What 2100 looks like

Very likely in hand

Inferred from the 2075 baseline.

Six things are very likely in hand by 2100 if the 2075 industrial baseline holds.

Beam arrays at 10 to 100 gigawatts operating from collector constellations inside 0.4 AU. The physics is settled and the capital cost is a few hundred tons of film. The gating item is political will, not capability.

Gram to kilogram-class craft accelerated to 0.1 to 0.2c and launched in swarms of thousands, attrition-tolerant by design.

Solar gravitational lens nodes operating on the focal lines of several target stars, delivering multipixel imaging and spectra of nearby terrestrial planets.

Autonomy sufficient for decade-scale unsupervised operation with no possibility of correction. This is the item the architecture has argued hardest for and it is the least contentious thing here.

Direct characterization of every terrestrial planet within 20 light-years. By 2100 we will very likely know which of these worlds has an atmosphere, which has an ocean, and which shows chemical disequilibrium — without having gone anywhere.

Routine interstellar object intercepts, and at least one sample of extrasolar planetesimal material in a laboratory. This is the single most confident prediction in the interstellar case, because it requires no technology that does not exist and no institution that is not already funded.

Plausible but not assured

Modeled.

Deuterium–helium-3 fusion propulsion at flight scale. Gated not by the physics of ignition, which will likely be settled well before 2075, but by the ice-giant helium-3 supply chain and its economics.

Fission fragment propulsion demonstrated in-system at specific impulses near 10⁶ seconds. There is no physical obstacle and the concept is testable at small scale, which means its absence by 2100 would be an indictment of priorities rather than of physics.

Magsail or photon-brake deceleration demonstrated in-system. A magsail braking against the solar wind is in principle testable at 1 AU today.

Antimatter production at gram-per-year rates and stable long-term trapping.

Any permanent crewed presence beyond Ceres.

Not in hand, stated flatly

Three things.

Any craft that has arrived at another star. Even an optimistic 2085 launch at 0.2c reaches Alpha Centauri in 2106, and the first data reaches Earth around 2110.

Deceleration of any substantial mass at another star.

Crewed interstellar flight of any kind, by any architecture, at any speed.

The honest answer is that by 2100 we will have launched and will not yet have arrived. That deserves to be said without softening. A site that argues the solar system becomes an industrial civilization within 75 years should not, at the end, quietly imply that the stars follow. They do not follow.

The honest ledger

What each branch owes.

Two of those — the sail and the solar-pumped laser — are the only entries anywhere in the interstellar case that are not scale-ups of something demonstrated. Everything else is either an industrial program, an unfunded test, or a measurement that a funded instrument is about to make.

The real content of the problem

Administrative, not relativistic.

Alastair Reynolds is the novelist who took the constraint seriously. The Revelation Space books have no faster-than-light travel and no way around the light-lag, and their entire social architecture follows from that refusal: lighthuggers are the only real capital in that universe because they are the only thing that moves between stars, and the Ultras who crew them become a separate class, drifting away from planetary societies through simple accumulated absence.

The relativistic bookkeeping does not support that particular detail. At 0.2c the Lorentz factor is 1.0206, so a 21-year crossing costs the crew about five months of differential aging. Time dilation is not the mechanism.

But Reynolds is right about the outcome and wrong only about the cause, which is the more interesting kind of correct. Anyone who leaves stops being a member of the society that sent them, and the mechanism is not relativity. It is the round trip. An institution that launches a probe in 2085 receives its answer in 2110, from a machine designed by people who have mostly retired, aimed at a target selected on the basis of observations that will by then be 40 years old, and answering a question the field may have moved past. The separation is administrative, and it is unavoidable at any speed short of the impossible.

What has kept humanity in this solar system is not propulsion, which is an engineering problem with known solutions and known costs; not the medium, which is a materials problem with a hard number attached; and not communications, which is an aperture problem that outer-system infrastructure already solves. It is that no institution has yet existed that could hold a purpose for 80 years while receiving nothing back.

Breakthrough Starshot did not fail on physics. It failed because in 2016 there was no industry to attach it to. The argument of this site is that by 2075 there is one: a power infrastructure inside 0.4 AU that produces the beam as a side effect of producing everything else, a fusion fuel chain in the ice giants, a volatile stockroom at Pluto with an elevator that costs almost nothing to build, autonomous systems that have run unsupervised at Callisto for 30 years, and a set of instruments at 550 AU that can say which star is worth the trip.

The century closes with the corridor open, the beam operating, several thousand craft in cruise, and nothing arrived. That is not a failure. That is what the first quarter of a crossing looks like.