posted
A question for you space-niks: How far away would ships in space have to be from one another to be out of each other's visual range? Is it merely a matter of diminishing perspective, no different than if they were on the surface of the Earth? Or would it be farther, since there's no planetary curvature to complicate things in space? And would it be farther than in the atmosphere of Earth since they'd be looking through a vacuum?
Posts: 1810 | Registered: Jun 2002
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Well, it would seem to be considerably further than in an atmosphere of any kind. However, sources of light are not as abundant in space. I guess it would really matter how close you were to a star (read: lightsource) and how sensitive the other ships instruments were. If you're talking light in the visual range, probably no more than 25,000 miles. (disclaimer: all numbers have been pulled firmly from my butt)
posted
The further away something is from you in space, the smaller it will look, right?
Since you're looking at things all around you, then you've got this huge sphere of space to look at. (The size of this sphere is 4 times pi times the distance from the object squared.) So not only will the object look smaller, but it will take up a smaller fraction of the whole sphere.
That's not even taking into account how bright it is, or how much competition it has from all the other lights in space that you have to be able to separate it from to see it.
Consider how close a car has to be to you on a long stretch of Arizona highway before you realize that one is approaching you. Use that for a rule of thumb for your spaceship in space.
I'd say visual range in space, even for something really bright, wouldn't exceed 20 miles unless it was also really big.
(I've been on a cruise to Belize City, Belize. The ships have to stay about 10 miles out and the passengers have to take special boats in and out of Belize Harbor. When we got to the harbor and looked back at the ship, it looked small enough to cover with my luggage key held out at the end of my arm. It was a Carnival cruiseship that carried close to 2000 people, so maybe that will give you some idea.)
posted
It also depends heavily on the amount of visual clutter. Nearer the center of the galaxy, for instance, it would be much harder to spot any particular small dim object because you're surrounded by billions of small bright objects and many more billions of small dim objects. Sorting out which ones are ships can be a pain, even using the full EM spectrum, if you rely on passive sources.
Just register all those factors together. The illumination in the area, the size and distance of the other ship, its surface albedo and emitted radiation, and some other conditions like the amount of interstellar dust in the intervening space all give you the astronomical magnitude of that object to the observer. Things like the instruments used to search for the other ship and the number of stars or other objects with a similar or greater magnitude as well as the ability of the obeserver to tell one thing from another in space (a big black thing with little white bits, really ) will determine what the detection threshhold is going to be.
If you use active, like radar or something, then you're in a whole different ball game. But some of the same factors still apply.
It is a given that you can make any non-emitter pretty much invisible in space, if you have some idea of the sort of scanners that will be looking for it. Same is true of any environment, really, but in space the visual environment is less complex (I already used the "big black thing with little white bits" quote, though).
I think that most writers set it up so that the drive of an interstellar ship has a big enough and distinctive enough energy signature that it's nearly impossible to hide or disguise as anything else. Bending space/time probably would paint you like a whore, come to thing of it.
posted
I suppose a lot depends on whether your situation is one where both ships are trying to stay in visual range of each other or not. If you know where to look for the other ship, and it was shining a very bright light your direction, you could spot it thousands of miles away or more (depending on how bright the light was.)
On the other hand, something painted black could be a hundred yards away, and you might not spot it unless you noticed it obscuring background stars.
posted
At the risk of persecution by those with a hard sci-fi bent, I'm going to pull out a classic Star Trekism.
If two starships meet in deep space, it's no accident. Space is so fantastically big and empty, the chances of seeing anything you aren't aiming for is nil.
To be within visual range of another space craft, you have to either both stop in the same place or be on the same course and matching speed.
Ancientech space craft (like the space shuttle) navigate using complicated mathematics that essentially point-and-shoot the ship toward its destination. Misplace a decimal point or use feet instead of meters and everyone aboard is dead.
Magitech space craft (like the Enterprise) have long distance sensing instruments, so adjustments in course and speed can be made on the fly. Those sensors are the only way you could detect another ship passing in close proximity.
Of course you could come up with some sort of shipping lanes or trade routes through space. Maybe with a line of beacons to help travellers stay the course. That way if a ship broke down, another ship would probably notice it at some point.
Hmmmm... to give a more direct answer... not very far at all.
posted
I don't know if you've read any of the Honor Harrington novels, but theres a lot of thoughts about detection of other craft in space in those.
Essentially, they rely on two different mechanisms -- detecting radiation from engines, and detecting changes in gravity caused by large masses moving. Visual sightings are considered essentially impossible.
Although note that the Honorverse is a little peculiar in some respects -- ships travel very fast (holding accelerations of 200g for long lengths of time) due to the requirement that they cannot use hyperspace while too close to a source of large amounts of gravity.
I believe the limit on human optical discrimination is that something that appears as less than about 1/10th of a degree cannot be resolved by the eye unless it is especially bright (e.g. a star). Obviously with telescopes, this limit can be extended, but you'd have a lot of searching to do...
posted
With human eyeballs, I presume? A few points of reference may help:
I can see the planet Saturn with my naked eye when it is 1.4 billion kilometers from the sun. Saturn is in the relatively dark outer part of the solar system, but it's also 120,000 kilometers wide. Your ships are smaller than that, right?
Here in the inner solar system, you've probably seen the International Space station yourself. When it is directly above your head it's about 380 kilometers away and it appears as a dot bigger and brighter than Venus. When it's on the horizon the ISS is nearly 1,000 kilometers away (give or take, I'm doing the math in my head). It's visible at that distance, but it's much dimmer, about the same as the lights on an aircraft at high altitude.
Naked eye observers could see the Apollo spacecraft while they were in Earth orbit, but once they headed toward the moon we needed a telescope to see them. It's probably safe to say that no human could see them by the time they were about 2,000 kilometers out.
quote:I can see the planet Saturn with my naked eye when it is 1.4 billion kilometers from the sun. Saturn is in the relatively dark outer part of the solar system, but it's also 120,000 kilometers wide. Your ships are smaller than that, right?
That leads to an interesting thought: I personally can't see Saturn, because I don't know when or where to look for it. If somebody who does know points it out to me, then I will know. That to say it may depend on who is doing the looking. If your character is in an area of the galaxy with which he is very familiar, he may be more likely to notice an out-of-place tiny light source among the stars.
(Of course, now it sounds like we're getting WAY too picky. It's your story. If you don't want him seeing the ship, don't let him see it; and if he should see it, show it to him somehow.)
[This message has been edited by TheoPhileo (edited March 13, 2004).]
quote:It's your story. If you don't want him seeing the ship, don't let him see it; and if he should see it, show it to him somehow.
That's pretty much my whole writing philosophy: make it work. But all these responses give me a nice firm footing on which to base my elaborations. Kind of like public speakers being advised to have loads more material than they need -- make sure there's always something you don't tell -- so as to speak from a position of confidence and strength. This has been great. (And that's not to discourage any further comment. Doesn't hurt to really secure that position of strength. Besides, you never know when some bit of info takes on a life of its own, fictionally speaking. )
Posts: 1810 | Registered: Jun 2002
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) will determine what the detection threshhold is going to be.
