What’s up Doc?

Envisat, Europe’s most advanced satellite, has just embarked on a five-year mission to help scientists monitor the health of the planet.

Providing all went well with its launch in the early hours of this morning Envisat, the most complex earth observation satellite ever built, will today begin a major check-up of the planet’s health. From its orbit 800km above the earth Envisat will spend at least the next five years circling the planet 14 times each day, tracking changes to the environment.

The £1.4bn mission, launched by the European Space Agency (ESA), has taken the past 12 years to develop, and involved at least 200 companies across Europe. As well as being the most complicated, Envisat is also the biggest satellite ever built in Europe.

The spacecraft, launched on an Ariane 5 rocket, is so large that when it was shipped to the ESA’s spaceport at Kourou, French Guiana, the 300 tonnes of equipment had to be sent via a Russian Antonov transport plane, two Boeing 747s and a boat. In orbit and with its solar panels fully open, it spans the length of a tennis court. OK, so it’s big. But what is so special about this satellite that has taken hundreds of engineers throughout Europe so many years to build?

The environmental information likely to be gained by the satellite will more than match its size, says ESA’s Guido Levrini, Envisat’s instrument manager. Envisat will allow scientists to study the sea level, wave heights and winds at the surface of the sea, as well as the chemical composition of the atmosphere and depletion of the ozone layer. ‘The amount of data we will gain from the satellite will have no comparison with anything else. Nothing like this has been done before,’ he says.

Over half the cost of the project has been spent on developing the 10 information-gathering instruments. The most expensive, heavy and complicated of these is the Advanced Synthetic Aperture Radar (ASAR). Created by the project’s lead contractor, Astrium, in Portsmouth, ASAR will take pictures of the earth, and is able to see at night or through clouds. ‘It is a new generation of imaging radar. It is much more flexible and powerful than any instrument of its kind flown before,’ says Levrini.

To build the radar’s 10m-long antenna alone, 64 new technologies had to be developed, he says. ‘To make the project work we had to overcome a number of technological barriers. So we had to bring in technologies that were totally new, had not been proven or had never been commercialised for use in space applications.’Envisat has been installed with an active radar, allowing it to scan a 400km area.

Previous satellites had passive radars, meaning the craft itself had to be moved if they were to cover such a large area, says Keith Rowe, head of business acquisition at Astrium.

The active system gathers images by bouncing microwave signals off the earth’s surface and measuring the echoes. The radar is also bipolar, says Rowe. ‘When it scans an object it looks at it vertically and horizontally, so we can get a lot more information.’

As ASAR is a completely new design it required new assembly techniques, and the company even had to establish a new centre at Portsmouth to build it. ‘It took two companies to build it, Astrium in the UK and Alcatel in France, and we had to set up a completely new range at Portsmouth to test it, because we had never tested anything so large before,’ says Rowe.

While the technology used on ASAR is state-of-the-art as far as space exploration is concerned, Astrium is already working on developing it further for the next generation of radar. This will include making the radar itself much smaller. ‘The next stage will be to produce micro-synthetic aperture radars. This is a stepping stone: without Envisat we would never have been able to make the jump.’

Images taken by ASAR will be used to determine the direction of ocean waves and, combined with information gained from other instruments, will make it possible for international shipping traffic to be routed to gain the best speed and fuel economy possible. The radar will also study the extent and motion of sea ice, and monitor deforestation and disasters such as earthquakes and floods.

Astrium was responsible for integrating ASAR and the other nine instruments on to Envisat’s Polar Platform, or backbone, which the company also developed, says Rowe.

‘The 10 instruments were highly complex, and all built by different companies. The greatest challenge was that they all came in at the same time, so we had to integrate them and make sure they were working together.’

Once they were all fitted to the platform Astrium engineers had to test each instrument, individually and in tandem, and send them back to the companies for further checks when needed. ‘This all had to be done in a clean-room environment – it was a major project-management challenge,’ he says.

The government contributed around £300m towards the £1.4bn cost of the project, and the UK’s space industry carried out a large amount of the development and production work. As well as Astrium, firms such as BAE Systems and Marconi were involved.

BAE developed eight infrared detectors and pre-amplifiers for the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS), which will gather information on air pressure, temperature, concentrations of ozone, water vapour and trace gases. Marconi Applied Technologies supplied imaging components for two of Envisat’s instruments, to collect information on the chemical composition of the atmosphere, the oceans and land surfaces.

