The window opens on 24 September

On 24 September 2026 the next practical transfer window to Mars opens. It stays open until 5 March 2027 — 162 days — and the cheapest departure inside it falls on 30 October 2026.

Take that departure and the numbers are these:

• 294 days in cruise
• Arrival at Mars on 20 August 2027
• Launch energy C3 = 9.1 km²/s²

There is a faster door. Leave on the same date and push the throttle to a 161-day crossing and you arrive on 9 April 2027 instead — four months earlier. The bill for that is a C3 of 39.3, more than four times the energy. That trade is the entire argument about how you get to Mars, expressed in two numbers.

These are solved, not looked up

Nothing above came from a calendar or a table. The deck runs a universal-variable Lambert solver: it sweeps departure dates against flight times, computes the transfer orbit connecting where Earth is to where Mars will be, and reads the energy off each solution. The valleys in that surface are the windows.

That distinction matters more than it sounds. A looked-up date is only as good as whoever last updated the table. A solved date can be checked, and it moves when the planets do.

The round trip, if you were staying

The same solver run forward gives the whole mission. Land in August 2027, wait for Mars and Earth to come back into alignment — 383 days on the surface — then fly home. Total elapsed time from leaving Earth to returning: 1,015 days, a little under 2.8 years.

That surface stay is not a choice anyone makes. It is what the geometry hands you. Leave early and there is no return trajectory; the planets simply are not where you need them to be.

After this one

The next window after this opens 19 October 2028 and closes 29 April 2029. If this one is missed, that is the wait.

Every figure here is computed live in your browser on the deck — the countdown on the right-hand panel is the same solver, running on your machine, with no server involved.