Dumb question about gravity/orbit...

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relaisterre

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While I was writing a SF story last week, I realized I had problems with my spaceship battles, travel distance, etc. So I thought about changing the story and pulling the ships out of space. I experimented with ships that just fly in the sky, or on the surface of a habitable planet. I thought if I scaled down my story's world from space to just one planet, I would solve my problems.<br /><br />The thing is, I didn't want Earth. I wanted another planet. I wanted larger oceans, larger continents, stronger winds, a bigger sun, etc. So I thought "Hey, I'll just make the planet bigger than Earth!" But then I realized this made no sense because of gravity. If I'm not mistaken, a bigger planet means heavier gravity, right? So even though I want breathable air, cool temperatures, clear skies and water to sustain life, nothing could work on a bigger planet because of increased gravity. My human characters couldn't walk normally, the ships couldn't fly, etc. Am I correct in thinking I can't write a planet that's "like Earth, just bigger"? Or am I missing something?
 
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le3119

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A world with Earthlike qualities could have a mass 2, 3 or more times the mass of Earth. The ships would have a harder time entering the atmosphere without crashing and leaving again due to a higher surface escape velocity. If these humans are like us, they'd have a tough time without some gravity control power on their ships. Maybe these are colonists that wear implants that allow their skeletons and muscles to remain healthy under higher surface gravity. Or do aliens live on this bigger Earth? Say your planet has a mass = 2.5 Earth masses, then the escape velocity would be Earth's times a factor of 2.5. A hundred lbs person might weigh 250 lbs on the surface! This assumes that the density of your planet = density of Earth. A world with 2.5 Earth masses but with a density of 0.65 of Earth's might have a surface gravity close to 1 G like Earth. More surface "real estate" but not a big deviation from Earth's gravity.
 
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relaisterre

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<blockquote><font class="small">In reply to:</font><hr /><p>That gives you a hundred time the area to play with. <p><hr /></p></p></blockquote><br /><br />*lol* That's what I wanted to hear! Thanks for the info guys. However, could you explain that density thing a bit more, in simple terms, and why it allows me to have a bigger Earth-like planet without increasing the gravity and crushing whoever would land/live there? The only physics classes I took were in high school, and I've barely begun (re)learning space stuff again because I feel ignorant in such matters. I don't like that feeling and giving my writings a more solid scientific base would be a good place to start.<br /><br />This is why I love stuff like this. I ask one question, and I get 3 more! <img src="/images/icons/smile.gif" /> Why the density, the ships that have a harder time in re-entry, and that iron core... lol... I love this.
 
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yevaud

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That's fairly easy.<br /><br />Gravity is dependent on the mass of the object. Basically, what density means (sorry this is so basic, but it's just that...*basic*) is that so much matter occupies a given space.<br /><br />So what he was saying was, if the Earth was suddenly made of material that was less dense (on average, of course - the Earth is made up of many things), and had the same gravitational force (1G) at it's surface, it would (have to) be the dimensions mentioned. <div class="Discussion_UserSignature"> <p><em>Differential Diagnosis:  </em>"<strong><em>I am both amused and annoyed that you think I should be less stubborn than you are</em></strong>."<br /> </p> </div>
 
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Saiph

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Right, the earth has a good chunk of iron and nickle in it (the core). If that core was smaller, and the planet had less iron and some of the heavier elements (smaller elemental numbers than iron though, as bigger elemetns are less common anyway).<br /><br />Anyway, density is mass of an object divided by it's volume. Iron is quite massive for it's volume, hydrogen very light for it's volume. So if you have a planet made of less dense atoms, you can get to a larger volume (and thus surface area), before you reach the same mass.<br /><br /> <div class="Discussion_UserSignature"> <p align="center"><font color="#c0c0c0"><br /></font></p><p align="center"><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">----</font></em></font><font color="#666699">SaiphMOD@gmail.com </font><font color="#999999"><em><font size="1">-------------------</font></em></font></p><p><font color="#999999"><em><font size="1">"This is my Timey Wimey Detector.  Goes "bing" when there's stuff.  It also fries eggs at 30 paces, wether you want it to or not actually.  I've learned to stay away from hens: It's not pretty when they blow" -- </font></em></font><font size="1" color="#999999">The Tenth Doctor, "Blink"</font></p> </div>
 
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relaisterre

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Ah, I see. So, I could invent an Earth-like planet, just bigger, if its core was less dense? So I could still roughly have the same gravity? And it wouldn't have to be explained in millions of pages. Interesting.<br /><br />Does this mean ships in orbit would behave "normally" too? (ie: as they would around the Earth?)
 
