Drive

The headline stands at 0.5% by 2071. If one of these settled tomorrow and nothing else changed, the whole tree is played out again and the headline lands where the figure says. This is the site saying in advance what a milestone is worth, before it happens: when one of them lands, the entry that records it quotes what was predicted here. These come from playing the tree out 20,000 times per scenario, which is a throw of the dice: anything smaller than about 0.10 points is inside the noise, so read the order rather than the last decimal.

• A site beyond the Earth-Moon system is crewed without a break for a year +0.2 points if yes 0.5% today · 0.7% if it happened · 0.0% if it turned out no (−0.5 points)
• Nuclear propulsion is used on several missions a year +0.1 points if yes 0.5% today · 0.6% if it happened · 0.5% if it turned out no (too small to tell apart from chance)
• Megawatt solar-electric propulsion is used on several missions a year +0.1 points if yes 0.5% today · 0.6% if it happened · 0.3% if it turned out no (−0.1 points)
• A crew travels beyond the Earth-Moon system and returns to Earth +0.1 points if yes 0.5% today · 0.6% if it happened · 0.0% if it turned out no (−0.5 points)
• Three missions in one year reach the asteroid belt within twelve months of leaving Earth too small to tell apart from chance if yes 0.5% today · 0.5% if it happened · 0.0% if it turned out no (−0.5 points)
• A nuclear-powered engine produces a kilometre per second of velocity change in space −0.1 points if yes 0.5% today · 0.4% if it happened · 0.5% if it turned out no (too small to tell apart from chance)

1 other open step under this factor is worth nothing to the headline, because the headline does not rest on it at any remove: An engine with both high thrust and high exhaust speed fires in space. Most belong to the tiers above the one the headline asks about.

• A crewed site beyond the Earth-Moon system now means at least five million kilometres from Earth, so nothing at the Moon can count 2026-10-03 The tree itself changed
• The fast drive no longer has to be fusion-powered: any engine that meets the thrust and exhaust-speed test counts 2026-10-03 The tree itself changed
• Megawatt solar-electric propulsion becoming routine was raised when I checked the first estimates: it is further along than nuclear propulsion and needs no reactor approvals 2026-10-02 Changed my mind
• Forty-one steps had their criteria tightened so that two readers would agree when each has happened, with no number moved for that alone 2026-09-20 The tree itself changed
• A crew landing on Mars and coming home came down: refilling in orbit is one link of a long chain 2026-09-20 Changed my mind
• Nuclear propulsion becoming routine, given one demonstration, came down: a demonstration creates no demand by itself 2026-09-20 Changed my mind

• A nuclear-powered engine produces a kilometre per second of velocity change in space D-nuclear-propulsion-changes-orbit-in-space
  60%baseline scenario, as of 2026-09-19
  Within 20 years Not yet
• A crew lands on Mars and returns to Earth D-crew-round-trip-to-mars
  45%baseline scenario, as of 2026-09-19
  Whole window Replaced
• A crew travels beyond the Earth-Moon system and returns to Earth D-crew-round-trip-beyond-the-earth-moon-system
  52%baseline scenario, as of 2026-09-25
  Whole window Not yet
• A site beyond the Earth-Moon system is crewed without a break for a year D-permanent-crewed-site-beyond-the-earth-moon-system
  40%baseline scenario, as of 2026-09-19
  Whole window Not yet needed for Slow Expanse
• An engine with both high thrust and high exhaust speed fires in space D-constant-acceleration-drive-demonstrated
  38%baseline scenario, as of 2026-09-25
  Whole window Not yet Long shot needed for Full Expanse
• Megawatt solar-electric propulsion is used on several missions a year D-high-power-solar-electric-propulsion-in-routine-use
  50%baseline scenario, as of 2026-10-02
  Whole window Not yet
• Nuclear propulsion is used on several missions a year D-nuclear-propulsion-in-routine-use
  40%baseline scenario, as of 2026-09-19
  Whole window Not yet
• Three missions in one year reach the asteroid belt within twelve months of leaving Earth D-fast-transit-in-routine-use
  50%baseline scenario, as of 2026-09-25
  Whole window Not yet needed for Slow Expanse