Several years ago, NASA selected the Applied Physics Laboratory (APL) of Johns Hopkins University to develop a most ambitious mission to the Sun. This was dubbed the Solar Probe Plus mission, intended to study the streams of charged particles hurled into space from a never before visited vantage point, the Sun's corona.

The solar corona is one of the most puzzling features of the Sun. It is a region around the Sun, extending more than one million kilometers from its surface, where the temperature can reach two million degrees.

This is where the solar wind originates. The corona is a plasma that emits X-ray radiation at temperatures greater than one million degrees. In fact, the corona can be observed only during a solar eclipse or with special instruments such that the main radiation from the Sun's surface is blocked.

No one can explain how this corona can exist.

While the temperature in the core of the Sun reaches 15 million degrees, it drops to a mere 5,000 degrees at the surface. Above the surface, the temperature should lower farther as you move away from the Sun. However, the temperature of the corona is measured at more than a million degrees.

This phenomenon requires a permanent heating mechanism, or the plasma would quickly cool down. No one, as yet, has been able to account for the large rate of heating necessary to heat the corona to these temperatures. Thus, we have an ongoing heliophysical mystery.

Inquiring minds at NASA wanted to know what is driving this phenomenon. Thus, the Solar Probe Plus mission began with preliminary studies in 2008. A Mission Concept Review was completed in 2009, and NASA approved the start of design and development activities in late 2009. Solar Probe Plus is in the late design stages with a planned launch date of mid-2018.

The Solar Probe Plus spacecraft will be unique. It must be compact, solar-powered and not have a mass greater than about 610 kg. In order to survive a solar passage within 5.9 million km (the closest solar passage ever), the design includes an 2.4-meter-diameter, 11.4-cm-thick, special composite solar shield located atop the spacecraft body.

The solar arrays must retract and extend as the spacecraft swings toward or away from the Sun during several loops around the inner solar system in order to insure the panels stay at proper temperatures and power levels. Note that at its closest solar passages the spacecraft must survive heat intensities of more than 500 times that experienced while orbiting Earth.

Solar Probe Plus will use seven Venus flybys over nearly seven years to gradually lower its perihelion, coming well within the orbit of Mercury and about eight times closer than any spacecraft has come before. The Venus flybys are necessary because this mission represents the highest energy-per-kg space mission ever attempted.

Not only does the launch vehicle have to reach earth orbit, but it must add enough energy to the spacecraft to escape Earth's gravity and then reduce its heliocentric speed to well below 30 km/sec. Success will require the use of our largest launch vehicle and a very compact spacecraft.

The selected mission launch vehicle had been the Atlas V. However, just recently a decision was made to change this to either a Delta 4 or a Falcon Heavy.

This change was based, in part, on the fact that the Atlas V would have required a new, high-performance custom upper stage. Such a stage would have introduced significant risk for the project.

The decision makers feel that the other launch vehicle options would reduce the risk. The Delta 4 is a proven vehicle, but the Falcon Heavy has not yet flown. Nevertheless, NASA has left the door open for SpaceX to compete for the Solar Probe Plus launch contract.

Since Falcon Heavy has three launches on its manifest between now and 2017, it may be approved for Solar Probe in time for the 2018 launch.