# Spaceships a'turnin round and round - in space

1. May 17, 2004

### NMonter

Spaceships a'turnin round and round -- in space

If a spaceship with a large rocket was moving in one direction [For visualization purposes, we'll call it the x-axis] then with its maneuvering jets turned 90-degrees and then turned on the rocket would it be continuing moving in the previous direction [x-axis] at the same rate [even while its moving faster and faster on the y-axis], would it slow down on the x-axis at the same rate as it is accelerating at the y-axis, would it decelerate on the x-axis at a different rate the ship is accelerating on the y-axis, would it continue on the x-axis at the same rate until the rocket traveling on the y-axis equaled the velocity it was traveling on the x-axis, or [to be all inclusive] something else?

I guess I am a little confused by opposite and equal reaction and objects in motion in space. My knowledge of good old Newton’s laws is from High School. I am an undergrad student, other than a few biology courses, my major doesn't require any natural science cources - so I haven't taken any yet.

The reason why I ask is that ships act like fighter jets on earth dodging and weaving in sci-fi shows [Although Babylon 5 did a good job with the fighters] and video games [Wing Commander had arcade like physics but Independence War I & II did a great job of being realastic], and it is something that I have always been curious about. As well, I have been wanting to make a science-fiction boardgame/roleplaying game that involves game-rules for spaceship movements with Newtonian rockets [the game takes place exclusively within a made-up solar-system, with no humans and made-up aliens].

I should make another topic at a different forum on this site, but since I am here, I might as well ask. In the game I want to create, I want to have as many earth-like planets [that allow complex plant and animal life and intelligent aliens to evolve] as possible. I was thinking that a Jupiter or Saturn sized gas giant that would be around 1 AU to 1.5 AU to an identical star of our sun, could have two or three near-earth sized moons [8,000 to 12,000 km in diamete]. I want to try to make this as realistic as possible, so I need to make it seem possible, but unusual, and abides by physics as reasonably possible.

From what I understand during the creation of the Solar System, heavier elements kept near the Sun and lighter elements were pushed further away because of the intense heat. [What is the name of the theory anyway? I want to find out more information on the creation of the solar system but I am having trouble finding the right info. I kept finding creationist sights... Sigh...] Would a gas giant near a Sun-sized star be possible? And what would be any problems for habitable earth-sized moons around a Jupiter sized gas giant?

I know of some possible issues, and I am trying to see if there are some creative ways around it, or if there is other issues.

One: Gas Planets Magnetosphere

I was poking around and one problem with an earth-like moon around a gas giant is that the magnetosphere of the parent planet could strip the atmosphere of the moons. However, I also read that if the moons had a strong field of their own that their atmospheres would not be stripped. I am assuming that this is true? Also what could be done to a gas giant to reduce the size of the field – assuming that it is a good thing? Would slowing down the rotation of the planet reduce the field, or would that cause other problems? If slowing down things would help would two or three earth-sized moons slow down the rotation? Also assuming the gas giant was formed near the sun and didn’t migrate closer, wouldn’t it have a large iron-nickel core? From what I remember Earth’s own field is generated because of our large iron core, how would this affect a gas giant? [Also would a thick iron-nickel core cause other problems with gravity and mass?]

Two: Tidal Heating, Orbits, and Being on the Farside

IO is a rather active planet and this attributed to tidal heating. How would two or three earth sized planets be affected by tidal heating by the parent planet and from each other? Would they have to be spaced apart by a few million km or more to prevent an IO like planet? Maybe have orbits far from the parent planet? If they are spaced far apart would there be a chance an outer moon would escape, but maybe a moon next over could pull the planet closer, would any of the planets/moons speed up and slow down depending on the orbits? We are talking about massive moons, would the moons have to worry about the other moons affecting each other than the parent planet?

Another problem would be that if they have large orbits there is going to be the problem of being in the shadow of a parent planet for a few days. Although I am sure plant and animal life would find ways to adapt, maybe plants do what some do here during the winter, but it is timed when the moon travels on the farside of the gas giant. I could imagine that the plants wouldn’t be very nourishing or good tasting during this period. Maybe the planets/moons don’t have much of a tilt, allowing more stable climates, making up for the dark periods – would a 0' tilt help any? Would the tidal heating be an advantage keeping the planet a little warmer during the period? I wonder if you were on a moon around a gas planet would the sun wax and wane like our moon as it orbits around the gas giant?

Anyway those are some issues that I know about now, would there be any other issues that would need to be address to make a reasonbly realistic solar system, or at least based on what we know about the universe today.

Thanks for any help, and as always forgive my ignorance.
Nick

Last edited: May 17, 2004
2. May 17, 2004

### enigma

Staff Emeritus
Hi Nick,

Welcome to the forums!

It would keep going in the X direction at the same velocity and start increasing in velocity the Y direction. All components of velocity are independant of the other.

Yeah, Independence War wasn't bad for physics. I hated the actual gameplay... but the physics was OK. Except of course for gravity and relativity.

cretinists...

It's called stellar evolution. At this point, we don't know how exactly it works. 10 years ago we thought we did, but we've since found many many Jupiter (or larger) sized planets in really close to the star. Closer even than Mercury. To my knowledge we've only found a handful of Jupiter sized planets in Jupiter-like orbits. It's not that they're not there, per se... they're just harder to notice that far out.

