Mission Overview

Solar Probe Plus will come closer to the Sun than any spacecraft has ever flown - and what it finds could revolutionize what we know about our star and the solar wind that influences everything in our solar system.

Solar Probe observing sun
Artist's concept of the Solar Probe Plus spacecraft as it approaches the Sun.

NASA has tapped the Johns Hopkins University Applied Physics Laboratory (APL) to develop the ambitious Solar Probe Plus mission, which will study the streams of charged particles the Sun hurls into space from an unprecedented vantage point: inside the Sun's corona - its outer atmosphere - where the processes that heat the corona and produce solar wind occur. At closest approach, Solar Probe would zip past the Sun at 125 miles per second, protected by a carbon-composite heat shield that must withstand up to 2,500 degrees Fahrenheit and survive blasts of radiation and energized dust at levels not experienced by any previous spacecraft.

Experts in the United States and abroad have grappled with this idea for 50 years, usually running into seemingly insurmountable technological and budgetary limitations. But in February 2008 an APL-led team completed a Solar Probe engineering and mission design study at NASA's request, detailing just how the robotic mission could be accomplished. The study team used an APL-led 2005 study as its baseline, then significantly altered the concept to meet cost and technical conditions provided by NASA.

NASA named the new concept "Solar Probe Plus" because of the gains in science over its predecessors.

Solar Probe Plus began pre-phase A work in July 2008 and completed a successful Mission Concept Review in October 2009. NASA approved the start of Phase A in December 2009; the team completed a System Requirements and Mission Definition Review and entered Phase B in January 2012. Solar Probe Plus is currently in Phase B, with a Preliminary Design Review planned for January 2014.

Standing the Heat

APL is designing and will build the spacecraft, on a schedule to launch no later than 2018. The compact, solar-powered probe would weigh about 1,350 pounds; preliminary designs include an 8-foot-diameter, 4.5-inch-thick, carbon-carbon carbon foam solar shield atop the spacecraft body. The solar arrays will retract and extend as the spacecraft swings toward or away from the Sun during several loops around the inner solar system, making sure the panels stay at proper temperatures and power levels. At its closest passes the spacecraft must survive solar intensity more than 500 times what spacecraft experience while orbiting Earth.

Solar Probe Plus will use seven Venus flybys over nearly seven years to gradually shrink its orbit around the Sun, coming as close as 3.7 million miles (5.9 million kilometers) to the Sun, well within the orbit of Mercury and about eight times closer than any spacecraft has come before.
The Helios 2 spacecraft currently holds the honor of coming closest to the Sun, having passed about 27 million miles (approximately 44 million kilometers) of the Sun in April 1976.

Dancing Through Fire

Solar Probe Plus will employ a combination of in-place and remote measurements to achieve the mission's primary scientific goals: determine the structure and dynamics of the magnetic fields at the sources of solar wind; trace the flow of energy that heats the corona and accelerates the solar wind; determine what mechanisms accelerate and transport energetic particles; and explore dusty plasma near the Sun and its influence on solar wind and energetic particle formation. Details are spelled out in the 2008 NASA Solar Probe Science and Technology Definition Team study; the agency selected the science payload for Solar Probe Plus in September 2010.

Solar Probe Plus is a true mission of exploration; for example, the spacecraft will go close enough to the Sun to watch the solar wind speed up from subsonic to supersonic, and it will fly though the birthplace of the highest-energy solar particles. Still, as with any great mission of discovery, Solar Probe Plus is likely to generate more questions than it answers.

The Sun-Earth Experience

APL's experience in developing spacecraft to study the Sun-Earth relationship - or to work near the Sun - includes ACE, which has sampled energetic particles between Earth and the Sun for more than a decade; TIMED, currently examining solar effects on Earth's upper atmosphere; the twin STEREO probes, which snapped the first 3-D images of explosive solar events called coronal mass ejections; and the Van Allen Probes, which are examining the regions of energetic particles trapped by Earth's magnetic field.

Solar Probe Plus will be fortified with heat-resistant technologies developed for APL's MESSENGER spacecraft, which completed three flybys of Mercury and began orbiting that planet in 2011. Solar Probe's solar shield concept was partially influenced by designs of MESSENGER's sunshade.

Solar Probe is part of NASA's Living with a Star Program, created to gather more information about the Sun and its effects on planetary systems and human activities. NASA's Goddard Space Flight Center, Greenbelt, Md., manages the program for the Science Mission Directorate at NASA Headquarters, Washington.