
Traveling to the Sun: Why Won’t Parker Solar Probe Melt? - shreyanshd
https://www.nasa.gov/feature/goddard/2018/traveling-to-the-sun-why-won-t-parker-solar-probe-melt
======
newscracker
Towards the end, it says:

> “After launch, Parker Solar Probe will detect the position of the Sun, align
> the thermal protection shield to face it and continue its journey for the
> next three months, embracing the heat of the Sun and protecting itself from
> the cold vacuum of space.”

What a phenomenal piece of engineering! The article was not only fascinating
to read as a non-astronomer/lay person, but it also makes it all look like
child’s play, the way they decided what materials to use and how.

> “And to withstand that heat, Parker Solar Probe makes use of a heat shield
> known as the Thermal Protection System, or TPS, which is 8 feet (2.4 meters)
> in diameter and 4.5 inches (about 115 mm) thick.“

So is someone going to be bothering someone else about TPS Reports [1] over
the expected seven year span of this probe? Sorry, I couldn’t resist making
that reference! :)

[1]:
[https://en.m.wikipedia.org/wiki/TPS_report](https://en.m.wikipedia.org/wiki/TPS_report)

~~~
earenndil
Correct me if I'm wrong, but isn't space actually not cold at all -- there's
barely any matter there to have a temperature?

~~~
belorn
Good question. How does heat radiation work in space?

Seems the answer is that you don't need matter for heat radiation.

~~~
vkou
Convection doesn't work in space, so you only lose heat to radiation.

Space is cold, but it doesn't feel cold.

~~~
raattgift
(Natural) convection doesn't work in free-fall. That includes the Vomit
Comet[1], the ISS, the Apollo capsule between the Earth and the Moon, and so
on.

Fans/blowers can drive covection artificially, though.

Convection (natural or artificial) doesn't work in the absence of a convecting
fluid, even when not in free-fall.

Definitely not space:
[https://www.youtube.com/watch?v=xdJwG_9kF8s](https://www.youtube.com/watch?v=xdJwG_9kF8s)
from about the 3 min 40 second mark.

(FWIW, the slinky stuff is also really cool; weight -- in the contact[1] sense
but not in the _mg_ sense -- is dissipational, and it's nice to see that
demonstrated, so I'm glad your comment caught my attention.)

> space does not feel cold

If any part of you which you expose to space (if it's shielded from solar
heating, etc.) is moist -- your skin, your eyes, your tongue, the insides of
your nose -- you will feel that part getting cold very quickly thanks to
evaporative cooling, which works very well in free-fall and in the absence of
a convecting fluid.

\- --

[1]
[http://math.ucr.edu/home/baez/physics/General/Weight/whatIsW...](http://math.ucr.edu/home/baez/physics/General/Weight/whatIsWeight.html)

------
Sreyanth
Not a rocket scientist, but this article is very well written for an audience
with a minimal understanding of Physics and Chemistry. Articles like these
help high school students realise that what they're learning now is not really
a waste of time.

------
chiph
Dad joke: "They'll go at night"

But really, it's cool that they're using carbon-carbon protection similar to
that which was originally developed for the leading edges of the Space
Shuttle. And I really want to know how they built foamed carbon for the
interior.

I'm guessing that they're using white ceramic paint on top instead of a
reflective foil shield (like the Webb uses) because the foil would be shredded
by the solar particles.

~~~
kijin
I'm curious about the foam, too. Normally, foam contains a lot of air. But
that kind of foam will blow itself apart in a vacuum. How do they make foam
where all the air pockets are replaced with vacuum? Is vacuum-filled foam a
better or worse insulator than air-filled foam?

~~~
reaperducer
_Normally, foam contains a lot of air. But that kind of foam will blow itself
apart in a vacuum._

In the video, Thermal Protection System Engineer Betsy Congdon says it's 97%
"air."

I can't say whether it's actually air, or she's simplifying things for the
general public or not.

