Need some help with astronomy

efreysson · Nov 8, 2015

I'm putting together a space opera serial, and while I'm making no particular effort to make the technology realistic, there's one major thing I feel I have to be fairly accurate about.

My version of FTL travel is that ships can make extreme leaps across interstellar distances, provided there is nothing in the way. So spacefarers have to align their ships along clear routes before leaping.

But the galaxy is moving, isn't it? How fast do star systems and such swirl around? Could a route stay empty enough to fly through for a good long while or would my space-pilots need to constantly update their charts before each and every leap?

King Neptune · Nov 9, 2015

efreysson said:
I'm putting together a space opera serial, and while I'm making no particular effort to make the technology realistic, there's one major thing I feel I have to be fairly accurate about.

My version of FTL travel is that ships can make extreme leaps across interstellar distances, provided there is nothing in the way. So spacefarers have to align their ships along clear routes before leaping.

But the galaxy is moving, isn't it? How fast do star systems and such swirl around? Could a route stay empty enough to fly through for a good long while or would my space-pilots need to constantly update their charts before each and every leap?

Galaxies move at variable speeds. And the speed were higher early in the life of the universe. The Milky Way is moving toward "The Great Attractor" at "14 million miles per hour", so your astronauts would have to factor that into every jump. And most galaxies are being attracted at a similar rate but at different angles.
http://www.dailygalaxy.com/my_weblog/2009/03/mystery-of-the.html

Dennis E. Taylor · Nov 9, 2015

It sounds like you're wondering if a star might move into the path during the jump. You haven't indicated, though, how long the jumps would be or how long the jump takes.

The thing is, though, if it was even potentially a problem, the shipping companies would adjust by doing shorter jumps from local star to local star, to get where they wanted.

LDParker · Nov 9, 2015

I'd imagine there would be technology that would (hopefully) take star movements into account to avoid leaps into a planet or star. That being said, space debris, ships, or asteroids/meteors might potentially create a problem.

RKarina · Nov 9, 2015

That could be an interesting thing to play with - the space pilots would need to not only calculate the route itself, but do it in a series of hops (possibly with pauses at some stops) to ensure a clear path. Could make for some fun - short stops with a time crunch, longer stop overs at some stations.

RichardGarfinkle · Nov 9, 2015

Are you asking about the speeds of stars within a galaxy or the speed of galaxies relative to each other.
Wikipedia has a decent article on stellar speeds.
https://en.wikipedia.org/wiki/Stellar_kinematics
Average is around 100km/sec (motion of stars relative to the galactic center, I believe). This is unlikely to cause any difficulties for the technology you are talking about.
Speeds of galaxies are pretty slow on the human scale, so that wouldn't be a problem.

One thing I wonder about. What constitutes a "clear route" the vacuum of space in a galaxy still has stuff in it; how clear does it have to be? There's also Dark Matter, does that cause problems for your system?

efreysson · Nov 9, 2015

Angry Guy said:
It sounds like you're wondering if a star might move into the path during the jump. You haven't indicated, though, how long the jumps would be or how long the jump takes.

InitiaNova said:
I'd imagine there would be technology that would (hopefully) take star movements into account to avoid leaps into a planet or star. That being said, space debris, ships, or asteroids/meteors might potentially create a problem.

RKarina said:
That could be an interesting thing to play with - the space pilots would need to not only calculate the route itself, but do it in a series of hops (possibly with pauses at some stops) to ensure a clear path. Could make for some fun - short stops with a time crunch, longer stop overs at some stations.

I'll be honest: I've given the technology in my setting no particular thought. I'm basically out to write a fantasy story with spaceships. Ie, space opera. I don't have a good grasp of astronomical terms or facts. But while I can handwave ray guns and artificial gravity and whatever, I feel I can't just ignore the fact that star systems move around. So I feel I might need to refine my space travel a bit.

I have no number for how long a jump is, but an average one would pass by several star systems. The series opens with the protagonists escaping a coup by making a few long, uninterrupted jumps.

RichardGarfinkle said:
Are you asking about the speeds of stars within a galaxy or the speed of galaxies relative to each other.
Wikipedia has a decent article on stellar speeds.
https://en.wikipedia.org/wiki/Stellar_kinematics
Average is around 100km/sec (motion of stars relative to the galactic center, I believe). This is unlikely to cause any difficulties for the technology you are talking about.
Speeds of galaxies are pretty slow on the human scale, so that wouldn't be a problem.

Well, I don't have travel between galaxies, so I guess I'm asking about how much stars move around on the human scale. As in, would a certain route stay open for a decent while? I need to figure this out, as it greatly affects military strategy and such.
The limitation I have in mind for FTL is that it has to be in a straight line.

