I saw the plot below in this answer (40MB PDF), and it made me wonder, at what distance would Parker fail to function? Also, at what point would it become so hot that it wouldn't be recognizable as a man-made object (at what point would it begin to melt / turn to plasma)?

Temperature sensitivity.

Even a specification statement on the design limitations and absolute maximum temperature would be helpful. I guess I could rephrase the question as "what is the absolute temperature limit for parker solar probe" and also "how far away from the sun does it have to be to hit this limit while the shield is facing directly towards the sun"?

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    $\begingroup$ Note that the probe would probably start failing (non-structurally) before any part of it started melting. $\endgroup$ – Alex Hajnal Dec 26 '18 at 16:01
  • $\begingroup$ @AlexHajnal honestly, that's likely the more objective requirement, I've edited the question. I couldn't think of a better word than melt for some reason at the time of writing. Thanks :). $\endgroup$ – Magic Octopus Urn Dec 26 '18 at 17:37

The last three Parker passes will be at 3.8 million miles from the Sun.

That’s as low as the sun shield is designed to go and still be a shield. As Parker gets lower and lower, the Sun takes up more of the sky, and the fully-shielded cone behind the shield gets smaller. At the lowest design altitude, the probe just fits into that cone with a small margin. Below it, parts will inevitably start to become exposed to raw sunlight, starting with the solar cell support structures. Even a few-percent exposure to the Sun’s surface is fatal at that altitude for parts not designed to take it.

enter image description here

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    $\begingroup$ So, what is the distance at which the first critical component would be expected to fail? Any idea what that component might be? $\endgroup$ – Chris B. Behrens Sep 8 '19 at 21:11
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    $\begingroup$ The shield was designed with engineering margin. If that’s needed, the Parker can’t go significantly lower. If not, 15% on the altitude might be available (depends on lots of info I don’t have). The first item exposed depends on attitude control, etc, but it’s probably the inner mechanisms of the solar array structures. $\endgroup$ – Bob Jacobsen Sep 8 '19 at 23:39
  • $\begingroup$ Well the electric field antennas and Faraday cup are always exposed, so figure out the point where niobium (and the other rare metals used in these instruments) start to have problems. I am guessing ablation and heat conduction to other systems would become problematic quite quickly. $\endgroup$ – honeste_vivere Jun 10 '20 at 19:14

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