Navigation in space

[[Tycho?]]

How does one measure position/direction in space? Not how the acutal data is collected, but how it is written, and what refference point is used. Would position be maesured in refference to the earth, or to the sun? Is direction the same, such as degrees off of the axis of the earths orbit? Does such a system even exist?

 

I am considering writing a sci-fi short story, possibly for a school project, and I want to know how this would work in space.

[Sayonara]

I was discussing this with atinymonkey the other day.

 

He mocked my suggestion of using a navigational computer that tracked key star positions, and said he was going to do it all by eye.

 

Not very helpful at all, I know. But there we go.

[ydoaPs]

perhaps it should be a rotating grid with (0,0,0) being the center of the galaxy. the rotation would be so that the grid wouldn't appear to change much if you are in our galaxy.

[atinymonkey]

I was discussing this with atinymonkey the other day.

 

He mocked my suggestion of using a navigational computer that tracked key star positions' date=' and said he was going to do it all by eye.

 

Not very helpful at all, I know. But there we go.[/quote']

 

To be fair, you dismissed my navigation by visual reference in favor of using the navicomp*. I simply pointed out that the navicomp was a fictional device, and as such not much use in actual navigation. The sextant is the way to go, especially as all you need to do is find the correct star to head towards**.

 

 

*A device employed in Star Trek to explain how and why the ships manage to traverse the galaxy without getting lost.

 

**Yes, I know the current technology requires complex planning and orbital trajectories, but it's just to find where the planet is.

[Sayonara]

To be fair, you dismissed my navigation by visual reference in favor of using the navicomp*. I simply pointed out that the navicomp was a fictional device, and as such not much use in actual navigation.

You're missing out the fact that I added a caveat along the lines of "if we're assuming that we have the technology to go to other stars, I'm going to assume we have the technology to map where we are while we're doing it".

 

 

*A device employed in Star Trek to explain how and why the ships manage to traverse the galaxy without getting lost.

Actually the term navicom is a common contraction of "navigational computer". I've never heard it mentioned in Star Trek. Such devices exist - you can buy them for your cruise liner, 747, car etc. It's just a generic name for any device that computes a route to a point from the origin.

 

[edit] In fact you must be thinking of something else entirely, because Federation vessels navigate by means of the subspace beacon network. [/edit]

 

 

The sextant is the way to go, especially as all you need to do is find the correct star to head towards**.

Good luck with that.

[Martin]

']How does one measure position/direction in space? Not how the acutal data is collected' date=' but how it is written, and what refference point is used. Would position be maesured in refference to the earth, or to the sun? Is direction the same, such as degrees off of the axis of the earths orbit? Does such a system even exist?

 

I am considering writing a sci-fi short story, possibly for a school project, and I want to know how this would work in space.[/quote']

 

the bright star Canopus is often used to establish a direction

 

probes sent to places like Mars and Jupiter have to be navigated

with extreme precision

 

they use heliocentric coordinates, or what is essentially the same, solar system center-of-mass

 

Other bright stars besides Canopus are used too, I just can't remember the names.

 

I might be able to find a NASA website table of some of the constants used in navigation. It is kind of interesting.

 

they dont use the MASS of the sun, in kilograms, they use a more precisely known heliocentric gravitational constant which is expressed in cubic meters over square seconds.

this is known out to more than 8 decimal places. maybe 9. (fantastic accuracy anyway) I will try to check

 

probe navigation, especially when they use the gravity-assist from flyby maneuvers (to save fuel) is very beautiful

 

 

Yeah, I found a bunch of high-precision constants for navigation in solar system

 

http://ssd.jpl.nasa.gov/astro_constants.html

 

the "heliocentric gravitational constant" is given with over 10 decimal places, in cubic meters per square second

 

the accuracy they claim for it is in parts per billion, so one can say that it is good out to 8 places. but probably it is better than that

 

they hit incredibly narrow windows out there so they have to be using

high precision constants

 

(like the mass of the sun in kilograms is only known to no better than 5 places, because newtonian G is not accurately known, so forget it, the uncertainty in G will overwhelm everything and trash the computation)

[RICHARDBATTY]

You could use a circular array of photo sensors with a lazer pointing at the center one from the other side of the ship. Any movement forward/back up/down would result in a deflection of the beam. One front to back you get side ways movement and combined you get pitch and roll. With a computer and callibrated sensors you could tell if you are still, accelerating turning etc and by the rate of acceleration etc always know where you are.