She also says twice that "water" is used in the radiators. But I'd have to
believe that NASA's using something that absorbs/dissipates heat a little more
efficiently. Perhaps whatever it is will end up in desktop gaming rig cooling
systems eventually.

~~~
zaarn
They used deionized water, not unusual for gaming rigs either.

The temperature range is about 15C to 125C, at high pressure this is most
ideal for use with water and water itself is a rather good coolant.

------
jccooper
The part of the answer here that NASA always seems to skip (perhaps because
it's not as fun as talking about all the cool heat-resistant technology) is
that the vehicle is in an orbit that will only take it into the corona for a
short time. While the closest approach is _very_ close, its aphelion is (at
closest) about the orbit of Venus. This gives it time to cool down after each
corona encounter.

[https://www.youtube.com/watch?v=cMNQeCWT09A](https://www.youtube.com/watch?v=cMNQeCWT09A)

~~~
Nokinside
Thank you. I was wondering just this.

Heat resistant material will eventually reach equilibrium where the back side
is almost as hot as the front side unless it's cooled somehow.

~~~
lifeformed
Also, the cool side of the craft will stay cool by radiating its heat away
into space.

~~~
bdamm
Is there a limit to the amount of heat a body of space can transmit via black-
body radiation? I wonder if the performance of the heat transfer is going to
be a useful measurement for science on this mission. I would think so. I just
can't help but wonder how much heat an area of space near a star can have
added to it. Is it infinite? Is the limit so high that it's far beyond even
the atmosphere of a sun? And are there special areas of space where the heat
limit has reached its maximum and if so what does that mean for the properties
of that space?

~~~
jccooper
The limit is based on your radiators, how much "empty" sky you can point them
at, and how hot you can get them. Those are all fairly finite. Space being
vacuum, there's no particular "amount of heat" it can hold, when you radiate
heat it just goes away. But there is a difference in bodies that can send heat
to you.

In space, most of the environment is "cold" in that there's not much energy
coming from it, so if you show the distant stars a hot radiator, it'll cool
off pretty well, because it's all send and no receive. But nearby bodies are
different: Earth is approximately room temperature (and in low Earth orbit
takes up quite a bit of the sky); the Sun is pretty hot, for a much smaller
portion of the sky (depending on your distance.)

If you're not near a planet, and you can reflect away most sunlight, you get
pretty cold. See James Webb Space Telescope or anything else with sun shades,
including the PSP.

The corona will change this a little, because it's a somewhat denser plasma
than usual, but not by much. It'll still be a question of radiator size and
heat, which is fairy easy to calculate.

------
JonoW
>This all has to happen without any human intervention, so the central
computer software has been programmed and extensively tested to make sure all
corrections can be made on the fly.

I'd love to get some deeper insight into how NASA writes and tests software, I
can only guess it's a million miles from how most of us work. Anyone know of
any good talks, articles from engineers there?

~~~
firebacon
The part that I find the most intriguing is "corrections can be made on the
fly".

I can see how you would ensure reliability through proper requirements
specification, a good software development process, separate independent
implementations and extensive verification.

However, every time I read a popsci article about space flight software, they
talk about this capability to push new code to the spacecraft while it is in
flight.

I'm really curious to learn what this looks like in practice (technical
details). Do they really have the ability to do an "ad-hoc" upload and
execution of arbitrary code on these systems? If so, how are the ad-hoc
programs tested and verified?

~~~
marsRoverDev
There is usually a piece of software running on the machine which basically
just does this - allows you to command an image upload to the SSD, do a
checksum of the file, then install it if all goes well. There is also usually
a simpler version of the software on a redundant SSD or partition which the
onboard computer will install if it detects that the software that is
currently installed is malfunctioning.

My understanding is that some spacecraft launch with beta/alpha equivalent
software. Correct me if I'm wrong, but I believe that the rovers do this, with
simple software installed first, then more complicated versions installed once
they know everything is working.

It's somewhat similar to updating your iphone, but instead you use a huge dish
to do the transmission and the bitrate is pretty horrendous.

I'm going to need a definition of "ad-hoc" here; no-one "deploys straight to
production" on a spacecraft. Any patches have to be thoroughly tested on
simulators and models of the spacecraft on earth before they are transmitted.

