultra low current, conduction-cooled superconducting magnets. The management of the current flowing in the superconducting magnets achieves control of the ADR milli-kelvin temperature. ADRs are ideal for space, being gravity independent and having no moving parts. High-level magnetic shielding enables the ADR to be used with very sensitive cryogenic detectors, e.g. superconducting tunnel junctions (STJ) and transition edge sensors (TES).
The charge-coupled device (CCD) is currently the image sensor of choice for nearly all space science and Earth remote sensing and ground-based astronomical visible light imaging systems. Large-format sensors with many millions of pixels, low readout noise and excellent uniformity are readily available. However, the development of CCD instrumentation for the space environment poses a number of challenges due to the requirement to minimise the size, mass and power of the necessary off-chip readout electronics, and the susceptibility of both the CCD and readout electronics to radiation damage. The problems are amplified with the ever-increasing aspirations of space scientists for large, multiple CCD focal plane arrays, and increasingly high readout rates. The emergence of CMOS active pixel sensor (APS) technology promises solutions to these issues, and is arguably the most important development in solid-state imaging since the invention of the CCD in 1969.
CMOS active pixel sensors exploit the same silicon chip technology as used in desktop PC microprocessors. Their first advantage over the CCD is that all necessary readout electronics can be incorporated on the same chip as the image sensor, thus offering high functional integration, compact and low mass cameras. Low voltage operation results in low power consumption, and the latest CMOS processing can yield exceptional radiation tolerance. APS technology is still in its infancy, and so far targeting the cost-sensitive commercial digital camera markets. However, technologists at RAL are developing science-grade sensors to meet the needs of future space missions. A recent achievement has been the development of a 12-million pixel sensor, optimised for low readout noise and high dynamic range (Figure 9). These are the first prototypes for a spectrograph on ESA’s forthcoming Solar Orbiter mission to the Sun. The requirements call for a large format detector, with small pixels, and sensitivity optimised for extreme ultraviolet light. Specialist design and fabrication techniques are being developed to meet these goals.
In the future, we can anticipate CMOS technology being selected, rather than the CCD, in those missions where there are clear advantages to be gained from the better functional integration of on-chip readout electronics, or in those applications in which the radiation environment precludes the use of CCDs.
The Millimetre Technology Group at RAL is developing receiver technology for the terahertz frequency range, defined here as extending from 30 GHz to 3000 GHz. Terahertz receiver development is required to support UK scientific excellence in astronomy and atmospheric remote sensing. Major new projects such as ALMA (the Atacama Large Millimetre Array) will allow dust clouds and star-forming regions in the interstellar medium to be imaged at millimetre and sub-millimetre wavelengths with unprecedented accuracy. In addition, the need to measure atmospheric processes involved in ozone depletion and global climate change has highlighted a requirement for radiometers, which operate at frequencies up to at least 1000 GHz, with wide instantaneous bandwidth, good sensitivity and high beam efficiency. The terahertz region is also a part of the electromagnetic spectrum that offers exciting possibilities for exploitation in new applications as diverse as high-speed communications and thermal imaging for medical applications. Excellent facilities exist at RAL for the design, manufacture and test of components, subsystems and receivers, and work is undertaken for a variety of organisations including UK research councils, industry, universities and government laboratories worldwide. As an example of technology development, a series of space-qualified mixers (see Figures 10 and 11) has been developed by RAL, under contract to ASTRIUM-UK for the Humidity Sounder for Brazil (HSB) instrument. This is a space-borne instrument, provided by INPE (Brazil’s National Institute for Space Research) as part of an agreement between the Brazilian and US governments, and launched in 2002 on the NASA AQUA platform. The waveguide mixers (the critical receiver component) employ planar diodes and exhibit state-of-the-art noise performance at 183 GHz, measured over an instantaneous bandwidth of nearly 10 GHz. Measurements show that the design exhibits flat RF performance over a range of at least 150–200 GHz with minimal or no tuning and a very low local oscillator (LO) power requirement.
