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The Parker Solar probe will launch soon, and over a period of years and several planetary flyby's loose enough energy from its heliocentric orbit to reach an orbit with a perhihelion of only about 4 million km.

The Spacecoast Daily article Traveling to the Sun: NASA Describes ‘Why Parker Solar Probe Won’t Melt?’ links to the video Blowtorch vs Heat Shield which shows only dark carbon-like materials, but in the video Why Won't it Melt? How NASA's Solar Probe will Survive the Sun the surface of the heat shield is white, as it is in the photo below.

What material will make up the surface of Parker's heat shield that faces the sun, and what is its reflectivity in visible and near IR wavelengths where most of the Sun's intensity falls?

The caption for the image from here says:

Parker Solar Probe’s heat shield is made of two panels of superheated carbon-carbon composite sandwiching a lightweight 4.5-inch-thick carbon foam core. To reflect as much of the Sun’s energy away from the spacecraft as possible, the Sun-facing side of the heat shield is also sprayed with a specially formulated white coating. (NASA Image)

More about the heat shield in this answer.

Parker Solar Probe heat shield

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    $\begingroup$ I was planning on asking this exact question today! $\endgroup$ – Jack Jul 22 '18 at 7:35
  • $\begingroup$ @Jack "d'oh!" Sorry about that! $\endgroup$ – uhoh Jul 22 '18 at 7:40
  • $\begingroup$ No not at all! I’m interested to see some answers $\endgroup$ – Jack Jul 22 '18 at 7:41
  • $\begingroup$ Unobtainium, of course! $\endgroup$ – Sean Jun 22 at 22:43
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This was difficult to answer until I figured out that this mission used to be called "Solar Probe Plus" and all the early studies were done under that name.

From here:

As part of the TPS Risk Mitigation effort, two potential ceramic coatings were found that met the requirements of the Solar Probe+ mission. Ceramic materials that are visibly white
generally provide the optical characteristics compatible with the proposed shield passive thermal management strategy. These characteristics are low solar absorptivity and high IR emissivity. Thermodynamic stability and chemical compatibility with C-C are additional differentiators that further narrow the list of candidate ceramics. At the end of the study, both aluminum oxide (Al2O3), commonly called alumina, and pyrolytic boron nitride (PBN) were found to notionally satisfy these basic characteristics.

Annoyingly this paper doesn't say which of the two was chosen. It does touch on the optical properties though:

The key optical property is the ratio of the solar absorptance to the IR emissivity, α / ε , set at the design value of 0.6 in Figure 3.6-1, which shows that the shield temperatures are reduced as the Sun distance increases or the cone angle is decreased.

enter image description here

Happily, this one does:

The baseline coating for the Solar Probe primary shield (without modifications such as doping) is a thin (100 − 125 μm) coating of alumina (Al2O3) on a C-C composite.

Some articles about the mission state that this coating was applied via plasma spray, this news article appears to confirm that.

The white coating on the front of the heat shield, the part that will face the Sun, was sprayed on by Plasma Processes, Inc. in Huntsville, Alabama.

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    $\begingroup$ Oh of course! I had a brief search earlier, but forgot about the name change so came up empty handed! Great answer $\endgroup$ – Jack Jul 22 '18 at 16:35
  • $\begingroup$ Very nice detective work, and very concisely written and well-sourced as well. Thanks! $\endgroup$ – uhoh Jul 22 '18 at 16:50
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    $\begingroup$ The first linked reference is a pretty comprehensive one, although from rather early in the design process. $\endgroup$ – Organic Marble Jul 22 '18 at 17:03

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