Astronavigation Basics

Galactic Cartography:

Our Galaxy identified in the Solomani 2nd Impirium parlance as the “Milky Way,” is an unexceptional mid range spiral-bar galaxy, located in a cluster of other galaxies called the “Local Group.”  While the Milky Way is exceptionally larger than some of the smaller Globular Cluster type galaxies, we are dwarfed compared to others.

The Milky Way galaxy is approximately 100,000 light-years across, and contains between 100 and 400 billion stars.  The 11,000 systems of the Impirium consists of about one 18 millionth of the galaxy. Imperial space lies in the remote inner rim of the Orion Belt, about 27,000 light years from the Galactic Core.   At the center of the galaxy, hidden within the galactic core is a super-massive black hole, which all other stars and gasses in the galaxy orbit around.

Looking down from above, the galaxy rotates clockwise, referred to as “Spinward.”  Imperial space is conventionally oriented to be below the galactic core at 6 o’clock.  Directions toward the galactic core are called “coreward,” away from the core is called “rimward,” in the direction of the galactic rotation is called “spinward,” against the direction of the rotation is called”trailing.”

Distances:

Distances within the space of a star-system are measured in Astronomical Units (AU).  The AU was standardized in the 2nd Impirium as the distance from the Solomani home-word Terra (Earth) to its star called Sol.  All travel by sub-light vessels within systems therefore calculates distances and travel times in AU.  For reference, one light-year is about 63,241 AU.

Distances between star-systems are measured in a unit called “Parsecs.”  A parsec is equal to about 3.26 light-years distance.  The origin of the term parsec is also from Terran history, and involved using ancient methods of measuring the distance from Terra to other star-systems.

Travel Times In-System:

Traveling within a star system is accomplished by use of “reactionless thrusters.”  This means the maneuver drive of ships are not being propelled by ejecting matter in the form of expanding gasses or plasma out of the rear of the ship.  The ships drive is powered by a self contained nuclear fusion reactor within the ship.  This permits the ship to use its propulsion continuously while in transit.
This is a vast improvement over reaction thrusters that burned fuel.  In ancient times, transits within system involved brief accelerations to place the vessels in orbital tracks that would eventually carry the vessel to its destination through gravity and inertia.  With continuous acceleration the most efficient mode of travel is to accelerate continuously towards the destination for the first half of the passage.  The ship then turns 180 degrees to face away from the destination and accelerates away from the destination continuously for the second half of the journey.

In this method, the vessel reaches maximum relative velocity at the halfway point, and then decelerates to low relative velocity at the destination.  Care still must be taken in plotting the course to account for movement of the destination along its orbital path while in transit, and the influence of gravity on the vessel as it travels.  A mistake could result in overshooting the destination, having to slow to a relative stop, and accelerate back towards the destination’s new location on its orbit.  

Another side effect of this method is the very high relative velocity of the objects while in mid transit renders most ships sensors as useless for locating non-mapped smaller objects such as ships.  The ship is going so fast that by the time the vessel is close enough to make detection, the object has been overshot by hundreds of thousands of miles.  Pilots and navigators therefore rely on updated system charts, traffic protocols, and the transponders on other vessels to navigate while traveling in system. A ship cannot make an unplanned stop to deal with something during transit. Likewise, this makes intercepting a vessel away from its origin or destination very difficult.

Communications:

Radio, meson, and laser communications travel at the speed of light.  One AU is about 8 light minutes.  Therefore it is generally impossible to have a two way conversation at distance.  Each transmission and response is delayed by the distance of the communication in AU times 8 minutes.

Laser Communications is difficult at such distances, because the communications are in a tightly directed laser burst to avoid others intercepting the traffic.  Such traffic requires broadcasting at the exact future location of the recipient, rather than the location of the recipient at the time of the broadcast.  In this respect, using a laser communicator is quite similar to trying to hit a target with the ship’s laser cannons.

Looking down from above, the galaxy rotates clockwise, referred to as “Spinward.”  Imperial space is conventionally oriented to be below the galactic core at 6 o’clock.  Directions toward the galactic core are called “coreward,” away from the core is called “rimward,” in the direction of the galactic rotation is called “spinward,” against the direction of the rotation is called”trailing.”

Out of all the vital facts I constantly forget about in the Traveller mythos, these top the whole list. Thanks for the refresher.