Particle Accelerator

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Guru Vishnu

5:56 AM

@JohnRennie: Good morning sir :-)

John Rennie

@GuruVishnu hi :-)

How did your function go?

Guru Vishnu

@JohnRennie It went fine. I see, I didn't tell what the function was. It was a house warming ceremony. It's a tradition to do that when we build/move on to new houses.

John Rennie

We have house warming parties in the UK as well, but they are just an excuse for a party :-)

Guru Vishnu

@JohnRennie Cool sir. I didn't know that. Just now learnt from Wiki - en.wikipedia.org/wiki/Housewarming_party that it's international!

John Rennie

Anyway, did you have fun? Was it a good weekend?

Guru Vishnu

6:02 AM

@JohnRennie Yes sir. Definitely. We had a lot of stuff to eat :-) Meantime, with my cousins we designed a morse code with a science kit and had fun transmitting messages through it.

John Rennie

Good :-)

Guru Vishnu

Sir, did you know that today a satellite manufactured by Airbus UK, was launched from US on an Atlas V rocket?

It's going to orbit closer to our star.

John Rennie

I didn't, but satellites get launched all the time and I don't keep track.

Oh, wait, you mean the solar explorer?

Guru Vishnu

Hmm... But this one is a bit special according to me.

@JohnRennie Solar orbiter.

John Rennie

O0ps, yes :-)

It's 6 a.m. here and I'm still half asleep :-)

Guru Vishnu

6:05 AM

:-)

And I think, today will be a busy morning as usual.

3 hours later…

Guru Vishnu

9:29 AM

@JohnRennie: Hi sir. If you were to paint a spacecraft that would orbit close to the sun, which colour do you choose - black or white?

My choice: White as it's a good reflector of thermal radiation.

John Rennie

@GuruVishnu typically they wrap the spacecraft in gold foil to reflect away the sunlight.

The last solar mission had a separate heat shield I think. I forget what it was called.

Guru Vishnu

Yes sir. Noticed that for most of earth orbiting satellites. BepiColombo had a silver foil, however.

John Rennie

Solar Dynamics?

Guru Vishnu

@JohnRennie Solar Orbiter :-)

John Rennie

No, the one before Solar Orbiter.

Guru Vishnu

9:36 AM

@JohnRennie Parker Solar Probe?

by NASA

John Rennie

Aha, yes! That's the one! :-)

Guru Vishnu

It had a white heat shield which faces the sun. It's logical to use white heat shield over any other colour due to its high reflectance.

But see for instance Solar Orbiter:

It has a BLACK heat shield! :-)

Do you have any physics based answer to this anomaly, sir?

John Rennie

Apparently the front is titanium foil

That black might just be a cover to protect the foil before launching.

Guru Vishnu

@JohnRennie No not so, sir. See the first image.

@JohnRennie Is titanium transparent?

John Rennie

@GuruVishnu no, it's shiny like aluminium foil. They probably used titanium because of its high melting point.

@GuruVishnu aha. The article I linked says:

> Solar Orbiter’s heat shield is coated with a thin, black layer of calcium phosphate, a charcoal-like powder much like pigments used in cave paintings thousands of years ago.

“It’s funny that something as technologically advanced as this is actually very old,” Pacros said. But the coating resists degradation under the blow of intense solar ultraviolet radiation. Although the black powder does absorb some heat, it’s excellent at shedding that heat back out to space.

Guru Vishnu

9:45 AM

Yes sir.

> Although the black powder does absorb some heat, it’s excellent at shedding that heat back out to space.

Black colour is also a good radiator.

So, it doesn't matter whatever the colour of the spacecraft's heat shield is? White-good reflector; black-good radiator ; Am I right sir?

John Rennie

The problem is that the black calcium phosphate will heat up then transfer heat to the titanium foil by conduction.

And conduction is typically far more efficient as a heat transfer process than radiation.

Guru Vishnu

10:01 AM

Fine. Now I got confused how it maintains a safe operating temperature on the other side of the heat shield. On Oct 29, 2019, I asked the same question on Space.SE, where I received an engineering based answer, and that's why I asked you. Since then, I'm really puzzled by this question. For some time I had to leave it in order to prepare for JEE. But today it launched and restarted the doubt loop in my brain.

You might find my following question and answer useful:

Q: Why is the Heat Shield of ESA's Solar Orbiter Black in colour?

Solar Orbiter (SolO) is a planned Sun-observing satellite, under development by the European Space Agency (ESA). Due to its proximity to the Sun, it receives about 13 times more solar energy than we receive on Earth, causing the temperature of its sunward-facing side to soar to 600°C. So there mu...