NASA’s Solar Probe Plus will become the first spacecraft to plunge into the Sun. It won’t enter the Sun’s surface, but its strange outer atmosphere, the corona, where the temperature is millions of degrees and the solar wind is accelerated to supersonic speeds. Perhaps Plus will tell us why.
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A Trip Into a Star
Imagine a spacecraft purposefully being sent into the volatile craw of a star and expected to survive and send back data. That simplistically describes the mission of the Solar Probe Plus. This unique spacecraft will visit our nearest star—our Sun—and hopefully answer many of the questions we have about some of the strange phenomena that occur in and around it.
Plus will enter the solar corona, the outer layer of the Sun’s atmosphere, not just once, but 24 times, each time going deeper and deeper into its seething plasma. At its deepest penetration, it will come to within 8.5 solar radii of the Sun’s photosphere—the visible surface we see.
The corona extends more than 600,000 miles into space. Fortunately for Plus, it is extremely tenuous, so while its temperature in its outer reaches (its temperature increases as you go away from the photosphere—one of the mysteries Plus hopes to solve) gets to two million degrees F, the plasma is sufficiently tenuous that the engineers are certain their design will survive the journey. In such a tenuous gas, when we speak of temperature, we are speaking more of the motion of molecules than of heat as we know it.
Other mysteries scientists hope Plus will solve is the origin of the solar wind. The solar wind 'blows' out from the corona, in a torrent of electrons and other particles that sweep by Earth and flow on into interplanetary space. At times, the wind can create havoc on our planet, disrupting power grids and communications.
The wind in the corona actually comes in three sizes--slow, fast (the supersonic wind) and a wind specifically associated with Coronal Mass Ejections, much stronger than any other. It is hoped Plus will find the origins of each of these.
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The Plus Design
There have been other solar probes such as SOHO, Ulysses and TRACE, but never has one attempted to dip into the corona. The others were either in Earth orbit or barely inside Mercury’s orbit. To plow through the expected 2500 degree heat it will encounter, Solar Probe Plus will hold a new type heat shield at its bow. The heat shield will be a carbon matrix consisting of carbon insulating foam about 15 cm thick. This will be covered with a ceramic optical coating to reflect some of the radiation that will impinge upon it. Another coating provides micrometeorite protection.
This is called the Thermal Protective System, or TPS.
The illustration below is an early design and shows a circular heat shield. At this time the shield is basically a square with two sides rounded. There will undoubtedly be more design modifications as the 2018 launch date approaches.
Power for the instruments and transmitter is provided by two solar cell arrays. The primary array provides power during Plus’ long journey to the corona. Once the plunge into that maelstrom begins, it is folded against the craft, under the protection of the heat shield, and the secondary array takes over.
The secondary array is smaller and actively cooled. As Plus moves inside 0.25 AU of the Sun and the primary array retracts, the secondary array is extended. As the probe moves ever closer to the Sun’s photosphere, the secondary array gradually retracts, until at its closest approach, only one row of cells will be exposed to half the solar disk.
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The Probe’s Mission Profile
Plus is scheduled for launch in July of 2018. It will make a series of seven Venus flybys between then and October 2024 to get gravity boosts. After its last boost it will be traveling 125 miles per second. This speed helps protect the probe from the heat because the blunt heat shield produces a bow shock wave that flows back and envelopes the spacecraft in a plasma sheath like a thermal blanket.
Its first foray into the corona comes about three months after launch, shortly after its first pass by Venus. This is sort of a dipping a toe in the waters run at about 30 solar radii. Each swing around Venus will cause Plus to dip deeper into the corona. As it does, it will spend more time during each dip, swinging around the solar disk more times each dip. It will make 24 swings into the corona, each one deeper and deeper, until the last three are only 8.5 solar radii from the photosphere. On these swings Plus will sample the solar winds in all their variations, as well as make other measurements.
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The Probe’s Instrumentation and Experiments
Plus will carry a variety of instruments designed to delve into the mysteries of the corona. The experiments these will carry out are noted below the list.
A Fast Electron Analyzer
An Ion Composition Analyzer
An Energetic Particle Detector
A Plasma Wave Detector
A Neutron/Gamma Ray Spectrometer, and
A Coronal Dust Detector
A 3-D Telescopic Camera
These instruments will enable researchers to perhaps pull the shroud from several of the corona’s mysteries. The ones they hope to solve are these.
What are the most abundant particles in the solar wind—electrons, protons or helium ions? And what are their properties? Some of the particles will actually be ‘caught’ in a cup and analyzed.
3-D images of the Solar Wind showing the clouds and shock waves as they pass the spacecraft.
Direct measurements of electric and magnetic fields that course through the coronal plasma. It is suspected that it is magnetic fields that heat the plasma to its extreme temperatures.
Measurement of radio waves and shock waves in the plasma.
Measurement of the electrical signature of dust particles in the corona.
An inventory of all elements in the coronal plasma.
Solar Probe Pulse, by the time its mission ends in 2025, may well solve many mysteries of the Sun.