~~~
firebacon
Thanks for the reply! So what you're saying is that it's just a "normal" over-
the-air software update. I.e. you add some new functionality and then do a
full system test of all functions of the software before replacing the entire
image?

That makes sense, but is almost a bit disappointing. After all, that is
exactly how it works for the boring systems here on earth. From various wired
& co articles I had the impression that there was possibly something more; a
mechanism that would allow users to send elaborate "commands" to the
spacecraft to perform "ad-hoc" tasks at runtime. (What I mean by "ad-hoc"
tasks are tasks that are unknown at the time of validation/testing of the
software.)

~~~
marsRoverDev
Yes, we can send commands to the spacecraft once it's up there to do thing
like modify memory or hard drive contents directly, turn on/off or command
payloads and equipment. The full list is pretty exhaustive - anything you
could want to be able to do, you can command manually. These things aren't
100% autonomous (though they have autonomous elements in the software).

There is also a way to send pre-programmed task lists to them which are
executed sequentially, with delays if necessary.

That kind of thing is in the hands of operations, so it's not usually the job
of the software team to test in the normal manner.

~~~
firebacon
Very interesting! Is this kind of command capability (e.g. ability to modify
memory contents) something that is usually only available on "non-critical"
subsystems, or would you generally expect to also find it on critical
components, like the communication or navigation modules?

~~~
marsRoverDev
The ability to modify memory contents is pretty much universal; you can modify
things like eeprom contents, the RAM, hard drive, etc. There is no
differentiation between critical and non-critical; it's all just fairly
critical.

Ground won't send telecommands to a spacecraft to modify a piece of memory
without knowing exactly what they're doing first.

------
flashman
> If Earth was at one end of a yard-stick and the Sun on the other, Parker
> Solar Probe will make it to within four inches of the solar surface.

91cm and 10cm, to save anyone else doing the conversion. Also, it seems to
understate the closeness: Closest approach is 6.1 million km, which is 1/24th
of 1 astronomical unit, but four inches is 1/9th of a yard-stick.

------
MichaelMoser123
>Another challenge came in the form of the electronic wiring — most cables
would melt from exposure to heat radiation at such close proximity to the Sun.
To solve this problem, the team grew sapphire crystal tubes to suspend the
wiring, and made the wires from niobium.

are these wires on the outside of the spacecraft? but what about the silicon
of all the electronic stuff that this thing must be keeping? The cooling
surface would also get a bit hot (it would always get some more energy at some
rate), so how does the coolant transmit any heat away from the probe?

~~~
ourmandave
Melting points: sapphire - 3,722°F (2,050°C) niobium - 4,491°F (2,477°C)

~~~
SomeHacker44
Importantly, they likely have very different thermal conductivity and and
specific heats. They probably also need to have reasonably similar thermal
expansion coefficients so heating and cooling cycles do not cause them to
strain and break.

------
hliyan
As I understand, temperature is a measure of how fast atoms and molecules are
vibrating, and not a measure of how much energy per unit area of contact can
be transferred in a unit time.

~~~
guidedlight
Exactly. Stick your hand into an oven at 100 celsius ...OR... Stick your hand
into into a pot of water at 100 celsius.

The difference is that the water molecules are more tightly packed, than the
air molecules in the oven. In space, they are quite far apart.

~~~
mpweiher
> Stick your hand into into a pot of water at 100 celsius

..and: please don't! :)

