Hacker News Reader

Comment by threefour | Hacker News Reader

If you’re curious why it won’t melt: https://www.nasa.gov/feature/goddard/2018/traveling-to-the-s...

Relevant part: the capturing cups are of metals with high melting points, plus

> The corona through which Parker Solar Probe flies, for example, has an extremely high temperature but very low density. Think of the difference between putting your hand in a hot oven versus putting it in a pot of boiling water

And for the rest of the craft there is a highly reflective heat shield.

are you saying this protects you from Corona? why isn't this in the news?

Putting your head in a hot oven continuously will probably prevent you from dying of the coronavirus indeed.

As does flying to the sun, for that matter. Can't catch corona if you're ~150 million kilometers away from the nearest other person.

The vacuum probably also helps. I'd also expect a few mm of vacuum might do the trick of avoiding contracting covid, though I'd not (as the Dutch say) put my hand in the flame for it.

(Equivalent expression in English might be that I'd not wager a lot of money on it, i.e. that I'm not completely certain given the consequences if it's wrong.)

Corona is dangerous.

I can’t believe you want to make light of it!

Sorry, it’s not sunny.

You'll probably die from the vacuum before any corona will kill you. or is that not 'prevent'?

Also spending most of it's orbit pretty far away from the sun. You can move your hand though an open flame with no issues, but if you keep it in the flame it will get burned.

Not sure I get the oven analogy. If I stick my hand in for a few seconds, sure, but if I was in the oven, I'll burn and die there too.

It's in an elliptical orbit that also flies by Venus so it's like putting your hand for a few seconds twice a year.

> if I was in the oven, I'll burn and die there too.

Eventually. Not momentarily.

It would take you much longer to die in the oven than if you would dive into boiling water. I didn't tried both so I cannot be sure, but I suspect that one can live in a boiling water just for a several seconds, while it is possible to live a few tenths of seconds in the oven.

Finnish saunas regularly go to 100°C (210F). I can personally attest that fifteen minutes in dry 100°C air is very survivable, you just get sweaty. Meanwhile 100°C water burns you almost instantly.

Though it's worth noting that when talking about humans in air the humidity plays a huge role. Sweat cools your body through evaporative cooling, which works better the drier the air is. At 100% relative humidity you can't cool down and eventually overheat, at 10% humidity we can survive some ridiculous temperatures for as long as we can keep sweating.

Yes, some sauna's need you to wear the key of your dressing locker on an arm bracelet. While relaxing nicely in the heat you can instantly burn yourself severely the moment the iron part of the key presses into your skin. (Talking from experience here)

You got it actually, it's all about how long you leave your hand in. And that is why the probe has a very elliptical orbit, so it "touches" the sun briefly and goes back out into space to cool down.

Temperature is not what burns you, it's heat flow that does the burning. A boiling pot has a lot of molecules densely packed together, so heat transfer happens fast. The air inside the oven is hot but will not be as effective in transferring heat to your hand. There is more "thermal resistance" between your hand and the air inside the oven than your hand and the water inside a boiling pot.

Imagine sticking your hand in the oven for 1 second. Now imagine sticking it in a pot of boiling water for 1 second. One you do every day, the other is an immediate trip to the hospital. What’s the difference? Water is far more dense than the air in the oven thus will transfer much more heat to your hand over that same duration.

I like to remind people that if you could suddenly create a mass of living bodies the size of the sun, it would have a higher power density :D

Basically not much heat is transferred because there's not many particles to do the transfer, like a hot oven.

What I didn't understand was the coolant. It's got to dump whatever heat it picks up somewhere? Just a big radiator?

> It's got to dump whatever heat it picks up somewhere? Just a big radiator?

Yes. The absence of any medium in space makes radiating the only way to get rid of heat.

The article puts some numbers on it, as well:

> That means that while Parker Solar Probe will be traveling through a space with temperatures of several million degrees, the surface of the heat shield that faces the Sun will only get heated to about 2,500 degrees Fahrenheit (about 1,400 degrees Celsius).

I'm a little curious how they were able to determine this before even sending a craft there and measuring it and have a high degree of confidence in its accuracy.

The thing about space is, heat transfer is all radiative. The "dark" side of the probe, which is in its own shadow, faces the darkness of space (which is very cold indeed). Therefore you can put insulation on the side facing the sun, and a radiator on the dark side to remove any heat that gets through the insulation.

But space is a vacuum which is a pretty good thermal insulator. How are they dumping the heat?

Pick a material for the surface of the radiator which has high emissivity.

That is, when the material gets warm, it will convert the thermal energy to electromagnetic energy (light). Some of this light will escape, leading to a cooling effect.

This is known as “incandescence” or “black body radiation”, and it’s why metals glow red hot when they’re heated to a certain temperature. (Though it’s not always red, it can be white, blue, or even UV. Animals are warm enough to glow in infrared, which is what thermal cameras detect).

The physical mechanism is radiative cooling: https://en.m.wikipedia.org/wiki/Radiative_cooling

More details about the specific mechanism on the Parker Solar Probe’s cooling system: https://blogs.nasa.gov/parkersolarprobe/2018/09/14/parker-so...

IR radiation, in a nutshell.

They're going at night, duh

Reply on news.ycombinator.com