Two-Parter: Stationary Bodies in Space and Orbits

[JEB33]

Summary: (1) Would it be possible (relatively speaking) for a spaceship to "sit" stationary in space, i.e., not in orbit around a planet; and (2) if possible, how long would it take a planet, viewable from the spaceship's window (humor me) to pass through your field of vision?

Part One

I'm writing a science-fiction novel. The setting is space...shocker, I know. I'm playing with a portion of the plot that involves (hopefully) a spaceship "sitting" in space. Specifically, a spaceship sitting between Earth and Mars. I want to place the ship in space so that it's not orbiting either planet.

I realize that the planets revolve around the sun, that the sun is moving within the Milky Way, and that the Milky Way has its own vector through space, etc.

But, relatively speaking, would it be possible for a spaceship to "sit" in space between the orbits of two planets?

Part Two

If the answer to Part One, above, is "Yes, it's possible," then assume the spaceship is pointed away from Earth toward where Mars would be. Further assume that a human is looking out a window of the spaceship (I know...windows in space...humor me). Finally assume the position, angle, etc. is such that the planet Mars, following its orbit around the sun, will at some point come into the field of vision.

How long will it take for Mars to pass through the field of vision? I'm not looking for an exact number, but would it be hours, days, longer?

Many thanks in advance for the responses. Feel free to shoot down any of my assumptions; I'm new at this!

 

[DaveC426913]

Can you further define "sitting in space"?
Relative to what?
The sun? The stars?

How long does it need to sit? You can work out its slow fall toward the sun. For a short enough time, it could be considered stationary to the naked eye.

(Edit: after examining the second half of the question, I assume about 18 days?)

 

[DaveC426913]

How long will it take for Mars to pass through the field of vision? I'm not looking for an exact number, but would it be hours, days, longer?

Again, define "field of vision".
If the passenger stuck her nose against a curved porthole, she could see quite a bit of star field to left and right.Earth orbit and Mars orbit are very roughly 48 million miles apart. If your spaceship is halfway between them, that's 24 million miles from Mars. If you assume a 60 degree field of view out a porthole, that's an equilateral triangle, so you'll get a field of view of 24 million miles at Mars' distance.

Mars travels at about 54,000 mph along its path (which can be considered straight here). Moving at 54,000 mph, Mars will take - again, very roughly - 18 days to cross 24 million miles.

At 24 million miles, Mars -being 2100 miles in diameter, but subtend .5 minutes of arc. That would be large enough to look disc-like against a background of points. And it would be bright. And full.

To summarize, without hard numbers:

To the naked eye a passenger might see a very bright dot making its way across the star field by a few degrees every day - say, a couple of finger's widths. Over that time it would shrink and dim somewhat as the spaceship fell toward the Sun.Finally: does her craft have even the slightest rotation relative to the background stars? It would take deliberate effort to ensure it has zero rotation over a period of 18 days. Unless it is perfectly fixed, she won't be able to use the porthole to measure movement. She'd have to check it relative to nearby stars to see motion.

And this would be complicated by the fact that her ship is falling away from it. Due to parallax, Mars would initially appear very slow or stationary wrt nearby stars but then appear to accelerate as the days pass.
BTW, if you're going to write science fiction, you should try to have at least this level of estimation.
I was able to do it and all I did was draw a mental map, Google some numbers and come up with an estimate.

 

[JEB33]

Wow! Awesome stuff! Thank you SO much for this response.

The field of vision (how much does she have?) was something that bothered me as I was writing the post. It's comforting to know someone else also thought it was an issue to address.

As I do more research, I learn more. Would the ship have to "fall toward the sun"? I'm out of my league here, but is there a Lagrangian point between Earth and Mars where the ship (or an object) can "sit" and not necessarily fall toward the sun? Or are Lagrangian points not really relevant to this discussion? If they are relevant is the Lagrangian point always (or in this case) the halfway point of the distance between the two planets' orbits?

Again, thank you! There's so many more questions to address, but that's not a bad thing.

 

[DaveC426913]

The field of vision (how much does she have?) was something that bothered me as I was writing the post.

Unless she's immobile, she can get as close to the porthole as she wishes, while looking left or right. Ideally, her field of view out a flat porthole approaches 180 degrees. At 24 million miles from Mars' orbit, by pressing her cheek against the porthole, she could keep it in sight through about 68 degrees of Mars' orbit, or about 118 days.

Again, I am doing this with nothing more than Google and calculators like this one:
http://www.1728.org/circsect.htmTry it.

As I do more research, I learn more. Would the ship have to "fall toward the sun"?

If it's literally stopped in space relative to the sun, yes.
But that depends on your story. Any reason it couldn't have a boost once in a while? Or even a initial outward velocity before going dead, so that the fall is partially canceled over the observation period?

I'm out of my league here, but is there a Lagrangian point between Earth and Mars where the ship (or an object) can "sit" and not necessarily fall toward the sun? Or are Lagrangian points not really relevant to this discussion?

