Coordinates in space

RebelButtMunch

Hi.

Down here we use GPS but in space how do space vehicles know where they are and how accurate is it.

Would the same system work outside the solar system?

rolion

Good question...

Is mostly same system like out ancestors, using stars and time.
I guess is a huge database of stars and their exact location from our humans perspective.

Im using on my telescope a similar system,based on stars' database and exact time.
See H E R E .

How it Works
The self-alignment technology behind our award-winning SkyProdigy series is now available for most Celestron's current computerized telescopes and compatible with many older models as well. (See compatibility list below.)
A small digital camera takes the place of your finderscope and attaches using one of two provided mounting brackets. The included StarSense hand control, with a database of over 40,000 celestial objects, takes the place of your NexStar hand control. The camera automatically captures a series of images of the sky. StarSense identifies the stars in the images, matching them to its database. Once a positive match is confirmed, StarSense calculates the coordinates of the center of the captured image, thereby determining exactly where the telescope is pointed.
Before StarSense, using a computerized telescope required a lengthy alignment process of finding and centering at least two bright stars in the telescope’s eyepiece. But StarSense automatically aligns itself with minimal user input. Just enter your time, date, and location and let StarSense do the rest!

BDI

Did you mention starsense?

ps200306

RebelButtMunch wrote: »

Down here we use GPS but in space how do space vehicles know where they are and how accurate is it.

Would the same system work outside the solar system?

It's actually quite a bit different from how we do things on Earth. Down here, before the advent of GPS, we oriented ourselves with respect to the surface of the Earth. The rotation of the planet gives us the polar axis and equator, and we arbitrarily choose a zero of longitude at Greenwich. For directions in space (from here on Earth) we likewise have the pole and celestial equator, and our orbit round the Sun gives a reference direction in the equatorial plane, the First Point of Aries. That's the point (which is actually in Pisces due to precession) at which the Sun crosses the equator from south to north on the vernal equinox. It provides a way to cross-reference the terrestrial reference frame with the celestial reference frame.

Nowadays we do things much more precisely. The International Celestial Reference Frame (ICRF) is a set of direction measurements from the barycentre of the solar system to a set of quasars which are fixed (to all intents and purposes) extra-galactic radio sources. The International Terrestrial Reference Frame (ITRF) is a geocentric reference frame, co-rotating with the Earth, which is correlated to the ICRF by taking into account precession, nutation, and a number of essentially random processes which affect the Earth's spin rate. The International Earth Rotation and Reference Systems Service (IERS) is responsible for measuring the Earth's orientation as a function of time and regularly publishes tables for the random elements that can't be computed.

Now, that was all a bit longwinded as none of it is any use in space! On Earth if you can measure your orientation and you have a time reference, you can figure out where you are. Finding your three-dimensional location in space is considerable more tricky. The short answer to the question is that you start with where you expect to be. Within the solar system you're always in orbit around something, either a planet, moon or Sun. To get where you are going you had to compute an orbit and to a first approximation, that's tells you where you are... or at least where you ought to be.

Most of the rest is done by radar ranging and Doppler measurements. This is very accurate for measuring radial velocities with respect to Earth. A combination of widely separated tracking stations on the surface of the Earth, along with the expected orbital parameters, can be used to determine the 3D position in space. Clearly this would be of little use outside the solar system. On rare occasions, onboard measurements from the spacecraft itself are used. This would involve measuring star positions with respect to some reference object such as a moon or planet. You can read more about all of this here.

rolion

ps200306 wrote: »

It's actually quite a bit different from how we do things on Earth.

Now.. Another challenge !
What about travel at light speed then.
How did they "manage" to travel so fast and still not "scratching" any star, not going through any physical object and, always arriving at the destination in few kilometres away !?
Thanks

PS
Happy Christmas in whatever location you are in this time, location and coordinates !

RebelButtMunch

rolion wrote: »

Now.. Another challenge !
What about travel at light speed then.
How did they "manage" to travel so fast and still not "scratching" any star, not going through any physical object and, always arriving at the destination in few kilometres away !?
Thanks

PS
Happy Christmas in whatever location you are in this time, location and coordinates !

