> We are very concerned with the fact that all the satellites which re-enter the Earth's atmosphere burn and create tiny alumina particles which will float in the upper atmosphere for many years
It is baffling to me that the BBC, a fairly respectable organization, allows this sort of terrible journalism. The author clearly does not understand the subject matter at all, which is not surprising since he's a business reporter regurgitating (presumably) a press release from a university and/or a forestry company.
“Briefly stated, the Gell-Mann Amnesia effect is as follows. You open the newspaper to an article on some subject you know well. In Murray's case, physics. In mine, show business. You read the article and see the journalist has absolutely no understanding of either the facts or the issues. Often, the article is so wrong it actually presents the story backward—reversing cause and effect. I call these the "wet streets cause rain" stories. Paper's full of them. In any case, you read with exasperation or amusement the multiple errors in a story, and then turn the page to national or international affairs, and read as if the rest of the newspaper was somehow more accurate about Palestine than the baloney you just read. You turn the page, and forget what you know.”
It's guaranteed to appear in about 5% of the comments every time an inaccurate article is being discussed.
That's why the leading edge, where the air is moving the slowest, is the hottest.
(This was one of the exercises in fluid mechanics class in college.)
The bit about the air cannot get out of the way fast enough comes from its inertia, not friction.
You'll find the same temperature rise in high speed aircraft like the SST. The max temperature rise is at the stagnation point where the air velocity is zero.
(Compress any gas and it warms up, this is how refrigerators work.)
At high velocities, this intuition breaks down. The communication time between molecules is limited to the speed of sound, the speed at which air molecules travel and bump into each other. If you are traveling faster than the speed of sound, (or several times faster, as is the case for re-entry), the air molecules don't have any information that an object is coming to them, because there hasn't been enough time for molecules hitting the object to then hit them.
It's a common example in fluid mechanics textbooks to derive the flow of a zero-viscosity gas around a cylinder, you can get an exact solution e.g. here: http://brennen.caltech.edu/fluidbook/basicfluiddynamics/pote... . Note that in this example there is no drag, but there are two zones of high pressure in front of and behind the cylinder. Those zones would have higher-than-ambient temperature.
When you squeeze a gas into a smaller volume it heats up because the heat energy of the gas which was spread out over the larger volume is now packed into a smaller one.
See Boyle's Law: https://en.wikipedia.org/wiki/Boyle%27s_law
What does it mean for a gas to heat up? Is the quantity of heat energy increasing, or the temperature?
A thermometer generally measures the temperature of molecules adjacent to the thermometer, which is a function of the amplitude of those molecules' vibration. More molecules within a given space increases the heat capacity, and the total heat energy represented by a temperature of the total quantity of molecules, but the temperature is a function of the average heat energy per individual molecule.
The temperature increase described by Boyle's law is a consequence of the work required to compress a gas—somewhere there has to be a compressor imparting kinetic energy to individual atoms.
Think of it this way, you have two containers of heat energy represented by 1L of an ideal gas at 1 atmosphere and 20°C. If you magically transported the contents of both containers into a single 1L container, and they continued to express a temperature of 20°C, you would have twice as much stuff, and contain twice as much heat energy in 1L. If on the other hand you had twice as much stuff and a higher temperature, then you would have increased the total amount of energy present in the universe.
The temperature comes from the extra energy put into the system to compress the gas rather than expressly from the density of the heat energy.
Because (2) is wrong. Compression occurs at a shock, as does heating, but that doesn't mean the heating is due (entirely) to compression.
Is the difference that hearing due to compression is a normal head-on collision, and heating via friction is a parallel "scratching" collision?
If you dropped a cube from space, the heat would be on the front face due do battery from air particles, not on the sides?
A temperature increase resulting from compression is a temperature increase resulting from friction. In other words, it's friction all the way down (until it's electromagnetism and other fundamental forces).
The other user claimed that physicists are making a mistake when they explain with friction rather than compression.
Looking back, I think your first comment was meant for the other user.
Unfortunately, the BBC is not what it used to be. In fact, it has probably never been as unpopular.
To be fair to the journalist, so does wikipedia, the National history museum and the FAA.
On balance, the journalist (and perhaps the editor) appears to be correct here.
I don’t know enough to say if you’re mistaken or not but i can’t find a good definition for your term “re-entry pollution”. Do you have any handy links for that term?
Wiki & the NHM re-define “space junk” as “space debris”. Google does this too but is just relying on these sources.
The wikipedia for space debris includes material that falls back to earth. It even lists some notable incidents. Wikipedia doesn’t appear to recognise the term “re-entry pollution”.
The FAA deals with debris here: https://www.ecfr.gov/cgi-bin/text-idx?SID=685669905c05d232fc... - it treats orbital and re-entry debris under the same provisions and doesn’t seek to further distinguish them.
>> clearly does not understand the subject matter at all
What sentences in the article lead you to this conclusion?
