Navigating in 3D

On 15 Sep 2006 08:41:16 -0700, chornedsnorkack DeleteThis @hushmail.com wrote:

>Asimov tells that it was hard to navigate through Galaxy because
>hyperspace jumps were unreliable, and moving by just a few lightyears
>would distort the starfield hugely. It took a lot of computing to
>identify where in space one actually was... like Ebling Mis did during
>their trip.
>
>The Dune universe also made the assumption that it is easy to get lost
>in hyperspace.
>
>Does it hold water?

It did many decades ago. As far as diffuculty in searching a
database of stars and computation of position, authors may have
substantially underestimated how much computing power would be
available. Moore's Law was published in the 1960's, and probably
wasn't well known or widely believed for a decade or so.
There should be little problem writing a database with known info
about tens of thousands of the brightest stars around the galaxy, and
using even a small telescope to look around. When you find a dense
background light in an encircling band, you've found the Milky Way and
know immediately the angle of the plane of the galaxy.
With a current laptop computer, the amount of time to find your
position should be seconds or at worst case minutes, and the main
limitation is the time the computer takes to move the motor-aimed
cameras and telescopes around and make measurements of the stars
found.

>Let´s have a look at what you could do by just measuring angles and
>estimating magnitudes of stars.
>
>If you identify correctly 1 star, you have no angles to measure.
>However, if you can estimate magnitude, you know your location on a
>sphere.

Yes but a fuzzy sphere. Even if you can measure magnitude exactly
and have a very good database with magnitudes, stars (including the
Sun) are known to measurably change their light output. In fact, the
database might have much raw data of several dated magnitude
measurements (for many of the brightest [in absolute terms],
most-visible-throughout-the-galaxy stars in the database) over years,
decades or centuries, so the computer can identify a possible trend
and extrapolate to current measurements. Once it positively identifies
a star (in conjunction with several other methods, for good
reliablility), it can add the current measurement to the database.
This of course requires calculating the actual 'date' at the star as
well (if you're 500 lightyears away, your measurement is of the star
as it was 500 years ago, if you're 16,000 lightyears away, it of
course appears as it was 16,000 years ago).
Another point, stars move over time, but of course their positions
at any time can be calculated.

>If you identify correctly 2 stars, you have constrained your location
>to a toroidal surface. And estimating their magnitudes can allow you to
>constrain your location to a circle, or else reject the identification.
>
>If you identify correctly 3 stars, you can find your real location in
>space from measuring angles. And estimating magnitudes can help confirm
>or reject the identification.

Also useful in identifying stars is their spectrum, and not just in
visible light. The database might have magnitudes at different
frequencies from microwave to x-rays. Though like magnitude, the
spectrum may change over a long period of time.

>If you identify 4 stars correctly, you can check the identification by
>angles alone.

The computer double-check and triple-check to be sure there's also
a close match of measured magnitues and spectra with those in the
database. This far from home with something this critical, I want to
wear both a belt and suspenders. I'd add recognizable "external"
things in the database too, such as the Andromeda galaxy.

>Now, while some of the brightest stars in the night sky are nearby,
>intrinsically dim stars (Alpha Centauri, Sirius...), some others are
>distant and intrinsically bright.
>
>If you move only a small distance from a known location, like the 50-pc
>trip from Terminus to Anacreon, then the distant bright stars have to
>be in your sky still, in rather similar locations. What does change is
>nearby stard. And some of them can become as bright as the distant
>ones. But if you can identify the distant stars, you know your
>approximate location and can make amn informed guess about what nearby
>stars to look for.
>
>In the worst case, where a nearby star projects near a distant star so
>that you cannot easily identify which is which, you can be aware of the
>possibility and just compute the possibilities. This is unlikely to
>happen to all distant spacemarks simultaneously...

That's why you have the database as large as possible, so you can
identify as many stars (and other objects) as possible, and more
reliably determine your position.

>
>How would a FTL ship travel if your FTL is unreliable and you need a
>low-tech system to input your actual location?

Badly... it would be bad if the destination could not be determined
precisely ahead of time or at least predicted precisely, even if your
exact position can be easily determined once your there. If you're ten
lightyears away, want to visit one of Mars' moons and a hyperjump puts
you randomly in a sphere around the Sun with the radius of Neptune's
orbit, this is bad (though in the sense of shooting a target, this may
be considered very good accuracy). You have to use conventional
rocketry to get to Mars or perhaps risk more hyperjumps.