Sure, but they may not suit your purpose:

If they are relevant is the Lagrangian point always (or in this case) the halfway point of the distance between the two planets' orbits?

(Also Googled.)
Earth's L2 is about 1 million miles out. So 23 million miles shy of halfway.
Mars' L1 is about 1 million miles in. So, again, 23 million miles shy of halfway.
(That's counter-intuitive. Needs to be verified.)

 

[JEB33]

Thank you!

 

[Bab5space]

Summary: (1) Would it be possible (relatively speaking) for a spaceship to "sit" stationary in space, i.e., not in orbit around a planet; and (2) if possible, how long would it take a planet, viewable from the spaceship's window (humor me) to pass through your field of vision?

Part One

I'm writing a science-fiction novel. The setting is space...shocker, I know. I'm playing with a portion of the plot that involves (hopefully) a spaceship "sitting" in space. Specifically, a spaceship sitting between Earth and Mars. I want to place the ship in space so that it's not orbiting either planet.

I realize that the planets revolve around the sun, that the sun is moving within the Milky Way, and that the Milky Way has its own vector through space, etc.

But, relatively speaking, would it be possible for a spaceship to "sit" in space between the orbits of two planets?

Part Two

If the answer to Part One, above, is "Yes, it's possible," then assume the spaceship is pointed away from Earth toward where Mars would be. Further assume that a human is looking out a window of the spaceship (I know...windows in space...humor me). Finally assume the position, angle, etc. is such that the planet Mars, following its orbit around the sun, will at some point come into the field of vision.

How long will it take for Mars to pass through the field of vision? I'm not looking for an exact number, but would it be hours, days, longer?

Many thanks in advance for the responses. Feel free to shoot down any of my assumptions; I'm new at this!

Can a spaceship stand still above a planet?

The answer is YES... but it is a relative one, since the Earth is not standing still, so standing still above the Earth implies you're only hovering above it while the gravity keeps you from floating off.

There is no known rocketry that has the propellant to hover above Earth for extended amounts of time. You can watch a video on youtube of a rocket that flies straight up into space instead of orbiting. Had it had enough velocity, it could escape Earth's pull altogether, but it did not. Result? In only TWO minutes it falls back to Earth. It had a camera abd parachute on it.

For you to have a vessel capable of hovering over a planet for extended amounts of time you either a need a very hot burning exhaust, lightweight vessel, which is impossible by our structural imitations today (would melt engine), or just ignore conventional rocketry altogether and make up fictional propulsion or antigravity system.
It need not be explained in detail, as it is a plot device, it only needs to work. Only bother with complicating how the drive works if it is a plot point, meaninf stuff occurs in the plot based off how the drive works, not simply that it gets you from poin A to B of the universe really fast.

EDIT: By the way, the gravity in low Earth orbit, the altitude where the ISS orbits, is actually about 96% of Earth gravity. If you hovered at that distance you would not float on your ship, you could walk around and feel normal, as it is near 1g.
The reason why crew on the ISS float is they are falling away from Earth, but not fast enough to escape it's pull completely... thus the orbit.
One more thing... the Earth would rotate beneath you since you're not attached, so do not expect to hover over the same spot all day, sooner or later yoy will be hovering over the Pacific.

One

 

[Janus]

As I do more research, I learn more. Would the ship have to "fall toward the sun"?

Without thrust, yes. However, this is not necessarily a total game ender. At a point halfway between Earth's and Mars' orbits, the acceleration due to the pull of the sun is only ~0.0037 m/s . Over the 18 day period mentioned above, this equates to the equivalent of a delta v of 5.8 km/sec. This is doable even with chemical rockets. (Though a great deal of your ship would have to be fuel; a 4 to 1 fuel to ship ratio just to hold position, not counting fuel needed to get there and back)
If, however, you assume you are using something like an Ion engine capable of producing the required thrust, with an exhaust velocity comparable to that of the Ion engines used for the Dawn mission, you would only need to use ~0.02 kg of reaction mass per kg of ship to maintain station keeping for that long.

 

[DaveC426913]

Over the 18 day period mentioned above, this equates to the equivalent of a delta v of 5.8 km/sec.

I've been assuming , based on the OP's mention of 'sitting in space' that s/he meant without engines.
Maybe we should just clarify that with the OP.

JEB: why is this person in this spaceship that's just sitting there for all this time?
Is it disabled?
Is there any attitude control? (Otherwise., as mentioned, you might have problems with rotation.)

 

[member 656954]

My second novel is orbital mechanics heavy, I use AstroGrav to do the calcs and visualize what's going on because it's not particularly intuitive to me. You can play with orbits and pick apart what the consequences are for different accelerations - I was surprised to find how little extra speed it takes to turn an orbit into a fly-by, and how complicated it is for an object to just 'sit' in space.

It's a great tool for getting the science right.