Been at the mulled wine? no such thing as faster than light.

ps200306

rolion wrote: »

Now.. Another challenge !
What about travel at light speed then.
How did they "manage" to travel so fast and still not "scratching" any star, not going through any physical object and, always arriving at the destination in few kilometres away !?
Thanks

I presume we're talking about fantasy, e.g. Star Trek. RBM is right, you can't travel at light speed, and if you could I would think even interacting with the tenuous interstellar gas might cause you problems, let alone hitting a star.

It's interesting though, to consider the likelihood of hitting a star at any speed. Perhaps it's easier to ask: how far could you travel through the galaxy before you're likely to hit a star. We can use the same sort of calculation as is used for the mean free path between molecular collisions in a gas. We need a small modification to cater for a tiny spaceship and big stars.

We also need a few approximations. Stars come in different sizes. The vast majority are smaller than 1 A.U. in radius. So let's ask about the chance of coming within 1 A.U. of the centre of a star. We'll call that the radius, r. Then we need to know the density of stars. It's highest in the galactic nucleus, and lower as you travel away from the plane of the galaxy. We'll take the average separation between stars in the thin disc, where the Sun is. That's 4 light years, which we'll call r*.

Then we'll define:

• the collisional cross-section, , and
• the average stellar number density,

Then, the mean distance between collisions is:



340 billion light years is more than three million times the diameter of the galaxy, and more than seven times the width of the observable universe. So the chance of colliding with a star (or even coming within one A.U.) in the thin disc of the galaxy is utterly negligible. You could cruise the galaxy at fifty percent of light speed for ten times the current age of the universe and still be unlikely to hit a star by random chance. Compared to their size, galaxies are as tenuous as smoke rings.

ps200306

I admit my comment about the interstellar medium (ISM) being a hazard when travelling at (near) light speed was off the cuff. I subsequently did some fact-checking of myself. It seems that at sufficiently high speed, even the photons of the cosmic microwave background would be a hazard!

At lower relativistic speeds, such as the 0.2c envisaged by Project Starshot, the main hazards would be dust although even heavy atoms could have an ablative effect. Dust grains of 0.1 microns would be dangerous while those bigger than 15 microns could be fatal.

While all this speculation about hi-tech is fun, I find myself more interested in actual processes in nature. So while trying to find out about mean free paths of atoms in the ISM, I came across this fascinating 1950 paper by Lyman Spitzer and Walter Baade, two of the luminaries of 20th century astronomy. (Spitzer in particular is well worth reading up on).

Spitzer and Baade consider collisions between galaxies in clusters. In a dense cluster such as the Coma Cluster they make an order of magnitude estimate of the number of collisions each galaxy might undergo over the course of several billion years of cluster evolution. They estimate perhaps 20 to 150 collisions. Even so, the separation between stars is such that there are no stellar collisions. Clusters are weakly gravitationally bound so galaxy collisions take place near cluster escape velocity, at several hundred to a couple of thousand kilometres per second. At these speeds, there are not even significant gravitational interactions between individual stars in the colliding galaxies -- the overall galaxy field is what produces tidal effects.

The story is quite different for the atoms of the ISM in these colliding galaxies. Even at a low ISM density of 0.1 atoms per cubic centimetre, the mean free path of ISM hydrogen atoms is only about a third of a light year, much less than the thickness of a disc galaxy. (This is the middle of the range of ISM densities, which span ten orders of magnitude). Much of the ISM gas is affected by a galaxy collision and the high speeds mean that the ISM becomes highly ionised. Charge separation in the plasma causes a very strong drag force, effectively leaving it behind as the colliding galaxies pass through each other.

In fact, you don't even need galaxy collisions. Inter-galactic space is not empty either, and cluster galaxies run into the intra-cluster medium. While rarefied, it's enough to tear the ISM from a galaxy by a process called ram pressure stripping.

The effects are fascinating. Galaxies stripped of their gas have nothing to fuel the formation of new stars. This is termed star formation "quenching". Yet the disturbance of the ISM by either collisions or ram pressure can also compress the gas, causing a flurry -- albeit sometimes brief -- of star formation. We can see the excess of blue light from "starburst galaxies" where this is ongoing, whereas quenched galaxies are "red and dead". Only actively star-forming regions have the most massive blue OB stars because they live fast and die young.

