# For Questions That Don't Merit Their Own Thread

## Recommended Posts

11 hours ago, Racescort666 said:

I don't know if this should be broken into its own thread but would you need a space suit on Titan? Aren't space suits more to provide pressure for our body (and protection from radiation and stuff) but would it be necessary on Titan since it has a dense atmosphere? Obviously you'd need some sort of protective equipment since it's -180 C and there's no oxygen to breathe but you presumably wouldn't need a full blown EVA suit.

11 hours ago, Skylon said:

Well at the surface the pressure is 1.45 times that on Earth, equivalent to being around 5m underwater (i think)

I imagine the suits could be lighter and less strong, but still required.

You could get away with an unpressurised suit, but other than that you'd definitely need one.

Edited by Steel

##### Share on other sites

How much would global sea levels have to drop for a circular object with a radius of 480 kilometres to fit onto the island of Madagascar?

Edited by ChrisSpace

##### Share on other sites
4 hours ago, ChrisSpace said:

How much would global sea levels have to drop for a circular object with a diameter of 480 kilometres to fit onto the island of Madagascar?

The amount that land area increases when sea levels fall is hugely dependent on the topology of the ocean floor around Madagascar. If it's on a gently sloping region then a large change coulld be achieved with a change of only a few of metres, if it's on a steep shelf then potentially tens or hundreds of metres would be required.

One question I have is wouldn't this mysterious object already fit? At it's widest (I'm assuming the part that bulges) it is 560 km coast-to-coast. [1]

Edited by Steel

##### Share on other sites
3 hours ago, Steel said:

One question I have is wouldn't this mysterious object already fit? At it's widest (I'm assuming the part that bulges) it is 560 km coast-to-coast.

RADIUS. Small typo, I meant Radius. So, 960km diameter.

##### Share on other sites

You can look at the chart viewer on navionics.com.

Seafloor depth is a function of age, as the floor cools over time as it drifts away from a middle ocean ridge where it began towards a continent and thus becomes denser and sinks into the earths lithosphere. The oldest seafloor is near the continental shelfs. It doesn't get (much) older than 180-200my as it becomes so dense that it starts to rip off and be subducted again (**).

Local anomalies not counted (trenches at subduction zones which can be deeper or lifted lithosphere over hotpsots which can be above sealevel) a seafloor of late jurassic/early cretaceous age is 4000-4500m deep.

So, around Madagaskar, the floor drops quickly (tens to one hundred km off the coast) towards 4000m and deeper towards the east, less towards Africa probably because a few km of sediments from the near continent are deposited on top of the the oceanic crust. As 5000m represents roughly the maximum depth of oceans(*) on earth you'd have to swallow the whole ocean away to fit a 1000km diameter plate, center on Madagaskar, totally on dry land there.

May i suggest to put the object in the Sahara desert ? You'd only have to sweep a bit of sand ...

:-)

(*) yeah, can be slightly deeper

Edit (**): very generalized view. Mainly for oceans with an active ridge, like the Atlantic.

Edited by Green Baron
Little correction of numbers

##### Share on other sites
On 7/10/2017 at 10:54 AM, ChrisSpace said:

How much would global sea levels have to drop for a circular object with a radius of 480 kilometres to fit onto the island of Madagascar?

You'd need bathymetry. Sometimes small sumbarine volcanoes or underwater plateau or extremely extended shelf could help your job; alternatively a series of trench could make it very difficult. The island where I live in (Java) is among the example - even a five metre drop could mean that the previous coasts could be located well inland, while in other parts it could just be a minor change.

##### Share on other sites

You are right, @YNM. The conditions near your place are different from those around Madagascar. The latter is surrounded by ocean crust, it is effectively a small continent of its own. Java otoh is only isolated because of a relative high stand of the sea level, otherwise it would form a connected landmass with Borneo, Sumatra and even be connected to the Asian continent; if sea level was 50m lower. Towards Australia the depth of the sea isn't more than 300m. But towards the south and east depths quickly drop to 5000m as well, just not as steep as around Madagascar.

