Gravity Question

[SteveD80]

If we define non-negligible gravity as: If a Kerbal jumps they will land. Can we create a body that is large enough to have gravity and small enough to move?

[Steel]

Not with anything short of degenerate matter, the densities required are enormous

Edited May 24, 2016 by Steel

[YNM]

The main problem here is actually "small enough to move". If by that you mean small enough object to be moved by the jump then gravity will love both of you and you'll "fall back" to the object in any way.

[Streetwind]

In Kerbal SPace Program, you cannot, because vessels don't generate gravity regardless of their size and mass.

And anything that is not a vessel cannot be moved.

[Shpaget]

Are you talking about real life?

Escape velocity of Deimos is 5,5 m/s, which means that if you jump, you fall down, eventually. However, don't try to run, you might not be able to get back down.

Deimos is, astronomicaly speaking, miniscule. It's barely a speck of dust, but it is still ~12 km in diameter. Even if just a pile of rocks, that is still a huge pile of rocks.

To put it into perspective, if all the iron ore the world produces was put on a pile, it would take more than 400 years to make a pile as large as Deimos. So no, we most certainly can not make a space object that would produce significant gravitational pull. As for moving it, we strugle with objects of few tons.

[PB666]

5 minutes ago, Shpaget said:

Are you talking about real life?

Escape velocity of Deimos is 5,5 m/s, which means that if you jump, you fall down, eventually. However, don't try to run, you might not be able to get back down.

Deimos is, astronomicaly speaking, miniscule. It's barely a speck of dust, but it is still ~12 km in diameter. Even if just a pile of rocks, that is still a huge pile of rocks.

To put it into perspective, if all the iron ore the world produces was put on a pile, it would take more than 400 years to make a pile as large as Deimos. So no, we most certainly can not make a space object that would produce significant gravitational pull. As for moving it, we struggle with objects of few tons.

You could do it more quickly if you used lead and other heavy metals. Since these have a higher density, and because a = u/r^2 if r is smaller you can reduce M, effectively reducing the amount need by (1-density of Iron/Lead) * 100%. Of course finding a source of lead to create a fake deimos might be challenging. BTW if you look into the sky, everthing that you see in miniscule on the cosmic scale, most of the really grand stuff requires a telescope to see (like galaxies) and the heaviest stuff is invisible. You would actually need less molten iron than deimos's mass because its made of carbonates and other impurities, and because its not completely gravitationally rounded or solid. The surface is rather loose. So if you did use molten lead you really could get the mass down.

[Shpaget]

I used iron ore example since it's abundant, probably among the top three most produced substances and has production data available.

As for the surface gravity and density, yes, you're correct.

[kerbiloid]

Voyager mass = 721 kg
Distance from Sun = 111 AU

Hill Sphere radius = a * (m/(3M))^1/3 = 111 * 1.5*10¹¹ * (721 / (3 * 2*10³⁰))^1/3 = 8216 m ~= 8 km.

I.e. both Voyagers are artificial celestial bodies with 8 km Hill sphere.
If you come close to a Voyager (< 8 km), you will become a satellite (or a double planetoid - depends on your body mass).

 

Edited May 25, 2016 by kerbiloid

[PB666]

3 hours ago, kerbiloid said:

Voyager mass = 721 kg
Distance from Sun = 111 AU

Hill Sphere radius = a * (m/(3M))^1/3 = 111 * 1.5*10¹¹ * (721 / (3 * 2*10³⁰))^1/3 = 8216 m ~= 8 km.

I.e. both Voyagers are artificial celestial bodies with 8 km Hill sphere.
If you come close to a Voyager (< 8 km), you will become a satellite (or a double planetoid - depends on your body mass).

 

Escape velocity would be in the mm/sec range starting from the tip of the TNG. 

[kerbiloid]

1 minute ago, PB666 said:

Escape velocity would be in the mm/sec range starting from the tip of the TNG

Of course.
v_escape = sqrt(2 * 6.67*10^-11*721/1) = 0.3 mm/s
v_circular = v_escape / sqrt(2) ~= 0.2 mm/s

Rotation period T = 2 π R / v = 2 * 3.14 * 8000 / 0.0002 / 86400 / 365.2422 ~= 8 years.

Also, you can easily make Oberth maneuver around the Voyager.

[G'th]

This introduces an offshoot question. Presuming you did mine enough to create a fake Deimos, would the removal of that mass from the Earth reduce gravity here on the surface?

[kerbiloid]

g = GM/R2 = G * ρ * 4πR³/(3 * R²) = (4/3)πGρR;

g ~ R

As you decrease the planet radius, g decreases.

[sevenperforce]

3 hours ago, G'th said:

This introduces an offshoot question. Presuming you did mine enough to create a fake Deimos, would the removal of that mass from the Earth reduce gravity here on the surface?

Technically? Yes. 

Measurably? Not even close.