How to know gravity of planets / moons / asteroids? (no wiki's help)

[Oldie]

Hi there!

I landed an ore miner on Ike and was surprised by its gravity; I thought its gravity was like minmus's gravity but Ike is a lot heavier O_o

I would like to explore the game without using the wikis so:  is there a way to know the gravity of a planet / moon / asteroid by sending a probe with some adequate module? 

[Markrc93]

Underneath the science section there's two instruments that can do that and there's one part of the UI:

Part wise: The accelerometer which reads out in g (when stationary on the surface, it'll give you the strength of gravity at that point). You can measure it on Kerbin's surface (ideally sea level) for your reference point. There's also the gravioli detector, which regardless of its motion will read out acceleration due to gravity at that point (again, you can measure it on Kerbin, ideally sea level)

UI wise: your g-force meter to the right of the nav ball, while not very precise of a measurement, does include the force of gravity, so a stationary probe there can give you some idea of the strength of gravity.

[bewing]

Is using the Tracking Station equally bad as using the wiki? In the Tracking Station you can focus any celestial body. In the Knowledgebase section in the bottom right is an icon that looks like Saturn (with a ring). Click that and it tells you what the ASL gravity is.

 

[Oldie]

Thanks a lot guys!

[ArmoredLipid]

Planets and moons have a level of gravity that can be measured with your G-force meter or the Double-C Seismic Accelerometer; I advice sticking the latter on an uncrewed probe to test the gravity before making a crewed landing. Asteroids have none; attach yourself to them with the Advanced Grabbing Unit/Klaw.

[BigFatStupidHead]

If you want to do some math, you can look at your orbit. Assuming a mostly circular orbit,

V=Sqrt((M*bigG)/R))

M is the mass of the planet and your ship - we'll solve for this. The mass of your ship is basically negligible.

bigG is the Gravitational Constant. I think KSP uses 6.67*10^-11

R is the radius of your orbit. This is not the same as altitude! If you have a very wide orbit, they are approximately the same.

Solving for M should give us -> M=(V^2)/(bigG/R)

With M, we can figure out the gravity.

F=G(M/R^2)

Same variables. That should give you your gravity!

 

[OhioBob]

1 hour ago, BigFatStupidHead said:

If you want to do some math, you can look at your orbit. Assuming a mostly circular orbit,

V=Sqrt((M*bigG)/R))

M is the mass of the planet and your ship - we'll solve for this. The mass of your ship is basically negligible.

bigG is the Gravitational Constant. I think KSP uses 6.67*10^-11

R is the radius of your orbit. This is not the same as altitude! If you have a very wide orbit, they are approximately the same.

Solving for M should give us -> M=(V^2)/(bigG/R)

With M, we can figure out the gravity.

F=G(M/R^2)

Same variables. That should give you your gravity!

KSP uses G = 6.67408E-11 m³/(kg-s²)

But really you don't need to know that, nor the mass.  Just solve for the product GM, often denoted by the Greek letter μ, which is known as the gravitational parameter.  All problems in orbital mechanics use GM, so there no need break it down further into G and M.  That just adds an extra operation and more numbers to remember.
 

[BigFatStupidHead]

1 hour ago, OhioBob said:

KSP uses G = 6.67408E-11 m³/(kg-s²)

But really you don't need to know that, nor the mass.  Just solve for the product GM, often denoted by the Greek letter μ, which is known as the gravitational parameter.  All problems in orbital mechanics use GM, so there no need break it down further into G and M.  That just adds an extra operation and more numbers to remember.
 

Oh? So if my math is good (which it often isn't), this can be simplified down to F=(V^2)/R?

[OhioBob]

12 hours ago, BigFatStupidHead said:

Oh? So if my math is good (which it often isn't), this can be simplified down to F=(V^2)/R?

Yes, but the variable F implies force.  Your equation computes gravitational acceleration.  Probably should write it, g = v²/r.

But if you use the orbital velocity and radius to compute g, then the above equation will give the value of g at the orbital radius, not at the surface.

What I would do is use the orbital velocity and radius to compute GM,

GM = r v²

(Note that GM is a constant for a given body.  For instance, for Kerbin GM = 3.5316E+12 m³/s².)

And then from that we can compute the acceleration of gravity at any radius using,

g = GM / r²

If we want the surface gravity, then just set r equal to the radius of the planet.

--------------------------------------

Note, however, that the equation GM = rv² only works if we are in a perfectly circular orbit.  If the orbit is elliptical, as it likely is, then we can compute GM using the Vis-viva equation,

GM = v² / (2/r - 1/a)

where v is the orbital velocity at radius r, and a is the semimajor axis.  The values v and r can be taking at any point along the orbit; and we can get a by averaging the minimum and maximum values of r.

 

Edited June 27, 2018 by OhioBob

[BigFatStupidHead]

30 minutes ago, OhioBob said:

Yes, but the variable F implies force.  Your equation computes gravitational acceleration.  Probably should write it, g = v²/r.

But if you use the orbital velocity and radius to compute g, then the above equation will give the value of g at the orbital radius, not at the surface.

What I would do is use the orbital velocity and radius to compute GM,

GM = r v²

(Note that GM is a constant for a given body.  For instance, for Kerbin GM = 3.5316E+12 m³/s².)

And then from that we can compute the acceleration of gravity at any radius using,

g = GM / r²

If we want the surface gravity, then just set r equal to the radius of the planet.

--------------------------------------

Note, however, that the equation GM = rv² only works if we are in a perfectly circular orbit.  If the orbit is elliptical, as it likely is, then we can compute GM using the Vis-viva equation,

GM = v² / (2/r - 1/a)

where v is the orbital velocity at radius r, and a is the semimajor axis.  The values v and r can be taking at any point along the orbit; and we can get a by averaging the minimum and maximum values of r.

 

Fantastic! Thanks.

[nasa legolas]

you need a graviolli detector to measure the gravity's pull.