Windows
One Window per Synodic Period
A minimum-energy window opens when Earth and the destination line up, once per synodic period: the time Earth takes to lap the destination, or be lapped by it.
For Mars that is 26 months. For the outer planets it approaches one year, because they barely move while Earth goes round; the constraint there is transfer time, not window spacing.
The Moon orbits Earth, so departures are possible every day. Spending more Δv widens each window and shortens the transfer.
By destination
Windows, Transfer Time and Light Time
Minimum-energy (Hohmann) transfer time unless a flown gravity-assist route is named.
| Destination | Window every | Minimum-energy transfer | Light time, one way | Windows per decade | Rated |
|---|---|---|---|---|---|
| Moon | Continuous | 3-5 days | 1.3 s | Continuous | Observed |
| Mercury | 3.8 months | 3.5 months; 7 years by gravity assist (BepiColombo) | 5-12 min | 32 | Observed |
| Venus | 19 months | 4.8 months | 2-14 min | 6 | Observed |
| Mars | 26 months | 8.5 months | 3-22 min | 4-5 | Observed |
| Ceres | 15 months | 15.5 months | 15-31 min | 8 | Observed |
| Jupiter, Callisto | 13 months | 2.7 years | 33-53 min | 9 | Observed |
| Saturn, Titan | 12.4 months | 6.1 years | 67-92 min | 10 | Observed |
| Neptune, Triton | 12.1 months | 31 years; 12 by Jupiter assist (Voyager 2) | About 4 h | 10 | Observed |
| Pluto | 12.0 months | 45 years; 9.5 flown by New Horizons | 4-7 h | 10 | Observed |
Gravity assists from Jupiter cut outer-planet transfers to about a decade but need Jupiter in the right place: Voyager 2's tour past Jupiter, Saturn, Uranus and Neptune used an alignment that recurs about every 175 years.
Mars
The Next Four Mars Windows
Departure dates are approximate; each window lasts about two months.
| Window | Departure | Missions | Rated |
|---|---|---|---|
| 2026 | October-December 2026 | NASA ESCAPADE, launched on New Glenn in November 2025 | Observed |
| 2028-2029 | November 2028-January 2029 | CNSA Tianwen-3 sample return, two Long March 5 launches; ESA ExoMars Rosalind Franklin rover | Building |
| 2031 | February-April 2031 | Tianwen-3 samples leave Mars for Earth | Building |
| 2033 | April-June 2033 | The window most crewed Mars architectures target | Projected |
Cadence
A Shipped Spare Can Be Years Away
The worst case for a part that fails just after a window closes: wait for the next window, then fly the transit.
| Destination | Window wait | Transit | Worst case | Rated |
|---|---|---|---|---|
| Moon | About 1 month | 3-5 days | About 1 month | Modeled |
| Mars | 26 months | 6-9 months | Up to 35 months | Modeled |
| Ceres | 15 months | 3-5 years by NEP | Up to about 6 years | Modeled |
| Callisto | 13 months | 5-7 years by NEP | Up to about 8 years | Modeled |
The window wait is the figure the Ship-or-Make Threshold uses. At Mars and beyond, a site that cannot make a failed part locally waits years for one; that schedule case is what The Machine Shop MVP sizes against.
China
Chinese Missions on the Windows
Tianwen missions to Mars, an asteroid and Jupiter, and the cadence of the Chang'e program.
| Mission | Window | Data point | Rated |
|---|---|---|---|
| Tianwen-1 · CNSA | Mars, July 2020 | Orbiter, lander and Zhurong rover in one launch | Observed |
| Tianwen-2 · CNSA | Asteroid Kamo'oalewa, May 2025 | Sample return, then on to comet 311P | Observed |
| Tianwen-3 · CNSA | Mars, 2028 | Sample return; samples back around 2031 | Building |
| Tianwen-4 · CNSA | Jupiter, around 2029-2030 | Ends in orbit around Callisto | Projected |
| Chang'e-7 · CNSA | Moon, any month | Launch postponed 24 August 2026; the lunar window does not constrain it | Building |
Contested
Deliberately Not Claimed
The load-bearing uncertainties, tagged with the confidence they actually earn.
Faster transits cost Δv. A Starship-class vehicle refilled in orbit can reach Mars in four to six months by spending more Δv than the 4.90 km/s in the index; the window still opens every 26 months.
NEP loosens windows but does not remove them. Low-thrust vehicles can depart off-window at a Δv penalty; the worst-case waits above assume launch on the window.
Record
Provenance and Data Sources
- Confidence
- Observed for flown, fielded or measured - Building for hardware and programs in development - Projected for announced plans - Modeled for original analysis
- Status date
- Q4 2026
- Method
- Synodic period from orbital periods; Hohmann transfer time from semi-major axes; light time at minimum and maximum Earth distance
- Missions
- NASA ESCAPADE; ESA ExoMars; ESA-JAXA BepiColombo; Voyager 2; New Horizons
- China
- CNSA Tianwen-1 to Tianwen-4; Chang'e-7 postponement (24 August 2026)
- Weakest link
- Transit times past Mars, which assume NEP