[Cap'n Refsmmat]

Ever heard of a ring laser? Those have a fiber-optic ring and do exactly the same thing.

http://en.wikipedia.org/wiki/Inertial_platform#Laser_gyros

[RICHARDBATTY]

Ever heard of a ring laser? Those have a fiber-optic ring and do exactly the same thing.

http://en.wikipedia.org/wiki/Inertial_platform#Laser_gyros

Every day is a school day. I think these sound like they only sense rotation though or did I read it wrong.

[[Tycho?]]

Ok, I failed to articulate myself properly.

 

I have no trouble thinking up ways to navigate in space, refference points to use, ways to view it, things like this. I've already thought up a way to define the x,y and z axis from which to get coordinates for the solar system. x is the plane of the eleptic, y is going through the top of the (rotational) poles of the sun, and z is a line intersecting the sun and a planet of choice. It would probably be earth for convienence, but for other solar systems the second largest mass in the system would be used. Or you could use distant stars for refference points, galactic orientation etc.

 

I want to know if there is any sort of system in place for this already. Like can you point to a place in the solar system and name the coordinates there by some international system? Or is there no system currently in place for this?

[swansont]

'']I want to know if there is any sort of system in place for this already. Like can you point to a place in the solar system and name the coordinates there by some international system? Or is there no system currently in place for this?

 

Astronomers use right ascension and declination. The Earth is the center of the coordinate system. Most stars and galaxies are so far away that the parallax is negligible.

[Martin]

they use heliocentric coordinates' date=' or what is essentially the same, solar system center-of-mass

...[/quote']

 

google "heliocentric coordinates"

 

you get a description of "solar ecliptic coordinates" which is used for navigation in the solar system

 

there is an established sun-centered system. I found a list of probes with their orbit parameters given in terms of sun-centered coordinates.

 

do you want a description of the standard heliocentric coordinates used for spaceprobe navigation? or do you prefer to look it up by google?

it is a little different from what you conjectured, tycho, but basically similar (the plane of the ecliptic is used and the normal vector perpendicular to that plane is one direction, the spring equinox establishes a second direction which lies in the ecliptic plane, the third axis is defined by the first two)

[Martin]

']Ok' date=' I failed to articulate myself properly.

...

 

I want to know if there is any sort of system in place for this already. Like can you point to a place in the solar system and name the coordinates there by some international system? Or is there no system currently in place for this?[/quote']

 

You expressed yourself understandably enough. I already answered around 7 posts back in this thread. There is an "international system"---that is to say an established heliocentric system of coordinates for navigation.

 

there is a system currently in place.

 

It is called heliocentric coordinates. If you google that you get a description.

 

But watch out! there are actually two different heliocentric coordinate systems! One system (heliographic) is for studying the sun itself and uses the sun's equatorial plane and the sun's axis of rotation. this is slightly askew from the plane of the ecliptic. this would not be good for navigation!

 

the type of heliocentric coordinates good for navigation is called "solar ecliptic"

[[Tycho?]]

Thanks martin, I did indeed read your original post, but I think I looked for "heliocentric gravitational constant" instead of "heliocentric coordinates". Now I found a site that perfectly describes it, thanks a bunch.

[Martin]

'']Thanks martin,..

you are cordially welcome [T?] good luck with the short story if you decide

to write it!

 

the most dramatic spaceprobe story I know was the Galileo mission

because they did it on small delta-vee---apparently inadequate engine and load of propellant.

the mission was originally intended for mars and they didnt have enough of a propulsion resources to go to jupiter

 

but they changed the plan and went to jupiter anyway

 

they started by using some of their propellant to slow up(!), so they would fall in towards venus, then used venus gravity and earth gravity assist, instead of fuel, and these multiple flyby maneuvers gave them the boost to get to jupiter,

 

then when they got there they encountered jupiter going much faster than the probe, so they did another gravity-assist with jupiter, to get capture there (going in very close and firing retro at the nearest point)

 

then there was more gravity-assist using jovian moons, to round out the orbit and change the orbit around jupiter so they could visit various things.

