Visions of the Cosmos

Planetary Science


The Solar Wind

Our  star is a very violent place.  Not only is it sending out electromagnetic radiation across the whole energy band but  It is also  blowing out plasma in all directions into space. 

The Solar wind is the plasma of charged particles coming out of the Sun in all directions at very high speed.  On average the speed in the vicinity of the Earth is  about 400 km/sec, almost a million mph!    The charged particles are free electrons, free protons and the ionised atoms of most of the more massive atoms of the periodic table.


The illustration is an artist's view of Venus Express space-craft investigating solar wind conditions.  Credit ESA -AOES Medialab

The source of the solar wind is the Sun's hot corona. The temperature of the corona is so high that the Sun's gravity cannot hold on to it. The composition of the solar wind reflects the composition of the solar corona, modified by solar wind processes. The exact mechanism of solar wind formation is not known.

It is responsible for the anti-sunward tails of comets and the shape of the magnetic fields around the Earth and other planets with magnetic fields. The Solar Wind can also have a measurable effects on the flight paths of spacecraft.

The solar wind varies routinely through the 27-day rotation of the Sun, as well as sporadically, in response to solar flares and to violent eruptions in the corona - namely Coronal Mass Ejections.  These eruptions can result in geomagnetic storms on Earth.

Credit for illustration SOHO/LASCO/EIT (ESA & NASA)

This illustration shows a Coronal Mass Ejection blasting off the Sun’s surface in the direction of Earth. This left portion is composed of an EIT 304 image superimposed on a LASCO C2 coronagraph. Two to four days later, the CME cloud is shown striking and beginning to be mostly deflected around the Earth’s magnetosphere. The blue paths emanating from the Earth’s poles represent some of its magnetic field lines. The magnetic cloud of plasma can extend to 30 million miles wide by the time it reaches earth. These storms, which occur frequently, can disrupt communications and navigational equipment, damage satellites, and even cause blackouts.

During a flare the material in the flare may be heated to temperatures of 10 million degrees Celsius; matter at these temperatures emits copious amounts of UV and X-Ray, as well as visible light. In addition, flares tend to eject matter, primarily in the form or protons and electrons, into space at velocities that can approach 1000 km/second. These events are called coronal mass ejections.

The Earth's Magnetosphere

Credits for diagram ESA

The solar wind buffets the blue funnel shaped magnetosphere around the Earth  which however shields it from much of the solar wind. When the solar wind encounters Earth's magnetic field it is deflected like water around the bow of a ship, as illustrated in the adjacent image

The imaginary surface at which the solar wind is first deflected is called the bow shock.   The corresponding region of space sitting behind the bow shock and surrounding the Earth is termed the magnetosphere; it represents a region of space dominated by the Earth's magnetic field in the sense that it largely prevents the solar wind from entering. However, some high energy charged particles from the solar wind leak into the magnetosphere and are the source of the charged particles trapped in the Van Allen belts.

The Cluster Experiment

Credit for Diagram ESA

The 16 July 2000 a Soyouz-Fregat launch vehicle provided by the French-Russian Starsem consortium lifted off with FM 6 Salsa and FM 7 Samba, the first pair of Cluster II satellites.
ESA's Cluster II mission, consists of four identical spacecraft flying in formation between 19000 and 119000 km above the Earth. There, they are studying  the planet's magnetic field and electric surroundings in three dimensions. In particular, they are looking at the effects of the solar wind, the hot wave energy produced by the Sun, which buffets Earth's protective magnetosphere.
This wind often breaks through the magnetosphere at the poles, producing auroras.
Cluster II will examine this and many other phenomena associated with the solar wind.


A joint ESA and NASA mission, Ulysses (named after the hero of Greek legend) is charting the unknown reaches of space above and below the poles of the sun.

Ulysses was launched by the Space Shuttle Discovery in October 1990. It headed out to Jupiter, arriving in February 1992 for the gravity-assisted manoeuvre that swung the craft into its unique solar orbit. It passed over the sun's south pole in 1994, and the north pole in 1995.

