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By far most of the mass in the Solar System, 99.8 percent, is housed inside the Sun itself; the planets are mere aggregations of the dust and gas that were left over after the creation of this star. Fueled by nuclear fusion in its core, converting hydrogen into helium, the Sun provides heat and fight to Earth. Without it, life as we know it would not be possible.

The Sun is very large and stands relatively close to Earth, so with specially constructed ground-based telescopes there is a lot of detail we can see: the swirling bubbles on the visible surface, the cooler and therefore darker sunspots, and the jets of extremely hot gas that are ejected into space.

Luckily for us, the Earth's magnetosphere diverts, and the atmosphere filters, a lot of the deadly radiation from the Sun. However, this also means that astronomers cannot detect everything that is happening on and inside our star from the ground. Another limitation is that from Earth we can only have a good look at the equatorial regions of the Sun; what is happening on its poles is hard to see.

In 1990, ESA and NASA therefore launched the Ulysses probe to climb above the plane in which all the planets orbit (ESA build the spacecraft, while NASA provided the launch with the Space Shuttle and the RTG to power Ulysses). Using a gravity swing from Jupiter, it went into a wide orbit over the Sun's poles.

Ulysses discovered that the solar wind, the stream of charged particles sent out by the Sun, actually has two components: a slow one emitted by the equatorial regions and a faster one blowing from the poles.

FIGURE 7.21 Artist impression of the launch of Ulysses from the Space Shuttle. [ESA]
FIGURE 1.22 SOHO and the location of its various Sun-observing scientific instruments. [ESA]

In 1995 the Solar and Heliospheric Observatory mission, SOHO, was launched on board an Atlas HAS rocket from Cape Canaveral Air Force Station. As ajoint project ofESA and NASA, the Sun observation satellite was designed to continuously view the atmosphere, surface and interior of our local star. From its position far away from Earth, SOHO's view was never interrupted by our planet.

SOHO is orbiting the Sun in a very peculiar orbit. It is located at the so-called Earth-Sun Lj Lagrangian point, where it can observe its target without its view ever being blocked by the Earth.

Lagrangian points are locations in space where the gravitational forces on, and the orbital motion of, a body balance each other. They were discovered by the Frenchman Louis Lagrange in 1772, when he described the mathematical "three body problem" of how a small object would orbit around two more massive bodies such as planets and stars. There are five Lagrangian points in the Sun-Earth system, Lt to L5, and Lj is a point directly between the Earth and the Sun (such points also exist for the Earth-Moon system).

A spacecraft in this L} point always remains in the same position with respect to the Earth and the Sun, and therefore it provides SOHO with a continuous, uninterrupted view of the Sun and a direct line of communication with Earth. If SOHO was just orbiting the Earth, the Sun would often be eclipsed by the planet. If it was orbiting the Sun, it would be hard for ground stations on Earth to stay in constant contact with SOHO. The fixed Li position makes SOHO a stable observatory both with respect to the Sun and to Earth.

In April 1998 SOHO successfully completed its nominal mission, but it continued to work perfectly. On December 2, 2005, it celebrated its tenth anniversary in space, and at the time of writing it is still being used.

SOHO has allowed more than 3,200 researchers to make major advances in solar science. The mission revolutionized our ideas about the solar interior, the solar atmosphere and the dynamics of the solar wind. Major SOHO achievements include the detection of rivers of plasma beneath the surface, a complicated magnetic layer on the Sun's outside, and the first detection of flare-induced solar quakes ("earthquakes" on the Sun).

Moreover, it made many spectacular images and movies of eruptions called "solar flares." These are monumental explosions on the Sun that are

FIGURE 1.23 This is how ESA's SOHO spacecraft sees the Sun in ultraviolet, a wavelength invisible to the human eye. [ESA]

caused by sudden releases of magnetic energy, and emit as much energy as millions of atomic bombs. The flares accelerate the normal solar wind (the stream of energetic particles expelled by the Sun) to "storm force," and also trigger the expulsion of enormous amounts of gas from the Sun in what are called "coronal mass ejections."

SOHO can only see one half of the Sun at any time, but scientists used its data to develop methods to see what is happening on the other side. They found out that eruptions on the back of the Sun send ripples all the way around to the side visible to SOHO.

In this way we can get warnings about solar storms earlier than ever, even before they become visible to us. This is important, because severe geomagnetic storms (caused by the powerful burst of the solar wind hitting the Earth's magnetic field) can disrupt not only electronics on board satellites, but also radio communications and even the electrical power supply on Earth (and also cause beautiful aurora displays in the atmosphere).

SOHO also discovered more than 1,000 comets, which means that it has almost doubled the number of known comets. Comets are chunks of ice and dust that start to evaporate when they get close to the Sun, often developing beautiful long tails of expelled material. From its unique vantage point, SOHO is able to see comets grazing and sometimes even impacting the Sun.

Many comet discoveries have been made by amateurs using SOHO images on the Internet. SOHO and modern computer information technology has thus enabled people from all over the world, from the United States to Lithuania and Taiwan, to look for new comets.

Actual samples of the Sun were brought to Earth by NASA's Genesis mission. It captured solar wind particles in specially designed, ultra-pure silicon wafers. The Genesis capsule carrying the precious samples managed to return to Earth, but unfortunately crashed when its parachutes failed to open. Luckily NASA has still been able to salvage some of the wafers with their captured solar particles from the debris (more on this dramatic failure in Chapter 8, "Death of a spacecraft").

There are many other spacecraft observing the Sun from orbits around the Earth. Rather than exploring the Sun from a closer range, they are used as astronomical observatories above the distorting and filtering atmosphere to study the interaction of the Sun's radiation with the Earth's magnetic field.

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