Space Navigation

[daveid66]

I was wondering how a route is plotted to travel from Planet A to Planet B. I figure word pairs such as left/right, up/down, North/South, East/West don't apply beyond the Earth's atmosphere. So, what kind of terminology IS used? I do realize that these bodies are in orbit around a star so that even if there was a specific directional term the direction would be apt to change.

 

Thanks for taking the time to read.

[CraigD]

Welcome to hypography, David! Please feel free to visit the introduction forum and tell us a bit about yourself.

 

I was wondering how a route is plotted to travel from Planet A to Planet B. I figure word pairs such as left/right, up/down, North/South, East/West don't apply beyond the Earth's atmosphere. So, what kind of terminology IS used?

Navigation anywhere, including 3-dimensional domain in which spacecraft travel, involves 2 fundamental values: position (a point) and velocity (a vector). For either, all that’s needed is a coordinate system, which requires a zero, or origin, point, and a couple of reference, or basis, vectors.

 

For space navigation, the choice of coordinate system is one of convenience, based on where you’re navigating. As most spaceflight orbits the Earth, the center of the Earth is a common origin point, and by convention, the basis vectors are one called the right ascension (RA), a line from the center of the Earth to the center of the Sun on the spring equinox (March 20th at 5:32 PM GMT this year), and any one at right angles to it in the plane of the equator.

 

Though a simple XYZ coordinate system can, and in software and manual calculation commonly is used, position and direction is usually given using polar coordinates, these coordinates are usually given as polar ones – a RA angle, called ascension, and a vertical one above or below the equator, called declination, and a distance. Because what’s usually needed is to aim a radio or a telescope at the spacecraft or natural body, these coordinates are usually converted into ones centered at a particular observatory on Earth, known as ephemerides. Fortunately, it’s easy to translate between different ephemerides.

 

See the wikipedia article equatorial coordinate system for further explanation.

 

When navigating between planets, the center of the Sun is usually used as the origin, the spring equinox and the plane of the Earth’s orbit for the ascension declination.

 

Since the spring equinox and the plane of the Earth’s orbit changes with time, to be precise, its necessary to chose a specific date for a standard RA value. Several such standards are used, perhaps the most common one know as J2000, based on the common (Julian) calendar year 2000.

 

Since most spacecraft and natural bodies travel most of the time in orbits, it’s useful to describe them not with a coordinate and velocity, but with orbital elements. These are a bit complicated, though, so unless you need to use them for some specific calculations, you might want to stop at just understand the fundamentals of simple 3-D coordinate systems.

[Turtle]

this might help. > . .:moon: . . . . :smilingsun:

 

Astronomical Coordinate Systems

 

Galactic Coordinates

This coordinate system is most useful for considerations of objects beyond the solar system, especially for considerations of objects of our Milky Way galaxy, and sometimes beyond.

Here, the galactic plane, or galactic equator, is used as reference plane. This is the great circle of the celestial sphere which best approximates the visible Milky Way. For historical reasons, the direction from us to the Galactic Center has been selected as zero point for galactic longitude l, and this was counted toward the direction of our Sun's rotational motion which is therefore at l = 90 deg. This sense of rotation, however, is opposite to the sense of rotation of our Galaxy, as can be easily checked ! Therefore, the galactic north pole, defined by the galactic coordinate system, coincides with the rotational south pole of our Galaxy, and vice versa.

 

Galactic latitude b is the angle between a position and the galactic equator and runs from -90 to +90 deg. Glalactic longitude runs of course from 0 to 360 deg.

 

The galactic north pole is at RA = 12:51.4, Dec = +27:07 (2000.0), the galactic center at RA = 17:45.6, Dec = -28:56 (2000.0). The inclination of the galactic equator to Earth's equator is thus 62.9 deg. The intersection, or node line of the two equators is at RA = 18:51.4, Dec = 0:00 (2000.0), and at l = 33 deg, b=0.

 

The transformation formulae for this frame get more complicated, as the transformation is consisted of (1.) a rotation around the celestial polar axis by 18:51.4 hours, so that the reference zero longitude matches the node, (2.) a rotation around the node by 62.9 deg, followed by (3.) a rotation around the galactic polar axis by 33 deg so that the zero longitude meridian matches the galactic center. This complicated transformation will not be given here formally.