Each sentence in the article in itself is good. The sentences together reveal that the author had no understanding of the topic. (or decided to wilfully write a confusing mess)
Let me illustrate:
" Space junk travels at an incredibly fast speed of more than 22,300 mph, so can have cause considerable damage to any objects it hits.
In 2006 a tiny piece of space junk collided with the International Space Station, taking a chip out of the heavily reinforced window. " This is true! Objects on orbit have very high energies. When two of these objects collide they generate tiny shards which can collide into new objects, shattering them too. This can lead to a cascading problem where many satellites on a given orbit turn into mush at the same time. And not only that you loose these satellites, but their shards remain on the orbit posing risks for future satellites. You might heard of this effect as the Kessler syndrome.
You might be wondering, how does making the sat from wood would help with this? The answer is it wouldn't. But you wouldn't know that from the article. It has zero connective structure to illuminate this.
So why are these people working on a wooden satelite? To quote again from the article: "We are very concerned with the fact that all the satellites which re-enter the Earth's atmosphere burn and create tiny alumina particles which will float in the upper atmosphere for many years,"
Ah. So these people are not worried about the Kessler syndrome. They are worried that re-entering space craft break up, and particles from it can remain in the upper atmosphere. That's an interesting problem. Maybe worth addressing. But if that's the problem why is the article waxing about speed of objects in orbit and chips colliding with the international space station?
You say you can't find a good definiton of the term "re-entry pollution". And you are right. It's not a widely used term. But clearly an apt description of what the company is talking about. Is this a serious concern? Has there been studies about this effect? Is there a more commonly used term for this kind of pollution? Who knows. I don't know. You don't know. The article should be answering these questions for us. Instead all we have is a single sentence ""Eventually it will affect the environment of the Earth."" Nice. Not very well researched. And probably even the journalist felt this is not enough so they wrote multiple paragraphs about what he could find of the dangers of space junk. But sadly he found a totally irrelevant other kind of problem. If they realised this and wrote the article still as presented then they are wilfully confusing people. If they are just dazzled by all the terminology then so bad, I would still expect more from professionals.
"On balance, the journalist (and perhaps the editor) appears to be correct here." What do you mean by that? If someone writes a word salad they are correct just because others have wrote similar word salads before?
"Wikipedia doesn’t appear to recognise the term “re-entry pollution”." So bad for the company and the journalist. It sounds like they have to do more work to explain their problem then!
And your ecfr.gov link. It talks about making sure that your satelite does not explode on orbit. (To mitigate the problem of the Kessler syndrome.) It also talks about post-mission disposal in paragraph (4). It contemplates two scenairos. One where you put your sat to a lesser used orbit, or one where you deorbit your satellite. In case you choose to deorbit you have to show that you are not too likely to plonk anyone in the head. From the perspective of this second scenario the whole sat burning up into aluminium mist in the upper atmosphere is a success. The company disagrees and says that we should worry about other unspecified effects too. Are they right? Maybe! But the article sure didn't do a good job to explain the distinction, and thus failed to ask the pointy questions it should have asked.
That Wikipedia and the FAA do not have special terms for these distinct concepts does not mean they conflate them. Indeed, basically nobody cares about re-entry pollution because it's just an utterly trivial type of pollution (and it's the only thing this proposed tech addresses).
The article mentions tracking/estimation of debris greater than 1cm. This seems reasonable to me. I doubt particulate matter counts as debris. We can see this in other parts of life. For example, if you are doing a FOD walk, you don't care about dust and "invisible" particulate matter.
People who think space junk cannot polite the atmosphere are the confused ones, not the author.
Focusing on the environment is a good thing but we risk burning people out or worse having them assume everything they read if more scare and fiction that true issue.
we live in a world where the virtue signaling has gotten so loud people are deaf and just tune it out. those who cared will still care but we are losing an entire segment simply because we check ourselves
Going by the maxim of “Space is always more empty than one could possibly imagine.”.
>inb4 “if the moon were a pixel”.
It's true that even near-Earth space is crazy, crazy big so that the amount of volume occupied by debris is extremely tiny, but what's also true is that debris is moving fantastically fast so that the volume "effectively occupied" by debris is only small rather than extremely tiny. This cashes out in the empirical facts (which fit perfectly with our models) that there have been four unintended collisions between satellites and debris:
https://en.wikipedia.org/wiki/Satellite_collision
These have all taken place in the past 25 year (despite > 50 years of spaceflight) because the density is increasing. If the density keeps increasing without the forthcoming mitigation efforts, the rate of unintentional collisions would keep rising to bad levels.
Large quantities of such particles have actually been seriously proposed to help combat global warming. What are the downsides of them? When these particles finally migrate into the troposphere they quickly fall to Earth at relatively low velocity due to the very high drag.
I'm all against polluting the earth, but non-reactive aluminum particles in the quantities that we are talking about are such a non-issue in comparison to anything else.