The author would like to thank Professor Kyle Daun, expert in radiation heat transfer, for bringing this exciting mission to his attention.
NASA has recently renamed one of their upcoming space probes, generating a small media splash. This is a good thing because the Parker Solar Probe (previously the Solar Probe Plus) has not gotten too much coverage recently, and certainly less than such a daring mission deserves. When it arrives at its target in seven years time, the probe is set to revolutionize solar science and study the dangerous and fascinating phenomena of solar flares.
The Parker Space Probe has been on NASA’s budget since 2009, with an initial launch date in 2015 (which was, of course, not met). This has now been pushed to August, 2018. On May 31st, NASA announced that they were renaming the satellite from Solar Probe Plus to the Parker Solar Probe, after solar astronomer Eugene Parker. Parker was the first person to propose the idea of a wind emanating from the sun. It was a controversial theory that attracted much criticism; the paper would not have been published in the Astrophysical Journal because two of his peer reviewers rejected it, but for the journal editor’s decision to overrule the reviewers. In 1962, less than a decade after Parker published his theory, his predictions were confirmed by observations made by the Mariner 2 probe.
As the focus of its namesake would suggest, the Parker Solar Probe’s primary purpose is to study the solar wind and other solar phenomena. More generally, the probe’s task is to study the corona, the “atmosphere” of the sun. This region of the sun is very mysterious and scientifically intriguing, and of great practical importance. The corona is the source of various charged particles that leave the sun at extremely high speeds, capable of damaging satellites and communication devices.
On the scientific side, one of the most interesting questions about the corona is why is it so hot: the surface of the sun is a paltry 6000°C. The corona, on the other hand, is several million degrees. How the corona is heated is a great mystery since the source of its energy is not known. The Parker Space Probe is fitted with a large number of instruments to study the corona in great detail: 5 voltage sensors and three magnetometers will measure the magnetic fields, electron temperatures, and plasma density. WISPR will take wide-field images of the sun, SPAN A+ and SPAN B will measure the density of different particles in the corona, and the unfortunately-named ISIS instrument will attempt to correlate different particles from the sun with the structures that produce them.
The Parker Solar Probe will launch in 2018, but it will not reach its science orbit until 2024. The reason for this long internment period is that, from an orbital mechanics standpoint, a near-sun orbit is energetically very far away. When a spacecraft sets out from Earth, be it towards a more inner or more outer location in the solar system, it needs to change its speed. Most of these speed changes need to be done using fuel. The Voyager probes famously supplemented their fuel by getting gravity assists off of the gas giants, gaining extra speed without spending any fuel. The sun is, energetically, further away than the gas giants. As a result, Parker will use 7 separate gravity assists from Venus to slow down. Once it completed its slowdown, Parker will, counter-intuitively, become the fastest man-made object ever; at its closest approach to the sun, the probe will be moving at 200 km/s.
The science orbit that Parker will adopt brings it ten times closer to the sun than Mercury’s orbit. It will spend 11 days within 0.25 astronomical units (AU, the Earth-Sun distance) taking frantic observations while moving at incredible speeds, and then between 88 and 150 days traveling out to the distance of Venus. During this latter time, it will transmit all of its collected data back to Earth. This orbit is seven times closer to the sun than any other solar probe has come, and that has produced some unique challenges.
The most important challenge is heat. The Parker Space Probe is equipped with a carbon-composite sun shield to keep its instruments below 20°C, even as it is subjected to solar radiation that is 520 times greater than it would be subjected to on Earth. Even the probe’s normal solar panels are retracted. Small secondary solar panels with liquid cooling are used to power the probe during the science portion of the orbit.
The Parker Space Probe is sure to reveal some exciting new things about the Sun, an object that seems so familiar to us and yet has some major unanswered questions associated with it. In addition to the practical concerns of investigating solar radiation and what that means for humans on Earth, the Parker Space Probe’s website offers a unique insight into our relationship with the sun and corona: “We live in the atmosphere of the sun.” Surely it is worth studying the most important part of the solar system in great detail.
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