Could a Moon/Planet/DeathStar Spin Faster Than it's Own Orbital Velocity

[Awesomesauce1337]

title
If such a body existed how would one go about landing on it?

[intelliCom]

1 hour ago, Awesomesauce1337 said:

title
If such a body existed how would one go about landing on it?

Define "spin faster". 'RPM' and 'orbital speed' aren't exactly compatible. If you're talking about the 'orbital speed' of a planet being the RPM around the parent star, landing on such a planet is easy, we do it all the time; Earth, Mars, Venus, they all spin with higher RPM than their orbit's RPM.

Of course, we don't normally define orbital velocity as RPM, but we don't measure a planet's spin in km/s or km/h either. How did you think this was supposed to work?

Edited September 3, 2022 by intelliCom

[tomf]

I think the question is can a planet spin fast enough that it's equator is traveling faster than the velocity required to orbit that planet. 

The answer is no, the planet is simply going to disintegrate. On the scale of planets there is no force that is going to be able to hold the planet together.

An asteroid held together as a single rock could spin faster than its orbital speed. To land on it would be a matter of grabbing hold as it whizzed by, or landing at the poles.

[intelliCom]

42 minutes ago, tomf said:

I think the question is can a planet spin fast enough that it's equator is traveling faster than the velocity required to orbit that planet. 

The answer is no, the planet is simply going to disintegrate. On the scale of planets there is no force that is going to be able to hold the planet together.

An asteroid held together as a single rock could spin faster than its orbital speed. To land on it would be a matter of grabbing hold as it whizzed by, or landing at the poles.

Roche Limit. I suppose if 'spinning faster' is defined as 'the surface moves around the planet's center of gravity faster than the planet moves around its star', then the whole planet would just turn into an asteroid belt from the centripetal force being so high.

I have an interesting thing I've always thought about; what if a planet's rotation is barely under the roche limit? How would it feel to stand on such a world? How would it operate? Visually, it would definitely be much more of an oblate spheroid compared to other planets, and likely wouldn't exist for longer than a few years, but what would it be like anyway?

[tomf]

4 hours ago, intelliCom said:

Roche Limit

That isn't the Roche limit, the Roche limit is about how close a secondary body can orbit to a primary body before the tidal forces pulling it apart are stronger than the gravitational forces holding it together.

On the rapidly spinning body, close to breaking up - it would be very oblate, the apparent gravity at the equator would be very low while the gravity at the pole would be more normal for a body that size.

Aparent Gravity would always be perpendicular to the floor so things wouldn't roll away to the equator.

A throw from the equator with 0 inclination would return to hit you on the back of your head if you didn't dodge. Any other inclination would land somewhere though

[intelliCom]

4 minutes ago, tomf said:

That isn't the Roche limit, the Roche limit is about how close a secondary body can orbit to a primary body before the tidal forces pulling it apart are stronger than the gravitational forces holding it together.

Ah, sorry. I hope you can understand how I got them confused.

5 minutes ago, tomf said:

On the rapidly spinning body, close to breaking up - it would be very oblate, the apparent gravity at the equator would be very low while the gravity at the pole would be more normal for a body that size.

I have a feeling that the pole's net gravity would actually be weaker than a fully stable body, as a decent amount of mass is coming from the sides instead of directly beneath the person standing on the surface.

Here's another theorhetical; a moon lies in a very close orbit to the oblate body. What does this do to it? If the oblate spheroid is destroyed, surely the planet's spin rate can be reduced enough to avoid total destruction? Maybe something like Methone?

[magnemoe]

7 hours ago, tomf said:

I think the question is can a planet spin fast enough that it's equator is traveling faster than the velocity required to orbit that planet. 

The answer is no, the planet is simply going to disintegrate. On the scale of planets there is no force that is going to be able to hold the planet together.

An asteroid held together as a single rock could spin faster than its orbital speed. To land on it would be a matter of grabbing hold as it whizzed by, or landing at the poles.

This, something held together just by gravity can not rotate so fast, some asteroids are close to this limit and they will start loosing mass. 
As I understand sunlight will very slowly spin up an small rotating asteroid and since their gravity is so low they don't have to spin that fast.