 

it was amazing----done on remarkably little resources, essentially by creative planning and very precise navigation taking the place of fuel

 

Galileo was a tour de force navigation thing, like a pinball artist getting multiple hits on one shot. If you google "Galileo" you will probably find several accounts of the mission---might give you ideas for the story.

[JohnB]

Truly the Galileo multiple sling shot navigation was awe inspiring. Especially given the time lag to Mission Control. An amazing piece of work.

 

(Why is there no "cheers" smilie?)

[[Tycho?]]

you are cordially welcome [T?] good luck with the short story if you decide

to write it!

 

the most dramatic spaceprobe story I know was the Galileo mission

because they did it on small delta-vee---apparently inadequate engine and load of propellant.

the mission was originally intended for mars and they didnt have enough of a propulsion resources to go to jupiter

 

but they changed the plan and went to jupiter anyway

 

they started by using some of their propellant to slow up(!)' date=' so they would fall in towards venus, then used venus gravity and earth gravity assist, instead of fuel, and these multiple flyby maneuvers gave them the boost to get to jupiter,

 

then when they got there they encountered jupiter going much faster than the probe, so they did another gravity-assist with jupiter, to get capture there (going in very close and firing retro at the nearest point)

 

then there was more gravity-assist using jovian moons, to round out the orbit and change the orbit around jupiter so they could visit various things.

 

it was amazing----done on remarkably little resources, essentially by creative planning and very precise navigation taking the place of fuel

 

Galileo was a tour de force navigation thing, like a pinball artist getting multiple hits on one shot. If you google "Galileo" you will probably find several accounts of the mission---might give you ideas for the story.[/quote']

 

Yeah those slingshot maneuvers are pretty neat. Since my story would take place between planets only it doesn't have a large bearing on it, but I will keep it in mind.

[Martin]

'']Yeah those slingshot maneuvers are pretty neat. Since my story would take place between planets only it doesn't have a large bearing on it, but I will keep it in mind.

 

If it is just between two planets without any gravity assist then

look at this page:

http://www.space.gc.ca/asc/eng/youth_educators/educators/resources/packages/orbital/activity_mars_key.asp

 

It is easy highschool level, but is about something very important:

the Hohmann Transfer Ellipse.

 

It shows how to calculate the time needed for a one-way trip to various planets, for instance jupiter.

 

It gives a picture. I like the way they present the transfer ellipse idea, with pictures, a simple formula you can calculate with, and several examples.

[Martin]

if you write a short story and a highschool teacher sees it,

consider whether teacher is impressed or not if you are familiar

with HTE (some say HTO for hohmann transfer orbit, instead of hohmann transfer ellipse, same thing)

 

know how to calculate travel time, which is very easy

 

it is the most efficient way to get from one circular orbit to another

one always starts with the hohmann transfer ellipse as the basic idea and then may refine it.

 

using special AU Astron. Units where the AU is the sun-earth avg. dist. the calculation is real simple. to go earth to jupiter you say earth is R=1

and jupiter is R=5.5 and the major axis (customarily called 2a) is 6.5

and half the majoraxis (socalled "semimajoraxis", writting a) is 3.25

 

and kepler law says period P in years is related to semimajor a (in AU)

by this famous equation

 

P³ = a²

 

so you plug in the ellipse's a = 3.25 and square and do cube root and you get that P = 5.86 years!

 

and the trip out to jupiter is half that so half of 5.86 is 2.9 years.

 

If your teacher is not too weird he or she will be satisfied that you know how to apply kepler law P³ = a² to the ellipse for getting from one planet to another.

 

everybody should know kepler law and hohmann ellipse because it's the basic determinant of cost of getting from here to there in solar system.

 

but there is also the delta vee cost. leaving earth, how much do you have to boost, so as to get on the ellipse track to arrive at jupiter?

by how much do you have to increase your speed?

 

(the 2.9 years is the cost in travel time, now we are thinking about fuel cost for the delta vee)

 

well for extra credit here are some formulas for that---the perihelion speed which you have to achieve leaving earth. these formulas are messier and there is a way to simplify them, they use the heliocentric gravitational constant GM. so this is an extra credit-type thing, I would guess:

http://scienceworld.wolfram.com/physics/HohmannTransferOrbit.html