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Summary

For a close-up view of the Sun

Since the beginning of the 1990's, six unprecedented spacecraft built in Europe for the Ulysses, SOHO and Cluster missions have made many amazing discoveries about the Sun, and how its storms affect the Earth. Yet, scientists still cannot predict the unruly behaviour of the star on which our lives depend. A closer look at the Sun is required. 

Solar Orbiter is intended to brave the fierce heat and carry its telescopes to less than one quarter of the Earth's distance from the Sun, where sunlight will be twenty times more intense than satellites in the vicinity of the Earth feel it. The spacecraft must also endure powerful bursts of atomic particles from explosions in the solar atmosphere. The reward will come in the form of sharp images obtained together with unprecedented measurements of the local near-Sun phenomena.

The pictures of the weird solar landscapes, where glowing gas dances and forms loops in the strong magnetic field, will be stunning. They will show details down to 200 kilometres wide, with a tiny fraction of the width of the Sun's visible disc, 1.4 million kilometres.

After launch, Solar Orbiter will manoeuvre into an orbit around the Sun. It will perform a close approach every five months. Around closest approach, when travelling at its fastest, Solar Orbiter will remain for several days roughly positioned over the same region of the solar atmosphere as the Sun rotates on its axis. Just as geostationary weather and telecommunications satellites are stationed over particular spots on the Earth's surface, so the spacecraft will seem to 'hover' for a while over the Sun. Solar Orbiter will therefore be able to watch storms building up in the atmosphere.

Solar Orbiter will exploit new technologies being developed by ESA for the BepiColombo mission to Mercury, the closest planet to the Sun.

Ground controllers will repeatedly fly Solar Orbiter close to Venus, and use the planet's gravity to nudge the spacecraft into higher inclination orbits. This will enable the instruments to see the polar regions of the Sun clearly for the first time. That's one of the prime scientific goals of the project. After 7.4 years, it will view the poles from solar latitudes higher than 30°, compared with 7° at best from the Earth.

The scientific instruments

Into a mass of 180 kilograms, the scientists want to accommodate a set of in-situ and a set of remote sensing instruments. The in-situ instruments consist of detectors for observing particles and events in the immediate vicinity of the spacecraft: the charged particles and magnetic fields of the solar wind, radio and magnetic waves in the solar wind, and energetic charged particles flung out by the Sun.

The remote sensing instruments will observe the Sun's surface and atmosphere. The gas of the atmosphere is best seen by its strong emissions of short-wavelength ultraviolet rays. Tuned to these will be a full-Sun and high-resolution imager and a high-resolution spectrometer. The outer atmosphere will be revealed by ultraviolet and visible-light coronagraphs that blot out the bright disc of the Sun. To examine the surface by visible light, and measure local magnetic fields, Solar Orbiter will carry a high-resolution telescope and magnetograph.

Key questions

"Go where no one has been before - that's the way to make discoveries," says Eckart Marsch of Germany's Max-Planck-Institut für Sonnensystemforschung, who first proposed the Solar Orbiter mission. "We can expect to clear up many mysteries about the Sun's behaviour."

Here are some key issues to be addressed.

How exactly is the solar wind propelled?
Non-stop streams of electrons and atomic nuclei pour from the Sun in all directions, in slow and fast electric windstreams travelling at speeds of 300 to 800 kilometres per second. This solar wind and its variations have a profound effect on the Earth's space environment, as investigated by the Cluster and Double Star projects. The SOHO spacecraft has discovered sources of the solar wind in the Sun's atmosphere, and has watched its particles being accelerated. Solar Orbiter will detect the solar-wind particles close to where they escape through funnels and holes in the Sun's magnetic field. Simultaneously it will watch the sources and accelerators of the wind more closely than ever before.

How does the Sun rule interplanetary space?
The solar wind fills a gigantic bubble all around the Sun, called the heliosphere. It extends far beyond the realm of the planets. ESA's spacecraft Ulysses, in its unique flights over the poles of the Sun, has given a comprehensive impression of storms and shocks in the solar wind, but Ulysses carries no camera. Solar Orbiter will give the first clear pictures of the polar regions, from where a key component of the solar wind emanates. More generally, the new spacecraft will explore the region where the Sun's atmosphere peters out and the heliosphere proper begins. It will add to the value of Ulysses' results, and enhance the scientists' understanding of the empire of the Sun.

How does the Sun's dynamo work?
Another discovery of SOHO, made in collaboration with a network of ground stations called GONG (Global Oscillation Network Group), is a dynamo deep inside the Sun that is the probable source of the magnetic activity seen at the surface. Yet profound mysteries remain, not least the fact that the dynamo surges and slackens at a rate quite different from the surface activity - every 16 months or so compared with the 11 years of the well-known sunspot cycle. Better understanding should come from Solar Orbiter's closer observation of the outward signs of solar magnetism in an ever-changing 'carpet' of magnetized gas and magnetic loops seen by SOHO, and especially from the polar views which will allow measurements of the Sun's rotation and flows of gas close to the poles.

How can we predict eruptions on the Sun?
Bursts of high-energy atomic particles from the Sun can endanger astronauts, cripple unmanned satellites, and may even cause computers on the ground to crash. Outbursts of another kind are mass ejections of gas from the Sun's atmosphere. These can jolt the Earth's magnetic field and sometimes they disrupt electric power supplies. Scientists have discovered that magnetic explosions are to blame for solar flares, particle outbursts and mass ejections, yet the link between these different types of eruptions is still cryptic. To clarify why, how and when they happen is a high priority for Solar Orbiter's scientists, who hope to learn how to anticipate the outbursts. For future astronauts, that may be a matter of life and death.

Can we make long-term forecasts of solar activity?
The Sun's activity changes rhythmically in the 11-year sunspot cycle, but the overall vigour varies from cycle to cycle. Cool episodes in the Earth's climate are linked to weak sunspot activity, while strong activity has a warming effect. In order to predict the Sun's contribution to climate change, scientists want to be able to forecast what the next cycle will be like. The best indicator seems to be the strength of the magnetic field near the Sun's poles. It is hard to measure from the Earth's vicinity, because the polar regions are seen almost edge-on. Solar Orbiter's slanting view of the poles should greatly improve the prospects for long-term solar forecasts.

Last Update: 1 September 2019
28-Mar-2024 19:31 UT

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