Inner-System Travel

Compared to interstellar travel, movement within a solar system consists of extremely short journeys – relatively speaking. However, if these are not undertaken at faster-than light speeds, the journeys can still take a very long time. In 2006, the NASA space probe New Horizons left Earth, reaching the dwarf planet Pluto nine years later.

Distance

Distances within a solar system are measured in Astronomical Units (AU). 1AU is equal to 93 million miles, or the average distance from Earth to its Sun. Pluto orbits at about 40AU on average, meaning that at light speed (FTL-1), it would take 5.5 hours to get there.

SUB-L 0.1 is roughly 550,000 km/h, and is the approximate speed of a high speed early 21st century space vessel.

SUB-L 7 is 1 hour per AU. 

SUB-L 20 is equal to FTL-1, the speed of light. Under normal physics, a ship cannot ever reach SUB-L 20; the laws of relativity forbid an object from reaching light speed without special faster-than-light technology.

Note that while a starship’s SPEED rating (as shown in its stat block) is used for both tactical combat and in-system travel, the two uses are not directly equivalent. Navigational speed (which uses an exponential scale similar to the way FTL speeds are calculated) is much faster than tactical speed (the number of kilometers a ship moves in one round; see Starship Combat, later) despite the fact that both are based on the same basic SPEED rating.

A SUB-L journey is resolved in exactly the same way as an FTL journey. The only difference is the units used.

Time Dilation

The final column on the Sublight travel time table shows the effect of time dilation on sublight speeds as they approach that of light.

FTL speeds do not suffer from issues of time dilation. However, those travelling at high sublight speeds will find that time passes more slowly for them than for those not moving. For every day (or hour, or any other unit of time) spent travelling, multiply it by the Dilation column to determine how much time passes for those at rest.

As can be seen from the table, the effect rises sharply as lightspeed is approached, with most of the increase being found in the small range consisting of 0.99 lightspeed and above. However, the effect is still noticeable at lower velocities, especially at 0.5 lightspeed and above.

The dilation rises ever more steeply in the increasingly tiny range between the ship’s speed and the speed of light. Whileno appreciable journey time saving is made beyond SUB-L 19.9 for a journey of 1 parsec, at 0.999999c two years passes for those at rest for every day spent travelling; and at 0.99999999999999c, twenty thousand years passes for each day.

Sublight speeds beyond SUB-L 19 are not typically used, for obvious reasons.