Solar Probe Plus: A Mission to the Sun

“Deep beneath the surface of the Sun, enormous forces were gathering. At any moment, the energies of a million hydrogen bombs might burst forth in the awesome explosion…. Climbing at millions of miles per hour, an invisible fireball many times the size of Earth would leap from the Sun and head out across space.”

- Arthur C. Clarke, “The Wind from the Sun”

Figure 1: Million-degree magnetic loops arching high into the solar corona as viewed in X-rays by NASA’s Solar Dynamics Observatory. Image credit: NASA.

The solar wind is a continuous stream of plasma flowing outward from the Sun in all directions. Its effects are apparent throughout the Solar System. The solar wind shapes the magnetic fields of planets, causes the tails of comets to point away from the Sun, weathers the surfaces of airless worlds, etc. Various spacecraft have sampled the solar wind right up to the edge of the Solar System.

Surrounding the Sun is a multi-million-degree crown of plasma known as the corona. It extends a few million km from the Sun and it is basically the Sun’s outer atmosphere. In comparison to the hot corona, the average surface temperature of the Sun is a mere 5778 K. The continuous expansion of the Sun’s corona into space is the source of the solar wind. Although we now know more about the Sun’s corona and the solar wind than ever before, two fundamental questions still remain. Why is the Sun’s corona so much hotter than the Sun’s visible surface? And how is the solar wind accelerated?

In 2018, NASA plans to launch a robotic spacecraft called Solar Probe Plus to study the Sun’s corona. By coming closer to the Sun than any previous spacecraft, Solar Probe Plus is exploring a region of space where no spacecraft has gone before. After launch, the spacecraft will orbit the Sun 24 times during its 7 year nominal mission duration. It will make use of seven Venus flybys to shrink its oval-shaped orbit around the Sun. Each flyby decreases the spacecraft’s minimum distance from the Sun.

On the final three orbits of its nominal mission, Solar Probe Plus will fly to within 9.5 solar radii of the Sun. 9.5 solar radii is 9.5 times the radius of the Sun, which is equal to 6.6 million km or 0.044 AU, where 1 AU is the average Earth-Sun distance. It also means that the spacecraft is just 8.5 solar radii or 5.9 million km from the Sun’s surface at closest approach. That is almost 8 times closer than Mercury’s minimum distance from the Sun and 8 times closer than any spacecraft has gone before.

Figure 2: Artist’s impression of Solar Probe Plus during a Venus flyby.

Figure 3: Solar Probe Plus will use seven Venus flybys to shrink its orbit around the Sun.

By flying into the Sun’s outer atmosphere, Solar Probe Plus is quite literally a journey to the Sun itself. As mentioned in a report by the Solar Probe Plus Science and Technology Definition Team (STDT): “Solar Probe Plus will repeatedly sample the near-Sun environment, revolutionizing our knowledge and understanding of coronal heating and of the origin and evolution of the solar wind and answering critical questions in heliophysics that have been ranked as top priorities for decades. Moreover, by making direct, in-situ measurements of the region where some of the most hazardous solar energetic particles are energized, Solar Probe Plus will make a fundamental contribution to our ability to characterize and forecast the radiation environment in which future space explorers will work and live.”

The mission’s four primary science objectives are:

-  Determine the structure and dynamics of the magnetic fields at the sources of both fast and slow solar wind.

- Trace the flow of energy that heats the corona and accelerates the solar wind.

- Determine what mechanisms accelerate and transport energetic particles.

- Explore dusty plasma phenomena near the Sun and its influence on the solar wind and energetic particle formation.

Meeting these key science objectives require in-situ measurements of the solar wind down in the Sun’s corona. To perform these measurements, Solar Probe Plus will carry a suite of 10 instruments - a Fast Ion Analyzer (FIA), two Fast Electron Analysers (FEAs), an Ion Composition Analyser (ICA), an Energetic Particle Instrument (EPI), a Magnetometer (MAG), a Plasma Wave Instrument (PWI), a Neutron/Gamma-ray Spectrometer (NGS), a Coronal Dust Detector (CD), and a White-Light Hemispheric Imager (HI).

