Launched in 1991, Japan's Yohkoh (Sunbeam) satellite observed solar flares at X-ray wavelengths. Mission data allowed scientists to identify several different types of flares, and demonstrated that the corona away from regions of peak activity was much more dynamic and active than had previously been supposed. Yohkoh observed an entire solar cycle but went into standby mode when an annular eclipse in 2001 caused it to lose its lock on the Sun. It was destroyed by atmospheric re-entry in 2005.[166]
One of the most important solar missions to date has been the Solar and Heliospheric Observatory, jointly built by the European Space Agency and NASA and launched on 2 December 1995.[71] Originally intended to serve a two-year mission, a mission extension through 2012 was approved in October 2009.[167] It has proven so useful that a follow-on mission, the Solar Dynamics Observatory (SDO), was launched in February 2010.[168] Situated at the Lagrangian point between the Earth and the Sun (at which the gravitational pull from both is equal), SOHO has provided a constant view of the Sun at many wavelengths since its launch.[71] Besides its direct solar observation, SOHO has enabled the discovery of a large number of comets, mostly tiny sungrazing comets which incinerate as they pass the Sun.[169]
A solar prominence erupts in August 2012, as captured by SDO
All these satellites have observed the Sun from the plane of the ecliptic, and so have only observed its equatorial regions in detail. The Ulysses probe was launched in 1990 to study the Sun's polar regions. It first travelled to Jupiter, to "slingshot" past the planet into an orbit which would take it far above the plane of the ecliptic. Serendipitously, it was well-placed to observe the collision of Comet Shoemaker-Levy 9 with Jupiter in 1994. Once Ulysses was in its scheduled orbit, it began observing the solar wind and magnetic field strength at high solar latitudes, finding that the solar wind from high latitudes was moving at about 750 km/s which was slower than expected, and that there were large magnetic waves emerging from high latitudes which scattered galactic cosmic rays.[170]
Elemental abundances in the photosphere are well known from spectroscopic studies, but the composition of the interior of the Sun is more poorly understood. A solar wind sample return mission, Genesis, was designed to allow astronomers to directly measure the composition of solar material. Genesis returned to Earth in 2004 but was damaged by a crash landing after its parachute failed to deploy on re-entry into Earth's atmosphere. Despite severe damage, some usable samples have been recovered from the spacecraft's sample return module and are undergoing analysis.[171]
The Solar Terrestrial Relations Observatory (STEREO) mission was launched in October 2006. Two identical spacecraft were launched into orbits that cause them to (respectively) pull further ahead of and fall gradually behind the Earth. This enables stereoscopic imaging of the Sun and solar phenomena, such as coronal mass ejections.[172][173]
The Indian Space Research Organisation has scheduled the launch of a 100 kg satellite named Aditya for 2015–16. Its main instrument will be a coronagraph for studying the dynamics of the Solar corona
One of the most important solar missions to date has been the Solar and Heliospheric Observatory, jointly built by the European Space Agency and NASA and launched on 2 December 1995.[71] Originally intended to serve a two-year mission, a mission extension through 2012 was approved in October 2009.[167] It has proven so useful that a follow-on mission, the Solar Dynamics Observatory (SDO), was launched in February 2010.[168] Situated at the Lagrangian point between the Earth and the Sun (at which the gravitational pull from both is equal), SOHO has provided a constant view of the Sun at many wavelengths since its launch.[71] Besides its direct solar observation, SOHO has enabled the discovery of a large number of comets, mostly tiny sungrazing comets which incinerate as they pass the Sun.[169]
A solar prominence erupts in August 2012, as captured by SDO
All these satellites have observed the Sun from the plane of the ecliptic, and so have only observed its equatorial regions in detail. The Ulysses probe was launched in 1990 to study the Sun's polar regions. It first travelled to Jupiter, to "slingshot" past the planet into an orbit which would take it far above the plane of the ecliptic. Serendipitously, it was well-placed to observe the collision of Comet Shoemaker-Levy 9 with Jupiter in 1994. Once Ulysses was in its scheduled orbit, it began observing the solar wind and magnetic field strength at high solar latitudes, finding that the solar wind from high latitudes was moving at about 750 km/s which was slower than expected, and that there were large magnetic waves emerging from high latitudes which scattered galactic cosmic rays.[170]
Elemental abundances in the photosphere are well known from spectroscopic studies, but the composition of the interior of the Sun is more poorly understood. A solar wind sample return mission, Genesis, was designed to allow astronomers to directly measure the composition of solar material. Genesis returned to Earth in 2004 but was damaged by a crash landing after its parachute failed to deploy on re-entry into Earth's atmosphere. Despite severe damage, some usable samples have been recovered from the spacecraft's sample return module and are undergoing analysis.[171]
The Solar Terrestrial Relations Observatory (STEREO) mission was launched in October 2006. Two identical spacecraft were launched into orbits that cause them to (respectively) pull further ahead of and fall gradually behind the Earth. This enables stereoscopic imaging of the Sun and solar phenomena, such as coronal mass ejections.[172][173]
The Indian Space Research Organisation has scheduled the launch of a 100 kg satellite named Aditya for 2015–16. Its main instrument will be a coronagraph for studying the dynamics of the Solar corona
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