Here’s the thing about orbits. Orbits are like fingerprints. No two are exactly alike, even when they’re pretty close together. And here’s the thing about planets. Planets are really, really far apart. Even if the planets started at the same speed, Mars is further out than the Earth. That means Mars has a much bigger circle (or circumference) to travel around in a single orbit, so it would take it much longer than the earth. By the time Earth has completed one orbit, Mars would only have done about two thirds of its own.

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But here’s the thing about orbits. Orbits are like fingerprints. No two are exactly alike. The Earth and Mars are not travelling at the same speed. It turns out Mars is going a lot slower than we are. So actually, one Mars year is just under two Earth years long. It takes almost twice as long for Mars to complete an orbit as Earth. So while we have whizzed around the sun once (at an insane 70,000 mph!) Mars is still languishing behind us on the opposite side of the sun (travelling a measly 50,000 mph), and is about as far away from us as it can possibly get. Fortunately, we will continue our impressive pace and catch up with it in just over another year.

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So, every two years and 2 months (more or less) the Earth passes very close to Mars, and this is the best time to be launching missions there, because the distance they have to travel is so very much short. In fact, this alignment is going to take place any day now, on May 22! Which is why ExoMars was launched earlier this year. And they’ll be launching another mission in four years’ time, when Mars will be even closer still, because, well here’s the thing about orbits…

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