At closest approach, 8.5 solar radii from the Sun’s surface, Solar Probe Plus will experience more than 500 times the intensity of insolation Earth gets from the Sun. Also at closest approach, the spacecraft will be hurtling around the Sun at a terrific speed of 200 km/s. It would make Solar Probe Plus the fastest spacecraft ever flown, beating Helios 2’s speed record of 70.22 km/s by almost a factor of three. At 200 km/s, getting from London to New York would take less than 30 seconds.

To withstand the intense radiation environment and the energetic dust particles near the Sun, the spacecraft is protected behind an 8-foot-diameter, 4.5-inch-thick carbon-carbon composite sunshield. The front surface of the sunshield includes an optical coating to refect part of the incoming solar energy. When the spacecraft is at its closest approach to the Sun, the sunshield receives a total incident flux of ~4 MW and its front face is expected to heat up to a temperature of ~1700 K while the backside is not expected to exceed 350°C.

The sunshield is attached to the rest of the spacecraft via the transition structure assembly (TSA). The TSA mechanically supports the sunshield but thermally isolates it from the spacecraft in order to protect the spacecraft from the high temperatures of the sunshield. Both the sunshield and the TSA keep the heat flow that is transmitted to the spacecraft to no more than 50 W, or ~0.001 percent of the original flux at closest approach. Nearly all of the spacecraft will be kept within the shadow created behind the sunshield.

Figure 4: A digital depiction of Solar Probe Plus.

Solar Probe Plus derives its power from two separate sets of solar arrays. To ensure the arrays are kept at proper temperatures, they will retract and extend as the spacecraft swings toward or away from the Sun. The primary arrays are used when the spacecraft is further from the Sun (i.e. beyond 0.25 AU). When the spacecraft is nearer the Sun, the primary arrays are stowed into the shadow created by the sunshield and a set of smaller secondary arrays is used instead.

Deployed at distances between 0.044 AU and 0.25 AU from the Sun, the secondary solar arrays consist of two small retractable panels. The solar cells used for the secondary arrays are triple-junction gallium arsenide (GaAs) based cells. Each retractable panel consists of GaAs-based solar cells mounted on an actively-cooled substrate to keep the cell junction temperatures bellow 120°C. Active cooling is done by pumping a liquid-based coolant in a loop, transporting heat from the panels to the radiators where the heat is being dissipated.

Throughout its nominal mission, Solar Probe Plus will spend 2149 hours within 30 solar radii, 961 hours within 20 solar radii, 434 hours within 15 solar radii and 30 hours within 10 solar radii. It is possible that the mission might be extended beyond the nominal 24 orbits around the Sun. Starting from the last three orbits of its nominal mission and onwards, the spacecraft’s orbit is such that it will circle the Sun once every 88 days, making closest approach to the Sun with an average frequency of 4 times per year.

Figure 5: Artist’s depiction of the spaceship ‘Icarus II’ in the film “Sunshine”.

Figure 6: Artist’s depiction of the observation portal on board the spaceship ‘Icarus II’ in the film “Sunshine”.

The Solar Probe Plus mission brings to mind a 2007 British science fiction film directed by Danny Boyle entitled “Sunshine”. Set in the year 2057, the Sun seems to be ‘dying’, plunging Earth into a permanent winter. A crew of seven astronauts on board a spaceship named ‘Icarus II’ is sent on a desperate mission to drop a massive thermonuclear device into the ‘dying’ Sun to ‘re-ignite’ it. The film’s science adviser, Dr. Brian Cox of CERN, mentions that the premise behind the ‘dying’ Sun is that the Sun has been ‘infected’ with a ‘Q-ball’ - a theoretical particle left over from the Big Bang that is disrupting normal matter. Hence, the thermonuclear device is meant to blast the ‘Q-ball’ to its constituents. The constituents then decay naturally, returning the Sun back to normal.

The spaceship, ‘Icarus II’, has a massive heat shield to protect it from the incinerating blast of radiation as it approaches the Sun. The film brilliantly depicts the sheer enormity of the mission. At the centre of the colossal parasol heat shield is an observation portal that allows the crew on board ‘Icarus II’ to observe the Sun. In the film, one crew member was viewing the Sun at 2 percent brightness and asks the onboard computer to let him see what the Sun truly looks like. The computer replies that just 4 percent brightness would do irreversible damage to the eyes. After all, the film “Sunshine” is fiction, but hopefully it will make the audience think a little more about the Sun, our star.