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In this answer the survival of the Parker Solar Probe spacecraft passing as close as 9 solar radii from the Sun's surface is discussed. Using guestimates for apoapsis and periapsis of 1.1 $a_{venus}$, 10 $r_{sun}$. I get velocities of about 11,000 and 190,000 m/s using the vis-viva equation.

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above: screenshot from page ES9 of the Solar Probe Plus Fact Sheet.

The heat shield is described here:

The main feature of the TPS is an 8-foot-diameter, 4.5-inch-thick, carbon-carbon, carbon foam shield that will sit atop the Solar Probe Plus spacecraft body. The system will protect the Parker Solar Probe from temperatures exceeding 2,500 degrees Fahrenheit and impacts from hypervelocity dust particles as it flies through the Sun’s outer atmosphere.

In the GIF below, the shield and a stand-off between the shield and the body of the espresso maker-shaped spacecraft can be seen. As described in the linked answer and here, the "side panels" are radiators to help cool the back of the shield, and the long legs connecting the shield to the satellite designed to have a very high thermal impedance to minimize heat flow.

I've asked about the origin of the dust near the Sun in this question If I understand correctly, the dust is slowly falling in to the Sun because particles of a certain size range experience a tangential deceleration force via the Poynting Robertson effect. It's related to astronomical aberration - moving transversely through a photon flux causes a slightly enhanced radiation pressure on the forward facing side of the particle. Since the force is velocity dependent, it is greater at periapsis, and thus tends to circularize the orbits.

My question is: If the heat shield is facing directly towards the Sun so that this long narrow spacecraft can remain in the shadow so that it is "moving sideways" through this dust field, how does the this shield "protect the spacecraft from impacts from hypervelocity dust particles as it flies through the Sun's outer atmosphere?"

Are there "hypervelocity dust particles" coming straight out of the Sun? Sun soot?

The question is further highlighted by this paragraph from Section 4.5.3 of this 2008 report:

The Coronal Dust instrument is mounted on the –Y panel looking in the ram direction to collect the maximum number of dust impacts.

edit: The constant, slow infall of particles/dust results in a higher concentration near the sun - a collapsing spherical shell has smaller and smaller area. This cloud has been historically observed as the source of Zodiactal Light. Most of the particles are tens or hundreds of microns in size, and one theory for the origin is the collisions of asteroids or from comets. When the cloud gets too close to the Sun, the particles disintegrate, and this "hole" in the cloud is called the Solar Dust Corona. There is much more to read in the paper nicely titled Dust Near The Sun.

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above: a long exposure from Paranal showing zodiactal light - this and larger versions available in Wikipedia.

Solar Probe Plus

above: GIF made from six screenshots, cropped from the YouTube video shown separately below, from the Johns Hopkins University Applied Physics Laboratory. The JHU APL has designed and will build and operate the spacecraft for NASA.

You can also see details of the articulation of the solar panels - note how they become increasingly shadowed behind the shield as they are withdrawn, until only a tiny section remains highly obliquely illuminated in the incredibly intense flux of light (and everything else) from the Sun.

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above: screen shot from NASA's Solar Probe Plus Fact Sheet

