21 Feb 2000
The first of the STM's trials to be carried out last week was the so-called 'shogun' test. This was a joint operation by technicians from CNES, Arianespace, and ESA's European Space Research and Technology Centre (ESTEC) in The Netherlands. (Arianespace operates the Ariane 5 launch vehicle that will send Rosetta on its way to Comet Wirtanen in 2003.)
The shogun test was a simulation of the shock transmitted to the spacecraft when its protective fairing separates from the rocket's upper stage. Using a copy of the actual spacecraft adapter which will attach Rosetta to the upper stage, a set of pyrotechnic devices was attached around the underside of the adapter. With all staff withdrawn to safe positions behind protective glass, these explosive charges were set off instantaneously to rupture the aluminium plate on the adapter.
"It only lasted a millisecond, but it went off with quite a loud crack," commented Rosetta Assembly and Integration Verification Engineer Alan Moseley.
The next day, the STM was subjected to a 'clamp band separation test'. The clamp band is a metal strip that attaches the launch adapter to the spacecraft. After checking that the clamp band fitted properly, technicians once again had to discover whether its explosive separation would damage Rosetta. This time, two pyrotechnic charges were set off. As the band was split in two, both sections were pushed away from the spacecraft interface by springs and caught by special 'catcher' brackets.
"Although the tests themselves were extremely short, the shocks imparted to the STM were quite severe. The detailed analysis of the impact on the spacecraft's units will data take several days, to come to a realistic conclusion," said J. C. Salvignol, Rosetta mechanical systems engineer.
"I'm pleased to say that the STM passed both tests without any damage," added his colleague, Jacques Candé.
Today saw the deployment test for one of Rosetta's 16 metre-long solar arrays. These enormous arrays are needed because Rosetta will be operating at five times the Earth's distance from the Sun, where levels of sunlight are only 4% those on our planet. Rosetta will be making history as the first spacecraft ever to use solar arrays to generate electrical power during a deep space mission beyond the asteroid belt.
The check-out began when six 'thermal knives' were used to melt through the attachments which held the array to the side of the spacecraft. Using a special jig, springs on the giant panel caused it to slowly open out to its full, impressive length. After 3 minutes 47 seconds, the array was fully extended, allowing engineers to check its alignment and condition.
Later in the day, yet another shock test took place. This time, the dish- shaped high-gain antenna was shaken by three pyrotechnic devices which exploded one after another. During a subsequent performance check of the antenna motor, the dish was successfully moved more than 40 degrees from its fixed position.
"Once again, everything was nominal," declared a highly satisfied Alan Moseley.
Structural Thermal Model (STM) of the Rosetta spacecraft in the ESA/ESTEC test centre.
ESA Science News
01 Jul 1999
The ceremonial unveiling was also witnessed by representatives of the Particle Physics and Astronomy Research Council (PPARC), which funds the UK contribution to the Rosetta mission, members of the media, and more than 100 scientists and engineers from around the world.
"The expertise of European scientists in cometary exploration has been absolutely outstanding, from the early days of astronomy with Halley and Newton, through to Ludwig Biermann who discovered and explored the solar system and its interaction with comet tails." said Prof. Bonnet. "Then Giotto flew to Halley's Comet 13 years ago. Now we have Rosetta, which builds on this experience."
A 1:4 scale model was chosen for today's unveiling because of the impossibility of finding a venue large enough to contain the full-size version. The flight model of the Rosetta Orbiter spacecraft is 32 metres across, so large that it would stretch the entire width of a football pitch. Since Rosetta will have to operate more than 720 million km (450 million miles) from the Sun, where light levels are only 4% of those on the Earth, it has to carry giant solar panels to provide electrical power in the dark depths of the Solar System.
One of the main themes of the event was the importance of Rosetta as a Cornerstone mission in ESA's Horizons 2000 science programme. "Rosetta is a flagship mission for European solar system exploration. It is one of four missions that will land on a foreign body in the first 10 years of the next century. ESA spacecraft will lead the way with landings on Mercury, Saturn's moon Titan, and even on a comet", said Prof. Bonnet.
Speakers also explained that, apart from its scientific importance, Rosetta is also a significant source of technological innovations which can be used for other missions such as Mars Express. This enables ESA to make efficient use of resources and reduce development costs for other science projects.
"Rosetta will be a tremendous technological challenge," said Project Manager Bruno Gardini. "We have less than four years to build the largest, most sophisticated spacecraft ever to visit a comet."
"There are many challenges ahead," he added. "Rosetta will have to survive a hazardous eight year trek across 5.25 billion km of space. It will then have to rendezvous with a comet which is travelling towards the Sun at over 130,000 km per hour. After releasing a lander onto its tiny nucleus, it will have to fly alongside the comet as it swoops towards the Sun."
"Space exploration is all about discovering the unknown," commented Rosetta Project Scientist, Dr. Gerhard Schwehm. "Just as, 200 years ago, the discovery of the Rosetta Stone eventually enabled Champollion to unravel the mysteries of ancient Egyptian hieroglyphics, so Rosetta will help scientists to unravel the mysteries of comets."
