Mars needs a bigger moon

One of the factors stabilizing Earth's climate is its large moon in a relatively close orbit; it acts like a gyroscope to keep Earth spinning the same direction without wobbling (much, anyway).

Mars, on the other hand, has two dinky moons (captured asteroids, actually). Without a big moon, its axial tilt swings around wildly.

When we terraform Mars -- and I hope it's a when, not an if -- we need to plan for the long term. To keep the climate of our newly terraformed planet stable over millions of years, we're going to need to stabilize Mars's spin.

Conveniently, the planetoid Ceres is in the asteroid belt right next door to Mars. It's big enough that we could move it in a close orbit around Mars, and it would have the same stabilizing effect on Mars that the moon has on Earth.