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  • $\begingroup$ One can assume they are coming directly out of the sun. That doesn't matter when the velocity tangential to the sun will be so high. Hmmm... $\endgroup$ – PearsonArtPhoto Jul 29 '16 at 12:36
  • $\begingroup$ @PearsonArtPhoto What does "one can assume" actually mean? Do you know this is happening - particles of dust are really coming straight out of the sun? I've never heard of that. Or are you just guessing? $\endgroup$ – uhoh Jul 29 '16 at 12:40
  • $\begingroup$ It makes sense that the main direction will be directly away from the Sun. They will probably be at some kind of an angle, but as I said, it doesn't really matter, the spacecraft is moving fast enough that the direction hardly matters. $\endgroup$ – PearsonArtPhoto Jul 29 '16 at 12:43
  • $\begingroup$ @PearsonArtPhoto It certainly matters quite a bit! And beyond protons and an occasional helium atom or ion, I've never heard of dust particles streaming out of the sun. The spacecraft will be moving much faster than any dust is orbiting since the dust is expected to be mostly circularized, so it should be getting hit in the side by the dust, not the front. Basic orbital mechanics I learned right here in SXSE! $\endgroup$ – uhoh Jul 29 '16 at 12:50
  • $\begingroup$ I'm picturing more something like the solar wind, which is on a near escape velocity, which would thus be going much higher speed than the spacecraft. $\endgroup$ – PearsonArtPhoto Jul 29 '16 at 13:00
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@uhoh - Mistake, no. Poor phrasing, perhaps yes.

It looks like the APL fact sheet is implying that the Solar Probe Plus heat shield must survive extreme temperatures, radiation, and dust impacts otherwise the the rest of the spacecraft will ablate near perihelion at closest approach.

If the heat shield is facing directly towards the Sun so that this long narrow spacecraft can remain in the shadow so that it is "moving sideways" through this dust field, how does the this shield "protect the spacecraft from impacts from hypervelocity dust particles as it flies through the Sun's outer atmosphere"?

It's not that the heat shield is meant to protect the spacecraft from dust, since at perihelion it's velocity transverse to the heat shield normal (i.e., several hundred km/s with respect to the sun) will vastly exceed the typical dust speeds expected (i.e., several tens of km/s with respect to the sun). In other words, in the spacecraft frame at perihelion the trajectory of impacting dust will basically be into the side of the spacecraft, not normal to the heat shield surface.

However, micron-sized dust hitting something at several hundred km/s, even at glancing angles, is not something to ignore. The teams have done considerable modeling to try and deal with this issue, but the dust distribution inside ~0.3 AU is not well known.

Fun Side Note: The FIELDS instrument is planning to look for dust by detecting the electric field pulses resulting from the plasma clouds produced by the impacting particle. This has been done before with missions like Wind and STEREO (and a few others) and was kind of discovered by accident.

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  • $\begingroup$ Thanks! So considering the sentence "The system will protect Solar Probe Plus from temperatures exceeding 2,500 degrees Fahrenheit and impacts from hypervelocity dust particles as it flies through the Sun’s outer atmosphere." there may be some fraction of dust with a positive radial component, but it really will not protect it from hypervelocity dust particles to any large degree? $\endgroup$ – uhoh Aug 3 '16 at 23:38
  • $\begingroup$ I've asked a follow-up question. $\endgroup$ – uhoh Aug 4 '16 at 10:16
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The heat shield (TPS) is not meant to protect the spacecraft from dust impacts. The statement is not quite accurate and (from a recent search) seems to have been removed from site.

As an engineer on the Parker Solar Probe team, I have been involved in the Preliminary and Critical Design Reviews for the mission which include presentations on Dust Protection and the TPS.

The dust protection studies evaluate potential damage to all spacecraft surfaces, including the TPS, but do not assign any unique role to any one surface as a protective device.

The TPS design requirements are focused on the thermal management of the spacecraft; there are no requirements for protection of the other spacecraft elements from dust damage.

This was something of an overstatement and seems to have been removed.

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  • $\begingroup$ Super! Thank you for taking the time to add this answer. There's another unanswered question in Physics SE you might be interested in; Parker Solar Probe passing extremely close to the Sun; what relativistic effects will it experience and how large will they be? $\endgroup$ – uhoh Jan 7 '18 at 7:29
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    $\begingroup$ Unfortunately, I do not have the basic knowledge or experience to answer that question - but I know people who do! If I get a chance I will pass the question along. The area in which this is most important is spacecraft timekeeping, and we have a very knowledgeable person on the team who specializes specifically in this area. $\endgroup$ – That60sKid Jan 7 '18 at 18:46

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