The good news is even within the planetary systems, it is still
overwhelmingly empty space so you're very unlikely to drop out of
hyperspace inside a star or planet. But the possibility is a bit
unsettling.

Happy hyperjumping.
 >> Stay informed about: Navigating in 3D 

Ben Bradley wrote:
> It did many decades ago. As far as diffuculty in searching a
> database of stars and computation of position, authors may have
> substantially underestimated how much computing power would be
> available. Moore's Law was published in the 1960's, and probably
> wasn't well known or widely believed for a decade or so.
> There should be little problem writing a database with known info
> about tens of thousands of the brightest stars around the galaxy, and
> using even a small telescope to look around. When you find a dense
> background light in an encircling band, you've found the Milky Way and
> know immediately the angle of the plane of the galaxy.
> With a current laptop computer, the amount of time to find your
> position should be seconds or at worst case minutes, and the main
> limitation is the time the computer takes to move the motor-aimed
> cameras and telescopes around and make measurements of the stars
> found.

The problem at the current time is that star positions are not known
well enough in three dimensions, except for our solar neighborhood, for
the laptop to be effective (or necessary). That will change within the
next few decades, though, as more improved parallax measuring satellites
are launched. I think the ESA is planning a mission (GAIA?) that will
measure parallaxes to microarcsecond accuracy. That would give decent
distance estimates (of, say, 1-parsec precision) out to, oh, about 1,000
parsecs. To get all the way across the galaxy, about 30,000 parsecs,
would require nanoarcsecond accuracy.

The reason you need all that is that stars are pretty anonymous (as far
as we know), so the problem is one of identification, not measurement.
Identification only becomes pretty simple if your positions are fairly
precise (like down to the parsec).

Fortunately, there are plenty of objects in the galaxy that are *not*
anonymous. The eta Carinae nebula, for instance, would be pretty easy
to identify from anywhere it could be seen. I think such objects would
make reasonable signposts even today (if we could travel fast enough).
Then, too, there are the other galaxies, which would get us into the
right area of the galaxy--certainly to within a few dozen parsecs, if
one has good enough measurements.

Someone once wrote to me to tell of a story called Dimension of
Miracles, written by a fellow named Robert Sheckley. Many of you have
probably heard of it. I was intrigued enough by the problem of finding
one's way back home from the center of the galaxy to write an Asimovian
essay (one of a series of about 30 thus far) called "The Dimension of
Our Galaxy," in which I explain how one could navigate back to the Earth
from the center of the galaxy, using a spaceship capable of travelling
at about 100,000 c (so that the straight trip would take a few months),
with only today's knowledge about galactic and extragalactic objects.
Those interested in the topic can find my essay at

http://astro.isi.edu/games/dimension.html

--
Brian Tung <brian.RemoveThis@isi.edu>
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
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My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.html
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Ben Bradley wrote:

> It did many decades ago. As far as diffuculty in searching a
> database of stars and computation of position, authors may have
> substantially underestimated how much computing power would be
> available. Moore's Law was published in the 1960's, and probably
> wasn't well known or widely believed for a decade or so.

Moore's law is simply an _empirical_ observation of the increase in
transistor density on chips. It was never a prediction, so there was
nothing to "believe."

--
Erik Max Francis && max.RemoveThis@alcyone.com && http://www.alcyone.com/max/
San Jose, CA, USA && 37 20 N 121 53 W && AIM, Y!M erikmaxfrancis
Come not between the dragon and his wrath.
-- King Lear (Act I, Scene I)
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Erik Max Francis wrote:
> Moore's law is simply an _empirical_ observation of the increase in
> transistor density on chips. It was never a prediction, so there was
> nothing to "believe."

Perhaps it was not initially meant to be a prediction, but it is quite
often used as one now. "Assuming Moore's Law continues to hold..." is a
frequent refrain in planning architectures for long-term development.

However, as I say, computing power is not the bottleneck here. You
still need travel on the order of tens of thousands times the speed of
light.

--
Brian Tung <brian.TakeThisOut@isi.edu>
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.html
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In rec.arts.sf.science Erik Max Francis <max RemoveThis @alcyone.com> wrote:
> Ben Bradley wrote:
>
>> It did many decades ago. As far as diffuculty in searching a
>> database of stars and computation of position, authors may have
>> substantially underestimated how much computing power would be
>> available. Moore's Law was published in the 1960's, and probably
>> wasn't well known or widely believed for a decade or so.
>
> Moore's law is simply an _empirical_ observation of the increase in
> transistor density on chips. It was never a prediction, so there was
> nothing to "believe."

