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Astronomie - Startvorbereitung für ESA Solar Orbiter Proba-3 Mission

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22.08.2016 

 

PROBA-3: SEEING THROUGH SHADOW TO VIEW SUN’S CORONA

Proba-3 formation flying satellites
22 August 2016

Every 18 months or so, scientists and sensation-seekers gather at set points on Earth’s surface, to await awe-inspiring solar eclipses. The Moon briefly blocks the Sun, revealing its mysterious outer atmosphere, the corona. Though what if researchers could induce such eclipses at will?

That’s the scientific vision behind ESA’s double-satellite Proba-3, the world’s first precision formation-flying mission, planned for launch in 2019.

An ‘occulter’ satellite will fly 150 m ahead of a second ‘coronagraph’ satellite, casting a precise shadow to reveal the ghostly tendrils of the solar corona, down to 1.2 solar radii, for hours on end.

“We have two scientific instruments aboard,” explains Damien Galano, Proba-3 Payload Manager. “The primary payload is ASPIICS, a coronagraph to observe the corona in visible light while the DARA radiometer on the occulter measures the total solar irradiance coming from the Sun – a scientific parameter about which there is still some uncertainty. 

Proba-3 revealing corona

“The corona is a million times fainter than the Sun itself, so the light from the solar disk needs to be blocked in order to see it. The coronagraph idea was conceived by astronomer Bernard Lyot in the 1930s – and since then has been developed and has been incorporated into both Earth-based and space telescopes.

“But because of the wave nature of light, even within the cone of shadow cast by the occulter, some light still spills around the occulter edges, a phenomenon called ‘diffraction’.

“To minimise this unwanted light, the coronagraph can be positioned closer to the occulter – and therefore deeper into the shadow cone. However the deeper it is, the more the solar corona will also be occulted by the occulter. 

Coronagraph on single satellite

“Hence the advantage of a larger occulter and the maximum possible distance between the occulter and the coronagraph. Obviously a 150-m-long satellite is not a practical proposition, but our formation flying approach should provide us with equivalent performance.

“Furthermore, the ASPIICS coronagraph itself contains a smaller, secondary occulter disk, to cut down on diffracted light still further.

Coronagraph across two satellites

“Precision is all – the aperture of the ASPIICS instrument measures 50 mm in diameter, and for corona observation performance it should remain as much as possible in the centre of the shadow, which is about 70 mm across at 150 m.

"So we’ll need to achieve millimetre-scale positioning control between the two spacecraft, effectively forming a single giant instrument across space.”

ASPIICS (Association of Spacecraft for Polarimetry and Imaging of the Corona of the Sun) is being developed for ESA by a consortium led by Centre Spatial de Liège in Belgium, made up of 15 companies and institutes from five ESA Member States.

Diffraction of light

“Many of these companies are new to ESA, and they’ve proved to be very motivated and eager to show their capabilities,” remarks Damien. “We’ve produced various prototypes of instrument elements, and our first complete ‘structural and thermal model’ should be complete in the autumn, ahead of our end-of-year Critical Design Review.

“We’re also looking into various optical aspects, such as the best occulter edge shape to minimise diffraction.”

There’s a lot of broader interest in this external occulter approach – especially for the imaging of Earth-like exoplanets, which would require the blocking out of their parent stars.

“It’s a similar challenge, the main difference being that the star in question is a point source of light rather than the extended source that our Sun is.

“So it could be that formation-flown external occulters become versatile scientific tools, opening many new vistas in astronomy.”

Quelle: ESA

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Update: 13.09.2016

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Proba-3 satellites form artificial eclipse
 

PROBA-3: SET THE CONTROLS FOR THE VERGE OF THE SUN

By converging in orbit, a pair of small satellites will open a new view on the source of the largest structure in the Solar System: the Sun’s ghostly atmosphere, extending millions of kilometres out into space.

The two satellites together are called Proba-3, set for launch in late 2019. Through precise formation flying, one will cast a shadow across the second to open up an unimpeded view of the inner area of the ‘corona’, which is a million times fainter than the blindingly brilliant solar disc.

“When I first heard of the idea I said ‘Wow! That’s just what we need’,” said Andrei Zhukov of the Royal Observatory of Belgium, serving as Principal Investigator for Proba-3’s solar instrument.

“The best way to observe the corona from the ground is during a solar eclipse, although we still have to cope with stray light – we cannot correct for the influence of Earth’s atmosphere. 