But it is not just hardware that the UK is contributing. Many of the country’s academics will be involved in studying and interpreting the data Envisat produces. Envisat’s radar altimeter will measure changes to the level of the sea’s surface to an accuracy of around 1cm in 10,000km, allowing climate change experts to monitor where ice caps are melting and causing sea levels to rise, says Duncan Wingham, professor of space and climate physics at University College, London. ‘We will be using the altimeter over the ice sheets to study the exchange of masses between the Antarctic ice and the ocean, to determine if the ice sheet is getting smaller.’

Envisat’s altimeter has a much more sophisticated control system than that of previous satellites, allowing the research team to capture echoes from areas of the 2km-thick Antarctic ice sheet that have been impossible to reach before. The team will also use information gathered by the altimeter to study the thin Arctic ice sheet, in the hope of discovering whether it is thinning, and if so for how long it can survive. ‘In the poles things can take decades to change so we need long-term observation to really understand what is going on, and what the causes are,’ he says.

The information will be entered into its on-board mass-memory device, and transmitted to one of two ground stations in Kiruna, Sweden, and Svalbard, Norway. From there it will be forwarded to ESA’s research centre at Rome.

The huge amount of environmental information collected from this five-year medical check-up will provide a complete picture of the planet’s state of health. Once the EU and Europe’s academic institutions have used this to diagnose the problems, they will be able to make decisions on their treatment, such as conservation programmes and restrictions on the use of chemicals.

But unlike most doctors, they will also be able to check whether countries are taking their medicine – the take-up of the Kyoto agreement being just one of the treaties likely to come under close scrutiny from EU officials.

Sidebar: Relay race

Envisat’s mission is so broad that another satellite, Artemis, will be needed to help relay the information to the ground.

But unfortunately Artemis, heralded at its launch last July as the most advanced telecommunications satellite ever to be developed in Europe, is in the wrong place. Due to a malfunction on the upper stage of its Ariane 5 launcher, the satellite was left at an altitude of 17,000km, though its intended orbit was 36,000km.

This could have been a disaster for ESA, which was expecting the satellite to remain in operation for at least five years.

But using its own propulsion system Artemis has now reached an altitude of 31,000km above the earth, and is on target to make its way to its intended orbit by the summer.

As luck would have it, ESA engineers had installed a newly designed ion propulsion system on board, which will provide enough acceleration to raise its orbit using only 20kg of the inert gas xenon as fuel.

Under this system the xenon is ionised by applying an electrical potential to it, and the ionised atoms are then accelerated out of the engine.

Expelling this stream of electrically-charged gas produces only a tiny amount of thrust, but enough to send a spacecraft across vast distances when free from the earth’s gravitational pull and air resistance.

Ion engines are expected to make it possible to reach regions of outer space unattainable using traditional propulsion systems. While slow to build up speed, the systems can produce 10 times as much thrust per kilogram of fuel as traditional engines.

To raise Artemis’ orbit using the ion drive its development team, including engineers from Astrium, had to re-orientate the satellite in a new direction, by developing new control software and transmitting it to the satellite.

‘This will be a very striking European success if Artemis does manage to move all the way to its correct orbit,’ says ESA’s Guido Levrini.

Sidebar: Probes of the future

After years of simply sending satellites into orbit, the European Space Agency is expanding into planetary probes and extra-solar planet hunting.

Already under way is the Huygens probe, currently travelling to Titan, Saturn’s largest moon, on the back of NASA’s Cassini spacecraft.

In November 2004, Huygens will parachute through Cassini’s atmosphere, to analyse its atmospheric composition, measure winds and global temperatures, determine surface properties and detect its internal structure.

Another mission under way, but not yet in space, is Mars Express, which will survey the red planet and search for water. Due for launch in June 2003 to arrive in December, it will carry the UK-designed Beagle Two probe that will analyse soil samples for signs of past life.

Four years later, ESA is planning the Bepi Colombo mission to Mercury. Named after an Italian professor, it is to be launched by 2009 and will use a new solar-electric propulsion system to help it reach the innermost planet. It will have to cope with sunlight 10 times as intense as normal, and surface temperatures of 400°C.

Still on the drawing board are missions designed to identify earth-like planets that could contain life outside our own solar system. The Eddington mission, named after the British astrophysicist Arthur Eddington, is planned for launch in 2008, and will examine distant suns in an attempt to find earth-sized worlds around other stars.

Finally, the Darwin mission will aim to detect extra-solar planets capable of sustaining life. A flotilla of six space telescopes, each at least 1.5m in diameter, will work together to scan nearby solar systems for signs of life.