The spacecraft are near-identical
It is hoped the first 3D images of the Sun will be provided by twin Nasa spacecraft launched on Thursday.
The Stereo mission (short for Solar TErrestrial RElations Observatory) will help scientists understand powerful solar eruptions, the "space weather" they create and the potentially serious effects for life on Earth.
THE NEED TO SEE THE SUN IN 3D
Stereo will allow more accurate study of the Sun's violent outbursts - known as coronal mass ejections (CMEs).
Violent eruptions occur on the Sun
These typically throw into space a billion tonnes of material from the Sun's atmosphere, at speeds of about 400km/s (about one million miles per hour).
They can damage satellites and could expose astronauts to dangerous levels of radiation. In the Earth's atmosphere, they can trigger serious magnetic storms and disrupt power supplies and communications.
The $520m (£280m) Stereo mission will provide new understanding on what triggers the eruptions, and allow scientists to follow their progression through space. At present, missions like Soho, which has been in operation for 10 years, are unable to show whether a CME is heading for Earth or away from it.
Stereo should give a couple of days' notice that a CME is heading towards Earth.
HOW STEREO WORKS
Stereo will provide 3D images
By positioning one of the Stereo spacecraft ahead of the Earth's orbit around the Sun and the other just behind, it will be possible to get 3D images of the Sun.
Scientists say this will work in a similar way to the slight offset between people's eyes - which allows depth perception.
The UK is providing the Heliospheric Imager, which will follow the progression of the CMEs through space. It has also produced the "eyes" of the mission - the scientific cameras that are attached to all imaging systems.
The UK contribution amounts to £1.9m, with the major scientific input coming from the Rutherford Appleton Laboratory and the University of Birmingham.
Each Stereo spacecraft is about the size of a fridge or, with its solar arrays extended, the length of a bus.
Each observatory is equipped with 16 instruments. They are nearly identical.
Minor differences include adjustments allowing one unit to fly upside down, to point its antenna at Earth. The "B" observatory's main structure is also a little thicker so that it can support the weight of the "A" observatory during launch.
Timing systems will ensure images are taken on each spacecraft within one second of each other, allowing them to be blended into 3D pictures.
Secchi: The Sun-Earth Connection Coronal and Heliospheric Investigation is a suite of instruments that includes the UK Heliospheric Imager. Secchi will picture and study coronal mass ejections from their origin on the Sun to their impact at Earth
Impact:In-situ Measurements of PArticles and CME Transients is another instrument suite which will measure energetic ions and electrons accelerated in coronal mass ejections and solar flares
Plastic: The PLAsma and SupraThermal Ion Composition instrument will also study CME particles - protons, alpha particles and heavy ions
S/Waves: The Stereo/Waves instrument monitors radio disturbances travelling from Sun to Earth
LAUNCH AND ORBIT
Stacked one on top of the other, the spacecraft will be launched from Cape Canaveral, Florida, on a Delta II rocket.
Launch is on a Delta II rocket
There is a considerable cost-saving in using a single launch rocket but it means Nasa has to use lunar swingbys to put the spacecraft in their correct orbits.
Although the Moon's gravity has been used to position spacecraft before, this will be the first time a double slingshot manoeuvre has been employed.
One Stereo observatory will run slightly ahead of the Earth in its orbit, taking some 345 days to go around the Sun. The other spacecraft will lag the Earth slightly, taking 385 days to go around the Sun. The satellites will also drift further apart each year, widening the angle between themselves and the Earth as viewed from the Sun.
The spacecraft will be in position about three months after launch. The mission is expected to last at least two years, with the observatories drifting slightly further from the Earth each year.