One thing I wonder about. What constitutes a "clear route" the vacuum of space in a galaxy still has stuff in it; how clear does it have to be?

Basically, no large solid matter, such as asteroids. So no sane spacer jumps through an area that has not been charted by scouts and beacons.

RKarina · Nov 9, 2015

I don't think you need to have a scientifically accurate answer to that... nor am I sure you could have one.
It sounds like what would work - in the context of your story - is the basic premise you've outlined:
There are charted areas and uncharted areas.
Space pilots navigate by plotting a course through these areas, calculating their passage (speed, direction, whatever) to avoid passing through large/solid matter.
No sane space pilot would jump an uncharted area.

That's really all that needs to be there I think.
So long as you acknowledge that things are in motion and the pilots are accounting for that motion you should be good. Maybe that's even part of the charting and beacon system - time stamps of some sort that indicate when it's safe to pass, or how long the "open" cycle is.

Add in some chatter about what happens if you don't calculate, or you go off chart - y'know, that crazy guy who attempted a hop while running away from the intergalactic police. Figured he'd jump across an uncharted area to escape and wham! Slammed right into an asteroid belt.

Dennis E. Taylor · Nov 9, 2015

On the human time-scale, stars will not move enough to change any routes. You won't notice any difference in the sky for hundreds of years, barring occasional outliers like Barnard's Star. If the society maintains star charts, they will have routes marked where the right-of-way is getting kind of tight, and they will publish updates (on the web?).

kuwisdelu · Nov 9, 2015

efreysson said:
I'll be honest: I've given the technology in my setting no particular thought. I'm basically out to write a fantasy story with spaceships. Ie, space opera. I don't have a good grasp of astronomical terms or facts. But while I can handwave ray guns and artificial gravity and whatever, I feel I can't just ignore the fact that star systems move around. So I feel I might need to refine my space travel a bit.

I don't think you need to worry too much about handwaving it, actually.

Ben Kenobi: How long before you make the jump to lightspeed?

Han Solo: It'll take a few moments to get the coordinates from the navicomputer.

Luke Skywalker: Are you kidding? At the rate they're gaining—

Han Solo: Traveling through hyperspace ain't like dusting crops, boy! Without precise calculations we could fly right through a star or bounce too close to a supernova and that'd end your trip real quick, wouldn't it?

That seems sufficient.

King Neptune · Nov 9, 2015

How much a star might move would depend on how far the jumps were and how much time they would take. If your ship were hopping from Earth orbit to a star on the opposite side of the Milky Way, say 50,000 light years (half the width of the Milky Way) away. Observations of the star's location would be 50,000 years out of date. Just for the sake of discussion, say that the jump is perpendicular to the direction to the Great Attractor, the star would have moved something around 1022 light years from the position observed. Adjustment could be made in the direction of the jump, but it would be difficult to come out within one light year of the star, but it's worth trying.

benbenberi · Nov 9, 2015

If your ships are traveling by means of the standard technology of your setting, on routes that ships before them have taken where the hazards/potential obstacles are already known & mapped, I would assume that the ship's navigation systems are capable of performing all the requisite calculations to determine a safe path and will automatically incorporate into those calculations the known movements of all the bodies involved between Here and There. And that they will regularly update their database with the latest navigational details to keep their calculations valid & their routing safe/efficient. Barring unplanned disasters, I wouldn't expect the ship's crew to ever need to manually align anything, or to manually perform any calculations or programming essential to their safe passage. The weakest point in any automated system is the human operator, and the goal of automation is to eliminate human intervention and manual steps everywhere they're not essential.

TL, DR: Ship's navigation systems do everything by themselves, once you tell them where you're going and when you have to be there. The only reason not to hand wave the entire problem out of the way is if you WANT it to be a problem for story-related reasons.

efreysson · Nov 15, 2015

I appreciate all the help so far.

I have an additional, minor question, about viewing things in space. I have a ship exploring an area well away from any stars. Wouldn't that mean nothing can be viewed visually unless it's close enough to shine lights on?

Oh, and is a fourteen-kilometre long object large enough to have its own gravitational pull?

King Neptune · Nov 16, 2015

efreysson said:
I appreciate all the help so far.

I have an additional, minor question, about viewing things in space. I have a ship exploring an area well away from any stars. Wouldn't that mean nothing can be viewed visually unless it's close enough to shine lights on?

No, there would be distant galaxies visible; they would look like stars. There may be areas on the universe in which nothing would be visible, but they would be unusual. The Universe is only about 27 billion light years across.

Oh, and is a fourteen-kilometre long object large enough to have its own gravitational pull?

It would have a very slight gravity. How much would depend on the mass and the other dimensions. A fourteen kilometer sphere of lead would have enough gravity that something would need force to get off it; that is, things wouldn't just drift away.

