Celestia Forums

I'm writing a scientifically plausible sci-fi story and need to know how to map a route of G2V stars (what I accept to be earth-bearing solar systems) from our sun to the side edge of our Galactic outer spiral arm (not the end of our spiral arm... just to the inside or outside side edge, equidistant from the galactic centre/root of our outer spiral arm). My tale involves nine G2 (luminosity class V, aka 5*--thank you Hungry4info) stars. My specific quest is to find out (1) how many are there in reality, (2) how far apart are they, and (3) how many other stars are between each of them?

What are the steps of finding this out? I figure I have to isolate the G2V stars, and also know where the inside edges of our galactic outer spiral arm at our location is to do this.

I've only had experience with Jim Smith's Free Virtual Galaxy for a simulator and that was a long time ago, so I don't really know how to use Celestia all that well.

Any help would be appreciated.

rrrraygun wrote:(luminosity class V, aka 4)

V is the Roman numeral for 5.
I = 1 = Supergiant
II = 2 = Bright giant
III = 3 = Giant
IV = 4 = Subgiant
V = 5 = Dwarf
VI = 6 = Subdwarf


A major limitation to what I think you're trying to do is the uncertainty in the distances to stars (of any type). Stars 200 light years away can easily have distance uncertainties on the order of 30 or 40 light years in either direction.

Based on the solar neighborhood, I'd say G-type stars are, on average, 10 - 15 light years apart. That's just pulling from memory though, and could be off a little.

The technique I would use would be to write a CelX function to Mark all G2V stars (maybe expand that to include +/- a few sub-classes)
(See viewtopic.php?f=9&t=11546 for a script that could be used as a start.)
look at the galaxy from above
and decide which ones I want to use

G stars are relatively dim. I suspect the Hipparcos dataset (which is what Celestia uses) doesn't include all that many really distant ones.

Um. Yeah. That's what I was afraid of.

This shows all the G2 stars known to Celestia against a background of Celestia's model of the Milky Way. There aren't any as far away as the Outer Arm. Sorry.

However, the Sun is usually considered to be in the Orion Spur, between the Sagittarius and Perseus Arms. If one selects instead the adjacent outer arm, the Perseus Arm, that should be doable.

Ah, I'm getting a clearer picture now. I will have to narrow my search for G2V stars to just the Orion Spur then... possibly even within the vicinity/range of the Gould Belt.

Okay, so instead now I need to know how far the Sun is to the closest (and also to the furthest) edge of the Orion Spur we are in?

Also, I'm looking for a string of nine G2V stars (excluding ones in double or triple star systems) where the furthest one would lie just on the edge of the Orion Spur. If G-type stars are roughly 10-15 light years away from each other, how far away would G2V-class stars be from each other?

Thanks again for any input. Learning a lot.

I'm sorry to say that I don't think we have distances for the stars you need.

I found what looked like a reasonable map of the local galactic arms, but I can't get it to match the positions of the arms in Celestia's Milky Way model at the same time as matching the positions of the objects marked on the map. I came across several papers about recent re-evaluations of some of the distances, making some of the objects closer than previously thought, but I don't think those differences are large enough to account for the discrepancies. I'm coming to the conclusion that whoever drew the pointers to objects got many of their distances wrong. They don't say where they got them.

*sigh*

The map I found is on Wikipedia:
http://en.wikipedia.org/wiki/File:OrionSpur.png

The distances that I found for the objects that it shows came from almost as many different sources as there are objects. They're documented in the DSC file of the Addon that I assembled while trying to match things.

Here's a wide angle view of the map overlaying Celestia's Milky Way.


Here's a closeup


The addon includes a URL to place the viewpoint at the viewpoint of the first screengrab above. It also includes the CelX script I used to mark the G2V stars known to Celestia.
The addon is large because it includes the map mentioned above.

http://www.lepp.cornell.edu/~seb/celest ... _stars.zip (4MB)

Ok...as I understand it, you are looking for a more or less 'straight line' of nine G2V stars, extending from here to the edge of the Orion Arm, correct?

Your intent, I believe, is to find suitable stars for earth analogues: worlds where we humans can step out, breathe the atmosphere, drink the water, while not having to worry about being deep frozen (except at the poles, maybe) or deep fried.

