Solar System Simulator · What if? · Mars
Why Mars launch windows come every 26 months
Mars is sometimes only three light-minutes away, yet a rocket can only leave for it every 26 months and spends about seven months getting there. Take the launch controls and find out why.
The rocket is fixed; the timing is free
A launcher can add only so much speed beyond what Earth already gives it — about 3.7 km/s for a Mars-class rocket, roughly what the Atlas V gave Perseverance. From Earth's orbit that is enough to coast out to Mars's distance from the Sun, and no more. It is not enough to go wherever you like: the probe follows a long curved transfer, and it arrives wherever that curve ends, about seven months later.
So the question is never "how do we get to Mars?" but "will Mars be there when we arrive?" During a seven-month transfer Mars moves well over a hundred degrees around its orbit. For the probe and the planet to meet, Mars has to be tens of degrees ahead of Earth at the moment of launch — in the simulator, windows open with Mars somewhere between about 40° and 75° ahead, depending on where Mars is on its eccentric orbit. Leave with the wrong geometry and the probe reaches Mars's distance to find nothing there.
Why 26 months
Earth takes a year to go round the Sun; Mars takes 687 days. Earth therefore laps Mars — comes back to the same relative position — every 780 days, about 26 months. The right geometry repeats on that cycle and no other, which is why Mars missions cluster into launch seasons a couple of years apart and why a delay of a few weeks can mean waiting two years.
When the next windows open
Run the same physics across the calendar: place the planets where they really are on each date and price the cheapest transfer that actually arrives. The stretch where that price fits the 3.7 km/s rocket is the window.
| Window | Cheapest launch | Transfer |
|---|---|---|
| 8 Oct – 7 Dec 2026 | 2 Nov 2026, 2.98 km/s | 326 days |
| 1 Nov 2028 – 1 Mar 2029 | 6 Dec 2028, 2.60 km/s | 300 days |
| Dec 2030 → | the cycle continues |
Computed with the simulator's physics core: real planetary positions by date, a verified minimum-energy transfer priced for each. The cheapest transfers are slow, ten to eleven months; real missions spend a little more fuel to arrive in about seven, which is why Perseverance made it in 203 days (launched 30 July 2020, landed 18 February 2021). Launch dates for actual missions also depend on the rocket, the payload and the arrival conditions, so treat these as the physics' opening and closing bell, not a manifest.
The porkchop plot, one row at a time
Mission planners draw a "porkchop plot": launch date on one axis, arrival date on the other, fuel cost as contours. The simulator computes one row of it every time you press launch — for the moment you chose, it scans transfer times, solves the exact departure velocity for each (a Lambert problem), throws away any solution that does not really fly to the aim point when re-checked, and keeps the cheapest. If the rocket can afford it, you fly it and rendezvous. If not, the probe gets the best kick the rocket can manage and you watch the miss, with the overlay showing where Mars was when you arrived.
What the simulator does
- The rocket is fixed at 3.7 km/s over Earth; the only thing that changes is when it leaves. Watch the simulator wait for the window, or take the timing yourself.
- A live readout shows how far ahead Mars is and what reaching it from here would cost.
- The simulator prices the cheapest real transfer from that moment. Inside the window it flies it: rendezvous within 0.05 AU, day count on the card. Outside, it flies the kick the rocket can afford and draws the miss.
- Fast-forward to the next window and try again; the pattern repeats every 26 months.
Questions people ask
When is the next Mars launch window?
In the simulator's physics it opens on 8 October 2026 and closes on 7 December 2026, with the cheapest launch around 2 November. The one after runs from 1 November 2028 to 1 March 2029. Real missions pick dates inside these stretches to suit their rockets and arrival plans.
How long does it take to get to Mars?
About seven months for a typical mission — Perseverance took 203 days. The very cheapest transfers take ten to eleven months; paying a little more fuel buys the faster arrival.
Why can't we just launch a bigger rocket whenever we like?
You can shorten the transfer with more speed, but outside the window the geometry is wrong by tens of degrees, and the extra speed needed grows into many kilometres per second — far beyond what any launcher can give a useful payload. The window is a physics limit, not a budget one.
How far ahead does Mars need to be?
The textbook answer is about 44° for circular orbits, but Mars's orbit is noticeably eccentric, so the real figure shifts from one window to the next — the simulator's windows open with Mars between roughly 40° and 75° ahead. The live cost readout is the honest indicator: when it dips under the budget, you are in the window.
Keep going
Every number on this page comes from the simulator's own physics or the real mission record it is checked against. Where the simulation and reality differ, the page says so.