Solar eclipses
Solar corona seen during terrestrial eclipses

“The next best method is by using ‘coronagraphs’ to create an articifical eclipse, either on ground telescopes or inside Sun-watching satellites such as SOHO and Stereo.

“The problem is that stray light bending around the edge of the occulting disc limits our view of the most important inner portion of the corona. SOHO’s coronagraph, for instance, can observe no closer in than 1.1 Sun-diameters. Others can see closer, but with strong stray light making detailed observation impossible.

“With Proba-3 we aim to see extremely close to the solar surface in visible light, by flying the occulter and coronagraph on separate satellites some 150 m apart.

A fiery solar explosion 

“This should give us a ringside seat on the most interesting segment of the corona, where a lot of interesting physics is going on, where the solar wind is born and ‘coronal mass ejections’ originate – gigantic solar eruptions with the potential to affect our terrestrial infrastructure.”

While the Sun’s surface is a comparatively cool 6000ºC, the corona averages a sizzling million degrees. The mystery is how energy travels from the cool Sun to the hot corona, in apparent defiance of the laws of thermodynamics.

“By mapping the fine structure of the inner corona for a prolonged time – we are targeting around six hours – our hope is that we gain insight into the kind of energy flows that are taking place,” notes Dr Zhukov. 

Proba-3
Proba-3's pair of satellites

“Our standard observing mode will be once per minute, but we could speed that up to a few seconds within a selected field of view, for instance when tracing the rapid evolution of a mass ejection.

“The ultimate goal is to be able to solve the physics of space weather, in order to forecast coronal mass ejections, which are known to have dramatic effects on terrestrial electricity grids and other infrastructure.”

Proba-3 is first and foremost a technology demonstration, exploring the potential of precise formation flying in orbit, but achieving meaningful scientific results will also help to prove its approach works.

Quelle: ESA

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Update: 20.08.2017

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Pioneering ESA mission aims to create artificial solar eclipses

 
Due to launch together in 2020, the two satellites making up Proba-3 will fly in precise formation to form an external coronagraph in space, one satellite eclipsing the sun to allow the second to study the otherwise invisible solar corona. Credit: ESA

As skywatchers and scientists converge on a transcontinental band of totality for Monday’s solar eclipse in the United States, engineers in Europe are building a unique pair of satellites to create artificial eclipses lasting for hours — a feat that that could be a boon for solar physicists but will escape the view of Earth-bound spectators.

The European Space Agency’s Proba-3 mission, scheduled for launch in late 2020, is made possible by two satellites, one about the size of a refrigerator, and another slightly smaller spacecraft with the rough dimensions of a coffee table.

The basic idea is to fly the smaller satellite directly between the sun and the field-of-view of cameras and instruments mounted on the bigger spacecraft, blocking the sunlight and revealing the glow of the sun’s corona, or super-hot atmosphere, and filament-like eruptions called solar flares.

The light coming from the surface of the sun is a million times brighter than the corona, requiring special measures to see the solar atmosphere.

The concept of obstructing the brightest light emanating from the sun to study activity around it is not new. Scientists have made observations of the corona for centuries during solar eclipses, and there are other space missions that carry coronagraphs, light-blocking discs buried inside telescopes used to make the relatively dim solar atmosphere visible.

But coronagraphs mounted inside telescopes are prone to stray light, a common problem in optics. Light escaping around the coronagraph disc can distort or mask views of the corona.

One simple way to think of the stray light problem is to compare an image of a total solar eclipse, a spectacular phenomenon where the faint corona suddenly springs into view. Holding your thumb over the sun at arm’s length does not produce the same result because sunlight has already been scattered by particles in Earth’s atmosphere.

“One of the science goals of Proba-3 is to reproduce the conditions of a total solar eclipse as much as possible,” said Andrei Zhukov, principal investigator for Proba-3’s coronagraph at the Royal Observatory of Belgium, in response to questions from Spaceflight Now.

Artist’s concept of an artificial eclipse created by the Proba-3 mission. Credit: ESA – P. Carril, 2013

In general, the longer the distance between an observer or a camera and the object obscuring the sun, the better the result. Scientists also do not have to worry about atmospheric distortions in space.

“This problem can be minimized by extending the coronagraph length, the distance between the camera and the disc, as far as possible – but there are practical limits to coronagraph size,” Zhukov said in an ESA press release.