SETI did something roughly like this as part of their search for stars to aim their radio telescopes at. Turnbull and Tarter turned out two versions of 'HabCat', the first based on the Hip Catalogue, the second based on Tycho. As part of this, they defined the characteristics of 'Hab-Stars'; that is stars which meet the criteria which they deemed necessary for the formation of earthlike worlds. As I recollect, their criteria included:

Spectral type of F5V down to about M0V (ideally should probably stop along about K5-K7V). Stars brighter than F5V don't last long enough for planets to form and earth style life to develop; and while the fainter K's and M's can last a really long time, you also tend to get into problems with tidal locking because the planets, in many cases would have to orbit too close in. (Under some conditions, they were willing to tolorate class IV stars - subgiants, though).

Fairly high metallicity. Because, at the time they prepared HabCat, the belief was that stars with low metallicity were unlikely to have planets. This view might have changed somewhat since then, but I cannot say for certain.

Age of at least 3 billion years (ties in with the spectral criteria).

Little or no variability - I seem to remember they set the limit here at 3%, which is still quite a bit.

They were willing to accept the possibility of habitable worlds in binary systems. Their conclusions here were that the stars in question would have to orbit *very* close to each other, or be fairly far apart (possibly 'common proper motion' pairs with separations on the order of hundreds or thousands of astronomical units, rather than actually orbiting one another). My view here is they might have gone a step too far in that they calculated orbits for many binary systems using seriously inadequate data.

As I recollect, their first Hib based HabCat catalogue had about 17,000 stars - again, though, these range from F5 to M0, and are not limited to G2 alone. If you hunt around enough, you might be able to find a copy of it on the web somewhere.

Their second HabCat, based on Tycho, was done with far less information on the various stars (not even including distances or spectral types) and hence is much rougher. They used a combination of B-V and proper motion cuts to select about a quarter million stars they deemed possible HabStar candidates, but were unable to refine this list further.

(I am engaged in a project of sorts in which I hope to be able to assign rough distances and spectral types, as well as double star info to about 40,000 of these stars - see the ASCC thread).

Some general thoughts you may or may not find relevant: the Hip parallaxes are, hands down, the best distance info currently available for most stars. The old line parallax astronomers were doing pretty good to get a distance accurate to within 10% for a star 50 parsecs out; the Hip will typically give a distance accurate to within 2% for that same star. However, out past about 100 parsecs, even the Hip info degrades somewhat; get out past 200 parsecs and much of the time the Hip parallaxes are have errors of 15-20% or better. Hence, if it is a 'line' of HabStars you are looking for...then your distance info is likely to become highly unreliable before the end of said line is reached.

Another thing to keep in mind with the Hip, is that while it includes many of the nearby stars, it is far from complete - I have reason to believe it misses something on the order of several hundred relatively sunlike (G5V - K7V) stars within 50 parsecs of the sun, as well as several tens of thousands of fainter stars within that same distance.

Okay, so to be clear...

1 parsec = 3.26 light-years.

The Oort cloud is estimated to have a diameter of 0.6 of a parsec, which is 1.96 light years.)

Stars that are "measured" to be at 50 parsecs (a.k.a. 163 light-years) have a 2% distance accuracy. Which is the best we can get by using the Hipparcos Catalogue... a high-precision catalogue of more than 100,000 stars collected from the "High precision parallax collecting satellite" the European Space Agency (ESA) launched in 1989 and operated between 1989 and 1993. Okay. Good.

And we figured the G-type stars were roughly 10-15 light years (a.k.a. 3.26-4.6 parsecs) apart. Strictly G2s or even G2Vs though... I'd have to go through the catalogue. Doing the permutation calculation: G-type (yellow) stars could be one of ten classes (ours being 2, meaning a 'yellow' two tenths towards 'orange'), multiplied by 5 luminosity classes (ours being Roman #5 = a dwarf, or more properly, a main sequence star), and you get fifty variations of G-type stars. Ergo, 10-15 light years becomes looking more like 500-750 light years (163-230 parsecs) for distances between just two earth-bearing solar systems of strictly G2V type.

500-700 light years is waaay more than the 2% accuracy zone which, again, is 163 light-years. Even at 200 parsecs (652 light-years) where the errors are 15-20% or worse, that's still only the first planet away. Ha.

The Orion's Spur is ~3,500 light-years thick...