The 1000km plate might fit on and between the islands with a moderate drop of 60-70m in your area :-) "C'est pas la mer a boire" french "One doesn't have to drink the sea" meaning "it's not impossible"

Well, it is practically ...

Edited by Green Baron

##### Share on other sites

Can blood type compatibility be changed?

That is, can you either lyse the protein flags from existing blood, or add the flags to the leukocyte "whitelist?" Basically everything short of gene therapy of the entire subject is fair game.

##### Share on other sites
16 hours ago, 0111narwhalz said:

Can blood type compatibility be changed?

That is, can you either lyse the protein flags from existing blood, or add the flags to the leukocyte "whitelist?" Basically everything short of gene therapy of the entire subject is fair game.

Sort of. AFAIK, the closest people got is taking blood, centrifuging it to separate out the plasma, then introduce artificial red blood cells. I don't know if it's done as a procedure, but we have this capability.

This is rarely necessary though. There are few cases where you can't get the right blood type these days. The one case where incompatible blood types can be a very serious issue is in mother/fetus incompatibility. It's rare that it causes issues, but when it does, they can kill both mother and child. And in this particular case, in vitro fertilization with gene therapy can solve the problem, making child's blood compatible with mother's. It is currently illegal almost everywhere, but hopefully, we'll start doing that in the near future.

##### Share on other sites

What time of the year is the earth's night side rotated towards the centre of the galaxy?

##### Share on other sites
6 hours ago, Mighty1 said:

What time of the year is the earth's night side rotated towards the centre of the galaxy?

This happens near the (northern) summer solstice.

##### Share on other sites

Not a question, but never forget we have frickin rovers driving around Mars! (click image for full-size)

Edited by tater

##### Share on other sites

I'm not sure I'll understand the answers, but I will ask...

Does a photon drive violate the theory of relativity?

Maybe I need an example here. Someone launches a spacecraft that uses solar panels for power, most of which powers some form of energy-efficient bulb (say florescent or LED here.) This bulb is surrounded by a mirrored chamber to focus the light out of the back of the craft (similar to conventional craft.) I am told that photons can be a source (although very small amounts) of thrust. Yet, until the bulbs burns out, this sounds like one of those 'reactionless' drives.

Does that mean this set-up violates either general relativity/special relativity? Or do scientists have something else in mind when mentioning photon drives?

##### Share on other sites
1 hour ago, 55delta said:

I'm not sure I'll understand the answers, but I will ask...

Does a photon drive violate the theory of relativity?

Maybe I need an example here. Someone launches a spacecraft that uses solar panels for power, most of which powers some form of energy-efficient bulb (say florescent or LED here.) This bulb is surrounded by a mirrored chamber to focus the light out of the back of the craft (similar to conventional craft.) I am told that photons can be a source (although very small amounts) of thrust. Yet, until the bulbs burns out, this sounds like one of those 'reactionless' drives.

Does that mean this set-up violates either general relativity/special relativity? Or do scientists have something else in mind when mentioning photon drives?

No that setup would be an photon rocket, downside is that light has an very low trust, you would be better of making an solar sail who could be much larger for the weight and would reflect almost all light.
You would get more trust of the solar push on the solar panels than your light engine anyway.

##### Share on other sites
13 hours ago, 55delta said:

I'm not sure I'll understand the answers, but I will ask...

Does a photon drive violate the theory of relativity?

Maybe I need an example here. Someone launches a spacecraft that uses solar panels for power, most of which powers some form of energy-efficient bulb (say florescent or LED here.) This bulb is surrounded by a mirrored chamber to focus the light out of the back of the craft (similar to conventional craft.) I am told that photons can be a source (although very small amounts) of thrust. Yet, until the bulbs burns out, this sounds like one of those 'reactionless' drives.

Does that mean this set-up violates either general relativity/special relativity?

Reactionless drives are primarily prohibited by conservation of momentum, not by general or special relativity. I mean, general and special relativity factor into conservation of momentum, but that's neither here nor there.

And the setup you describe does not violate conservation of momentum. Each time a photon is released by the bulb, the total mass-energy of the spacecraft decreases ever so slightly. The loss of mass-energy is the energy of the photon; the change in momentum is the momentum of the photon.