RichardGarfinkle · Nov 16, 2015

efreysson said:
I appreciate all the help so far.

I have an additional, minor question, about viewing things in space. I have a ship exploring an area well away from any stars. Wouldn't that mean nothing can be viewed visually unless it's close enough to shine lights on?

If you're asking about objects in your lightless area of space, then yes little would be visible without brought sources of illumination. But You would be able to spot the presence of objects that are between the viewer and distant stars.

Oh, and is a fourteen-kilometre long object large enough to have its own gravitational pull?

The amount of gravitational force exerted depends only the mass of the object not on its size. Although, obviously a small object made of standard matter does not exert much gravitational force. The general formula for calculating the force exerted on an object of mass m by another object of mass M is
GMm/r[SUP]2[/SUP]
where r is the distance between the objects and G is Newton's constant of universal gravitation.

The acceleration due to gravity exerted by an object of mass M is GM/r[SUP]2[/SUP].

The above assumes no relativistic effects.

efreysson · Nov 16, 2015

King Neptune said:
No, there would be distant galaxies visible; they would look like stars. There may be areas on the universe in which nothing would be visible, but they would be unusual. The Universe is only about 27 billion light years across.

RichardGarfinkle said:
If you're asking about objects in your lightless area of space, then yes little would be visible without brought sources of illumination. But You would be able to spot the presence of objects that are between the viewer and distant stars.

Ah, I see I wasn't clear enough. Yes, I do mean objects being visible within that particular area. The protagonist comes across a gigantic derelict ship. The idea is his radar tells him SOMETHING big is up ahead but he doesn't know what until he shines lights on it.

It would have a very slight gravity. How much would depend on the mass and the other dimensions. A fourteen kilometer sphere of lead would have enough gravity that something would need force to get off it; that is, things wouldn't just drift away.

The amount of gravitational force exerted depends only the mass of the object not on its size. Although, obviously a small object made of standard matter does not exert much gravitational force. The general formula for calculating the force exerted on an object of mass m by another object of mass M is
GMm/r[SUP]2[/SUP]
where r is the distance between the objects and G is Newton's constant of universal gravitation.

The acceleration due to gravity exerted by an object of mass M is GM/r[SUP]2[/SUP].

I'm not good at this kind of math, but it's the derelict ship I'm curious about. So it's made mostly of metal and would be quite heavy.

He would need gravity boots to walk within it, right?

Jozzy · Nov 16, 2015

efreysson said:
Ah, I see I wasn't clear enough. Yes, I do mean objects being visible within that particular area. The protagonist comes across a gigantic derelict ship. The idea is his radar tells him SOMETHING big is up ahead but he doesn't know what until he shines lights on it.

A ship far away from any stars would be hard to detect with non-future technology. If it was emitting something (radio, infrared, light, x-rays, netrons, etc.) these could conceivably be seen by a passive scan.

A dark chunk of metal that has cooled to near absolute zero would be almost impossible to detect without an active process (lidar, radar, etc.)

RichardGarfinkle · Nov 16, 2015

efreysson said:
Ah, I see I wasn't clear enough. Yes, I do mean objects being visible within that particular area. The protagonist comes across a gigantic derelict ship. The idea is his radar tells him SOMETHING big is up ahead but he doesn't know what until he shines lights on it.

I'm not good at this kind of math, but it's the derelict ship I'm curious about. So it's made mostly of metal and would be quite heavy.

He would need gravity boots to walk within it, right?

It depends. In terms of just gravity, yes he would need something to hold him to the ship. But if the ship were spinning, he would experience acceleration toward the outer hull which would feel like gravity (even though it isn't).

By the way, if he's using radar, he already is shining a kind of light on it, since radio waves are also,electromagnetic radiation.

Dennis E. Taylor · Nov 16, 2015

A spaceship wouldn't be a really good source of gravity. By nature, they're hollow, so 14 km worth of ship might only be a half-km worth of metal. And it would be quite spread out.

Need some help with astronomy

efreysson

Closer than ever

King Neptune

Dennis E. Taylor

Get it off! It burns!

LDParker

RKarina

RichardGarfinkle

Nurture Phoenixes

efreysson

Closer than ever

RKarina

Dennis E. Taylor

Get it off! It burns!

kuwisdelu

Revolutionize the World

King Neptune

benbenberi

practical experience, FTW

efreysson

Closer than ever

King Neptune

RichardGarfinkle

Nurture Phoenixes

efreysson

Closer than ever

Jozzy

Please, sir, I want some more.

RichardGarfinkle

Nurture Phoenixes

Dennis E. Taylor

Get it off! It burns!