...and the ninth earth-bearing solar system in my fictional story could very well be [8 huge distances to travel, times 500-700 light-years] between 4000-6000 light-years away from earth.
...so maybe my ninth earth-bearing fictional solar system (Frog Planet... where X-files-ish greylings in a state of pre-metamorphosis as a kind of tadpole thing that resembles a newt and a monkey and comes from from giant "queen bee " toads come from) might reach the edge of Orion's Spur after all.

I think I'll go with lining up the nine G2Vs down the length of the Orion spur then.

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My new questions now are:
1. How many light-years (or parsecs) are we to where the Orion spur connects to the Persius arm?
2. How many light-years (or parsecs) are we to where the Orion spur connects to the Sagittarius arm?
3. How many light-years (or parsecs) are we to "Turner 5" and are there GVs there just beyond "Turner 5" at the end of what seems to be a sort of mini-spiral arm's terminating end?
and
4. What major unique sights are to be found 4,000-6,000 light-years along the Orion spur? I mean, what exists only once (in either direction up or down the Orion spur or toward the direction of "Turner 5") in 4,000-6,000 light-years (which is 1,227-1,840.5 parsecs)?

Wow, that was like doing my taxes. Good old calculator.

rrrraygun,

Unfortunately, the answers to your questions are still topics of ongoing research. How one defines the various components of the Milky Way's spiral arms is also controversial. The colorful maps on Wikipedia are "artists' conceptions" and should not be considered strictly factual. If you haven't already, I suggest you peruse the Web site
http://galaxymap.org/ Its author discusses some of the issues in his blog, although he also provides copies of those colorful maps

1. How many light-years (or parsecs) are we to where the Orion spur connects to the Persius arm?
2. How many light-years (or parsecs) are we to where the Orion spur connects to the Sagittarius arm?

Your guess is almost as good as anyone's Using a ruler or equivalent on the (highly speculative) maps that are available on Wikipedia probably would be as accurate as any other method.

3. How many light-years (or parsecs) are we to "Turner 5"

I found three very different answers.

According to the WEBDA galactic cluster database at
http://www.univie.ac.at/webda/cgi-bin/o ... r=turner+5
it's about 400 parsecs from us.
That's corroborated in the catalog of open clusters (clusters.txt) on the page http://www.astro.iag.usp.br/~wilton/
but I don't know where that distance originated. This may be a foreground cluster, not associated with the possible distant cluster that's actually of interest to you. I.e. the distant cluster (if it exists) actually has some other designation, and is not Turner 5.

According to Simbad at
http://simbad.u-strasbg.fr/simbad/sim-b ... BAD+search
there's a Cephid variable (T Ant [or T Antliae], HIP 46924) very close to it which has a parallax of about 0.6 milli-arc-seconds. If my numerology is right, that would correspond to a distance of 1666 parsecs. Unfortunately, its parallax measurements have large errors, so it's not included in Celestia.

One should be able to deduce the Cephid's distance from its period. Turner's original paper describing T Ant and the putative adjacent cluster (Turner 5?) is available at
http://www.aanda.org/index.php?option=c ... 7..325TFUL (Turner & Burdnikov, 2003)
They postulate that T Ant actually is a double star and initially describe the adjacent stars as a "poorly populated open cluster". Reading the paper in detail suggests that there might be two clusters in the field of view, one less than 1kpc away and another one closer to the variable, deduced from the stars' reddening. Unfortunately, Table 2 of the paper does not include any stars in the Hipparcos catalog, although quite a few are in the Tycho catalog. At any rate, they deduce a distance for T Ant of 2.3 kpc and suggest that adjacent stars in the field of view might be about 1.5-2kpc from us. They're also very inconclusive about the actual existence of a cluster, saying that more observations would be required.

and are there GVs there just beyond "Turner 5" at the end of what seems to be a sort of mini-spiral arm's terminating end?

Of course there are appropriate stars on the other side of the cluster, regardless of where it's located (or if it exists). Whether any of their distances have been measured accurately is another matter entirely. To put it another way, any G2V stars in that area of the sky were too dim for Hipparcos to measure.

4. What major unique sights are to be found 4,000-6,000 light-years along the Orion spur? I mean, what exists only once (in either direction up or down the Orion spur or toward the direction of "Turner 5") in 4,000-6,000 light-years (which is 1,227-1,840.5 parsecs)?

