Mars, Phobos and Deimos

Mars is the destination most often discussed for cyclers, and the Red Planet Line route in Ultimate Cycle Catcher is built around it: a Sun-centered cruise followed by quick work between the two Martian moons.

The red planet

Mars is about half the width of Earth, roughly 6,800 kilometres across, with about 38% of Earth's surface gravity. It has a thin carbon dioxide atmosphere, polar ice caps, the largest volcano known in the solar system and a canyon system that would stretch across a continent on Earth. A Martian day is only about 40 minutes longer than ours.

Orbit and launch windows

Mars orbits the Sun at an average of about 1.52 times Earth's distance and takes about 687 days to go round once. Earth, on the inside track, catches up with Mars and laps it roughly every 780 days, about 26 months. That synodic period sets the rhythm of every Mars mission: the favourable alignment for a low-energy trip comes around only once every two years or so, and missions cluster around those dates.

A minimum-energy transfer takes about eight and a half months one way. After arriving, a crew on such a trajectory would have to wait over a year at Mars for the planets to line up again for the trip home.

Earth–Mars cyclers

Earth–Mars cyclers try to remove that waiting by keeping a large habitat permanently on a loop that encounters both planets. The best-known family of designs takes a little more than two years per orbit, matched to the 26-month synodic period, and uses a close pass of Earth on each cycle to rotate the orbit so that it keeps lining up with Mars. In some versions the outbound leg from Earth to Mars takes around five months, while the return leg is much longer; a pair of cyclers running in opposite directions can provide a fast trip each way.

The trade-off is speed: a cycler passes each planet at high velocity, so taxi craft must accelerate hard to meet it and brake hard on arrival. That is the moment the game turns into a timing challenge. You are the taxi, and the cycler will not slow down for you.

Phobos

Phobos, the larger moon, is an irregular lump about 22 kilometres across. It orbits only about 6,000 kilometres above the Martian surface, closer than any other known moon to its planet, and goes round in about 7 hours 39 minutes. Because that is faster than Mars rotates, an observer on Mars would see Phobos rise in the west and set in the east, sometimes twice a day. Tidal forces are slowly pulling it inward, and over tens of millions of years it is expected to break apart or fall.

In the game, Phobos is the target that whips around Mars every couple of seconds, which is why its rings close so quickly in the Mars-centered view.

Deimos

Deimos is smaller, about 12 kilometres across, and much farther out, at around 23,500 kilometres above the surface. It takes about 30 hours to orbit, a little longer than a Martian day, so from the ground it drifts slowly across the sky. Both moons are dark, cratered and may be captured asteroids or debris from an ancient impact; scientists are still debating their origin.

Three frames on one route

The Red Planet Line departs in an Earth-centered view, cruises in a Sun-centered one and arrives in a Mars-centered one, then reverses the sequence on the way home. Real mission designers use the same approach, switching their reference body as a spacecraft passes from one world's sphere of influence into another's.

Useful numbers

• Average distance from the Sun: about 228 million km.
• Orbital period: about 687 days.
• Earth–Mars synodic period: about 780 days.
• Phobos orbital period: about 7.65 hours. Deimos: about 30.3 hours.

Next: the asteroid belt. Or read the science overview.