Galactic Survey Timescales

NB: This is a gathering of thoughts for a “Centauri Dreams” article and a possible Journal submission. Thus copyright is asserted.

How would a Civilization with starflight explore/visit/colonise every star in the Galaxy? How many ships are needed and how long would it take?

Consider: the Galaxy has 100 billion stars that are, on average, 3.5 light-years apart. If they were all in a line that’s a trajectory ~350 billion light years long. At ~0.1c that’s 3.5 trillion years of travel time. If we use one ship.

Thus many ships will be needed to achieve the goal in minimal time. Imagine a million ships launch forth from the Galactic Core spiraling outwards. They cover paths ~350,000 light-years long in a Galaxy ~50,000 light-years in radius, thus something like a logarithmic spiral trajectory is required. At 0.1c each Ship must be built to last 3.5 million years of mission-time. Clearly self-repair and/or self-reproduction in some form will be required. Can each ship be reasonably expected to last ~3.5 million years, even with that built in?

To minimise energy/reaction mass required, I’d suggest the vehicles use near-stellar flybys to deflect their courses to their next targets. Just how fast could a Survey vehicle go and remain in the Galaxy? Surprisingly the answer is a significant fraction of the speed of light.

To complete the mission – a complete Galactic Survey – requires a strategy and programme that can last millions of years. In Earth’s biosphere, only “species” not individuals – organisms or even genes – can last that long unchanged. Genomes continually evolve, though some parts are too critical to change much. Self-repairing/reproducing systems, due to their complexity and need for adaptability, may mutate and change. How many “generations” of duplication can they undergo and remain on task?

Multiple authors have pondered these questions – Sagan, Tipler, Freitas, Forgan, Bjoerk, Wiley, Landis Barlow and others – but there are always implicit or explicit assumptions that need re-examination.

For example, just how far can a starship travel through our quite dusty Galaxy before it *must* stop? Forgan argued that a minimum distance – just ~10 light-years – is sufficient to reach every star. Thus to span from our part of the Galaxy to the far side at 75,000 light-years, the number of hops is ~7,500. If each hop is a generation of self-replicating probes, will their initial purpose remain unchanged?