I'm sure there are some. Further research would be required.

Selden wrote:The colorful maps on Wikipedia are "artists' conceptions" and should not be considered strictly factual.

The Milkyway template that I have prepared for the Celestia distribution as part of my 10000+ NGC/IC galaxies was carefully shaped by implementing all known scientific constraints. It is not strictly factual, since a number of facts are simply still unknown. I still consider the amazing coincidence of Selden's add-on of known pulsar locations with the Milkyway arms as highly remarkable.



The green cross denotes Sol's location.
Here are more views




Fridger

Fridger,

Thanks for posting that! The addon is slightly out of date, since more have been found since then. I'm sure there must be a handful of pulsars in the region of interest. Several planetary nebulae must be in there, too. Each of the latter would be a unique visual experience. I'm not sure how close one would like to get to one of them, but they wouldn't be nearly as dangerous as pulsars. And, of course, the region is littered with gas and dust clouds. (Viz the "reddening" comment above.) They'd cause serious problems for spacecraft traveling at a substantial fraction of the speed of light.

A few more items to take into account...

First, the differing conditions of the 'galactic environment' for want of a better term at different points. Very roughly speaking the central third of the galaxy is not really a good place to find life bearing planets, earthlike or not. Lots and lots of nasty radiation floating about being the main reason.

Along the outer fringe of the galaxy we have another problem...though I'm not entirely certain if it is as severe of one as once was thought. Most of the stars in these fringe areas (or at least large numbers of them) are class VI or 'subdwarf' stars, which if memory serves are both underluminious (not that big of a problem) and also possessed of very low metalicity compared to 'normal' class V stars. This is important because a stars metallicity was thought to be (and might still thought to be) critical to determining whether or not planets would actually form there. Basically, the higher the metallicity, the greater the chances that star would have planets.

I believe there was a report of a subdwarf with a planet not all that long ago, though, so this might not be a really major concern. However, an earth like world orbiting such a star might be seriously deficient in heavier elements, barring arrivals (large meteors) from outside that solar system.

One other thing about subdwarfs - they can, at least in theory - last a *really* long time. I seem to remember seeing some estimates going as much as 100 billion years, or about 10 times longer than a normal class V star.

Another thing that would have potential major long term effect on a possible extra solar earthlike world would be the Galactic Orbit of its host star. Many stars have extremely eccentric galactic orbits; I know of several stars (mostly subdwarfs) with orbits which take them from almost the center of the galaxy (really really crowded) to what amounts to intergalactic space (really really empty).

I almost hesitate to bring this up, but upon reflection, there have been a number of photometric distance surveys, usually dealing with galactic star distribution, (light) extinction, or metallicity, which do give 'close enough' distances to hundreds of 'normal' stars as much as a couple thousand parsecs off.

Also:

G2 IV = Subgiant. Any earth like worlds orbiting a star like this are, well, doomed. At this point the star is starting to expand, and in so doing begins to fry and/or disrupt the orbits of any planets that would have been within its habitable zone. Residents of such a world have the option of remaining and perishing, or heading elsewhere (but also many many millenia to decide which). After that, it turns into a Class III giant star.

G2 V = Main Sequence. The type of 'normal' sun like star your really looking for, though in a pinch other Class V stars of F5 down to around K5-7 would also work.

G2 VI = Subdwarf, with the low luminosity and metallicity issues I was going on about earlier.

And now that I take another look at your post:

And we figured the G-type stars were roughly 10-15 light years (a.k.a. 3.26-4.6 parsecs) apart. Strictly G2s or even G2Vs though... I'd have to go through the catalogue. Doing the permutation calculation: G-type (yellow) stars could be one of ten classes (ours being 2, meaning a 'yellow' two tenths towards 'orange'), multiplied by 5 luminosity classes (ours being Roman #5 = a dwarf, or more properly, a main sequence star), and you get fifty variations of G-type stars. Ergo, 10-15 light years becomes looking more like 500-750 light years (163-230 parsecs) for distances between just two earth-bearing solar systems of strictly G2V type.

Not quite that bad. The SETI crew did come up with better than 17,000 possible 'HabStars' within about 400 lightyears - and given how many sunlike stars the Hip missed within that range, the actual total is probably several times that. Really ideal candidate HabStars, though (G2V or otherwise)...yes, they would be much fewer, but still not as few as your calculations suggest.