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zavvy

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Hi, Saiph...<br /><br /><font color="yellow">Right, the earth has a good chunk of iron and nickle in it (the core).</font><br /><br />Do we actually know this for a fact or is it just theory?<br /><br />Thanks!
 
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Saiph

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We've got a pretty good idea that's what it is from siesmology. We know the solidity and density of a solid core by how seismic waves bounce around the interior. <div class="Discussion_UserSignature"> <p align="center"><font color="#c0c0c0"><br /></font></p><p align="center"><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">----</font></em></font><font color="#666699">SaiphMOD@gmail.com </font><font color="#999999"><em><font size="1">-------------------</font></em></font></p><p><font color="#999999"><em><font size="1">"This is my Timey Wimey Detector.  Goes "bing" when there's stuff.  It also fries eggs at 30 paces, wether you want it to or not actually.  I've learned to stay away from hens: It's not pretty when they blow" -- </font></em></font><font size="1" color="#999999">The Tenth Doctor, "Blink"</font></p> </div>
 
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majornature

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What is the name of this story that you are writing?<br />I'm creating a graphic novel dealing with "Aliens" from other worlds. Of, course, Earth is included in my action packed sequel. I've been working on it for over five years and I'm close to 100 episodes! <br /><br />Now, about the spaceships, increase the speed and power of a spacecraft and it may as well survive in any condition. It also depends on the gravity of your planets that you created in your story and the description on how your spacecraft will function in your story. <br /><br /><br /><br /> <div class="Discussion_UserSignature"> <font size="2" color="#14ea50"><strong><font size="1">We are born.  We live.  We experiment.  We rot.  We die.  and the whole process starts all over again!  Imagine That!</font><br /><br /><br /><img id="6e5c6b4c-0657-47dd-9476-1fbb47938264" style="width:176px;height:247px" src="http://sitelife.space.com/ver1.0/Content/images/store/14/4/6e5c6b4c-0657-47dd-9476-1fbb47938264.Large.jpg" alt="blog post photo" width="276" height="440" /><br /></strong></font> </div>
 
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larper

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<blockquote><font class="small">In reply to:</font><hr /><p>So, I could invent an Earth-like planet, just bigger, if its core was less dense?<p><hr /></p></p></blockquote><br /><br />Jack Vance, "Big Planet"<br />http://www.infinityplus.co.uk/nonfiction/bigplanet.htm <div class="Discussion_UserSignature"> <p><strong><font color="#ff0000">Vote </font><font color="#3366ff">Libertarian</font></strong></p> </div>
 
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nexium

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If you double the diameter of an Earth the area increases by 4 times and the mass by 8 times unless you reduce the average density. Halving the density is difficult, as materials in the core get compressed to higher density forms under very high pressure. ie metalic hydrogen has a density of about 1.5. Halving the average density is a realistic and believable goal. I believe this results in a gravity less than twice that of Earth. Selected humans can likely tolerate twice earth's gravity long term, but injury from falls is greatly increased. A modest technology increase will allow spacecraft to take off and land, but with reduced pay load.<br /> A small amount of iron and nickel in the core may allow the planet to have a strong magnetic field that may provide sufficient protection from a class F sun which is bigger and hotter and white instead of the yellow class G2 sun that our solar system has.<br /> The surface can be rich in heavy metals, provided the mantle is mostly low density elements. In theory dissolved and chemically combined hydrogen can account for half of the volume of the mantle and core, thus reducing the average density by two or more. A planet with one hundred times the diameter of Earth needs to be hollow with thin walls (made of unobtainium) and vacuum inside to stay under two g surface gravity. The unobtainium is especially unbelievable. Neil
 
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