Theoretically, how close could any probe get to a star?

[Ultimate Steve]

Using every trick in the book, like high temperature alloys, sunshields, lots of radiators, liquid mirrors (to reflect light, because a normal mirror would melt), vacuum insulation, and anything else, assuming delta-v is not a problem, how close could one conceivably get a probe to a star the size of the sun? Or Kerbol?

[mikegarrison]

You might want to specify some other conditions. With enough delta-v I could dive a probe right into the sun. But it seems like that's not the answer you are looking for.

Send the probe out far away and continue to accelerate it until it reaches some significant fraction of the speed of light. Then dive it at the sun. I'm pretty sure you could build up enough momentum to blow through the radiation pressure and penetrate reasonably deeply into the sun.

Edited May 23, 2018 by mikegarrison

[Ultimate Steve]

10 minutes ago, mikegarrison said:

You might want to specify some other conditions. With enough delta-v I could dive a probe right into the sun. But it seems like that's not the answer you are looking for.

Send the probe out far away and continue to accelerate it until it reaches some significant fraction of the speed of light. Then dive it at the sun. I'm pretty sure you could build up enough momentum to blow through the radiation pressure and penetrate reasonably deeply into the sun. 

Sorry, probably should have clarified - assuming we're going at reasonable velocities, with an apoapsis near Earth's orbit, how low can we get the periapsis and not have the probe melt?

[Brotoro]

Zero meters. Just encase it in enough ablative material.

[kerbiloid]

Just on first glance:

A probe doesn't have any icold holdover: neither internal (like a cooling tank in a car), nor external (like the air for a steam engine, or a lake for a nuclear plant) one.
So, it should radiate as much energy, as it receives from the Sun.

Albedo, mirrors, etc, would effect in three ways:

• It would decrease rate of heating, so instead of melting in an hour, the probe would melt in two hours.
  So, if the probe doesn't stay near the Sun (say, passes near it at hyperbolic velocity) — it could escape before it melts.
  Unlikely this can effect on elliptic orbits, as anyway the passage time is at least hours.
  So unless the probe has mass and heat capacity of a big rock/small asteroid, enough big to escape before get heated, unlikely a mirror can help in this case.
   
• A case of that: if the probe has something to evaporate from the rear end, while its front side is a mirror (i.e. kind of managed ablation), this would slow the probe heating enough to pass the perihelion.
  But:
  1) it should refill the ablator tank between the passes;
  2) Near the equilibrium temperatures comparable to melting point of construction materials, radiation is more effective way of energy transmission than atomic movement, so the required ablator mass would be at least comparable, better - much greateer than the probe own mass. So, the probe would be an artificial comet, with instrumental module in the rear side.
   
• A mirror front surface is a must have just because any "dark", non-reflecting spot would be getting warmed much faster than the surface around it.
  So, the probe would start locally melting faster than when its fron surface is a mirror.

So, as the pass lasts enough long the probe in whole would anyway get a near-equilibrium temperature.
It can redistribute it with combination of a front mirror and rear radiators/ablators, but as the probe itself should solid, at least one part of the craft (between the solid probe and non-solid "heat exhauster") should be heated up to the equilibrium temperature. (It shouldn't if the probe were passing enough fast).

The most heatproof alloy is HfC*TaC (1:1), its melting point is ~4500 K.

So, as (R²T⁴ = const), the closest radius is ~= Rstar * (Tstar / 4500)².

Sun: T = 6000. R ~1.8 Rsun.

Blue giant: T = 30000. R ~25 Rstar, ~10^-1 AU.

Coolest red star: T = 2500, R ~= ... oops, 0.3, i.e. inside the star.
But inside the star the probe is surrounded by the star material.
So a radiator panel should be >2500 K hot (actually hotter because its efficiency is defined by the temperature difference), and is not an option.
Additionally, it's getting heated by drag surrounding gas flow.
So, only uncontrolled ablation from the whole surface would help a little, but
1) The probe  is not an asteroid, its mass and heat capacity is small.
2) Uncontrolled ablation would be greater from the front, where it pushes the probe retrograde together with drag.
So, actually, "inside the star" depth would be just ~0, scratch the star surface.

(Asteroid would melt and crash even earlier, unless it's made of tungsten).

Ablation is nice, but the ablator gas cloud keeps being heated and radiates, too. Heating the craft from the outer side.
Also, mass ablation flows make pressure inside and thrust outside, making the whole system unstable and unpredictable.

So, probably ~Rstar is minimal altitude for small stars.
Tens-up-to-hundred for giants.

[sevenperforce]

All the answers:

https://what-if.xkcd.com/89/

[Ultimate Steve]

Okay, so vital parts behind a very strongly insulated box, which is mounted on a tungsten box of the less vital parts which uses 1 or 2 series of liquid ablator/coolant, which will be ejected to the sides of the spacecraft (not the back, that would radiate to heat the vitals). Then, attach the whole thing to a temperature resistant reflective mass of tungsten alloy a hundred meters or more across. Get the thing going fast enough that flyby time will be minimal and "drag" won't be much of an issue, perhaps with an outer planet flyby, and the probe could theoretically touch the photosphere.

Okay, that's interesting. Thanks!

[p1t1o]

How big can a ball of iron get before it stops being a ball of iron (and starts being something like liquid iron, or neutronium?)

And how long would it last if we flung it into the sun (with a probe at the very centre)?

Tungsten is fine, but it might start fissioning at some point in its journey.

Edited May 23, 2018 by p1t1o

[Nuke]

the operating words arent how but how long. we could send one now if we were willing to wait a million or so years for the return data, assuming we could build something to survive a million years in space. solid state everything and the ak-47 of radhard computers. if we want to use nukes as rocket fuel we could get that down to 100s of years. not you got to build the ak-47 of radhard computers that can also survive the repeated jolting of nuclear warheads going off in the tail pipe.

Edited May 23, 2018 by Nuke

[sevenperforce]

8 minutes ago, Nuke said:

the operating words arent how but how long. we could send one now if we were willing to wait a million or so years for the return data, assuming we could build something to survive a million years in space. solid state everything and the ak-47 of radhard computers. if we want to use nukes as rocket fuel we could get that down to 100s of years. not you got to build the ak-47 of radhard computers that can also survive the repeated jolting of nuclear warheads going off in the tail pipe.

The nearest star is a good deal closer than all that.