“Instead, Proba-3’s coronagraph uses two craft: a camera satellite and a disc satellite,” Zhukov said. “They fly together so precisely that they operate like a single coronagraph, 150 meters (492 feet) long.”

The duo will launch together into an highly elliptical, oval-shaped orbit around Earth taking the satellites as high as 37,611 miles (60,530 kilometers) and as low as 372 miles (600 kilometers).

In that orbit, the satellites will complete one lap around the planet every 19.6 hours. For six of those hours, cameras on Proba-3’s larger satellite will have an artificial eclipse.

Proba-3 will see the features down to 34,500 miles (55,600 kilometers) from the sun — about 8 percent of the solar radius — resolving activity closer to the solar limb than any current space mission. Zhukov said ground-based observers looking at a total solar eclipse can still see more of the corona than Proba-3, but the advantage of a space mission is the eclipse’s longevity.

“During two years of its nominal mission, Proba-3 will provide around 1,000 hours of coronal observations,” Zhukov wrote in an email to Spaceflight Now. “This has to be compared with several minutes of duration of ‘natural’ eclipses during the same time.

“Proba-3 will also be free from disturbances produced by the Earth’s atmosphere in all astronomical observations,” Zhukov wrote.

ESA is developing the Proba-3 mission as an experimental demonstration, with scientific observations of the sun a secondary goal.

Engineers want to test out technologies for autonomous formation flying on Proba-3, which will use ranging measurements with the help of GPS navigation signals and optical sensors.

The two spacecraft will be connected with an inter-satellite radio link, and the so-called occulter satellite — the smaller of the pair — will carry low-power micro-thrusters for fine maneuvers, keeping the two vehicles positioned with millimeter precision.

Proba-3 will create an eclipse when the satellites are farthest from Earth. The satellites will passively drift apart during the rest of each orbit, a fuel-saving measure to minimize consumption of the mission’s limited supply of propellant.

The capabilities to be proved out on Proba-3 could be used on future missions to repair satellites in orbit or return samples from Mars, according to ESA.

Already approved for development as a tech demo mission, Proba-3 won the backing of ESA’s science program committee earlier this year. The agency’s scientific division will pay for Proba-3’s science operations center to ensure astronomers get the most out of the project.

Proba-3 was scheduled for launch in 2019, but officials recently pushed back the mission’s liftoff to the fourth quarter of 2020.

“The complexity in the development of the formation flying technology does not allow the launch in late 2019 as was planned earlier,” Zhukov said. “The project schedule is now consolidated, and the launch in the fourth quarter of 2020 is the new baseline. That does look feasible.”

Quelle: SN

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Update: 28.08.2017

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Solar Orbiter on track to launch before next total solar eclipse

airbus-solar-orbiter-final-sta

Stevenage, 21/08/2017 - Today’s total Solar Eclipse across the United States of America will provide a spectacular view of the mysterious Solar Corona, the one million degree “halo” around the sun, which can only be seen from Earth when the Moon passes in front of the bright solar disk, which otherwise completely drowns out the faint light of the corona.

Scientists lucky enough to be able to see the Solar Eclipse from Earth will be studying this rare glimpse of the corona during the one hundred and sixty seconds or less that the eclipse lasts, to try to answer the many unanswered questions about this mysterious corona. Noone yet knows for instance, why the corona is more than 100 times hotter than the surface of the Sun.

Meanwhile the European Space Agency spacecraft Solar Orbiter is in the final stages of spacecraft integration at the Airbus spacecraft assembly hall in Stevenage, UK.

Solar Orbiter will be launched in February 2019 into a close orbit around the Sun and will allow scientists to study the solar corona in much more detail, for much longer periods, and at a much closer distance that can ever be reached here on the ground, or for that matter, by any spacecraft circling the Earth. 10 instruments will be flown that will study not only the corona but the Sun’s disk in great detail, the solar wind and the solar magnetic fields which will give us unprecedented insight into how our star works, and how we can better predict periods of stormy “space-weather” that the Sun throws our way from time to time.

The last of the ten instruments is being installed this month and the next step is system testing before the heatshield, antennas and boom are added towards the end of the year. In addition, the first instrument end-to-end electrical test has been performed successfully showing that the system works completely as expected.