"The complexity for minimum component costs has increased at a rate of
roughly a factor of two per year ... Certainly over the short term this
rate can be expected to continue, if not to increase. Over the longer
term, the rate of increase is a bit more uncertain, although there is no
reason to believe it will not remain nearly constant for at least 10
years. That means by 1975, the number of components per integrated circuit
for minimum cost will be 65,000. I believe that such a large circuit can
be built on a single wafer."

Sounds like a prediction to me. Of course, there was no concrete reason to
believe it was going to come true at the time it was made, but it was a
prediction and amazingly enough it's remained true up to the present day.

--
Michael Ash
Rogue Amoeba Software
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chornedsnorkack.RemoveThis@hushmail.com wrote:

> The Dune universe also made the assumption that it is easy to get lost in hyperspace.

Sort of a random thought, but since we're assuming FTL in this
discussion, a comment made in, IIRC, "Glory Road" may be apposite.
Again IIRC, it was something like "FTL is the easiest way to get lost
in the Universes."

Given what special relativity has to say about the possible
consequences of FTL, plus the current semi-respectability of
many-worlds theories, that may be right.

Getting lost in space sans FTL seems fairly easy to avoid. FTL may be
the problem.
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JRS: In article <eeenst$7su$1@praesepe.isi.edu>, dated Fri, 15 Sep 2006
10:29:01 remote, seen in news:rec.arts.sf.science, Brian Tung
<brian RemoveThis @isi.edu> posted :
>
>The problem at the current time is that star positions are not known
>well enough in three dimensions, except for our solar neighborhood, for
>the laptop to be effective (or necessary). That will change within the
>next few decades, though, as more improved parallax measuring satellites
>are launched.

At least within the Galaxy, the only problem is if you want to go much
further away than anyone has gone before, *and return*.

To go to the Crab Nebula, for example, just jump that way; and after
each jump look for it and calculate the next jump. The Crab should be
more and more easily identifiable; after all, it's an atypical "star",
which is presumably the reason for going.

To navigate to the Other End of the Galaxy is easy enough; it's a big
target. Coming back to exactly here is not so easy.

To reach Alpha Centauri, or Barnard's Star, or Sirius, and to return,
should be navigationally easy with present knowledge. And ISTM that the
first ships there will carry equipment to measure the angles between the
nearer stars and identifiable remote objects. Combining those with
measurements from Earth, we have baselines of the order of 10 LY instead
of about 1/36500 LY - and that should then give positions good enough to
cover a fair fraction of the Galaxy.

--
© John Stockton, Surrey, UK. ?@merlyn.demon.co.uk Turnpike v4.00 MIME. ©
Web <URL:http://www.merlyn.demon.co.uk/> - FAQqish topics, acronyms & links;
Astro stuff via astron-1.htm, gravity0.htm ; quotings.htm, pascal.htm, etc.
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Dr John Stockton wrote:
> JRS: In article <eeenst$7su$1@praesepe.isi.edu>, dated Fri, 15 Sep 2006
> 10:29:01 remote, seen in news:rec.arts.sf.science, Brian Tung
> <brian.DeleteThis@isi.edu> posted :
> >
> >The problem at the current time is that star positions are not known
> >well enough in three dimensions, except for our solar neighborhood, for
> >the laptop to be effective (or necessary). That will change within the
> >next few decades, though, as more improved parallax measuring satellites
> >are launched.
>
> At least within the Galaxy, the only problem is if you want to go much
> further away than anyone has gone before, *and return*.
>
> To go to the Crab Nebula, for example, just jump that way; and after
> each jump look for it and calculate the next jump. The Crab should be
> more and more easily identifiable; after all, it's an atypical "star",
> which is presumably the reason for going.
>
Bad example, actually. A supernova remnant is one of the few
conspicuous object that by definition does change fast. Crab is visible
because it is just 1000 years old - but it is 5000 LY away. So, if you
try to approach, you could see it physically spreading apart, dimming
and vanishing into the background illumination even as you approach FTL.
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Allen Thomson wrote:
>
> Getting lost in space sans FTL seems fairly easy to avoid. FTL may be
> the problem.