Also, the distribution of sunlike stars is not uniform. They appear to be significantly more numerous in the southern declinations as viewed from earth.

I am going to have to choose between going in the direction of Cygnus X, The Perseus Transit, or toward Turner 5/Vela Molecular Ridge Clouds.

I found out that Turner 5 and the Vela Molecular Ridge Clouds are found in the night sky in the constellation of the southern sails.

Questions:
1.The Perseus Transit is in the direction of what constellation?
2. Cygnus X is found in what constellation?
3. The galactic central point is found in what constellation?

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I am also curious about long-period comets. In my book, people are solidified with a sub-periodic table comet fungus that grows in the space between their atoms, placed into a giant comet-mining robot made out of sub-periodic table elements that nuclear blast-accelerates to catch up and latch onto a comet, and then once outside of the Oort cloud, catches a supercomet in the direction of the next earth-bearing solar system. Also, in book 4 of the 5 books in the series, 2 Megacomets come and take out 4, then 5 earth-bearing solar systems as they go by (one at the beginning of book 4, and the other at the end of book 4).

What are your thoughts of this happening regarding the supercomets and Megacomets? Do you think they could be from matter coming in from long-ago ancient events from outside of the Galaxy?

These are the last questions I plan on asking on this forum thread. I've gotta get moving on tidying up the rest of the book. Thanks everyone for participating. It has been illuminating and also frustrating figuring this stuff out.

rrrraygun wrote:1.The Perseus Transit is in the direction of what constellation?
2. Cygnus X is found in what constellation?
3. The galactic central point is found in what constellation?

For your first question, Perseus is a constellation, and I would like to assume that the "Perseus Transit" is in Perseus, but I don't know for sure.

If by your second question, you mean Cygnus X-1 (or another x-ray source in Cygnus), then the answer is Cygnus.

For your third question, it's in Sagittarius.

rrrraygun wrote:What are your thoughts of this happening regarding the supercomets and Megacomets?

What's the difference between the two?

Regardless, the odds of what I think you're proposing are pretty darned close to zero. But since it's clear you're writing a fictional novel, that shouldn't be a problem.

rrrraygun wrote:Do you think they could be from matter coming in from long-ago ancient events from outside of the Galaxy?

No. Why would such an explanation be necessary when we know planet formation here formed comets long ago? And if we need an extraterrestrial origin for comets, why extragalactic? Aren't solar systems (and especially galaxies) capable of keeping most of their own comets?

The Milky Way has been absorbing other, smaller galaxies for billions of years. It's a topic of current research.

Some people think that the outermost gassy arm recently detected might be the remnant of one. Certainly some streams of stars have been associated with some disrupted galaxies. Just as there are relatively small bodies expected to be associated with the Milky Way's star forming regions, one would expect some to have originated from one of those other galaxies. There's no reason to expect anything unusual about them, though, except for their origins. But, hey, who knows what might have happened to them on their way through interstellar space...

p.s. If you're really trying to make the story realistic, though, "sub-periodic" is just not believable. It's not valid scientific terminology. A fungus is made of the same kinds of atoms as everything else. You'll need to use some other way to describe whatever it is. Simply eliminating the made-up pseudo-scientific terminology would be a reasonable solution, I think. A parasitic fungus dormant in comets is quite good (bad?) enough on its own.

Disregard what I said about sub-periodic table fungus.

I'm now trying to figure out the scientifically plausible way to send a robot from just one star to the next.

I did some research on comets and it turns out that the Ikeya-Zhang comet has an orbital period of 366.57 years to go 101.9 AU (15,244,017,500 km). Therefore, a one-way trip starting from near the sun on its way to Pluto then will take 183.3 years to go 50.95 AU (7,622,008,750 km ...approximately 0.00008 of a light-year). This includes the distance and time it takes to travel from the sun to halfway between the orbit distances of Earth and Mars where the robot will attach itself.

I was thinking the robot ship could nuclear blast accelerate and latch onto this thing. Then it could mine it for ~180 years, and then at the furthest point detach itself.

Now here's the thing, we think that G-type stars are about 10-15 light years apart from each other. And seeing as there are 50 different types of G-type stars, average distance could be more like 500-750 light-years apart between the stars we catagorize our sun to be. Buuuut, between those G2V stars, there are many other stars.

