A NASA spacecraft set to launch in 2015 will come eight times closer to the Sun than any previous probe, operating within the star's scorching outer atmosphere, or corona. The $750 million Solar Probe will study the birthplace of the solar wind.

During its expected seven-year lifetime, Solar Probe will make seven gravity slingshots around Venus, each time getting closer to the Sun. At its closest approach, it will orbit the Sun from within the outer part of the corona, at a distance of between 8 and 10 solar radii from the centre of the Sun.

That is much closer than the previous record holder, the Helios spacecraft, which came within 67 solar radii of the star in the 1970s.

"Solar Probe is going to be our first visit to our mother star," said Manolis Georgoulis, the mission's deputy project scientist at Johns Hopkins University's Applied Physics Laboratory in Laurel, Maryland, US, which will design and build the probe. "The quality of the data that we hope to gather at such a close distance to the Sun is unprecedented."

NASA is putting together a team to decide what sensors and detectors should fly on Space Probe, and the agency is expected to put out a call for payload ideas later this year.

But scientists hope the probe will help solve two enduring solar mysteries: Why is the Sun's outer atmosphere, or corona, so much hotter than its visible surface, which lies beneath it? And what accelerates the solar wind, a stream of charged particles from the Sun, to supersonic speeds?

Heat shield

The idea of studying the Sun at close range was first proposed by the US National Academy of Science in 1958. But scientists have only recently been able to design heat shields for such a spacecraft within NASA's tight budgetary guidelines.

The mini-bus sized Solar Probe will be protected from the Sun's fierce radiation by a disc-shaped, carbon-composite heat shield that will be 2.7 metres in diameter and about 15 centimetres thick.

The heat shield technology is based on that used in Messenger, a NASA spacecraft that completed its first flyby of Mercury in January and that was also designed by engineers at APL.

The side of the shield facing the Sun will heat up to 1400 °Celsius (2600 °F), while the instrument-carrying payload behind the shield will remain at room temperature, said Solar Probe project manager Andrew Dantzler.

"It's not your run-of-the-mill spacecraft. The whole spacecraft is optimally designed to dissipate heat," Dantzler told New Scientist.

And because the spacecraft will be embedded inside the corona, instruments don't ever have to point directly at the Sun. "It's sort of like if you're sampling the atmosphere here on Earth – you don't need to point at a particular direction," Dantzler said. "All of Solar Probe's instruments are hidden behind the heat shield."

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