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A history of exploring the Sun

Paul Sutherland, Feature writer
Apr 29, 2012, 23:00 UTC

Sen—The Sun is vital to our existence, providing light and heat that allows life to thrive. But it is also the source of potentially harmful space weather that could knock out satellites and power supplies and pose a major radiation hazard to astronauts.

Growing awareness of the threat that the Sun poses has led to a number of space missions to study this, our nearest star. Last week a contract was awarded to Astrium in the UK to build one called Solar Orbiter that will fly in 2017. Today Sen looks at missions past and present, plus those yet to come.

Two probes, Helios 1 and 2, were launched in November 1974 and January 1976 to make the first studies of the Sun. Their looping orbits took them to within a third of the distance of the Earth from the Sun, sending back information on solar plasma, the solar wind, cosmic rays, and cosmic dust, plus data about its magnetic-field.

Ulysses was launched from the space shuttle Discovery in October, 1990. It flew via Jupiter to be sent on a path that would carry it to high solar latitudes from where it could look down on the Sun. Ulysses operated for 12 years, returning data that helped scientists understand the nature of the Sun's environment, called the heliosphere, and the forces active in its outer atmosphere, called the corona.

In December 1995, a joint NASA/ESA probe called SOHO (the Solar and Heliospheric Observatory) was launched from Cape Canaveral, Florida, to study the Sun from its core to the outer corona and solar wind. With an original two-year mission, this highly successful satellite is still going today, orbiting the Sun in step with the Earth. The latest views from its suite of different cameras are constantly updated to view on the web. An unexpected sideline has been the discovery of Sun-grazing comets, mainly by amateur astronomers scouring these images, making SOHO the most prolific discoverer of comets ever.

Cluster is a constellation of four spacecraft, launched in two pairs in 2000 by Russian Soyuz rockets to fly in formation around Earth. Together they sent back detailed 3D data about how the solar wind affects our planet.

In February 2002, NASA launched one of its Small Explorer Missions, called RHESSI (The Reuven Ramaty High Energy Solar Spectroscopic Imager) which focused on studying explosive solar flares.

One of the major space observatories currently observing the Sun is Hinode, a joint project between Japan, the US and the UK, which was launched in September 2006 from Uchinoura Space Center, Japan. The spacecraft carries three science instruments with which to watch our home star: the Solar Optical Telescope, X-ray Telescope and Extreme Ultraviolet Imaging Spectrometer.

They work together to study how magnetic energy is generated and travels from the visible surface, or photosphere, of the Sun to the corona plus how that energy is released into the Sun's outer atmosphere.

Our views of the Sun took a leap forward following the launch from Cape Canaveral in October 2006 of NASA's STEREO (Solar TErrestrial RElations Observatory) mission - two nearly identical observatories which are giving us, as their name suggests, a three-dimensional view of the Sun. This was achieved by placing one of the telescopes ahead of Earth in its orbit with the other trailing behind our planet. STEREO has helped scientists understand much more about the flow of energy and matter from the Sun to Earth by revealing the 3D structure of coronal mass ejections, those violent eruptions of matter from the sun that can disrupt satellites and power grids.

Launched in November 2009, ESA's low-cost Proba-2 was launched into orbit around the Earth from the Plesetsk Cosmodrome, Russia, carrying two instruments to observe the Sun - a Large Yield Radiometer (LYRA) and an extreme-ultraviolet telescope (SWAP) - plus two more Czech experiments to monitor space weather.

A French satellite called PICARD launched from Russia in June 2010 to measure solar irradiance, the diameter and solar shape, and to probe the Sun's interior using seismological techniques.

The last major space telescope to go into action was NASA's Solar Dynamics Observatory which launched in February 2010 from Cape Canaveral. SDO is studying how solar activity is created and produces space weather. In the process it is taking the most detailed and spectacular pictures of the Sun ever recorded with its high-resolution camera. Other instruments measure the Sun’s interior, its magnetic field, and the hot plasma of the solar corona.

ESA's Solar Orbiter is due to launch in 2017 on a seven-year mission to study the Sun and how it drives space weather. The spacecraft's orbit will take it closer to the Sun than the planet Mercury, at a distance of 42 million kilometres. Leading solar physicist Dr Lucie Green said: "It will fly into the Sun's heliosphere, or outer atmosphere, to find out how it formed."

Solar Orbiter will coordinate with a NASA mission to send six Solar Sentinels to watch the star. Four identical probes called Inner Heliospheric Sentinels will sit inside the orbits of Venus and Mercury to taste energetic particles streaming in the solar wind. They will be aided by a Near-Earth Sentinel, a single probe orbiting Earth which will observe the corona, and a Farside Sentinel that will watch the other side of the Sun.

Most daring mission is one planned by NASA for launch in 2018, called Solar Probe Plus. This spacecraft will fly closer to the Sun than any has before in a bid to find out how its corona is heated and how the solar wind is accelerated.