For Questions That Don't Merit Their Own Thread

[Green Baron]

I don't care what the wiki says, it is written by people like you and me ;-), i try to find papers on the matter. A difference of 1.4 alone would be at the lower range of the density of a small rocky asteroid. Yes, that is a lot, more than water. So there goes your sarcasm ;-)

A planet is much denser than an asteroid because of its gravity and not only because of the elements it consists of, it is much more compressed, one cannot compare these and say they have the same densities and so must have the same composition, that does not work.

What i was trying to explain is that the density in the wiki may be wrong. It is an extrapolation, a guess, inferred by comparison with other material. We don't know the density of Psyche, but we do know its albedo. You can find deduced densities from 1.8 from an encounter to 4.6 (all g/cm³). Density estimations depend on diameter estimations, porosity estimations and measured albedo (the only measured thing here apart from unlikely encounters), which (the albedo) can be a hint to estimate surface material.

Likewise, as you say, can the guess be nonsense that psyche is a pure metal core, as depicted above. It probably is not and all our arguing is about nothing because we defend estimations at the edges of what is observed that are just wrong. There are other explanations out for the apparent contradiction between Psyche's albedo and the density of comparable roids. Which one can go through if interested.

And btw., 3.3 would still be ok for a 50% metal asteroid of the size and porosity one can assume for Psyche. No problem here ...

Edited March 14, 2019 by Green Baron

[kerbiloid]

1 hour ago, Green Baron said:

I don't care what the wiki says

For sure, I should believe your words about 4.6 more than wiki's 3.7 (obviously taken from some book or article about the asteroids).

Wait... Or no.. It cannot be so! There is a link [3] to the source of information about density=3.7.

https://en.wikipedia.org/wiki/16_Psyche#cite_ref-Hanus2017_3-1

 Hanuš, J.; Viikinkoski, M.; Marchis, F.; Ďurech, J.; Kaasalainen, M.; Delbo', M.; Herald, D.; Frappa, E.; Hayamizu, T.; Kerr, S.; Preston, S.; Timerson, B.; Dunham, D.; Talbot, J. (2017). "Volumes and bulk densities of forty asteroids from ADAM shape modeling" (PDF). Astronomy & Astrophysics. 601 (A114): 1–41. arXiv:1702.01996. Bibcode:2017A&A...601A.114H. doi:10.1051/0004-6361/201629956.

 

1 hour ago, Green Baron said:

A planet is much denser than an asteroid because of its gravity and not only because of the elements it consists of,

Even such heavy thing as Earth still has 12 inside against 5.5 in average.
So, a puny asteroid cannot be stretched twice in any case.
I can believe if the asteroid 3.7 in total has core of 4.6 (but that just means that an unstretched core would be less than 4.6, so honest crustal 3.7).

1 hour ago, Green Baron said:

What i was trying to explain is that the density in the wiki may be wrong.

Everything in our picture of the Universe maybe wrong. Does it mean that we should restart the discussion about the assumptions and empiric experience?

1 hour ago, Green Baron said:

We don't know the density of Psyche, but we do know its albedo. You can find deduced densities from 1.8 from an encounter to 4.6 (all g/cm³).

So, it is anyway several times less dense that a polluted ball of iron which we usually understand as a planet core.
It's average estimaton is 3+, while both 2 and 4 are estimated as marginal values.

1 hour ago, Green Baron said:

And btw., 3.3 would still be ok for a 50% metal asteroid of the size and porosity one can assume for Psyche.

As well as it is ok for a 0% metal asteroid.
No evidence that Psyche is something more than a one more rock with some amount of metal semi-extracted from the silicates.

UPD.
Long, long ago there was a planette, Protopsyche.
It was much smaller than Moon, but enough big to start slowly differentiating.
Then something crashed into it. The poor planette was broken. The inner piece of the planette is now known as Psyche.
If it was a metal core, it would be much denser, almost 7.8.
If it had a differentiated metal core inside, it would not be metal-looking on surface. It would have a dense iron core and a thick layer of rock above it.
But it's not a core. It's just an underdeveloped miscarriage of the core. 

Edited March 14, 2019 by kerbiloid

[Green Baron]

46 minutes ago, kerbiloid said:

As well as it is ok for a 0% metal asteroid.

No, it is not. Here's the problem: one cannot compare a planet like earth with an asteroid. A rocky asteroid can not have a density of 3.3. These densities for silicate crusts are found in earth's mantle. Basaltic material that has been brought up by tectonic processes e. g. at ocean ridges can have that density too on earth, until it weathers away or is subducted again. But earth's crust is much fluffier than that, around 2.7 - 3.0 in total, even under it's surface gravity. Density goes up to ~13 in the inner core because of gravitational compression (no pores, crystalline structure) which an asteroid does not have or only very little.

A rocky asteroid alone is too small for a density of 3.3, if it has not been chipped off from a much larger body where these densities can exist. An iron asteroid otoh can have a density of 3.7, or even 3.3, considering the much higher porosity and clastic rocky material in between the iron. Marsian crust, though mostly basaltic, may have a density of ~2.6 because of porosity, but still lower than earth's continental crust's median density. Because earth's continental crust is of less dense material than basalt, due to higher porosity, which is a function of gravity.

