Cosmic Distance Scales
Understanding the sheer scale of space is the first barrier to thinking about galaxy travel. Here’s how distances stack up from human-scale to cosmic.
The Ladder
| Unit | Definition | In km | Example |
|---|---|---|---|
| 1 AU | Earth-Sun distance | 149.6 million | Mars is ~1.5 AU |
| 1 light-minute | Distance light travels in 1 min | 18 million | Sun to Earth = 8.3 light-min |
| 1 light-hour | Distance light travels in 1 hr | 1.08 billion | Past Jupiter’s orbit |
| 1 light-year (ly) | Distance light travels in 1 year | 9.461 trillion | See concept-light-year |
| 1 parsec (pc) | 3.26 ly | 30.9 trillion | Standard astronomical unit |
| 1 kiloparsec (kpc) | 3,260 ly | — | Milky Way disk thickness ~1 kpc |
| 1 megaparsec (Mpc) | 3.26 million ly | — | Galaxy cluster scales |
| 1 gigaparsec (Gpc) | 3.26 billion ly | — | Observable universe ~28.5 Gpc diameter |
Key Distances for Travel
Within Our Solar System
- Earth to Moon: 1.3 light-seconds (384,400 km)
- Earth to Mars: 3-22 light-minutes (variable)
- Earth to Pluto: ~5.5 light-hours
- Edge of Oort Cloud: ~1-2 light-years
Interstellar (Within Milky Way)
- To dest-proxima-centauri: 4.24 ly
- To dest-alpha-centauri: 4.37 ly
- To dest-trappist-1: 39 ly
- Across the Milky Way: ~100,000 ly
- To dest-sagittarius-a (galactic center): ~26,000 ly
Intergalactic
- To dest-large-magellanic-cloud: 160,000 ly
- To dest-andromeda (M31): 2.537 million ly
- Across the overview-local-group: ~10 million ly
- To the Virgo Cluster: ~54 million ly
- Edge of observable universe: ~46.5 billion ly
The Sobering Math
At current top speed (mission-voyager-1 at 17 km/s = 0.000057c):
- Proxima Centauri: ~73,000 years
- Andromeda: ~44 billion years (longer than the universe has existed)
At 0.1c (achievable with advanced tech-solar-sail or tech-fusion-drive):
- Proxima Centauri: ~43 years
- Andromeda: ~25 million years
At 0.2c (mission-breakthrough-starshot target):
- Proxima Centauri: ~21 years
- TRAPPIST-1: ~195 years
At 0.9c (extreme concept-relativistic-travel):
- Proxima Centauri: ~4.7 years (ship time: ~2 years due to concept-time-dilation)
- Andromeda: ~2.8 million years (ship time: ~1.2 million years)
The Inescapable Conclusion
Intergalactic travel with known physics is effectively impossible for biological life without:
- Near-light-speed travel + concept-time-dilation (still takes millions of ship-years for galaxies)
- tech-generation-ship civilizations spanning millennia
- Spacetime manipulation like tech-alcubierre-drive (currently speculative)
- tech-cryosleep or digital consciousness transfer
Interstellar travel within ~50 ly is the realistic frontier for the next centuries.