For Questions That Don't Merit Their Own Thread

[Snark]

24 minutes ago, fredinno said:

How does the gravity of the Earth change as you go down into the planet? I heard that you'd be weightless in the core, but how about everywhere else? The mantle? The outer core?

If you're outside a hollow spherical shell, you experience the shell's gravity as if it were a point mass in the center of the sphere.  However, if you're inside a hollow spherical shell, the shell's mass contributes no gravity to you-- it all cancels out.

Therefore, to work out what would be the gravity for any given point inside a planet, at some depth:  basically, just consider that (spherical) portion of the planet that's below your depth, and treat it as a planet, and work out its gravity.  The part of the planet that's above you simply doesn't count.

That's why there's zero gravity at the earth's center:  because everything is above you.

If you were at a depth of, say, 1000 km, then the way you would work out the gravity would be:  imagine taking the Earth, and stripping off its outer 1000 km, and standing on the surface of what's left.  That's what the gravity would be.

For a planet of uniform density (which the Earth is not), then the maximum gravity will be on the surface, whereas the deeper you go, the lower the gravity gets.

In the case of the Earth, the density is very much non-uniform.  The center is a lot denser than the outer layers.  The Earth's overall density is something like 5.5 g/cm³ IIRC, but I think that varies from something like 12ish at the center to a whole lot less than that in the crust.

Therefore, whether the gravity would go up or down as you descend from the surface depends on the density profile.  As you go down, from the surface, you're losing the contribution of the mass above you... but you're moving closer to that really dense mass concentration at the core.  The former would tend to reduce gravity, the latter would tend to increase it, and just which one would predominate would depend on the density gradient.  Certainly the gravity will eventually have to go down as you approach the center, but I don't know off the top of my head whether max gravity would be on the surface or at some depth below it.

[peadar1987]

In case anyone's interested, I had a play around, and assuming the earth has a constant density (which it doesn't), the drop-off in gravitational acceleration seems completely linear, no weird curve to it at all.

[Snark]

56 minutes ago, peadar1987 said:

In case anyone's interested, I had a play around, and assuming the earth has a constant density (which it doesn't), the drop-off in gravitational acceleration seems completely linear, no weird curve to it at all.

Yes, that's correct.  For a sphere of uniform density, surface gravity is directly proportional to the radius.

While we're on the topic of fun things that fall out of the math:  If you have a sphere of average density ρ, then the time taken for a circular orbit at the sphere's surface is t = sqrt(3π/Gρ).  Note that there's no term for radius in that equation.  In other words, all spheres of a given density have the same surface orbital time.  For example, the Earth has an average density of 5500 kg/m³.  If you do the math, that works out to a surface orbital time of about 84 minutes (never mind the pesky atmosphere getting in the way).  If you had  a ball bearing of the same density, which is 1 centimeter in diameter... it would also have the same surface orbital time. 

[fredinno]

What is the difference between hard and soft docking?

[peadar1987]

3 hours ago, fredinno said:

What is the difference between hard and soft docking?

Soft docking is usually done using something with some degree of flexibility (like the probe of a probe and drogue design). This reduces the initial force caused by the motion of the spacecraft, and any misalignment. Once you're soft docked, you can use that connection to stabilise and bring the craft together, and make a stronger, more permanent connection using bulkier latches, which is hard docking.

If you went straight for hard docking, you'd need a ridiculously precise docking procedure, and create a lot of force in the docking mechanism, but if you only went as far as soft docking, your connection wouldn't be particularly strong or rigid, and certainly wouldn't be able to maintain a pressure seal if that's what you were after.

[KerbonautInTraining]

How in the absolute %£€×@!₩ do you track a deep space probe?

With something like the ISS you can track it's motion across the sky and combine that with the doppler effect to get a state vector. 

With something approaching Mars you can't even dream of tracking it's motion across the sky, it'll just be noise. You can still use the doppler effect but that's only half of the equation. How do they do it?