##### Share on other sites
On 7/25/2017 at 10:21 AM, sevenperforce said:

Reactionless drives are primarily prohibited by conservation of momentum, not by general or special relativity. I mean, general and special relativity factor into conservation of momentum, but that's neither here nor there.

Conservation of momentum is a direct consequence of relativity. The details vary depending on which kind of relativity we are talking about. In Special Relativity and Galilean (Classical) Relativity there is a global translational symmetry, which leads to total momentum being a conserved quantity per Noether's Theorem. In General Relativity, there is a local Poincare Symmetry, which leads to a conserved current which momentum factors into.

So it's entirely valid to say that reactionless drive violates Special/General Relativity. That is where relevant conservation laws come from.

But yeah. Electromagnetic radiation, i.e. photons, has momentum. So all relevant quantities are conserved in a photon drive/rocket. You just need an obscene amount of power to get any useful thrust, and you still get a finite ISP, because energy has mass too.

##### Share on other sites
22 hours ago, K^2 said:

But yeah. Electromagnetic radiation, i.e. photons, has momentum. So all relevant quantities are conserved in a photon drive/rocket.

Just to be clear, do all forms of electromagnetic radiation have mass, or just some such as photons?

##### Share on other sites
8 hours ago, 55delta said:

Just to be clear, do all forms of electromagnetic radiation have mass, or just some such as photons?

All electromagnetic radiation (gamma, x-ray, microwave, visible e.t.c) is made up of photons. Photons do not have mass (rest mass, that is, which is what we classically think of as mass. I won't go into this now though, that's for another time.), but they do have momentum thanks to relativity.

Edited by Steel

##### Share on other sites

Sorry, I confused mass and momentum there.

But to me, this still leaves a bit of a question still and it's a tough one. If a photon drive conserves all quantities, how does EM/Canne drive designs violate the same?

Is something different with microwaves? Was the output too big for the input? Or was it that the explanation by the inventor/s was not scientifically sound?

##### Share on other sites
53 minutes ago, 55delta said:

Sorry, I confused mass and momentum there.

But to me, this still leaves a bit of a question still and it's a tough one. If a photon drive conserves all quantities, how does EM/Canne drive designs violate the same?

Is something different with microwaves? Was the output too big for the input? Or was it that the explanation by the inventor/s was not scientifically sound?

A photon drive expels photons as an exhaust which creates a thrust, thus satisfying conservation of momentum. EM drive has no exhaust, it just bounces microwaves up and down in a sealed chamber (but none of these photon leave the device), however it still supposedly creates a thrust, thus violating conservation of momentum.

Edited by Steel

##### Share on other sites

Also the supposed EM drive thrust is far higher than would be expected from a photon drive of the same input energy.

##### Share on other sites

Photons do have relativistic mass. They just are never at rest, so they have no "rest" mass.

Incidentally, all rest mass is, in fact, relativistic mass from the velocities of the subatomic particles; just wrapped up in particle-particle bonds.

##### Share on other sites

So the answers are much simpler than I expected them to be and it defines the difference neatly.

Thanks to everyone who answered.

##### Share on other sites
On 7/29/2017 at 7:48 PM, sevenperforce said:

Photons do have relativistic mass. They just are never at rest, so they have no "rest" mass.

Incidentally, all rest mass is, in fact, relativistic mass from the velocities of the subatomic particles; just wrapped up in particle-particle bonds.

That's not true. Not even all elementary bosons are massless, and none of the fermions are. More than 99% of the mass of the matter is dynamic, as you describe, but there is still some amount of inherent rest mass associated with individual fields. Also it's a bit more complicated than "relativistic mass from velocities," since a stationary electron still has a very high self-energy compared to the bare mass, but that's getting into details of Quantum Field Theory.

##### Share on other sites

Why did the space shuttle have such a huge, high vertical stabilizer on top of it? Aerodynamics Testing In KSP™™™ proves that the high torque makes spin recovery very hard, and just touching the rudder dangerous.

I think the rudder layout of the stock Dynawing would be much more reasonable for the space shuttle.

## Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

Only 75 emoji are allowed.