By the time of the next global total eclipse, across the Pacific Ocean and South America on 2nd July 2019, Solar Orbiter will have begun its three and a half year journey to the inner solar system to get close to our Sun.

solar-orbiter-2019

Quelle: Airbus

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Update: 19.09.2018

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Staring at the sun: Solar Orbiter telescopes will get closest view yet

European Space Agency’s seven-year, €1bn mission will investigate the effects of the sun on satellite technology

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ESA’s next-generation Sun explorer, Solar Orbiter, will be launched in February 2020, and will take three and half years to complete its journey. Photograph: ESA

A new space mission will carry telescopes closer to the sun than ever before, capturing detailed images of its surface landscapes and taking measurements of its atmosphere.

Once it reaches its destination – the sun’s orbit – the spacecraft will be able to investigate the origins of the solar wind – the stream of ionised gas emanating from the sun – and of fierce explosions on its surface called solar flares, as well as taking pictures of the polar regions of the sun for the first time.

The European Space Agency’s Solar Orbiter will launch in February 2020 using a Nasa rocket from Cape Canaveral in Florida, and will take approximately three and a half years to reach its vantage point, about one fifth as far as Earth is from the sun. Before it gets there, it must first head for Venus and then manoeuvre back towards Earth, before finally dropping very close to the sun and adjusting its position until it eventually reaches a distance of about 42 million kilometres from its surface.

On 12 August, Nasa’s Parker solar probe started a similar perilous journey towards the sun, aiming to reach within 7 million kilometres of its surface, but it lacks the ability to gather images that the onboard telescopes give the Solar Orbiter.

“Most [previous] missions have been more focused on sampling the environment around the spacecraft or on looking at the surface of the sample. This is doing both in a way that hasn’t been done before,” said Michelle Sprake, a systems engineer at Airbus Defence and Space in the UK, who has been working on Solar Orbiter for the past eight years.

The seven-year mission, which is costing about €1bn, aims to do 20 flybys of the sun. To do this, Solar Orbiter will need to withstand a huge temperature range, from 600C when close to the sun to -180C in deep space. “It’s been one of the most challenging missions that we’ve ever developed,” said Justin Byrne, head of science at Airbus.

Solar Orbiter will help scientists understand and predict the effects of solar wind and big solar eruptions on communications and navigation systems on Earth.

“The overarching science question we are trying to answer is about how the sun creates this plasma bubble around it and about how solar activity changes through time and also changes this bubble we all live in,” said Daniel Müller, ESA’s Solar Orbiter project scientist.

Although on Earth we are continuously being “gently buffeted by the what is called the solar wind”, a big solar storm can affect spacecrafts, satellites and the accuracy of GPS signals, said Louise Harra, a professor of solar physics at UCL Mullard Space Science Laboratory and part of the Solar Orbiter project. “It wouldn’t affect you and I when using our satnavs,” she said, but it could affect someone using high precision GPS, for example, in oil drilling. Similarly, during a solar storm, aircraft are not allowed to rely on GPS signals and have to reroute.

Solar Orbiter is “one piece in the puzzle” in terms of trying to understand and predict these events with a bit more accuracy, so we can work out whether we’re going to be affected by them, said Sprake.

The mission scientists hope to understand what drives the sun’s magnetic field and its 11-year cycle of activity. Being able to study the poles will be central to this, said Harra. At the moment, “because we don’t know what’s at the poles, we’re missing a big part of the picture,” she said.

“We have never had the telescopes to look down and measure the magnetic field at the poles, to see what the structures are. It’s not just going to be black darkness, we believe. There are going to be some interesting phenomena there. And that global magnetic field is what drives the solar activity and the solar wind,” she told the Guardian.

The spacecraft is fitted with 21 sensors and 10 instruments which will look at the sun and at the surrounding environment, as well as a 3.1m by 2.4m protective heat shield made of multiple layers of insulation materials designed to reflect as much heat as possible.

“The spacecraft’s job is to deliver a precious cargo of 10 instruments to the vicinity of sun,” said Ian Walters, Solar Orbiter project manager at Airbus. The instruments are so sensitive that the spacecraft has to be completely clean, he said. The cleaning process has included baking each unit on the spacecraft and subjecting it to a purge using dry nitrogen.

Now that all the instruments have been installed and tested at the Airbus facilities in Stevenage, UK, engineers are busy wrapping up the spacecraft in thermal blankets before it can be shipped to IAGB facilities in Munich for final tests ahead of its launch.

The Solar Orbiter launch window is set for between 6 and 24 February 2020, said Walters. “That three week slot is fixed by the geometry of Earth and Venus. We will not miss it,” he added.

Quelle: The Guardian

 

 

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