But we totally consent in the assumption that Psyche may not be a stripped core of something bigger. No problem here.

And yes, in some cases i do know more than the wiki. Edit: i would expect everyone with a special interest to do so as well.

Edited March 14, 2019 by Green Baron

[DDE]

Does sustaining a static magnetic field require energy input? Specificallu, how much battery power do I need to keep an electromagnet active?

[kerbiloid]

1 hour ago, DDE said:

Does sustaining a static magnetic field require energy input?

Resistivity anyway causes losses.
(If you mean the electromagnet),

https://www.engineersedge.com/calculators/magnet-lifting/magnetic_force_calculator.htm

The magnetic flux is caused by the current → circuit → resistivity → losses.

Edited March 14, 2019 by kerbiloid

[p1t1o]

3 hours ago, DDE said:

Does sustaining a static magnetic field require energy input? Specificallu, how much battery power do I need to keep an electromagnet active?

No. As an example, consider a permanent magnet.

An electromagnet has energy costs but that energy doesnt go into the field - as long as we are talking about a static field. Moving fields are another matter - eg: there will be some energy consumed on initial generation of the field.

The power required for a certain field strength will depend on the resistance of the wires. EG: superconducting MRI scanner has very strong field but low energy requirements (although the cooling system might have higher power requirements).

A magnetic field is like a spring, a spring does not require energy to sit there, but you can store energy inside it.

Edited March 14, 2019 by p1t1o

[Cunjo Carl]

13 hours ago, DDE said:

Does sustaining a static magnetic field require energy input? Specifically, how much battery power do I need to keep an electromagnet active?

You can get the power requirement using any of the following formulas based on what information you have available for your electromagnet.

Power = V²/R = I*V = I²*R

With Power in Watts, nominal voltage "V" in Volts, current "I" in amps, and resistance "R" in ohms. 2 out of the 3 should be provided by the manufacturer. On the other hand, if you're looking to make your own and want to know more let me know!

 

Separately, in regards to the planes @ARS asked about a while back, but a bit off topic...

Spoiler

The show they came from just showed up on my streaming service. I just watched the first episode and it's not my cup of tea, but it has some cool tech demonstrated like a pre 1st gen fighter startup sequence (at about the 8:00 mark), and some very kerbal moments like taking off from the inside of a remarkably impractical dirigible. So, maybe some will find it entertaining!

You can watch it here free, and if you feel the urge to binge watch it please feel free to use one of these codes to make it commercial free for a couple days.       RSVGTZMMARM   SAN4Y3CZJKE   UFUKBM7TYJX

Oh, I also wanted to mention Space Brothers! I was surprised the other day when a non-technical friend used the word 'regolith' in conversation after watching the show. It's apparently a surprisingly good near future look at an everyday fellow trying to become a JAXA astronaut. I'm personally saving it for a rainy day...

 

 

Edited March 15, 2019 by Cunjo Carl

[Cunjo Carl]

On 2/23/2019 at 12:11 PM, Aperture Science said:

It's me again, back with another question. This time about fluid dynamics.

The de Laval nozzle accelerates a gas while reducing its pressure, correct? If I reverse its orientation, will the result be a gas with a slower velocity, but higher pressure?

If not, is there any other device that increases fluid pressure without moving parts?

 

There is! All jet airplane air intakes actually do this, and it's referred to as "inlet pressure recovery".

In a rocket engine, the high pressure gas is choked at the de Laval nozzle throat, and allowed to expand through the nozzle where it trades its temperature and pressure energy for kinetic (flow) energy going out the back. It's an 'isentropic' process (ideally at least), meaning it can be reversed by reversing the situation. So if we instead start with a fast flowing gas (imagining the air around a supersonic airplane), and we intake that air through a nozzle and a choke it we'll get a high temperature and pressure gas. Unfortunately, we can't put enormous bell nozzles on front of our planes, so the real world pressure recoveries aren't that spectacular when traveling at high mach speeds. I wasn't aware before hand, but I just checked, and it appears that pressure recovery is quite good at lower speeds, especially sub mach.

This, and the expanding tube diameter you mentioned are the only situations I'm aware of that increase pressure without moving parts or an external energy input.

 

Edit: *All jet intakes except scramjets use inlet pressure recovery. On the other hand ramjets only use inlet pressure recovery with no moving parts to further boost the pressure like turbojets (the standard turbine engines) have.

Edited March 15, 2019 by Cunjo Carl

[Chel]

How exactly would a space elevator work? Wouldn't it just like flop around once getting up to a certain height? How would we even construct it?

[Lisias]

43 minutes ago, The_Cat_In_Space said:

How exactly would a space elevator work? Wouldn't it just like flop around once getting up to a certain height?

Traction, centrifugal and inertia. 

You need a really heavy weight up there, in a geosynchronous orbit,  to keep a very resistent and rugged tape tied to it tractioned so the flopness will not occur by mechanical resistence.