Dr. Schwehm went on to explain that comets are among the oldest, and least altered, objects in the Solar System. They are thought to have existed, almost unchanged, for the last 4.5 billion years, and are regarded as the building blocks from which the planets formed.
Comets are also important sources of information for scientists studying how our planet evolved and life began. One theory suggests that a comet collision wiped out the dinosaurs 65 million years ago. Such impacts were much more frequent when the Earth was young. It seems that comets arrived in such vast numbers that they may have delivered a significant fraction of the water in our oceans. Furthermore, some scientists believe that the organic (carbon-based) molecules found in comets were the 'seeds' from which life evolved on Earth 4 billion years ago.
Dr. Alan Fitzsimmons of Queen's University, Belfast, commented, "Through PPARC, the UK is funding the Open University's MODULUS instrument on board Rosetta and also supporting the Plasma Science Package. This will allow European scientists to decipher the physics and chemistry of comets in unprecedented detail, and ensures that the UK plays a key role in this exciting mission."
"The Rosetta Stone has been described as the most famous piece of rock in the world, and is one of the most visited objects in the British Museum," said Dr. Richard Parkinson of the Museum's Department of Egyptian Antiquities. "This month marks the 200th anniversary of its discovery by Napoleon's troops at Rosetta in Egypt. The Museum is celebrating the event with a revolutionary redisplay of the newly conserved stone, and a special exhibition, entitled `Cracking Codes', to show the full impact of the Stone on our understanding of the past."
"The use of the stone for decipherment of Ancient Egyptian was not a single event but a continuing process of scientific investigation. This is why we are particularly delighted to be able to include a model of the Rosetta space probe to show how decipherment is very much to do with the future and not just with the past," he added.
Despite a peppering from dust particles travelling faster than bullets, Giotto survived its encounter with Comet Halley to return the first detailed, close-up pictures of a comet nucleus. Six years later, the remarkably robust spacecraft made history once more when it visited a second comet.
During today's flyby, the now deactivated spacecraft swept to within 220,000 km of the planet (just over half the Earth-Moon distance). Travelling at a speed of about 3.5 km/s, Giotto's trajectory took it over the South Pole and southern South America before it headed once more into deep space.
"Giotto paved the way for Rosetta," said Gerhard Schwehm. "It was the Agency's first planetary mission and was a tremendous success, both technically and scientifically. It provided a wealth of scientific results and gave scientists the unique chance to study two different comets with the same set of instruments."
USEFUL LINKS FOR THIS STORY
More about Giotto
British Museum exhibitions
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ESA Science News
11 March 1999
Dignitaries present at the press conference included the German Parliamentary State Secretary, Wolf-Michael Catenhusen, and DLR Chairman, Professor Walter Kröll.
Also available to answer questions from the press were ESA Project Scientist Gerhard Schwehm, DLR lander project manager Stephan Ulamec, and lander lead scientist Helmut Rosenbauer from the Max-Planck-Institut für Aeronomie.
"The Rosetta lander will play a key role in our investigation of Comet Wirtanen," said Dr. Schwehm. "It will provide ground truth about the nucleus which will complement the orbiter's measurements of the comet's composition, surface and subsurface conditions."
The box-shaped Rosetta lander measures approximately 1 metre across and 80 cm in height. The flight version will be attached to the side of the Rosetta orbiter during the journey to Comet Wirtanen, then soft land on the surface of the icy nucleus. Data from the lander will be relayed back to Earth via the orbiter.
The structural and thermal model of the lander presented at the meeting will be used for vibration and thermal tests. These are needed to ensure that the probe will survive the hazards of shaking during launch and extreme temperature variations during its 9-year-long voyage to the comet.
Rosetta lander website
Rosetta lander instruments
Paris, 3 April 1997
While Comet Hale-Bopp steams away into the outer darkness, not to return to the Sun's vicinity for many centuries, the European Space Agency and multinational teams of space scientists are finalizing plans to examine another comet at very close quarters, in the Rosetta mission.
The scientific payload was confirmed by ESA's Science Programme Committee in February. Now the scientists must perfect the full range of ultra-sensitive yet spaceworthy instruments in good time for Rosetta's despatch by an Ariane 5 launcher in January 2003. And even as most of the world was admiring Comet Hale-Bopp at its brightest, dedicated astronomers were examining the comet that will be Rosetta's target.
Although too faint to be seen with the naked eye, Comet Wirtanen made its closest approach to the Sun on 14 March and a fairly close approach to the Earth on 24 March. This comet comes back every 5.5 years. Rosetta will dance attendance on Comet Wirtanen, not at the next return in 2002, nor even in 2008, but in 2013. The project is an ambitious and patient effort to achieve the most thorough investigation of a comet ever attempted.