~~~
sebazzz
Fast reflexes will prevent major damage. It won't prevent pain though.

~~~
jacquesm
As someone with extensive scarring on one arm from a water burn I'd prefer it
if you didn't put things like this out there.

Fast reflexes won't help your hand to recover from a bad burn, they won't
prevent it either, your hand is _much_ too large to be immersed fully and
retracted before substantial damage will occur.

------
TheSpiceIsLife
Previous discussion
[https://news.ycombinator.com/item?id=17569741](https://news.ycombinator.com/item?id=17569741)

------
gersh
They have a nice theory that the spacecraft won't melt when it gets close to
the sun, but do they really know for sure? NASA doesn't succeed on every
mission. It is possible something will go wrong, and the probe will melt.

------
austincheney
[http://earthsky.org/space/heat-shield-parker-solar-
probe](http://earthsky.org/space/heat-shield-parker-solar-probe)

It appears the heat shield is a carbon sheet sandwich. At first I was guessing
some form of tungsten-carbine, but that is the traditional material of NASA
heat shields.

> Why is the solar wind a breeze closer to the sun but a supersonic torrent
> farther away? Why is the corona itself millions of degrees hotter than the
> surface?

I suspect the answer to all those questions is simply _gravity_ , but it will
be nice to verify such things with data.

~~~
hutzlibu
"Why is the corona itself millions of degrees hotter than the surface?

I suspect the answer to all those questions is simply gravity, but it will be
nice to verify such things with data"

Can you explain your hypothesis a bit?

~~~
smueller1234
Not GP, but here's an article that talks about it. Note that this is
relatively recent. It's not something that was thoroughly understood in
ancient times exactly.

[https://www.nasa.gov/feature/goddard/sounding-
rockets/strong...](https://www.nasa.gov/feature/goddard/sounding-
rockets/strong-evidence-for-coronal-heating-theory-presented-at-2015-tess-
meeting)

~~~
superkuh
> [https://www.nasa.gov/feature/goddard/sounding-
> rockets/strong...](https://www.nasa.gov/feature/goddard/sounding-
> rockets/strong-evidence-for-coronal-heating-theory-presented-at-2015-tess-
> meeting)

It explains the temporal heating behavior at some scales. But it doesn't give
a mechanism of heating. It could be electron beam target heating. But it could
also be mediated by plasma waves.

The electron beams need acceleration and the most common suggestion is x-point
magnetic reconnection providing up and down voltage gradients due to changing
magnetic field. But the amount of electrons needed is unphysically large; the
entire electron contents of the relevant volume of the corona.

There are plasma wave models that don't require unphysically large parameters.

These two (and a couple other) options aren't clarified by the observation of
heating profiles. With the launch of Parker Solar Probe and the DKIST
(diffraction limited solar telescope) the two models above will finally be
testable. Spectroscopy of ion species by DKIST will tell what kind of heating
is happening and Solar Probe will be there to measure the input from the
corona.

------
msravi
> Particles may be moving fast (high temperature), but if there are very few
> of them, they won’t transfer much energy (low heat). Since space is mostly
> empty, there are very few particles that can transfer energy to the
> spacecraft.

That explains why energy is not transferred by conduction or convection to the
spacecraft. But what about energy (heat) transfer by radiation? Why won't the
spacecraft get all the energy from radiation and have its temperature shoot
up?

~~~
andbberger
Radiant shielding

------
cwmoore
A key sensor (Faraday cup [1]) for studying the solar wind, outside of the
shield, was constructed of not one but apparently four varieties of
_unobtainium_ : Titanium-Zirconium-Molybdenum alloy, acid-etched tungsten,
sapphire crystal tubes, and niobium wiring. All together these could make a
fascinating engagement ring.

[1]
[https://en.wikipedia.org/wiki/Faraday_cup](https://en.wikipedia.org/wiki/Faraday_cup)

------
netfire
The article makes it sound like there is no possible way for the probe to
melt. Is this actually the case? Is there no possibility that manufacturer
defects or a solar anomaly that could cause unexpected problems?

I don’t want to downplay the good design and engineering that went into this,
but should we be so confident without actually having done something like this
thousands of times?

~~~
perlgeek
> Is there no possibility that manufacturer defects or a solar anomaly that
> could cause unexpected problems?

There sure is. At least two of the systems (positioning and water cooling) are
active systems that could fail.

> but should we be so confident without actually having done something like
> this thousands of times?

