Mars missions leave Earth in bunches, then nothing for two years. That rhythm isn't bureaucracy — it's the Solar System's own timetable, and it explains half the launch calendar.
You aim at where it will be
An interplanetary spacecraft doesn't fly toward Mars; it flies toward the empty point in space where Mars will be months from now. The standard route, called a Hohmann transfer, is an ellipse that kisses Earth's orbit on one end and Mars's orbit on the other — the minimum-fuel path, coasting nearly all the way. For Mars, riding that ellipse takes roughly six to nine months. The catch: it only works if Mars arrives at the far end of the ellipse at the same moment you do.
The planets set the schedule
Earth laps the Sun in a year; Mars takes about 1.9 years. The two planets swing into the right relative positions — Mars leading Earth by just the right angle — once every 26 months or so. That alignment is the launch window, and it's typically only a few weeks wide. Hit it and physics does most of the work; miss it and no budget on Earth buys it back — you wait. It's why Perseverance, and two other nations' Mars missions, all left within eleven days of each other in the summer of 2020.
Stealing speed from planets
For destinations beyond Mars, even the best ellipse is too slow, so navigators cheat — legally. Fly close behind a moving planet and its gravity slingshots you forward, the spacecraft stealing a sliver of the planet's own orbital momentum. The Voyager probes used a once-in-176-years lineup of the outer planets to hop Jupiter → Saturn → Uranus → Neptune in twelve years instead of many decades. Gravity assists are why mission trajectories look like pinball: each bend is a free engine burn the Solar System donated.
Reading the calendar like an insider
Once you know the rhythm, the launch schedule starts telling stories. A probe launching 'toward Venus' may really be aiming at Mercury or the Sun, using Venus as a brake or a slingshot. A Moon mission can leave almost any month — the Moon is always nearby — while a Mars window makes agencies sprint for a date the sky set decades in advance. When you see a planetary mission slip its date by two years rather than two weeks, now you know why: the bus doesn't wait.