"The hyperdrive is a miracle of advanced technology. Powered by incredibly efficient fusion generators, hyperdrive engines hurl ships intohyperspace, a dimension of space-time that can be entered only at faster-than-light speeds. The theories and realities of hyperspace travel are understood by few but highly-trained hyperspace technicians in the astrophysics communities, and even they admit that certina aspects remain a mystery.
Certain things are clear, though. Hyperspace is coterminous with realspace: each point in realspace is associated with a unique point in hyperspace, and adjacent points in realspace are adjacent in hyperspace. In other words, if you travel "north" in realspace then jump to hyperspace, you'll be heading "north" in hyperspace as well. Objects in realspace have a "shadow" in hyperspace. That is, there is a star (or star-like object) in hyperspace at the same location as it occupies in realspace, and this is a danger to those travelling in hyperspace.
This is why astrogation and astrogation computers are so important, and why they are standard aboard most hyperdrive-equipped ships. Careful calculations must be made to assure that a hyperspace-travelling ship doesn't smash into a planet or star while hurtling through this dimension; only the desperate - or foolhardy - attempt hyperspace jumps without up-to-date astrogation charts and astrogation Droids or computers.
Many Droids and astrogation computers used on starfighters are capableof containing data for only one hyperspace jump at a time; others, such as the Rebel Alliance Y-wing can hold up to 10 jumps without being reprogrammed.
Larger starships, such as Imperial Star Destroyers and similar models, have large onboard astrogation computers capable of virtually unlimited jump calculations and actually store jump coordinates for almost every forseeable destintion the ship may wish to reach.
Even with sophisticated astrogation machines, mistakes are not uncommon. There are millions of stars in the galaxy, and billions of planets (not to mention asteroids and other debris), and space is not static - what was a safe course a few days ago may now be filled with debris from an exploded starship or collision between large bodies. Authorities estimate that the locations of more than 90 percent of all large bodies in the galaxy are unknown! With all these variables, even the largest, most sophisticated computers, operated by the mostexperienced astrogators can plot a fatal path through hyperspace - even alongwell-traveled routes."
Addition from the Star Wars Sourcebook:
"Space is not a complete vacuum. Floating molecules are everywhere -only a few per cubic centimerter, but they exist. Larger objects, though rarer, are common too. There are many more "rogue" planets - which float in the interstellar void, unwarmed by any sun - than in star systems. There are uncountable asteroids, meteors and random chunks of ice and rock between the stars.
If a ship travelling at translight speeds hits an object of any size, it is instantly vaporized.
Even a close graze with a rogue planet or sizeable asteroid would throw a ship vastly off course.
Astrogation is a tricky business.
Han: Traveling through hyperspace ain't like dusting crops, boy! Without precise calculations we'd fly right through a star or bounce too close to asupernova, and that'd end your trip real quick, wouldn't it?"
And from the Second Edition RPG:
"A ship in hyperspace must still go around all objects in our dimension, called realspace. Therefore, ships can seldom travel straight from one star to another - they must avoid the mass shadows and gravitational affects of every star, planet, asteroid and other galactic phenomenon in the way.
The solution to this was the creation of established, known hyperspace routes. As these routes were traveled, the obstacles along the route became better known, and ships could risk going faster and faster. In simple terms,using an established, well known route allows for very fast travel, even between distant stars, while using a poorly traveled route, even if only over a shortdistance, takes longer and is often more dangerous.
Starships also have mass shadow sensors that allow them to detect massshadows and shut down the ship's hyperdrive to avoid collision...sometimes."