[fredinno]

2 hours ago, KerbonautInTraining said:

How in the absolute %£€×@!₩ do you track a deep space probe?

With something like the ISS you can track it's motion across the sky and combine that with the doppler effect to get a state vector. 

With something approaching Mars you can't even dream of tracking it's motion across the sky, it'll just be noise. You can still use the doppler effect but that's only half of the equation. How do they do it?

Either triangulation with GEO satellites, stars, or asteroids, all with known positions.

[Dfthu]

Could they reuse the falcon heavy boosters and then make them boosters for the core or cant they do that? How about taking a side booster and making it a core on a Falcon 9? (no boosters) How hard would it be to make them cores or vice versa?

[Kryten]

On 4/8/2016 at 4:04 PM, KerbonautInTraining said:

How in the absolute %£€×@!₩ do you track a deep space probe?

With something like the ISS you can track it's motion across the sky and combine that with the doppler effect to get a state vector. 

With something approaching Mars you can't even dream of tracking it's motion across the sky, it'll just be noise. You can still use the doppler effect but that's only half of the equation. How do they do it?

Long-baseline radio interfereometry, and sometimes celestial navigation (e.g. NH's approach to Pluto).

[sevenperforce]

11 hours ago, Dfthu said:

Could they reuse the falcon heavy boosters and then make them boosters for the core or cant they do that? How about taking a side booster and making it a core on a Falcon 9? (no boosters) How hard would it be to make them cores or vice versa?

Easily. Now, components will probably be reused in the same configuration at first, simply because that can be done at the launch site. But once a booster has been reused several times and needs to be shipped back to the factory for more extensive refurb, we will surely see reconfig.

[YumonStudios]

18 hours ago, Dfthu said:

Could they reuse the falcon heavy boosters and then make them boosters for the core or cant they do that? How about taking a side booster and making it a core on a Falcon 9? (no boosters) How hard would it be to make them cores or vice versa?

Probably easily. It seems the only changes on FH boosters to the F9 cores is the nozecone vs the 2nd stage engine fairing. It probably isn't ideal to interchange the two, but it should be possible.

[PB666]

On April 7, 2016 at 2:47 PM, Snark said:

Yes, that's correct.  For a sphere of uniform density, surface gravity is directly proportional to the radius.

While we're on the topic of fun things that fall out of the math:  If you have a sphere of average density ρ, then the time taken for a circular orbit at the sphere's surface is t = sqrt(3π/Gρ).  Note that there's no term for radius in that equation.  In other words, all spheres of a given density have the same surface orbital time.  For example, the Earth has an average density of 5500 kg/m³.  If you do the math, that works out to a surface orbital time of about 84 minutes (never mind the pesky atmosphere getting in the way).  If you had  a ball bearing of the same density, which is 1 centimeter in diameter... it would also have the same surface orbital time. 

Concerning density, Yeah i saw that on wikipedia

surface gravity is mu/r^2 That is to say surface gravity is the inverse square of the distance tonthe center of th point mass

 

if you reduce a body to a point mass, and at the point you have a light, then the intensity of the light and the pull of gravity absolutely correlate with radius. This is probably a reflection of the fact that both are transmitted as fields. 

Potential Energy is different. Energy, the  escape energy for any non-orbiting object is u/r. If you place the object in an orbit of say a, then the escape potential is 0.5mu/a. The problem with escpe energy is that its only relevant for objects in non-inertial reference frames or in the case of an object in an eccentrity =1 in which the mass is a point source. If we only look at non-inertia objects escape energy is 0.5mu/a. 

 

[Wjolcz]

How much cheaper would the ISS be if it was launched using the Falcon 9 rockets only?

[Kryten]

17 minutes ago, Veeltch said:

How much cheaper would the ISS be if it was launched using the Falcon 9 rockets only?

Not applicable. Falcon physically couldn't lift some of the modules, and many parts of the US segment required shuttle for transfer to station and it's arm for installation.

