Washington, April 9 (IANS): NASA's ambitious Solar Probe Plus mission came a step closer to reality when it successfully completed its Critical Design Review (CDR) recently.

An independent NASA review board met at the Johns Hopkins University Applied Physics Laboratory (APL) in Maryland to review the mission plan.

APL has designed and will build and operate the spacecraft -- that will fly closer to the sun than any other spacecraft has done before -- for NASA.

The main goals for the Solar Probe Plus mission are to trace the flow of energy and understand the heating of the solar corona and to explore the physical mechanisms that accelerate the solar wind and energetic particles.

The CDR certified that the Solar Probe Plus mission design is at an advanced stage and that fabrication, assembly, integration and testing of the many elements of the mission may proceed.

Solar Probe Plus is scheduled to launch aboard a United Launch Alliance Delta 4-Heavy rocket with an upper stage from Cape Canaveral Air Force Station, Florida, in 2018.

Over 24 orbits, the mission will use seven flybys of Venus to reduce its distance from the sun. The closest flyby will be just about 6.1 million kilometres from the surface of the star.

Scientists have long wanted to send a probe through the sun's outer atmosphere, or corona, to better understand the solar wind and the material it carries into our solar system.

Solar Probe Plus will carry four instrument suites to study magnetic fields, plasma, and energetic particles, and will image the solar wind.

The spacecraft and instruments will be protected from the sun's heat by a 4.5-inch-thick carbon-composite shield.

During the closest passes around the sun, temperatures outside the spacecraft will reach nearly 1370 degrees Celsius.