# Is it possible to use Magnetic refrigeration to cool down a sun probe, so that it could survive at least a few hours inside the photosphere?

The probe in my mind is unmanned, with a super-reflective surface to deflect most of the energy from the Sun on all wave length, only absorb enough to power its operation.

Please also elaborate if you think of a better solution. Thanks!

• If its surface already reflects enough energy, there would be no need to cool it, as it wouldn't heat up in the first place. Aug 25, 2017 at 18:06
• @Phiteros, super reflective is not enough. Let's say it is reflective at 99.999%, thus absorbing only 0.001%. The Sun emitts 3.8E26 W, with a surface of 6.1E18 square meter, which gives 60E6 W per square meter. 0.001% of that amount is still 6 kW per square meter (6 times the amount we get on Earth in a clear day)
– L.Dutch
Aug 25, 2017 at 19:27
• You can't cool anything if there is nowhere for the heat to go, if you ever feel the side of your fridge, it is warm. That's because the heat is being "pumped" out with a thermodynamic process. Your craft will not be able to cool itself below the surrounding temperature unless it is outputting more heat than it receive. The energy required to cool something to operating temperature while at the surface of the sun probably requires more energy than a spacecraft could ever hope to generate. Aug 25, 2017 at 19:39
• @A.C.A.C. what if my fictional craft can turn this extra heat into radiation energy and use it to propel itself?
– A.Z.
Aug 25, 2017 at 19:53
• If your "fictional craft can turn this extra heat into radiation energy and use it to propel itself" then you have built a perpetual motion machine of the second kind, that is, you have broken the 2nd law of thermodynamics. ("Heat cannot be spontaneously converted into mechanical work", or "a thermal engine needs both a hot reservoir and a cold reservoir".) As a bonus, in science-fiction terms you have built the holy grail of spacecraft propulsion, a reactionless drive. Aug 25, 2017 at 20:25

(Magnetic cooling is used in cryotechnology. Not useable here, even theoretically imo. You'd have to use some other technique. But ...)

No, impossible. The edge of the photosphere has a density similar to where meteors appear in earth's atmosphere. Atmospheric friction alone will burn up your spacecraft. Especially since it would be at a tremendous speed (ten times that of a typical fast meteor (~50 km/s) hitting earth).

Flying into the chromosphere is similarly impossible, simply because the shadow behind your sunscreen is too small to place enough radiators in it. Enough power for a heat pump would be available, but the sun has an angular diameter of nearly 180°. You can't hide from it.

In the corona, more than three solar radii away from the surface, it's possible for a few hours. It's over a million degrees hot, but the density is very low. Don't get fried in an eruption. 09 NASA's Parker Solar Probe is currently on the way there, minimum planned distance is seven solar radii. It would have been possible to get even closer (down to four solar radii, close to the inner edge of the corona), which is what an earlier mission design wanted to do. The closer you get, the faster you're out again. PSP will reach close to 200 km/s, the original Solar Probe mission would have got up to over 300 (and taken a slingshot around Jupiter to reach that orbit).

UPDATE: Perhaps I should take back my verdict on the chromosphere. Once you are at the inner edge of the photosphere (3 solar radii, which would be possible today), the radiation density does not increase that much any more if you get closer. Also, the time you spend there gets shorter, because your speed increases. Now you just need a sunshield with a higher heat capacity (=thicker), a radiator design that uses the whole dark side of your ship (a flat cone, the cone pointing away from the sun), and a well insulated interior that keeps your payload. And to power the heat pump, you use a cleverly designed thermoelectric generator. With a ship in the region of 10 tons (20 times heavier than PSP), I'll wager you can get as close as two solar radii, and get away.

• Thanks @Karl for the great answer. I was thinking of a probe flying through the photosphere when it was captured by a mysterious force, and that was how Helios obtained the insight on human technology. I'll change my plot to a flyby in the corona when the probe was caught in a flare. Like a fly shot down by an archer fish...
– A.Z.
Aug 28, 2017 at 18:11