 

Before 1959, the intersection line had been taken as zero galactic longitude, so that the old differred from the new latitude by 33.0 deg (the longitude of the node just discussed, but for the celestial equator of the epoch 1950.0):

 

l(old) = l(new) - 33.0 deg

 

For a transition time, the old coordinate had been assigned a superscript "I", the new longitude a superscript "II", which can be found in some literature.

For some considerations, besides the geo- or heliocentric galactic coordinates described above, galactocentric galactic coordinates are useful, which have the galactic center in their origin; these can be obtained from the helio/geocentric ones by a parallel translation.

[modest]

I was wondering how a route is plotted to travel from Planet A to Planet B. I figure word pairs such as left/right, up/down, North/South, East/West don't apply beyond the Earth's atmosphere. So, what kind of terminology IS used? I do realize that these bodies are in orbit around a star so that even if there was a specific directional term the direction would be apt to change.

 

Thanks for taking the time to read.

 

Think of the earth. You can get anywhere with north/south latitude, east/west longitude, and altitude. With those three numbers you can mark a position anywhere around the earth... even hundreds or thousands of miles above it.

 

If you want to move around the solar system then all you have to do is the same thing but with the sun rather than the earth. Give the sun a north/south latitude, east/west longitude, and an altitude (or distance from the sun). With that you can mark any position in the solar system.

 

~modest

[Tormod]

There is a more complex issue at hand here. Not only do we need to know where the planets are, but the time to get there may be prohibitive. Therefore methods like planetary slingshots (the get the benefits of gravity assist to boost or reduce speed) are used for spacecraft traveling out to Jupiter, Saturn etc.

 

A lot of celestial mechanics is involved, and like Craig mentioned, the orbital elements and actual position of objets related to each other.

 

The Sun is a difficult object to use as measurement since, while it has north and south poles, it has no fixed visible representations of east/west. One could always say that wherever you are, the Sun would be "inwards" and away would be "outwards". But the coordinates x, y, z in relation to the sun could be any point in a complete orbit of the star. So while it is easy to know where you are in relation to the sun, you'd need at least one more object (like the Earth) and it's relative position to the Sun, in order to triangulate your position in space.

[modest]

The Sun is a difficult object to use as measurement since, while it has north and south poles, it has no fixed visible representations of east/west. One could always say that wherever you are, the Sun would be "inwards" and away would be "outwards". But the coordinates x, y, z in relation to the sun could be any point in a complete orbit of the star. So while it is easy to know where you are in relation to the sun, you'd need at least one more object (like the Earth) and it's relative position to the Sun, in order to triangulate your position in space.

 

 

The coordinate system centered on the sun is the galactic coordinate system. The second object (or, really, direction) used to orient the equator and pole is the galactic center.

 

I think we might be aiming just a bit over David's head though.

 

I figure word pairs such as left/right, up/down, North/South, East/West don't apply beyond the Earth's atmosphere. So, what kind of terminology IS used?

 

On earth directions are given in up, down, north, south, east, and west. But, in space none of these directions would seem to make sense. How, after all, could there be an up or an east halfway between earth and mars? But, the truth is, those same concepts are used in space. They are worded a bit different, but the same idea applies.

 

The three coordinates that get a person around earth are north/south latitude, east/west longitude, and altitude. The three coordinates in the galactic coordinate system are north/south latitude, east/west longitude, and distance—all measured from the sun. So, it's really not that different from the directions you are used to. Just think of altitude as distance from the sun. North is the direction that the sun's north pole is pointing, and east is the direction that the sun is rotating.

 

~modest

[Qfwfq]

The difficult thing isn't so much how to say where things are, but working out an efficient way to get to destination (which includes knowing where it will be, when you will reach it).

[belovelife]

in my perspective

if - we have the ability to shrink and expand space

then - space travel would be different than we think

where as you plot a course and fly, you would shrink space and move

 

(consider a grain of sand in the core and you push itt magnetically where the grain of sand with a specific field bends space-time

and pushing the grain achieves movement )

 

then, for instance, if someone shot a bullet at you, you could expand space to where the distance is equivicated to light years

 

on the other hand, you could shrink space, where the vector involved is like watching a tv, only the "glare " would be atmosphere

 

so you could theoreticall from the comfort of your home, shrink to your parents living room, make a tiny crack (nM) in the roof, and stick a probe in to watch