[thisisbrians]

you're probably getting downvoted because there isn't really a temperature 4 million miles away from the Sun (it's mostly just empty space being bombarded by radiation)

2,500º F is merely the temperature the probe is expected to reach at that distance. if it were to stay at that distance indefinitely, it would grow much, much hotter as it absorbed more energy from the sun.

[tomnicholas1]

No not necessarily - it will keep growing hotter until the black body radiation emitted by the probe matches the power of the radiation hitting the probe. Then it will stay at constant temperature.

It's a standard undergraduate problem to work out what this equilibrium temperature is for a flat plate at a distance from the sun equal to the Earth's orbital radius.

Interestingly the result is only a few 10's of degrees less than the average temperature of the real Earth - the difference is due to the Greenhouse Effect.

For the probe one could easily do the maths but I could believe that at 4 million miles that equilibrium temperature is 2,500F.

[thisisbrians]

i definitely can't do the math myself, but excellent retort. thank you for the nuance

[feoren]

Temperature is so wibbly-wobbly. The probe will reach an equilibrium energy-in vs. energy-out temperature depending on its distance from the sun, its surface area facing the sun, and the materials being lit, vs. its surface area facing away, the thermal radiation rate of various materials, and other factors. You could give an aerospace engineer almost any temperature between the CMB and the surface of the sun and they could probably design a (at least theoretical) probe that would reach that temperature eventually* at almost any distance. My guess is that 2500 ºF probably is the equilibrium temperature of the probe at that distance.

* With "eventually" being "assuming a stable state for infinite years" which is of course not how astrophysics actually works.

[lazide]

Eh, not quite yeah?

You’re talking about heat (think ‘amperage’), where temperature is more like voltage.

You can’t get above a specific temperature merely by transferring more heat, or losing less heat, etc.

Upper bounds of temperature is still going to be limited by the temperature/frequency of the input energy, barring energy loss which can reduce it.

The solar atmosphere layers have specific maximum temperatures that limit the maximum temperature of objects exposed to them or the radiation from them.

[thisisbrians]

well, in space (perfect vacuum) you only lose heat to radiation

but you can keep absorbing radiation indefinitely

so the equilibrium temperature will depend on the incoming radiation and ensuing outgoing radiation as dictated by the material makeup of the thingy

[lazide]

Assuming passive systems? It doesn’t work that way.

Once the object has reached the temperature of the source of the radiation (assuming radiative heat transfer), it reaches equilibrium and it will radiate away at the same rate it is absorbing (as a black body). Per the 2nd law of thermodynamics.

It’s why there is a maximum temperature with concentrated solar too - regardless of magnification, you can’t exceed the temperature of the surface of the sun the light was emitted from. Attempted to do so will actually heat the sun (or some other thing) through radiative thermal heat transfer the other direction.

It’s also why radiative heat transfer can’t be used to produce infinitely high temperatures by having a large emitter near a tiny absorber (like a speck of dust) in a vacuum.

If there is some kind of heat pump or laser or the like which you a providing power, then that doesn’t apply of course, but for pure black bodies it does.

If you have some way to let an object absorb radiation, while emitting no radiation even when it is as hot as the source of that radiation, then you have something pretty special going on eh?

[lazide]

Edit window closed, but here is a more in-depth explanation. It’s decidedly non-obvious [https://www.reddit.com/r/AskPhysics/comments/1arh55g/why_can...]

Part of it is due to the ‘Principle of Etendue’ [https://en.m.wikipedia.org/wiki/Etendue#Conservation]

What confuses people I think (practically) is that the actual high temperature (~4500F) is far beyond the limits of a useful highest temperature in 99% of situations we might want in engineering. A spacecraft hitting that temperature is going to be a molten piece of scrap long before it hits that point.

But the limit does actually exist - it won’t somehow hit 10,000F for instance. That is also why we can’t produce infinitely high temperatures with a huge magnifying glass - the highest we can hit, regardless of how big it is, is still ~ 4500F. Higher temperatures need something like an electric arc furnace, or LASER.