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NASA To Make Announcement About First Mission To Touch Sun (nasa.gov)
NASA published the following media advisory moments ago: NASA will make an announcement about the agency's first mission to fly directly into our sun's atmosphere during an event at 11 a.m. EDT Wednesday, May 31, from the University of Chicago's William Eckhardt Research Center Auditorium. The event will air live on NASA Television and the agency's website. The mission, Solar Probe Plus, is scheduled to launch in the summer of 2018. Placed in orbit within four million miles of the sun's surface, and facing heat and radiation unlike any spacecraft in history, the spacecraft will explore the sun's outer atmosphere and make critical observations that will answer decades-old questions about the physics of how stars work. The resulting data will improve forecasts of major space weather events that impact life on Earth, as well as satellites and astronauts in space.
Project Icarus. This needs to be called Project Icarus.
There are a number of ways to define the depth of the Sun's atmosphere.
1. The largest is the Heliosphere which is very roughly a tear-drop shaped region varying between 121 AU (astronomical units) and a lot larger (the heliotail is of unknown length). The boundary between the heliosphere and the local interstellar medium is called the heliopause and is defined as the region where the "pressure" of the interstellar medium is balanced by the "pressure" of the solar wind. Fewer and fewer low speed particles are flowing away from the Sun past the heliopause, and fewer low speed cosmic rays (and cosmic dust) are able to penetrate past it. Voyager spacecraft have passed the heliopause and are in interstellar space.
2. The next logical definition is the "termination shock" where the solar wind's speed transitions from supersonic to subsonic (due to 'resistance' as well as the particles climbing out of the Sun's gravitational well). This is out at about 80 to 90 AU (twice to thrice the distance to Pluto).
3. Next logical "surface" is the "end" of the Corona, which is the region which looks like a halo around the Sun when photographed during a solar eclipse. It surrounds the Sun (but its distance varies with time and electric/magnetic field strength) for millions of kilometers (the Sun's diameter is about 1.4 Gm (giga meter = million km)). It's the region of space where the Solar Wind is so hot that it's visibly glowing.
4. The photosphere is the Sun's "usual" or generally accepted surface. It's a relatively thin region - optically a bit more transparent that Earth's sea level (dry, clean) atmosphere (but of course a lot hotter 4000 - 6000C and less dense (0.0002 (vs 1.25 kg/m^3 at 1 atm))). It's about 400-500 km thick and is defined as the region below which the Sun has become opaque to visible light. The chromosphere directly above it is much hotter, a inconvenient fact that we've yet to explain. This region is where the surfaces of sunspots can be seen, and below it is the convection zone where the Sun's inner material floats up and radiates heat. (It takes an estimated 170,000 years for a photon released in the core in a fusion reaction to bump it's way to the 'surface' of the Sun, (assuming you're ok with the idea that one gamma-ray photon scatteres into thousands of lower frequency photons but still is in some way the same thing... The heat from that same fusion reaction takes a lot longer, millions of years, to make its way to the surface...but none of this has anything to do with locating the "surface"...)
The Earth is immersed inside the Solar Wind, as far as I'm concerned we're inside the Sun, although our magnetic field protects us from a lot of radiation. "Touching" the Sun is a bit of hyperbole.