Launch windows: aim where it will be

Before the lesson — what this assumes

• Assumes: farther orbits are slower (Kepler's third law, at least by shape) — the planet pages' distance and year figures rebuild it in minutes.
• Assumes: comfortable algebra with fractions — the lesson derives 1/(1/T₁ − 1/T₂).
• Pre-work: read the launch-windows page top to bottom once, without trying to understand the solver — just collect what seems strange.

The plan — every step carries its minutes

The minutes are there for the teacher's pacing — and so that any single step can be handed to a student as their own five-minute lesson. See students teach the class.

What they should find — the teacher's key

• Synodic period Earth–Mars ≈ 780 days; Earth–Jupiter ≈ 399 days; Earth–Saturn ≈ 378 days — all three derivable and all three checkable on the page.
• The transfer geometry: launch when the target leads by the angle it will cover in half the transfer time.
• Model limits are stated, not hidden — the difference between a teaching model and a mission plan.

Standards: HS-ESS1-4 (orbital motions and Kepler's laws).

What the picture fakes: On every system view the planet dots are drawn far larger than scale, and where each moon sits along its orbit is illustrative — sizes, distances, speeds and directions are real, positions are not solved, and the pages say so.

Questions to chase on your own

For the student who wants more — each answerable with the tools, no teacher required:

• Compute the Earth–Venus synodic period (Venus's year: 225 days). Why might a Venus mission planner care about ~584 days?
• From the windows list: which target's windows are most frequent, and does your formula agree it must be so?

Questions the curious actually ask

Real questions, mostly from real kids — the kind that sound simple and open trapdoors. Worth raising in class before someone raises them for you:

If gravity pulls everything, why doesn't the moon fall on us?
Here's the secret: it is falling — right now, continuously — and missing. The moon moves sideways so fast that by the time it has fallen toward Earth, the Earth's surface has curved away beneath it. An orbit is a fall that never lands. Newton figured this out by imagining a cannon on a mountain firing faster and faster until the cannonball fell all the way around the world — and that thought experiment is every satellite, the space station, and the moon.

Why don't the planets ever crash into each other?
Mostly they can't — each is locked in its own lane by its own speed, and the lanes are separated by distances that make the planets themselves like grains of sand miles apart. But here's the part worth knowing: early on, they did crash. The leading theory for where the moon came from is a Mars-sized world hitting the young Earth. The quiet, orderly system in the simulator is the survivor of a demolition derby — the orbits you see are the ones that lasted.

How does the sun keep burning? There's no air in space!
Because it isn't burning — fire needs air, and the sun would have burned out in a few thousand years if it were a bonfire. The sun is crushing: its own gravity squeezes its core so hard that hydrogen atoms fuse together, and fusion releases millions of times more energy than fire. It has run this way for 4.6 billion years and is about halfway through its fuel. Nothing is on fire. Something much stranger is happening.

Go further — beyond this site

• NASA's Eyes on the Solar System — fly the real missions in 3-D — the professional version of today's drawing.
• NASA Mars Exploration — every launch in the class's lifetime landed inside one of these windows; check.

Same question, other grades

Each grade band re-asks this topic's question one level deeper — observe it, describe the pattern, measure it, explain the mechanism, quantify it and question the model. This page is the quantify it — and question the model rung.