Extreme radiation. If you're planning on making conflicting factions which evolved on the different moons, to make it any way realistic, the planets would need to be in pretty high orbits, I'd guess.

Tidal forces. Your planets would probably be tidally locked. They'll have one side facing the giant at all times, sort of like our moon is with us.

Meteorite impacts. The giant would constantly be pulling asteroids and comets into its gravity well. It seems to me that you'd have more large impacts more often than we do.

I'm sure there are other factors.

I'll leave this one to someone more knowledgeable than I.

Not unless they get dragged out by a major planetoid. If they're in a stable orbit, to escape you need to be going $\sqrt{2}$ faster.

Not so much. Draw a sketch to scale. You'd be surprised at the distances involved. Even with a gas giant, their umbra isn't THAT big (Darkness Falls was one of the movies I ended up yelling at the screen). You'd really only have a major eclipse once in a while. Our moon is about 400k km away, with an orbit of 28 days. It gets a full eclipse no more than 2 times a year, and it only lasts an hour or two.

Last edited: May 17, 2004
3. May 17, 2004

### NMonter

From what I understand that if a ship using it’s thrusters changed where the rocket was pointing it would be constant in the previous direction even as it is increasing in velocity in the new direction [assuming that the directions aren’t opposite]? Bummer, I’ll either have to have fake physics or require that a ship stop if wants to make a turn - if I understand you right.

As far stellar evolution is concerned that is good news that gas planets can be near a sun without problems, that I have some breathing room, at least for now. If I remember weren't some of those extrasolar systems red-dwarves?

As for the radiation, I heard about extreme radiation in relation to Jupiter, but admittedly I didn’t think too much about it. I would have to find where a safe orbit would be, assuming there is one. Another point is that I am assuming that if the moons have relatively strong magnetospheres that would protect the life on the moons, or am I giving too much credit to it.

Speaking of orbits, what would be the maximum distance to a Jupiter-sized planet that a earth-sized object would have for a stable orbit? This is a general question but how would two or more Earth-sized objects relate to each other being in such close proximity to each other?

You mentioned that moons would be tidal locked to the parent planet, I have to admit I am having trouble visualizing it. So I am going to have to make a model to help understand the relationship. I assuming that on any of the planets/moons each side would get sunlight as it rotates around the gas giant. What would the gas giant look like at night on the side that faces it?

As far as your point about the Umbra, I actually looked that up as soon as you mentioned it, and I felt a little silly, your right it would not be much of an issue for life but would be neat to watch.

Your right about the meteorites and comets, there I would take an artistic license, if three or more intelligent life-forms live in a single solar system, then they lucked out and haven’t had any major hits. Although Ganymede and Collisto don’t *appear* to have any more hits than our moon, does anyone know more about that? Another thing would be what kind of havoc if a comet didn’t hit the moons but hit the gas-giants, like what happened to Jupiter a little under 10 years ago.

One more thing, about your point about Independence War gameplay, oy was it HARD! Although I won’t pretend to understand the games take on LDS and Capsule drives. I guess there isn’t a truly good way to propose interstellar travel that doesn't seem hokey to someone, or at least one that I can understand. Which is why I am keeping it within a single solar system, plus I can keep the level of technology at a point where there is serious risks of space-travel. Although for the sake of convenience I am going to ignore any problems of clocks being different between a very fast moving ship and the space station it is moving to.

Thanks again,
Nick

4. May 17, 2004

### Gunni

5. May 17, 2004

### BobG

Your original question had the thrusters firing at a right angle to the spacecraft's path. If you want to eliminate your speed in the original direction while increasing your speed in the new direction, either change the angle of your thruster or use two thrusters.

6. May 26, 2004

### Cecil

I believe a strong magnetic field would almost eliminate any radiation recieved on the surface.

There is no upper limit on the semi-major axis of a stable orbit. The only thing you'd have to take into account would be gravitational influnces from the central star. Any distance from the gas giant is okay, as long as the star is far enough away.

To nitpick your terminology a bit, a moon "revolves" around a planet, and "rotates" on its axis.

Let's use Europa, which is tidally locked to Jupiter, as an example. Note that this means Jupiter is fixed in the sky with respect to the horizon (ie, it's only visible from one hemisphere). Europa's orbital period is 3 days, so lets trace the scenery for one orbit from the point of view of a Europian organism standing such that Jupiter is directly overhead.

We start at local noon (T+0) with an eclipse of the sun by a new Jupiter (completey dark). The sun passes out from behind Jupiter to the west, and Jupiter begins to be illuminated as a crescent from the west side. As the sun sets (T+1/4 orbital period) the west half of Jupiter is illuminated. The sky darkens and Jupiter continues to wax, until at midnight (T+1/2 orbital period) it is fully illuminated. The sun rises due east at T+3/4 orbital period and Jupiter's east half is illuminated and waning. Eventually, one orbit later, we have another eclipse of the sun by Jupiter.

The above description is a little simplified, assuming the orbit of your moon is in the same plane as the solar orbit of your gas giant. You can play around with the inclinations slightly - a slight inclination (~5 degrees) will do little more than eliminate the eclipses except for during two "eclipse seasons" - a twice annual period of time where Jupiter eclipses the sun every orbit. The duration of a "season" depends on the angular size of the gas giant and the exact orbital inclination.

A comet impact on the gas giant would have little to no effect on lunar life, except the chance to see some really cool special effects.

A description of interstellar travel is either physically possible, or easy to understand. Pick one.