A Mission to Touch the Sun in Jeopardy
We are living with an active star we hardly understand and have barely explored. More than 1.3 million Earths could fit in the same volume as our sun, the source of our world’s energy — with fossil fuels, wind and hydro power simply being converted into forms of solar energy. A single solar flare lasting only a few minutes generates a hundred-thousand times more energy than all of the world’s power plants combined over the course of a year.
With a temperature of 1 million degrees Celsius, the sun’s corona — its outer atmosphere — is the place where solar flares arise, and it is the continuing expansion of the outer corona that creates the solar wind and the heliosphere of our solar system. To the best of our knowledge, the million-degree coronal temperatures are produced primarily by the dissipation of plasma waves propagating out from a raucous bed of magnetic outbursts called microflaring, which are produced by localized, small but intense, magnetic fields lying underneath the corona on the active surface of the sun. It appears that the highest solar wind speeds — up to 700 kilometers per second and higher — are a result of extended heating and rapid divergence of the wind’s flow as it escapes the sun. The whole structure is sometimes punctured by violent magnetic coronal mass ejections and by intense flares with outbursts of highly energetic and dangerous electrons and nuclei. This is the primary source for the space weather experienced here at Earth.
To understand these transient dynamics, solar physicists need to measure detailed conditions directly in the corona to supplement the average temperatures that we obtain through remote observations. We must take the temperature of the sun where the solar wind arises and follow the outward extension of the magnetic and energetic particle outbursts that we observed far out in space.
For decades, solar physicists dreamed of a satellite mission to touch the sun. The original $1 billion to $2 billion nuclear-powered Solar Probe mission concept with one or two dives into our sun’s corona pushed the limits of technology, approaching to within four solar radii of the center of the sun where solar radiation is 2,500 times more intense than at Earth (200 solar radii). After careful review of the science requirements and an innovative approach to the mission design using seven gravity assists from the planet Venus, the Solar Probe team proposed the Solar Probe Plus mission to NASA headquarters in 2007, which backed off the closest approach to the sun to 8.5 solar radii, where the sunlight is only about 500 times more intense than at Earth. This allows more extensive instrumentation aboard the Solar Probe spacecraft — measuring magnetic fields, plasma velocity and density, ion and electron temperatures, and thermal anisotropies, as well as the energies and composition of the energetic solar particles — in coordination with related observations from several other heliophysics missions over an entire solar cycle.
Instead of a NASA Flagship-class mission, the cost estimates for Solar Probe Plus were at a New Frontiers-level price ($750 million in 2007 dollars, including launch), with the planetary flybys significantly cutting costs if Solar Probe Plus were launched in 2015, while also allowing for richer science return for the money with 24 dives into our sun’s corona.
In April 2008, NASA headquarters directed the Solar Probe Plus team to go forward with this planned cost and launch date, and work has been at full speed ever since. Significant progress was made in testing and fabricating processes for all of the critical components for the mission, including the 2.7-meter-diameter carbon-foam-filled solar shield and other thermal protection.
Solar Probe Plus was even recently graded with an “A” in a performance assessment of NASA’s heliophysics program by the National Academy of Sciences earlier this year with the observation that “Solar Probe received the highest possible grade due to efforts to control costs via intelligent mission redefinition.”
Unfortunately, with the release of NASA’s budget to the Congress last May, NASA headquarters changed their direction and notified the Solar Probe Plus team that they intend to cut the funding and disperse the team for three more years, delaying the mission’s launch from 2015 to 2018. This was devastating news to the team who had been so focused and innovative. But what is worse to us as taxpayers is that NASA’s current approach with a 2018 launch will lose the opportunity for coordinated observations with other spacecraft. Even more important, the mission now will cost far more money (back to a Flagship-class mission) because a larger launch vehicle will be needed when the planets are no longer aligned for gravitational assists as they will be in 2015. Losing focus, good scientists and engineers will find other work and may not be available when the mission gears up again, adding significant risk to the already-challenging Solar Probe Plus mission.
For these reasons, I hope that Congress will remain steadfast in their support for the Solar Probe Plus mission launching in 2015, as they did in the omnibus appropriations bill that President Barack Obama signed into law last March. This approach offers better science sooner, at a lower cost and with less technical risk. With Congress holding fast in their support, I further hope this spurs the president’s science advisers and NASA’s new leadership to keep Solar Probe Plus on course for launch in 2015, so we may carry out this inspiring and scientifically important mission at a price our nation can afford.
Gene Parker is professor emeritus in the department of physics and the department of astronomy and astrophysics at the University of Chicago.