Clamping wheels would pull the vehicle up by friction.

Since to each action there's a reaction, that weight upthere must be such that the weight of the vehicle going up accelerating against the gravity doesn't disturb his orbit (not to mention the weight of the "tape").

Edited March 15, 2019 by Lisias
Hit "Save" too soon.

[ARS]

What modifications required to turn land-based aircraft into carrier-based aircraft? If a purpose-built carrier-based aircraft is assigned as land-based aircraft, does it need modifications too? Because I think taking off and landing on carrier is much harder compared to landing on land-based airbase

[DDE]

2 hours ago, The_Cat_In_Space said:

How exactly would a space elevator work? Wouldn't it just like flop around once getting up to a certain height? How would we even construct it?

You’d tow an asteroid into geostationary orbit, and ise it as a counterweight.

58 minutes ago, ARS said:

What modifications required to turn land-based aircraft into carrier-based aircraft? If a purpose-built carrier-based aircraft is assigned as land-based aircraft, does it need modifications too? Because I think taking off and landing on carrier is much harder compared to landing on land-based airbase

Significant reinforcement of the overall structure. Folding wings. Landing tailhook, modifications to the nose gear for the catapult system. Hardening against salt water corrosion.

None of these negatively impact operation from conventional airstrips, they’re merely excessive. I understand that the most recent MiG-29 upgrade was built from the groundhp as a two-seat carrier aircraft, and then the airframe would be downgraded from there (e.g. replace copilot with fuel tank).

[Green Baron]

5 hours ago, The_Cat_In_Space said:

How exactly would a space elevator work? Wouldn't it just like flop around once getting up to a certain height? How would we even construct it?

Hypothetically, one would build a platform in geostationary orbit, put a counterweight above it and connect equator, platform and counterweight with a "rope".

Practically there is no material that could hold the stress, no propulsion to tow a counterweight in high earth orbit and hold it there until connected and anyway no means to build such a thing. Ignoring the problems with coriolis forces as well as a "rope" sweeping through the cloud of satellites and space junk once a day.

Edited March 15, 2019 by Green Baron

[kerbiloid]

Spoiler

6 hours ago, The_Cat_In_Space said:

How exactly would a space elevator work?

You get into the lift cabin, press the button with the floor number, and it moves.

A short video instruction.

Spoiler

 

 

-

 

Edited March 15, 2019 by kerbiloid

[Lisias]

9 hours ago, ARS said:

 Because I think taking off and landing on carrier is much harder compared to landing on land-based airbase

Take off from carriers are more dangerous, but not necessarily harder. Landing there, on the other hand, it's almost suicidal.

[Green Baron]

The GTC telescope has opened a live website:

http://grantecan.es/en/gtc-live

Local timezone is UTC. I'll have a look later to see what they are aiming at :-)

 

Edited March 15, 2019 by Green Baron

[DDE]

2 hours ago, Lisias said:

Take off from carriers are more dangerous, but not necessarily harder.

Well, it is, which leads to a lighter weapons and fuel load all else being equal. Even catapults are of limited help.

[Lisias]

12 minutes ago, DDE said:

Well, it is, which leads to a lighter weapons and fuel load all else being equal. Even catapults are of limited help.

Not harder than doing the same from civil roadways or pretty short airstrips. Carriers are not he only ones in need of short take-off procedures.

If you want a way more dangerous approach, give a peek on this:

Spoiler

 

I'm not saying that taking off from Carriers is easier. I'm just saying that it's not necessarily harder. Taking off from any short length strips, being a carrier or not, is harder.

Edited March 15, 2019 by Lisias

[DDE]

14 minutes ago, Lisias said:

Not harder than doing the same from civil roadways or pretty short airstrips. Carriers are not he only ones in need of short take-off procedures

I’d say they set a very short bar.

[kerbiloid]

Say, we have SETI.
How can we insult/offend an extraterrestrial civilization to make them send a fleet here?
(Unlikely we can use harsh words, gestures, or anything biology-related, as we have no idea what are biological and social normality for them.)

[Gargamel]

It depends on the carrier.   If the carrier is big enough, no modifications are necessary.  Smaller carriers though can only be accessed by VTOL aircraft.   So your mileage may vary. 

[Lisias]

24 minutes ago, kerbiloid said:

Say, we have SETI.
How can we insult/offend an extraterrestrial civilization to make them send a fleet here?

Send "All your base are belong to us" to them.  

[adsii1970]

2 minutes ago, Lisias said:

Send "All your base are belong to us" to them.  

No, write a program that rewrites their computer systems and replaces it with Windows 10, subjecting them to the fun of Windows Updater. Guarantee they'd show up... in a rather angry mood...

[Gargamel]

Just send them a list of the primes

2 3 5 7 11 13 16 19.....

[kerbiloid]

13 minutes ago, adsii1970 said:

Guarantee they'd show up... in a rather angry mood...

That's the aim.
With the fleet of guaranteed angry aliens moving to us, the humans will be forced to rapidly develop the thermonuclear energetics and the space program regardless of money, asteroid mining, and so on.