By Jonathan Amos
Science reporter, BBC News
Nasa launched two near-identical spacecraft in late 2006
Scientists say they have demonstrated the principle of a very effective early warning system that would give notice of huge eruptions on the Sun.
Two Nasa spacecraft have been used to track massive clouds of energetic particles thrown off our star.
These eruptions, when they hit the Earth, can damage satellites, disrupt communications and harm astronauts.
The Stereo probes have shown how two widely separated vantage points can be used to forecast an impact's arrival.
Although Stereo is only a scientific research mission, the project team says its work illustrates clearly how an operational "space weather" early-warning system could work.
"For the event we describe [in an upcoming scientific paper], we would have been able to give 24 hours' advance notice," said Dr Chris Davis, of the UK's Rutherford Appleton Laboratory, a key partner on the mission.
"That's ample time to power down a satellite until the worst of the storm has passed; and if you're an astronaut on the space station, you would have had plenty of time to get into an area that has much better shielding."
The Stereo (Solar TErrestrial RElations Observatory) spacecraft were launched on 25 October 2006.
The Stereo data allows scientists to model the shape of CMEs
One probe was put just ahead of the Earth as it moves around the Sun; the other was stationed just behind. They have been allowed to gradually drift apart, and that separation has enabled the orbiters to construct 3D images of the Sun-Earth system.
Scientists are using these pictures to model the structure and movement of Coronal Mass Ejections (CMEs).
These are colossal "bubbles" of high-temperature (hundreds of thousands of degrees) plasma that occasionally break away from the Sun.
The plasma is formed mostly of electrons and ions of hydrogen and helium. A CME will contain typically a billion tonnes of matter and billow into space at about 400km/s.
When directed at Earth, the CME particles, and the magnetic fields they carry, can have undesirable effects, such as introducing glitches into satellite electronics and damaging the DNA of astronauts.
Now, after two-and-a-half-years of research on CMEs, the Stereo scientific team says it has a much better understanding of the speed, trajectory, and three-dimensional shape of these powerful solar explosions - a shape one scientist likened to a French breakfast croissant in the way the bubble of particles spread out from the Sun.
"You can see how the 'croissant' grows in time," explained Angelos Vourlidas, from the Naval Research Laboratory in Washington.
"We've [modelled] 40 CMEs and we can do it for pretty much any CME the Sun is going to throw at us. We're very positive this is going to lead to great space predictions."
Not only can the Stereo pair see which CMEs are likely to hit or miss the Earth, they will also give a far more precise timing for any impact.
"If you use the hurricane analogy, if you were able to predict landfall to only plus or minus 12 hours that wouldn't be considered very good," said Michael Kaiser, the Stereo project scientist. "With Stereo, I think we can narrow that down to just two, three or four hours, which is a great step forward in our business."
The Stereo probes are about to pass through what are termed Libration points, or Lagrangian points, in space. These are gravitational "sweetspots" in the Sun-Earth system where spacecraft can maintain their positions with relatively cheap (in fuel terms) manoeuvres.
It is at these Lagrangian points that a fully operational forecasting system based on the Stereo model might be stationed.
"I think we've shown this could be a valuable addition to the [forecasting] 'tool box'," Dr Davis told BBC News.
"There is other information you'd need to know like the orientation of the magnetic field within the cloud because that has a big effect on how it is impacting on the Earth. We can't tell that from our cameras so there's still a need for other measurements; but there's a way to put all this extra information into the models to make them much more accurate."
The UK has major interest in the mission, having built the camera detectors on all the imaging instruments. It also produced a Heliospheric Imager (HI) for each platform. This instrument is used to follow the progress through space of a CME cloud by tracing its reflected light.
The spacecraft were launched on a trajectory that went past the Moon
The lunar swingby positioned the spacecraft in widely spaced orbits
One now leads the Earth in its orbit; the other lags behind
Over the course of their mission, the twins will continue to separate
Their different views are combined to make 3D movies of CMEs