Carbon fibre optical benches
The highly advantageous stiffness-to-mass ratios achievable with composite materials such as carbon fibre reinforced plastic (CFRP) make them an attractive choice in the design of high performance structures for space instruments. Optical benches are common in space optical instruments to mount components such as mirrors and detectors and position them with very high precision. The very low thermal expansion coefficients achievable with CFRP are particularly helpful in meeting the extremely exacting dimensional stability requirements in such instruments.
Unfortunately, the CFRP matrix usually includes resins that absorb and capture moisture from the atmosphere as well as volatile residues from the manufacturing process. In space, the latter outgas from the CFRP matrix and are preferentially deposited on the coldest elements, frequently the mirrors that are exposed to cold space. The University of Birmingham, in partnership with Maclaren Composites, has recently completed the design and assembly of a low outgassing CFRP optical bench for flight on a Japanese mission, Solar- B. By judicious choice of materials, including the use of cyanate resins, and the very careful implementation of design procedures to avoid trapped volumes, the 2 m CFRP structure has achieved extremely low outgassing levels. Important in the manufacture of the structure was the detailed cleaning processes and material test programmes supported by the Space Science & Technology Department at RAL.
The phase measurement of electronic beams
Space instrumentation has always been limited in size by the dimensions of the launcher fairing or nose-cone. As instrumentation has become more ambitious these limitations have been challenged in order to achieve the scientific progress desired, to the extent that science payloads are being conceived with scales well beyond that of the launch vehicle. The concept being developed is that of launching different parts of the instrument separately and positioning them in space using laser or microwave beams to measure and control their separation. In the most extreme case, that of the gravitational wave mission LISA, the separations are 5 million km and have to be measured to a precision of 10 picometres. Exchanging coherent electromagnetic beams between the various instrument packages can achieve these performance goals provided that the phase of the electromagnetic beam can be measured to a precision of 10–6 radian/root Hz.
The University of Birmingham is developing an electronics unit to perform this phase measurement by a signal chain involving very high performance analogue conditioning and filtering followed by conversion to digital form for Fast Fourier Transformation. The electronics package is being developed in conjunction with SEA Ltd and the University of Glasgow and will be flight-tested on the ESA SMART-2 mission. These techniques will be of value in instruments as diverse as X-ray telescopes and searches for extra-solar planets, two cases where the various optical elements of the whole instrument are to be mounted on separate satellites.
As seen from the previous examples – in themselves only a small sub-set of current UK contributions to the global space programme – the UK is a major power in world space. The UK has Principal Investigator status on many projects (including forthcoming missions to Mars, to the Moon and to a comet) and on future missions such as the replacement for the Hubble Space Telescope.
In all these examples, plus numerous other past and present space missions involving the UK, the advancement of critical technology has been of fundamental importance. UK universities, research laboratories and industry are at the forefront of many of these developments; something of which we can be immensely proud. The future, however, is even more exciting and once again the UK is in an excellent position to take a leading role.
The giant three-tonne Rosetta mission will undertake an 11-year odyssey of the solar system to rendezvous with a comet and place a lander onto its surface
The ADR is intended to cool cryogenic photon detectors, which function in the region of 30–100 mK
There are many individuals and groups within the UK space community who contribute successfully to the UK space programme. My thanks go to the following for their contributions to this article: Richard Davies (Manchester University), Audrey Nice (SSTL), Tony Dean (Southampton University), Mike Cruise (Birmingham University), Andrew Coates and Ian Hepburn (MSSL), Nick Waltham and David Matheson (RAL), and David Parker (Astrium-UK).
Prof Richard Holdaway FREng
Director of Space Science & Technology, Rutherford Appleton Laboratory
Richard Holdaway is Director of Space Science & Technology at Rutherford Appleton Laboratory. He is a Fellow of the American Institute of Aeronautics and Astronautics (AIAA), and was elected FREng in 2001. He is a Visiting Professor at Southampton University and University of Kent, and has worked on numerous space projects over the last 30 years.