Another phenomenon is starting to be understood: galaxy mergers can also drive gas into the central black hole at the heart of each galaxy. A feeding black hole produces an active galactic nucleus (AGN) and these can produce incredibly intense outflows of material which can also strip the interstellar medium. This is another mechanism thought to be behind star formation quenching.

On our human timescales the galaxies look like stately, unchanging "star cities", but they have incredibly complicated, and often violent, pasts (and futures).

oriel36

ps200306 wrote: »

It's actually quite a bit different from how we do things on Earth. Down here, before the advent of GPS, we oriented ourselves with respect to the surface of the Earth. The rotation of the planet gives us the polar axis and equator, and we arbitrarily choose a zero of longitude at Greenwich.

This is a long, long way from the actual technical details between planetary geometry and the Lat/Long system.

To explain how clocks, including the GPS system, measure distances on the surface of the Earth thereby pinpointing location requires a study of timekeeping from its foundations.

ps200306

Would welcome corrections on anything that's wrong.

oriel36

It is more a question of being incomplete for the whole issue covers four different types of reference frameworks from start to finish. We would not be looking for something wrong but expanding the history of heliocentric astronomy and timekeeping as taking two separate routes.

The original framework for timekeeping begins, at least in written form, with the creation of the 1461 day/4 year calendar cycle formatted as three years of 365 days and 1 year of 366 days using a specific reference and framework -

".. on account of the procession of the rising of Sirius by one day in the course of 4 years,.. therefore it shall be, that the year of 360 days and the 5 days added to their end, so one day shall be from this day after every 4 years added to the 5 epagomenae before the new year" Canopus Decree 238 BC

The ancient astronomers realised they could not base their year on a cycle of 365 days otherwise their festivals would quickly drift away from the equinox and solstice points so they used the fact that Sirius skips a first annual appearance by one day/rotation after the fourth cycle of 365 days hence the close proximity between timekeeping and the daily/annual cycles.

The Egyptians followed a system based on 36 different constellations rising with the Sun every 10 days with a 5 day interlude (epagomenae). In 21st century terms, using a satellite tracking with the Earth's orbit, it represents the transition of a star from left (evening appearance) to right (morning appearance) of the central/stationary Sun due solely to the orbital motion of the Earth -

https://sol24.net/data/html/SOHO/C3/96H/VIDEO/

This system is more productive and creative for linking the motions of the planet to Earth sciences while the later system of Ptolemy where the Sun moves directly through the constellations is more productive for astronomical predictions and timekeeping -

http://community.dur.ac.uk/john.lucey/users/sun_ecliptic.gif

The seasonal appearance of individual stars or heliacal risings as distinct from the position of the Sun within the 12 constellations is a subtle but important technical detail before moving on to more recent times with the appearance of the 24 hour and Lat/Long systems along with accurate clocks.

Sadly, I believe, I will lose my privilege to this forum.

ps200306

Interesting. I think I mentioned the heliacal rising of Sirius in a previous post, about how the Egyptians used it to predict the Nile flood. Though I think it would take us very far afield in a question about how spacecraft keep track of the their positions. :pac:

oriel36

ps200306 wrote: »

Interesting. I think I mentioned the heliacal rising of Sirius in a previous post, about how the Egyptians used it to predict the Nile flood. Though I think it would take us very far afield in a question about how spacecraft keep track of the their positions. :pac:

A heliacal rising is nothing more than Sirius moving far enough to the right side of the central Sun and its glare to be seen as a morning appearance. It is now possible to see the stars change position from left to right and parallel to the orbital plane as a function and demonstration of the Earth's orbital motion -

https://sol24.net/data/html/SOHO/C3/96H/VIDEO/

This framework makes it possible to convert the Egyptian system into a heliocentric system, after all, the heliacal rising of a star or its first annual dawn appearance is ultimately an orbital trait using a stationary/central Sun and a moving Earth to account for the observation.

https://apod.nasa.gov/apod/ap181123.html

(Sirius is that bright star beneath the meteor)

The change in position of the stars parallel to the orbital plane is more a demonstration than a proof that the Earth orbits the Sun as the language of 'proof' is far too severe and the development of timekeeping begins at this juncture using the original cyclical references supplied by the Earth's motions.