The tree · Drive · World event · Root: on the scale of the whole window
A drive that could cross the solar system in weeks is fired in space
The Epstein drive in plain terms: an engine whose exhaust is fast enough and whose thrust is high enough that a ship can accelerate the whole way and reach the Belt in weeks rather than months. Nothing like it exists. Chemical rockets have the thrust but not the exhaust speed; electric thrusters have the speed but push like a breath. This is the node that separates the Slow Expanse from the Full Expanse, and it is the project's clearest long shot.
What counts as it having happened
A propulsion system operating in space sustains, for at least one hour, an exhaust speed above one hundred kilometres per second (chemical rockets manage under five) together with a push of more than one kilonewton (roughly the weight of a hundred kilograms on Earth, which today's high-speed electric thrusters miss by a thousandfold), confirmed by the operator and by independent measurement of the vehicle's acceleration.
Settled by: The operator's test record and independent tracking of the vehicle's acceleration
Probability, by longevity scenario
| baseline2071 ± 8 | moderate2080 | strong2095 | radical2136 | openno deadline |
|---|---|---|---|---|
| 3% | 6% | 15% | 35% | 50% |
Each number is the chance this resolves yes before that scenario's window closes, given that everything it depends on resolves first. Estimated 2026-09-19.
Why these numbers: A hundredfold weight gap and no thrust ever produced from fusion; a coin flip with no deadline is the founding rule at work. An outside model's 50 percent for a 90-day belt transit by 2070 is a much lower bar and does not transfer.
How it could happen, and where physics would have to give way
Long shots belong in the tree by the first founding rule; the second says they must be written plausibly all the way to the edge of the impossible, with the mechanism named and the breaking point stated plainly.
The mechanism: A fusion reaction whose products are steered out the back directly, rather than used to boil water and spin a turbine, gives exhaust speeds of thousands of kilometres per second in principle. The path runs through fusion plants on the ground (proving the reaction can be sustained), then compact magnet-based reactors made cheap by superconducting wire, then a reactor light enough to fly, then a nozzle that turns its output into thrust. Pulsed designs that detonate small fuel pellets are the leading route on paper because they avoid holding a steady plasma.
The breaking point: The reactor has to be light as well as working. Ground fusion designs come to tens or hundreds of tonnes for each megawatt of electricity; paper drive concepts need about a kilowatt for each kilogram, a gap of a hundred- to a thousandfold that no built device has closed, and fission-electric systems top out near a fifth of a kilowatt per kilogram. No fusion device has ever produced thrust. Everything past a heavy fusion plant on the ground is engineering nobody has done.
Notes
Weight-gap figures checked 2026-09-08 against a European Space Agency assessment of open magnetic fusion for propulsion and NASA fusion-rocket studies: conceptual fusion drives are projected at one to ten kilowatts per kilogram, fission-electric near 0.2, and the practical requirement is quoted as above one kilowatt per kilogram.
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The record itself is data/D-drive.toml in the public repository.