Here is thought, I often heard that traveling by FTL could be a kin to
Time travel,
just suppose when you misjump with FTL, not only do you displace
yourself in space, but also displace your self in 'Time' as well.
Would your typical stellar database be able to handle any changes in
time? Would it be very easy or hard to determine if you are lost in
time as well as space?


Just my $0.02

Space Cadet

Moon Society - St. Louis Chapter

http://www.moonsociety.org/chapters/stlouis/

There is only one (maybe 2) basic core reasons for humans to go
beyond LEO, That is for the establishment of space settlements or a
space based civilization. Everything else are details.

Gary Gray 11/9/2005
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Space Cadet wrote:

> Allen Thomson wrote:
>> Getting lost in space sans FTL seems fairly easy to avoid. FTL may be
>> the problem.
>
> Here is thought, I often heard that traveling by FTL could be a kin to
> Time travel,

It's not quite that straightforward. With a faster-than-light drive,
you can make multiple transits in such a way that you will end up back
in time. But it's not like travelling faster than light results in you
steadily travelling further and further into the past.

--
Erik Max Francis && max.RemoveThis@alcyone.com && http://www.alcyone.com/max/
San Jose, CA, USA && 37 20 N 121 53 W && AIM, Y!M erikmaxfrancis
The little I know, I owe to my ignorance.
-- Sacha Guitry
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Erik Max Francis wrote:
> Space Cadet wrote:
>
> > Allen Thomson wrote:
> >> Getting lost in space sans FTL seems fairly easy to avoid. FTL may be
> >> the problem.
> >
> > Here is thought, I often heard that traveling by FTL could be a kin to
> > Time travel,
>
> It's not quite that straightforward. With a faster-than-light drive,
> you can make multiple transits in such a way that you will end up back
> in time. But it's not like travelling faster than light results in you
> steadily travelling further and further into the past.

Okay, I misspoke, I guess my underlining assumption was that if the FTL
malfunctioned in a bad way, that there was a (small) chance of being
displaced in time as well as space.

But for the sake of the argument, lets say that there is a very rare
occurance where taking a bad FTL jump takes you back(or even forward)
in time as well as space. Would your typical Astronomical Data Base be
able to handle a displacement in Time(even a small one) when
determining your location?

Just my $0.02

Space Cadet

Moon Society - St. Louis Chapter

http://www.moonsociety.org/chapters/stlouis/

There is only one (maybe 2) basic core reasons for humans to go
beyond LEO, That is for the establishment of space settlements or a
space based civilization. Everything else are details.

Gary Gray 11/9/2005
>
> --
> Erik Max Francis && max.DeleteThis@alcyone.com && http://www.alcyone.com/max/
> San Jose, CA, USA && 37 20 N 121 53 W && AIM, Y!M erikmaxfrancis
> The little I know, I owe to my ignorance.
> -- Sacha Guitry
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Brian Tung wrote:
> Nyrath the nearly wise wrote:
> > But if the displacement in time approaches
> > a non-trivial fraction of the period
> > of one galactic rotation, the calculation
> > becomes vastly more difficult.

Is this long enough to greatly alter the orbital
periods of binary star systems?

> It doesn't even really have to be that. If the wish is to be able to
> distinguish star from star, then one measure of how long before things
> get confusing is the time it takes for two stars to switch positions.

Well, that could be an extremely short time period if
you count binary star systems.

> Of course, that's more a problem for G, K, and M dwarfs. The brighter
> O, B, A, and F stars are fewer and further between, and it would take
> correspondingly longer for them to be lost in the shuffle.

I don't understand the popularity in this thread of using
these relatively rough distinctions, when binary orbit
period should be pretty fine.

A decent fraction of stars out there are in binary
star systems, right? And the orbital period can be
calculated very precisely from a wide variety of angles.
For orbits fast enough for spectral analysis, any direction
except roughly along a pole will do.

Isaac Kuo
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Space Cadet wrote:
> just suppose when you misjump with FTL, not only do you displace
> yourself in space, but also displace your self in 'Time' as well.
> Would your typical stellar database be able to handle any changes in
> time? Would it be very easy or hard to determine if you are lost in
> time as well as space?

Kinda sorta.

Star catalogs not only document a star's position,
but also its "proper motion". Using those two
pieces of information it is straightforward, though
tedious, to calculate the correction required to
counteract limited time travel.

But if the displacement in time approaches
a non-trivial fraction of the period
of one galactic rotation, the calculation
becomes vastly more difficult.
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