[StrandedonEarth]

It couldn't be done. Ignoring the fact that many ISS modules were designed to be lifted by the Shuttle, many of the more ungainly items (Canadarm 2 for one) would have required its payload bay. Without that, some sort of support structure would have been needed, adding more up-mass. The payload bay also allowed sending up a few smaller components in one go, without having to pack it into a shipping container (the Shuttle was the shipping container)

Aside from that, the Shuttle acted as a work platform, supporting the extra (specially trained) bodies involved in the construction, and providing crane services (Canadarm 1).

Oh sure, it could have been built without the Shuttle, but it would have been engineered completely differently. Cost is hard to estimate, it would have needed many more launches. And it probably would have had a different form. Shuttle could lift 16 tons to the ISS.. Falcon 9v1.1 can lift 13 tons. So some components would have needed to be smaller.

[Nibb31]

To launch on a rocket, ISS modules would have needed their own propulsion and power to actually rendez-vous with the station, either with some sort of disposable tug, or built-in like the Russian modules. That would have made them much much heavier, and for parts like the solar panels, truss segments, or the CMGs, it simply might not be possible.

[wumpus]

On 4/8/2016 at 1:13 PM, fredinno said:

Either triangulation with GEO satellites, stars, or asteroids, all with known positions.

And really good radios with tons of filtering.  NASA still tracks Voyager I/II (or at least did for decades) and was getting virtually zero signal from them (for shocking low values of zero).

[RainDreamer]

How cheap does rockets have to be to be viable as suborbital transportation vehicle between locations on earth, assuming we perfected the landing technology like spaceX booster, and ignoring security/legal stuff?

[Nibb31]

1 hour ago, RainDreamer said:

How cheap does rockets have to be to be viable as suborbital transportation vehicle between locations on earth, assuming we perfected the landing technology like spaceX booster, and ignoring security/legal stuff?

Suborbital point-to-point transportation isn't viable. It wouldn't be faster than a supersonic transport and requires more energy.

[Wjolcz]

Why are the water outbursts (or fountains, or whatever they are called) on Enceladus seen mainly at the poles? Is it because the gravity of Saturn/spin of the moon makes ice on the equator too thick for water vapours to shoot into space?

[shynung]

11 hours ago, Nibb31 said:

Suborbital point-to-point transportation isn't viable. It wouldn't be faster than a supersonic transport and requires more energy.

It is viable, but only for very specific payloads. Something that needs to be delivered fast, and be virtually untouchable in transit. Nuclear missiles, in case it wasn't obvious.

So the question turns to how much a single Minuteman missile costs. Wikipedia says $7,000,000, which I assume includes the payload.

[RainDreamer]

Is there any uses for high earth orbits -commercial, scientific, or otherwise - aside from being trash can orbits for defunct geostationary sats?

[Kryten]

48 minutes ago, RainDreamer said:

Is there any uses for high earth orbits -commercial, scientific, or otherwise - aside from being trash can orbits for defunct geostationary sats?

There's the Vela Hotel nuclear test detecting sats and various magnetospheric missions, but that's about it for stuff entirely in HEO. The Russian RadioAstron space radio telescope uses an elliptical orbit which causes to be in the HEO region most of the time, this giving a large distance relative to earth-based telescopes to use for radio interferometry with them. In theory you could station an IR telescope there to reduce interference from the earth's thermal radiation, but in practice all those so far placed above LEO have gone with sun-earth Lagrange points or outright heliocentric orbits.

[KAL 9000]

Which is faster: a duck riding a bicycle, or a running horse?

[WinkAllKerb'']

how will being (including human) will react to long multigenerationnal space travel without a moon

tag: evolution, dna, magnetic field, gravity etc.

https://en.wikipedia.org/wiki/Portal:Neuroscience

https://fr.wikipedia.org/wiki/Rayonnement#Exemples_de_rayonnements

https://en.wikipedia.org/wiki/Mirror

Edited April 25, 2016 by WinkAllKerb''