There are two ways to travel in space given a limited amount of fuel: one way is to use half the fuel to accelerate as much as possible initially to reach a decent rate of travel in the frictionless vaccuum of space out there, and then using the rest to decelerate once within a certain distance of the destination (docking with another Ikeya-Zhang comet to mine it as it rides into the solar system #2); the other way is to use other great chunks of mass out there as gravity wells to slingshot off of. Just like the space probes NASA has sent, but using the event horizons of stars instead of planets for this effect.


I'm thinking that a combination of the two would be cool, using "Project Orion"-type successive nuclear blasts to accelerate the bot from the Ikeya-Zhang comet drop-off point to reach a maximum speed, then possibly slingshoting from stars between us and the next G2V star as an artifical comet itself.

It would be handy to know approximately how many stars are between two G2V stars on average. What is the range of star types found in Orion's Spur?

It takes a year to travel a light-year at the speed of light. I don't know about the robot actually going the speed of light... maybe taking into consideration the mass of the giant robot, the mass of fuel it can carry and the acceleration it can achieve the limit would be evident. In the story I had initially figured 1000 years would be the time it took the robot to go from outside of one earth-bearing solar system to just outside of another one. That would mean the robot went roughly half to 75% the speed of light to get there. And that's not accounting for the amount of time and distance it would take to accelerate to that speed. I've gotta dig out my physics formulas so I can work this out.

[The physics would be as follows in four equations...done twice to calculate for both the minimum and maximum estimated distances of 500-750 light-years:
1. To find the length of time and distance travelled from starting at a speed of zero in a frictionless vaccuum to get to maximum speed dependent on acceleration from successive nuclear blasts given the mass of the bot, the rate of fuel used to half of the supply, and total mass of fuel.
2. The same as above, but with the rest of the fuel being used at the fuel use rate to decelerate the robot back to zero.
3. The entire distance minus the above distances to give you the distance cruising the middle bit with zero acceleration at a specific speed in order to find out how long that part takes.
4. The amounts of time accelerating and decelerating, plus the amount of time spent cruising the middle bit with zero acceleration to find out how long the entire trip would take.]

Maybe the idea of using other stars to slingshot around is bogus. Maybe it could help a little. There's a formula that tells you that at a higher speed, an amount of mass has a greater amount of potential force or something like that...which equates to mass (e=mc squared i believe). Maybe if we knew the speed and mass of the bot from the above equations, we'd be able to know how much it compares to the masses of certain sized stars, or their larger orbiting planets (like Jupiter, which I learned that other one-time-only comets whip around and are ejected from our solar system for good by) that orbit stars.

I'll do the physics myself once I review the documentary "Code Name Project Orion", do some more internet browsing, and find those physics formulas. Then I'll post what I get.

I don't know about the robot actually going the speed of light... maybe taking into consideration the mass of the giant robot, the mass of fuel it can carry and the acceleration it can achieve the limit would be evident. In the story I had initially figured 1000 years would be the time it took the robot to go from outside of one earth-bearing solar system to just outside of another one. That would mean the robot went roughly half to 75% the speed of light to get there. And that's not accounting for the amount of time and distance it would take to accelerate to that speed. I've gotta dig out my physics formulas so I can work this out.

It's impossible to travel at light speed. Be sure you're incorporating relativity into your equations if you're aiming for maximum believability.

There's a formula that tells you that at a higher speed, an amount of mass has a greater amount of potential force or something like that...which equates to mass (e=mc squared i believe).

That equation applies only to objects at rest. If you're applying it to things at relativistic velocities, try the full verison.

One of the more phausible ideas for (relatively) high speed interstellar travel has been around for quite a while - the Bussard Interstellar Ramjet. Basically, the spacecraft uses a giant electromagnetic 'scoop' to collect free floating hydrogen which is then used to power the vessels fusion drive.

Biggest problems are -

- that until you get up to speed (about 5-6% C), the scoop will produce more drag than the drive output will be;

- and that once you are in true interstellar space, there won't be enough hydrgen to provide much in the way of motive power anyhow (the energy used for the scoop and 'other stuff' will be almost completely offset by the gain).

