WASHINGTON, Sept. 3 (UPI) -- NASA has started development of Solar Probe Plus, a mission to study the sun more closely than ever before, with a target launch date of 2018, the agency says.

The spacecraft will plunge directly into the sun's atmosphere at approximately 4 million miles from the sun's surface, into a region that no other probe has ever encountered, an agency release said.

The mission will carry five separate science investigations hoping to discover more about our sun than any previous mission, NASA said.

"The experiments selected for Solar Probe Plus are specifically designed to solve two key questions of solar physics -- why is the sun's outer atmosphere so much hotter than the sun's visible surface and what propels the solar wind that affects Earth and our solar system? " Dick Fisher, director of NASA's Heliophysics Division in Washington, said.

"We've been struggling with these questions for decades and this mission should finally provide those answers."

A revolutionary carbon-composite heat shield will withstand temperatures exceeding 2,550 degrees Fahrenheit and blasts of intense radiation as the spacecraft approaches the sun.

NASA has earmarked $180 million for preliminary analysis, design, development and tests of the mission's science experiments.