Generally, the maximum difference between displacements in realspace and hyperspace is a factor of about twenty million. A skilled astrogator with computer assistance can usually come close to this limit, thus minimising trip time. An expansion factor of twenty million means a 0.1 AU hyperspace trip can cover 10 parsecs in realspace, a typical distance between colonised systems.
Mistakes in astrogation usually result in traversing a lesser realspace distance than intended, and sometimes in a completely different direction. This is seldom immediately dangerous, since it almost always places the ship in interstellar space. From there a new fix can be made by observing the positions of nearby stars and another hyperspace trip made. Such a second trip may be difficult, however, if the ship is low on reaction mass. A severe astrogation miscalculation is often less of a problem, since it is likely to result in a lesser realspace displacement, putting the ship within easy reach of the same starport or docking facility from which it came.
Hyperspace itself resembles realspace in all important features except that what little matter is naturally present is spread evenly, rather than gathered into galaxies and in turn stars and planets. So there are no physically noticeable features or landmarks in hyperspace. For this reason, navigation within hyperspace is dependent on dead reckoning from the last known realspace position and orientation of the starship. The crew of a ship entering hyperspace without a good realspace fix has no choice but to re-enter realspace at a random location, and hope they don’t re-appear within a star or planet.
Entering hyperspace can be done anywhere. It is normal procedure for planetside ships to skew while sitting on the landing site. This saves blasting into orbit with engine power. Skewing requires a large energy discharge, usually provided by capacitors. Once the ship is in hyperspace, it remains there until it skews back to realspace, again requiring an energy discharge. Power is not required to keep a ship in hyperspace; indeed if the power plant is damaged after a skew the ship may be stuck in hyperspace! To obtain the realspace fix required for reliable navigation through hyperspace, a planetside ship must either get coordinates from the starport traffic control computer, or make a manual fix by sighting several stars. If a ship lands far from starport facilities, it must wait until nightfall and clear weather to obtain a fix(!), unless the pilot wishes to make a short random trip.
Leaving hyperspace can also be done anywhere, but is dangerous if the corresponding realspace location is inside a star, planet, or otherwise occupied. In such a case, the ship will intersect with the object, and both will deform enough to allow all the matter to exist as close to its original position as possible. This generates considerable heat, but any living beings intersected in this way will almost certainly die instantly anyway.
Typical interstellar astrogation error (assuming a good realspace fix) is about 0.01 AU, which is a comfortable distance to cover on reaction drive. Astrogators will typically aim at a point about 0.02 AU from the destination world, to minimise chances of an intersection accident.
By far the largest chance of an intersection accident is with the star of the target system. Actually appearing within the star is unlikely. A G star like Sol occupies only a ten millionth of the volume of space out to a radius of 1 AU, so a 1 AU astrogation error (essentially making the ship appear at random within the system) has about a one in ten million probability of a ship-destroying accident. Good astrogators rarely make an error that large. Planets are much smaller than stars, so appearing at random within a system gives only about a one in a billion chance of intersecting with a terrestrial-type planet. Intersecting with the atmosphere of a gas giant is only slightly more likely, and is not even instantly deadly, although it will kill the crew quickly if they can’t do something about the poisonous gases suddenly inside their ship.
Once a ship returns to realspace it will have to cover any remaining distance to its destination. If it intends landing on a planet it should either be streamlined for gliding through an atmosphere or have enough thrust for a soft landing.
Skewing into and out of hyperspace is a stomach-churning experience. Any character on board a skewing ship must make a HT roll. Failure results in nausea identical to being space sick, except that a recovery roll may be made 10 minutes later. If the recovery fails, the character is space sick as per the rules in GURPS Space, only getting additional recovery rolls every 24 hours. If a character is already space sick when the skew occurs, the character will remain space sick, but must still roll to check for critical failure. Immunity to Space Sickness provides no protection against skew sickness.