So...to use this method, I'd start by heading into the Oort cloud in the *opposite' direction from your target star, and latching onto a nice big juicy comet head for your initial full source to get up to 0.5 C - probably have to strap on a bunch of fusion engines to offset the increase in mass. This is your 'booster'. Now...do this right, by the time your in the middle of the solar system (about where earth is, give or take a few astronomical units), you should be going fast enough to where you can use the scoop on its own...and with a bit of luck, by the time you have to stop using the scoop for motive power, you should be coasting along at....maybe 75% - 95% of C. (yes its a wild guess). (`time dilation' at .95 C is about 3 to 1 if memory serves).

At this point, you still *really* want to keep the scoop turned on, even though your not using it as a drive. The reason is because of all the other stuff floating around out there; hitting a grain-of-sand sized micrometeor would be about like taking a direct hit from a very powerfull H Bomb. The scoop could be used to deflect some of that. But it still ain't enough. My (simplistic) recomendation would be a 'pathfinder' type probe several dozen to several hundred au infront of the main spacecraft to detect and provide a few minutes warning (maybe) for the larger debrie, with 'screens' - very large, very thin, very light disks in layers a few au apart in front of the craft, also replaced fairly often. That way you won't loose the whole dang ship to a collision from a speck of spacedust.

Slowing down at the far end...depending on the design of the vessel, you could repeat the process that got you moving at significant percentage of C to start with...or you could just turn the scoop onto its widest possible setting, at let it act as a 'parachute' of sorts.

Ramscoops have been used by a number of SF authors, including Niven, and most recently Brotherton.

Or you could just cheat and go with some form of warp drive.

rrrraygun wrote:Disregard what I said about sub-periodic table fungus.

But I like the idea of a nasty, intelligent fungus wanting to take over! (Although at least a couple of authors have written about something similar to that. Neal Asher comes immediately to mind.)

I'm now trying to figure out the scientifically plausible way to send a robot from just one star to the next.

I did some research on comets and it turns out that the Ikeya-Zhang comet has an orbital period of 366.57 years to go 101.9 AU (15,244,017,500 km). Therefore, a one-way trip starting from near the sun on its way to Pluto then will take 183.3 years to go 50.95 AU (7,622,008,750 km ...approximately 0.00008 of a light-year). This includes the distance and time it takes to travel from the sun to halfway between the orbit distances of Earth and Mars where the robot will attach itself.

The idea of "hitching a ride" in itself has never made any sense to me. The spacecraft would have to use an amount of energy to catch up to and fly alongside (more to land on) a body which is comparable to what would be needed to make the trip by itself. Grabbing a comet along the way to mine its raw materials and to use its mass for shielding or housing, sounds quite reasonable, though.

I was thinking the robot ship could nuclear blast accelerate and latch onto this thing. Then it could mine it for ~180 years, and then at the furthest point detach itself.

That seems plausible.

Now here's the thing, we think that G-type stars are about 10-15 light years apart from each other. And seeing as there are 50 different types of G-type stars, average distance could be more like 500-750 light-years apart between the stars we catagorize our sun to be. Buuuut, between those G2V stars, there are many other stars.

I don't know where you got the "10-15 light years apart". I'm not saying it's wrong, just unfamiliar to me. I'm not sure I'd trust that value enough to extrapolate to other star densities. There's a lot of local variability. No matter the spacing, though, there are always lots of dim stars in between the bright ones.

There are two ways to travel in space given a limited amount of fuel: one way is to use half the fuel to accelerate as much as possible initially to reach a decent rate of travel in the frictionless vaccuum of space out there, and then using the rest to decelerate once within a certain distance of the destination (docking with another Ikeya-Zhang comet to mine it as it rides into the solar system

yup.

#2); the other way is to use other great chunks of mass out there as gravity wells to slingshot off of. Just like the space probes NASA has sent, but using the event horizons of stars instead of planets for this effect.

a) slingshoting doesn't work so well in the interstellar situation. To first approximation, the speed a spacecraft has at a particular distance from a star while falling in toward it is the same speed it'll have later when it get to that same distance from that star while falling away from the star (although it'll be going in a different direction) ... unless the spacecraft accelerates itself while down in close to the star. To second approximation, it could gain some speed since both star and spacecraft are orbiting around the galactic center. The spacecraft could be gravitationally accelerated into a higher energy orbit around the galaxy while the star is slowed to a lower energy orbit around the galaxy. I don't know how much that would help, but I don't think it would significantly reduce interstellar travel times.

b) The term "event horizon" only applies to black holes. A comparable term for normal bodies is the Roche limit. That's the distance from a planet or moon where the tidal force it generates is greater than the forces holding together whatever object is approaching it, causing it to come apart. Usually spacecraft are structurally strong enough that this distance is irrelevant -- unless it's orbiting a neutron star, perhaps.