Exploring our star's environment is vital if scientists are to build a complete picture of the sun, how it works and its effect on the solar system. In particular, the satellite is studying the solar wind that blows non-stop from the sun and carves a huge bubble in space called the heliosphere.

Ulysses is providing the first-ever map of the heliosphere from the equator to the poles.

Ulysses is equipped with a comprehensive range of scientific instruments. These are able to detect and measure solar wind ions and electrons, magnetic fields, energetic particles, cosmic rays, natural radio and plasma waves, cosmic dust, interstellar neutral gas, solar X-rays and cosmic gamma-ray bursts. This combination of experiments will help scientists understand the sun and its heliosphere, and perhaps the sun's influence on Earth's climate.











  Illustration  on the left Ulysses at Jupiter Credit ESA -Dave Hardy                 Illustration  on right Ulysses at the Sun Credit ESA


ESA's 370 kg Ulysses probe, launched in October 1990, continues to investigate the Sun and interplanetary space.


Ulysses Project  Scientist Dr Richard Marsden

Ulysses in ESTEC's Test Centre. Ulysses is a joint ESA mission with NASA studying the interplanetary medium and solar wind in the inner heliosphere beyond the Sun's equator for the first time. Its high-inclination solar orbit took it over the Sun's south pole in 1994 and then the north pole in 1995. It is now heading back towards the Sun for a second southern solar pass above 70 deg latitude during September 2000 to January 2001, and a similar northern passduring September to December 2001. [Image Date: 1989/11] [89.11.009-007]








Ulysses embarked on third set of polar passes  on 17 November 2006

On 17 November 2007 the joint ESA-NASA Ulysses mission reached another important milestone on its epic out-of-ecliptic journey: the start of the third passage over the Sun's south pole. Launched in 1990, the European-built spacecraft is engaged in the exploration of the heliosphere, the bubble in space blown out by the solar wind. Given the capricious nature of the Sun, this third visit revealed new and unexpected features of our star's environment.

The Heliosphere

The Sun is more than just a glowing ball of gases. It is constantly sending out charged particles that are the parts of the solar wind. The solar wind moves out from the Sun and creates a large bubble called the heliosphere. Every star in the universe has a heliosphere. At its edge, there is an area where our stars charged particles meet the particles from other stars. That place is the heliopause.

The Heliopause

The heliopause is the outer edge of the heliosphere. Think of it as the surface of the bubble that surrounds our Solar System. The heliopause is the part of our Solar System that is exposed to the ions and particles of deep space. It is over 100 times the distance from the Earth to the Sun. In a sense once you leave the heliopause you have left the solar system.

The Structure of the Heliopause.

Just inside the heliopause is the termination shock. The termination shock is where the solar winds slow down to speeds slower than the speed of sound. They slow because of the effect of particles from other stars in the area. Eventually the solar wind speeds will drop to nothing because of the winds from other stars. The region between the heliopause and the termination shock is called the heliosheath. Once you cross the heliopause, it is a solar wind dead zone. Both Voyager probes are reaching the termination shock and will eventually cross the heliopause and move into deep space.

The mission objective of the Voyager Interstellar Mission (VIM) is to extend the NASA exploration of the solar system beyond the neighborhood of the outer planets to the outer limits of the Sun's sphere of influence, and possibly beyond. This extended mission is continuing to characterize the outer solar system environment and search for the heliopause boundary, the outer limits of the Sun's magnetic field and outward flow of the solar wind. Penetration of the heliopause boundary between the solar wind and the interstellar medium will allow measurements to be made of the interstellar fields, particles and waves unaffected by the solar wind.

Interstellar Mission


Mission Characteristic

The VIM is an extension of the Voyager primary mission that was completed in 1989 with the close flyby of Neptune by the Voyager 2 spacecraft. Neptune was the final outer planet visited by a Voyager spacecraft. Voyager 1 completed its planned close flybys of the Jupiter and Saturn planetary systems while Voyager 2, in addition to its own close flybys of Jupiter and Saturn, completed close flybys of the remaining two gas giants, Uranus and Neptune.

