( I came up with a date system, rather arbitrarily, for this scenario/site.)
The Starship Wanderer is a generation ship, miles in size, and rather cumbersome. Carrying capacity is in the thousands of people. Some can carry over 1,000,000 people and have a bit of room left over.
Courier ships are small dart-like ships carrying 1 to 3 people.
They have the same type of engines. Bussard ram jets.
For some ( I don’t know any physics I’m just making this up as I go along.) reason the couriers go faster than the generation ships. Much faster. 0.99 percent the speed of Light rather than 0.87 percent the speed of Light. May not seem like much, but couriers get there faster than the passenger ships. In some instances the couriers seem to go faster than the speed of light.
A scientist from Vega 5 started a project to determine travel times from both types of ships, for the same solar system to same solar system travel. His name is lost to Antiquity.
He, or rather those who carried on the project in the years after he died of old age awaiting results back, soon found the time it took varied.
A courier and a generation ship going from Vega to Altair took 300 days for the courier, and 900 days for the generation ship.
But going from Vega to Kochab took the courier 150 days and the generation ship more than 200 years.
Something was a bit odd.
Information finally came back to Vega. Sometimes a courier crew notices a small flash outside the hull at some point and they believe they get to the destination faster. Scientists didn’t believe them. Rather like millenia ago the Sea Captains back on Old Earth were ignored when they talked about rogue ocean waves, which were finally proven.
However, due to the centuries it takes to get data not much has been done with this information.