I'm thinking that a combination of the two would be cool, using "Project Orion"-type successive nuclear blasts to accelerate the bot from the Ikeya-Zhang comet drop-off point to reach a maximum speed, then possibly slingshoting from stars between us and the next G2V star as an artifical comet itself.

It would be handy to know approximately how many stars are between two G2V stars on average.

Sorry: I can't help there.

What is the range of star types found in Orion's Spur?

All types of stars are present, from brown dwarfs to supernovae -- although the latter aren't around for long -- just an expanding shell of gases and a neutron star are left afterward.

It takes a year to travel a light-year at the speed of light. I don't know about the robot actually going the speed of light... maybe taking into consideration the mass of the giant robot, the mass of fuel it can carry and the acceleration it can achieve the limit would be evident. In the story I had initially figured 1000 years would be the time it took the robot to go from outside of one earth-bearing solar system to just outside of another one. That would mean the robot went roughly half to 75% the speed of light to get there. And that's not accounting for the amount of time and distance it would take to accelerate to that speed. I've gotta dig out my physics formulas so I can work this out.

I think "the rocket equation" is the one you need. It takes into account the mass that's lost as fuel is expended.

[The physics would be as follows in four equations...done twice to calculate for both the minimum and maximum estimated distances of 500-750 light-years:
1. To find the length of time and distance travelled from starting at a speed of zero in a frictionless vaccuum to get to maximum speed dependent on acceleration from successive nuclear blasts given the mass of the bot, the rate of fuel used to half of the supply, and total mass of fuel.

More than half the mass needs to be expended in order to get up to speed. Remember that you have to accelerate all the fuel that'll be thrown away during the later stages of acceleration.

2. The same as above, but with the rest of the fuel being used at the fuel use rate to decelerate the robot back to zero.
3. The entire distance minus the above distances to give you the distance cruising the middle bit with zero acceleration at a specific speed in order to find out how long that part takes.
4. The amounts of time accelerating and decelerating, plus the amount of time spent cruising the middle bit with zero acceleration to find out how long the entire trip would take.]

Your description is a little too simplified, since so much fuel mass is needed to get up to speed compared to the total mass that it started with.

One place to learn about some of the complications of atomic-powered spaceflight is http://www.projectrho.com/

Maybe the idea of using other stars to slingshot around is bogus. Maybe it could help a little. There's a formula that tells you that at a higher speed, an amount of mass has a greater amount of potential force or something like that...which equates to mass (e=mc squared i believe).

Umm. Well, not quite. That's the equivalent energy of a body's rest mass.

Maybe if we knew the speed and mass of the bot from the above equations, we'd be able to know how much it compares to the masses of certain sized stars, or their larger orbiting planets (like Jupiter, which I learned that other one-time-only comets whip around and are ejected from our solar system for good by) that orbit stars.

I'll do the physics myself once I review the documentary "Code Name Project Orion", do some more internet browsing, and find those physics formulas. Then I'll post what I get.

You might want to try to get a copy of the book _Project Orion_ by George Dyson . It has a lot more usable information than the video and is available from your favorite book store or library. Some links to other references about Project Orion are available at http://www.lepp.cornell.edu/~seb/celest ... index.html

(I never finished the Addon because I kept stumbling over the trajectory calculations needed to actually get it into orbit. I could have fudged it, I suppose, but that was just too depressing a thought. Maybe someday.)

I hope these thoughts help a little.

Selden wrote:I don't know where you got the "10-15 light years apart". I'm not saying it's wrong, just unfamiliar to me.

It was a guestimate I threw out earlier.

If it is horrifically off, I'll take the responsibility for that

I'm going to have to change the title of my book from "The Scientifically Plausible Spaceship Adventure Story" to something else now, or seriously gut the thing so the farthest the characters get to is the Oort cloud. Depressing.