As the successor to ESA's highly successful Giotto mission to Halley's Comet and Comet Grigg-Skjellerup (which took seven years) Rosetta will spend eight years positioning itself. It will manoeuvre around the planets until it is shadowing Comet Wirtanen far beyond Mars, on nearly the same path around the Sun. In 2011 it will rendezvous with the comet and fly near it. In April 2012 Rosetta will go into a near orbit around Comet Wirtanen, and escort it for 17 busy months, as it flies in to make its closest approach to the Sun in September 2013, at the climax of the mission.
"The Giotto mission placed us at the forefront of cometary exploration," comments Roger Bonnet, ESA's director of science. "The motivation came from European scientists with a sharp sense of the special importance of comets for understanding the Solar System. The same enthusiasm drives us onward to Rosetta, which will ensure our continued leadership in this important branch of space science."
Rosetta will drop a lander on to the comet's surface, for close inspection of its physical condition and chemical composition. The lander is a venture led by Germany, France and Italy, with participation from Austria, Finland, Hungary, Poland and the UK. As a box packed with scientific instruments and standing on three legs, the lander will be capable of anchoring itself to one spot and drilling into the surface.
It may also be able to hop like a flea to visit another part of the nucleus. A combination of solar energy and electric batteries will enable operations to last for several months.
"The combination of Rosetta in orbit around the comet and the lander on its surface is very powerful from a scientific point of view," says Gerhard Schwehm, ESA's project scientist for Rosetta. "We shall watch Comet Wirtanen brewing up like a volcano as it feels the heat of the Sun. In place of hazy impressions of the nucleus of a comet half hidden by its dust clouds, we shall see all the details with unprecedented clarity."
Just as the Rosetta Stone was the key that unlocked the meaning of Egyptian hieroglyphs, so the Rosetta spacecraft is intended to decipher the meaning of comets and their role in the origin and history of the Solar System. Here are a few of the main puzzles.
The comet specialist Uwe Keller of the Max-Planck Institut fur Aeronomie, Germany, is one of the Giotto veterans who has helped with the planning of Rosetta. He was in charge of Giotto's camera.
"Rosetta is the mission we are all waiting for," Dr Keller comments. "After I spent six years analysing our images of the Halley nucleus, I say that basic scientific assumptions about the nature of comets are still contradictory. We shall settle the arguments only by the close, prolonged inspection that Rosetta will make possible."
The spacecraft will therefore have a high degree of robotic self-reliance. It will also be capable of hibernating for more than two years without attention -- a technique devised by ESA for the later stages of the Giotto mission.
Rosetta will rely on solar power, even when more than five times further than the Earth from the Sun. Special low-intensity solar cells are under development for Rosetta. Conditions in this farthest phase of Rosetta's voyage will be very chilly, but ESA's engineers are satisfied that the temperatures inside the spacecraft can be kept within limits by black paint, multilayer insulation and electric heaters. Despite its originality and sophistication, Rosetta will be just a flying box with solar arrays like wings, looking rather like a telecommunications satellite.
"Keep it simple," is the motto of John Credland, ESA's project manager for Rosetta. "Simplicity brings reliability," he explains, "and that is my overriding concern for the engineering of a spacecraft that has to survive and operate far from the Earth for nearly eleven years."
To command Rosetta, and to receive its signals carrying new of the comet, ESA will use a new 32-metre deep-space tracking antenna at Perth in Australia, and a 15-metre antenna in Spain. The spacecraft operations, especially in the near-comet phase of the mission, will be a novel experience for the controllers at the European Space Operations Centre in Darmstadt, Germany.
The gravity of the comet will be weak, and Rosetta's manoeuvres around it will be like a ballet in slow motion. At around 10 kilometres distance, the spacecraft will travel at only 1-2 kilometres per hour in relation to the comet and take about a week to circle once around the nucleus. Sometimes Rosetta will swoop even closer to the comet's surface, to inspect possible landing sights and to drop the lander. The spacecraft's thrusters will adjust the orbit. To keep manoeuvres to a minimum, and so conserve fuel and avoid polluting the comet's environment, computer simulations will help the spacecraft navigators to predict the consequences of any manoeuvre for weeks in advance.
From among several short-period candidates, the mission team chose Comet Wirtanen as Rosetta's target comet because it offered the quickest timetable between the launch of the spacecraft and the completion of the mission.
The comet was discovered by chance by Carl Wirtanen in 1948 on photographic plates at the Lick Observatory in California. In 1972 and 1984 encounters with the planet Jupiter reduced the size of Comet Wirtanen's orbit, and shortened the interval between its visits to the Sun from 6.65 to 5.5 years.
Despite many observations no one really knows the comet's mass, size and shape. The uncertainties are reflected in the computer simulations of manoeuvres near the comet. These cover a wide range of possibilities from a lightweight comet to a massive one, and from a small comet 1 kilometre in diameter to a large one 20 kilometres wide. The best estimate may be 1.5 kilometres. But it is in the nature of a voyage of exploration like Rosetta's that you don't know what you will find!
Photos are available on the ESA home page on Internet: