A rocket cannot point at Mars and fire. It has to leave on an orbit around the sun that meets Mars where Mars will be — and that is only possible for a few weeks every couple of years.
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Distances from the sun are in AU — one AU is the Earth’s average distance, 149,597,870 km — and the angle is where the body sits around its orbit, measured from the March equinox direction.
None of these dates is written into this page. Each one is solved when the page is built, and again in your browser when it loads, by asking a single question of the real orbits: when is the target positioned such that, after the time a minimum-energy transfer takes, it will have arrived at the far end of that transfer?
| Target | Leave | Arrive | Flight | Speed change | Burn from low Earth orbit |
|---|---|---|---|---|---|
| Mars | Nov 4, 2026 | Jul 27, 2027 | 265 d | 3.08 km/s | 3.63 km/s |
| Jupiter | Oct 27, 2026 | Sep 14, 2029 | 1,053 d | 8.88 km/s | 6.35 km/s |
| Saturn | Jun 27, 2027 | Feb 9, 2033 | 2,054 d | 10.31 km/s | 7.30 km/s |
Click a planet to fly it in the simulator above. The last column is what the rocket actually has to do from a parking orbit 300 km up — always less than the middle column, because you are already moving fast up there.
A rocket cannot point at Mars and fire. It has to leave Earth on an orbit around the sun whose far side touches Mars’s orbit — half an ellipse — and it has to leave at the moment when Mars will have arrived at that far side by the time the ship gets there. That is what a launch window is, and it is why they come round only every 25.0 months.
Turn on Flight path to Mars above to see the ellipse, where Mars is on launch day, and where it will be on arrival. Click the date to fly it.
A rocket cannot point at Jupiter and fire. It has to leave Earth on an orbit around the sun whose far side touches Jupiter’s orbit — half an ellipse — and it has to leave at the moment when Jupiter will have arrived at that far side by the time the ship gets there. That is what a launch window is, and it is why they come round only every 12.9 months.
Turn on Flight path to Jupiter above to see the ellipse, where Jupiter is on launch day, and where it will be on arrival. Click the date to fly it.
A rocket cannot point at Saturn and fire. It has to leave Earth on an orbit around the sun whose far side touches Saturn’s orbit — half an ellipse — and it has to leave at the moment when Saturn will have arrived at that far side by the time the ship gets there. That is what a launch window is, and it is why they come round only every 12.5 months.
Turn on Flight path to Saturn above to see the ellipse, where Saturn is on launch day, and where it will be on arrival. Click the date to fly it.
It is a minimum-energy transfer: the cheapest possible path, half an ellipse with Earth’s orbit at one end and the target’s at the other, solved against where the planets really are — so the arrival lands on the planet’s real distance from the sun and its real position on the day it gets there, not on a circle standing in for its orbit.
It is not a mission plan, and three things are left out on purpose. The orbits are treated as flat, so the real inclinations — Mars is tilted 1.85°, Jupiter 1.3° — cost a plane change this ignores. Real missions trade fuel for a faster arrival, so they leave within days or weeks of these dates rather than exactly on them. And anything going past Jupiter usually steals speed from a planet on the way instead of buying it with fuel, which changes both the date and the path.
What it gets right is the thing worth teaching: why the window exists, why it comes round on the cadence it does, and why the cost of the trip is set by where two planets happen to be rather than by how far apart they are.
Draw two circles round the sun: Earth’s orbit and Mars’s. Now draw the smallest ellipse that touches both — it touches Earth’s circle at one end and Mars’s at the other, and that half-ellipse is the cheapest route between them. Fire the engine once, at the right moment, and you coast the whole way. The catch is that the trip takes 265 days, and Mars has to be at the far end when you arrive, not where it was when you left. Mars moves about 139° while you are in flight, so it has to start roughly 41° ahead of you — and it is only in that position for a few weeks every 25 months.
Everything else about interplanetary flight is a refinement of that picture, including the reason a probe to Saturn usually goes past Venus first.
The date, the zoom, the span, the speed, the layers and any flight path are all in the address bar, so copying the URL shares exactly what is on screen. Set it up above, then take the link — it is the quickest way to hand a class one specific thing to look at.
This one is about the whole system. If the question is where the sun and the moon are from where you are standing — what time the sun comes up, why tonight's moon is the shape it is — that is the Sun, Earth & Moon movement simulator, which has a page for every city and a slider over a day, a week or a month.
Also: the classroom guide · sunrise & sunset by city · moon phase & moonrise · lunar eclipses · how the positions are worked out
Why can't a rocket just fly straight at Mars? Because it is already moving at about 30 km/s sideways, along with the Earth. A spacecraft does not leave the solar system's rules when it leaves the launch pad: it stays in orbit around the sun, and the only thing an engine can do is change the shape of that orbit. Getting to Mars means enlarging the orbit until its far side reaches Mars's — and then arranging to be there when Mars is.
Why do launch windows come round every 26 months for Mars? Earth goes round the sun faster than Mars, so it laps it. The time between one lap and the next — the synodic period — is about 26 months for Mars, and only then are the two planets in the right relative position again for a minimum-energy transfer. For Jupiter it is 13 months and for Saturn about 12.5, because the outer planets barely move while Earth goes round.
Is the next window on this page the date NASA would use? Close, but not identical. This is the cheapest possible path with no plane change and no gravity assist, so it is the reference every real plan starts from. Real missions leave within days or weeks of it, trading extra fuel for a better arrival geometry, a shorter flight or a launch site's constraints.
Why is Saturn so much more expensive than Mars? Because the change in speed you need is set by how much you have to enlarge your orbit around the sun, not by the distance. Leaving Earth's orbit for Mars costs about 3.1 km/s; for Saturn it is about 10.3 km/s, and the flight takes 8 times as long. That is why almost everything sent to the outer solar system steals speed from a planet on the way instead of buying it.