"we" are confident enough that we rely on it to protect a > 1 billion USD
probe. What's the use in adding a lot of ifs and maybes to some piece of
marketing/explanation?

If it fails, adding some ifs and maybes to a marketing video won't really
change anything.

~~~
netfire
Doesn’t it undermine the credibility of NASA if something goes wrong? The
public who is the audience for this article is also paying the bills (through
taxes). I’d settle for just changing “won’t melt” to “shouldn’t melt”. I think
that appropriate for something that’s never been attempted.

~~~
dawnerd
Either way we’re chucking lots of money at the sun so if it fails we still
learn something, not to mention developed a lot of technologies along the way.

~~~
netfire
For sure, but it would be a shame if NASA lost confidence and trust (and
possibly funding) with their stakeholders (the public), because they weren’t
more upfront about the potential risks. As a scientific organization that has
experienced significant (and expensive) failures before, I expect better.

~~~
natep
It seems like you want to discredit NASA now ("shouldn't melt") vs some
imagined possibility("won't melt" but it does). With the amount of design,
analysis, testing, and independent review and verification of the systems,
backup systems, triply redundant systems, and autonomy, we can be as sure of
it not melting as we can be sure of anything. And we certainly spent a large
amount of money on this (about 1/10th of the World Cup), but in performance
and value per $, it's a great deal.

~~~
netfire
Not at all. I expect NASA has done as much due diligence, planning, testing,
and verification as possible. I just don’t think they are being upfront as
they should be about the possible risks for a previously unattempted
scientific endeavor. We’ve had massive failures (including NASA itself), in
environments that are much better understood and with systems that have
actually had exposure to those environments.

------
close04
Any reason why they're using water for cooling? There's no other liquid with
lower density and perhaps better thermal properties?

I imagined they would try to save weight in some places if it allows them more
freedom in others. Although I have no idea how much water is used in the first
place so it might be a moot point.

~~~
ComputerGuru
> The coolant used for the system? About a gallon (3.7 liters) of deionized
> water. While plenty of chemical coolants exist, the range of temperatures
> the spacecraft will be exposed to varies between 50 F (10 C) and 257 F (125
> C). Very few liquids can handle those ranges like water. To keep the water
> from boiling at the higher end of the temperatures, it will be pressurized
> so the boiling point is over 257 F (125 C).

~~~
close04
I just imagined that for an object sent _to the Sun_ they'd want to... overdo
it a little. Go for something synthetic that would behave even better or would
save 1/2 Kg.

I'm just the inquisitive type. Explaining that something is used always makes
me wonder "why not something else" :).

~~~
tesseract
Water has a higher specific heat than most substances, and probably the
highest of anything that's a liquid across the operating temperature range for
the electronics.

------
otto_ortega
It blows my mind to think that we DO have materials that can withstand such
temperatures, even after understanding the part about heat transfer.

I hope that kind of materials can be mass-produced on the short term future to
be used as insulation for homes!

------
trhway
With 200km/s deep orbit we should have probably brought several tons of fuel
to do Oberth burn say for a piece of the probe which would then pass by the
Vojager in a few years and be on track to get to alpha centauri in several
thousand years :)

------
jeandejean
The article and the videos it contains are amazingly clear. Cannot wait for
the first results of that mission! I'm always amazed by NASA great endeavors,
and how these scientists can be so sure their probe are not going to melt...

------
jogundas
This mission is also very cool in the sense that the period from launch to
first interesting data is short. There is going to be a Venus flyby on 28 Sep,
and a Sun flyby on 1 Nov.

------
callumprentice
Some of the most incredible astronomy shots I've seen are of close(r) ups of
the sun - I can't wait to see what Parker sends back as it closing in on the
Sun.

------
known
Can we use same probe to travel to Earth's core?

------
FigmentEngine
go at night...

------
nextstep
Inches? A yard stick? Millions of degrees Fahrenheit?

I’m American and I’m embarrassed by this. This is science, make it easy for
people to understand. Use SI units, please.

~~~
trgv
It's written for a general American audience so it makes sense to use those
units.