[cubinator]

There are certain configurations that result in a toroidal shape when you approach those limiting speeds. They wouldn't last very long, though, because they're unstable.

[kerbiloid]

We forgot about the pulsar moon.

[Awesomesauce1337]

Tomf is correct about how to interpret my question, lets say with the power of a planet made of an alloy of handwavium and unobtainium it can spin faster than the orbit velocity required to orbit it. How would you land on it?

[Leganeski]

19 hours ago, tomf said:

An asteroid held together as a single rock could spin faster than its orbital speed. To land on it would be a matter of grabbing hold as it whizzed by, or landing at the poles.

This is also true for small enough moons, such as Lili from Galileo's Planet Pack. Lili is on a very tight orbit around a much denser planet which it is tidally locked to, causing it to spin faster than orbital velocity at its equator. However, only the top 1-2 km of the equatorial ridge is actually above the synchronous orbit altitude, so the rock's internal strength can hold everything together.

1 hour ago, Awesomesauce1337 said:

How would you land on it?

As @tomf said, land near the poles. I've done it successfully on Lili without much difficulty.

 

[KSK]

On 9/4/2022 at 1:02 AM, Awesomesauce1337 said:

Tomf is correct about how to interpret my question, lets say with the power of a planet made of an alloy of handwavium and unobtainium it can spin faster than the orbit velocity required to orbit it. How would you land on it?

Carefully.

[Shpaget]

Such a planet can not form naturally, so your handwavium shell would need to be built before the spin up.

In any case, if you match your spacecraft speed so it's zero relative to the surface, and fly on a tangential path with periapsis at the height of the docking port, you're good to go.

Then again, a civilization that can shrink wrap a planet in unobtanium and spin it up, probably won't have much trouble just brute forcing the approach.

For the curious visitors who don't have such capability, landing on the poles would be much easier.

[Hannu2]

On 9/3/2022 at 2:38 PM, tomf said:

That isn't the Roche limit, the Roche limit is about how close a secondary body can orbit to a primary body before the tidal forces pulling it apart are stronger than the gravitational forces holding it together.

It is practically the same thing. Calculation of Roche limit assumes that bodies have no structural strength. It is very good approximation for planet size objects, because structural strength of solid materials are negligible compared to gravitational forces present in near flybys. But smaller objects can rotate much faster. For example rotating space stations are based to that.

 

On 9/4/2022 at 3:02 AM, Awesomesauce1337 said:

Tomf is correct about how to interpret my question, lets say with the power of a planet made of an alloy of handwavium and unobtainium it can spin faster than the orbit velocity required to orbit it. How would you land on it?

You have to have some means to grab on surface or use engines to push ship against surface. Net force to object on surface is out from center.

However, you can land on pole if you have same roll rate than planet and can somehow manage controls. I would not like to try it with hand control id spinning rate is noticeable.

[Superfluous J]

Land on the poles.

QED

EDIT: Aww people already said that

Edited September 5, 2022 by Superfluous J

[Shpaget]

3 hours ago, Hannu2 said:

if you have same roll rate than planet and can somehow manage controls. I would not like to try it with hand control id spinning rate is noticeable.

Orbital period at Earth surface is about one and a half hours. You can safely ignore it.

[magnemoe]

On 9/5/2022 at 4:59 PM, Shpaget said:

Orbital period at Earth surface is about one and a half hours. You can safely ignore it.

However docking to the outside of an rotating space station is more interesting. 

In Freefall they have this pretty unique setup
http://freefall.purrsia.com/ff3500/fc03457.htm
In short they fly at a bit below the rotation speed of the space station, the station come around and catches you.  

Not sure how well this will work, suspect the floor will come up pretty fast even if you move almost as fast as the station rotation you are not rotating with it, the lip will not help as it its angled the wrong direction. Now I would put this inward near the center there the centrifugal force is much weaker and still keep the center docking hub free for huge ships, would work better at .1 g. 
Now they have room temperature superconductors so they can grab the ship but I still feel its a bit of an aircraft carrier landing. 

Edited September 6, 2022 by magnemoe

[sevenperforce]

A planet spinning so fast that its tangential rotation speed at the equator was close to its own orbital speed would rapidly morph into an extremely oblate spheroid.