At the start of the VIM, the two Voyager spacecraft had been in flight for over 12 years having been launched in August (Voyager 2) and September (Voyager 1), 1977. Voyager 1 was at a distance of approximately 40 AU (Astronomical Unit - mean distance of Earth from the Sun, 150 million kilometers) from the Sun, and Voyager 2 was at a distance of approximately 31 AU.

As of July 2003, Voyager 1 was at a distance of 13.3 Billion Kilometers (88 AU) from the sun and Voyager 2 at a distance of 10.6 Billion kilometers (70 AU).

Voyager 1 is escaping the solar system at a speed of about 3.6 AU per year, 35 degrees out of the ecliptic plane to the north, in the general direction of the Solar Apex (the direction of the Sun's motion relative to nearby stars). Voyager 2 is also escaping the solar system at a speed of about 3.3 AU per year, 48 degrees out of the ecliptic plane to the south.


Both Voyagers are headed towards the outer boundary of the solar system in search of the heliopause, the region where the Sun's influence wanes and the beginning of interstellar space can be sensed. The heliopause has never been reached by any spacecraft; the Voyagers may be the first to pass through this region, which is thought to exist somewhere from 8 to 14 billion miles from the Sun. Sometime in the next 5 years, the two spacecraft should cross an area known as the termination shock. This is where the million-mile-per-hour solar winds slows to about 250,000 miles per hour—the first indication that the wind is nearing the heliopause. The Voyagers should cross the heliopause 10 to 20 years after reaching the termination shock. The Voyagers have enough electrical power and thruster fuel to operate at least until 2020. By that time, Voyager 1 will be 12.4 billion miles (19.9 billion KM) from the Sun and Voyager 2 will be 10.5 billion miles (16.9 billion KM) away. Eventually, the Voyagers will pass other stars. In about 40,000 years, Voyager 1 will drift within 1.6 light years (9.3 trillion miles) of AC+79 3888, a star in the constellation of Camelopardalis. In some 296,000 years, Voyager 2 will pass 4.3 light years (25 trillion miles) from Sirius, the brightest star in the sky . The Voyagers are destined—perhaps eternally—to wander the Milky Way. For current distances, check: Mission Weekly Reports


It is appropriate to consider the VIM as three distinct phases: the termination shock, heliosheath exploration, and interstellar exploration phases. The two Voyager spacecraft began the VIM operating, and are still operating, in an environment controlled by the Sun's magnetic field with the plasma particles being dominated by those contained in the expanding supersonic solar wind. This is the characteristic environment of the termination shock phase. At some distance from the Sun, the supersonic solar wind will be held back from further expansion by the interstellar wind. The first feature to be encountered by a spacecraft as a result of this interstellar wind/solar wind interaction will be the termination shock where the solar wind slows from supersonic to subsonic speed and large changes in plasma flow direction and magnetic field orientation occur.


Interstellar MIssion


Passage through the termination shock ends the termination shock phase and begins the heliosheath exploration phase. While the exact location of the termination shock is not known, it is very possible that Voyager 1 will complete the termination shock phase of the mission between the years 2001 and 2003 when the spacecraft will be between 80 and 90 AU from the Sun. Most of the current estimates place the termination shock at around 85 ± 5 AU. After passage through the termination shock, the spacecraft will be operating in the heliosheath environment which is still dominated by the Sun's magnetic field and particles contained in the solar wind. The heliosheath exploration phase ends with passage through the heliopause which is the outer extent of the Sun's magnetic field and solar wind. The thickness of the heliosheath is uncertain and could be tens of AU thick taking several years to traverse. Passage through the heliopause begins the interstellar exploration phase with the spacecraft operating in an interstellar wind dominated environment. This interstellar exploration is the ultimate goal of the Voyager Interstellar Mission.