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Article 2006

From The Guardian, July 18, 2006

Stereo satellites will let scientists tune in to the Sun's mood music

By Alok Jha, Science Correspondent
July 18, 2006

Scientists want to create the first three-dimensional model of the Sun in an effort to protect the Earth from its most violent eruptions, which can affect everything from global positioning systems to mobile phone networks. The Stereo mission, due to be launched next month, will map the Sun's mood swings and the dangers they pose to the solar system.

The $500m (£275m) space mission is funded by Nasa and the UK's Particle Physics and Astronomy Research Council. It will consist of a pair of satellites - each more than half a tonne in mass and the size of a large deep freezer - which will monitor activity on the Sun's surface for the first time.

"While the Sun may appear to be one of the most familiar objects in the sky, it is constantly changing," said John Zarnecki of the Open University.

"We need to understand its mood swings more fully in order to predict and, more importantly, protect [the Earth from] the potentially devastating effects - particularly of the coronal mass ejections."

Coronal mass ejections (CMEs) are events in which more than 1,000m tonnes of charged particles are hurled into space at an average speed of a million miles an hour. If the particles reach Earth, they can damage orbiting satellites and disrupt electronics and power grids at the surface.

"In 1859 a giant solar storm erupted, hurling hot plasma - a coronal mass ejection - towards the Earth. Fortunately that happened before the space age, so the effects were fairly tame," Professor Zarnecki said. "Had that storm happened today, communications and navigation satellites would have taken the full impacts. Forecasts suggest that a major outburst such as that would have had a similar economic impact to a category five hurricane - that's Katrina level."

Chris Davis of the Rutherford Appleton Laboratory in Oxfordshire, part of the UK team working on Stereo, said as people became more dependent on spacecraft technologies such as GPS car navigation systems and satellite TV, the need to understand and predict CMEs became more urgent. "We need to understand the space weather so we can build spacecraft that are robust enough to survive in this environment," he said. "[With Stereo], we'll be much more able to predict the speed and direction of these coronal mass ejections and have a much earlier warning of when such an event is heading towards our planet."

In 1989 a CME caused magnetic disruptions which played havoc with the power network in Quebec. A series of transformers tripped and 6 million people were left without power.

Despite their potentially devastating effects, little is known about how CMEs are formed. "Whilst our Sun may seem a calm, familiar object in the sky, in reality it is rather more manic," said Richard Harrison of the Rutherford Appleton Laboratory. "It generates constantly changing knots of magnetic fields that twist and churn and, occasionally, snap like an over-stretched rubber band."

Eventually the energy stored in these tightly wound magnetic fields explodes out of the Sun, taking large amounts of gas with it. A typical CME contains 100 times the energy of the world's nuclear arsenal.

"At the moment, we cannot recognise the telltale signals that precede an outburst, but we expect Stereo will change that," Professor Harrison said.

Via satellites such as the Solar and Heliospheric Observatory (Soho), launched in 1995 to study the Sun, scientists receive an hour's notice of a stream of plasma caused by a CME heading for Earth. Stereo will increase that warning to about two and a half days, which could give enough time to prepare.

"With space stuff, you would power down high voltages temporarily," Dr Davis said. "That costs money but it's cheaper than losing a spacecraft. For power networks, what you need to understand are the extreme events so you can engineer the networks to be able to withstand those events."

Once launched into space, Stereo's probes will eventually separate and move apart at a few hundred thousand kilometres a year. "One spacecraft will slowly move ahead of the Earth, the other lag behind - the resulting offset will allow the two spacecraft to have depth perception and give them stereo vision such as humans have," said Chris Eyles of the University of Birmingham.

Scientists hope the first measurements from the mission will emerge by November.

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