Earth and the Moon

The first two routes of Ultimate Cycle Catcher, Lunar Shuttle and Free Return, take place entirely around Earth. They are where the game teaches its core idea: every target is moving, so the catch has to be timed.

The Moon's orbit

The Moon orbits Earth at an average distance of about 384,400 kilometres, roughly thirty Earth diameters. It completes one orbit relative to the stars in about 27.3 days. Because Earth is meanwhile moving around the Sun, the cycle of lunar phases, from one new moon to the next, takes a little longer: about 29.5 days. That difference is a small everyday example of the synodic periods that govern interplanetary launch windows.

The orbit is not a perfect circle. The Moon's distance varies by around 40,000 kilometres over a month, and its path is tilted by about five degrees to Earth's orbit around the Sun. The Moon also keeps the same face turned toward Earth, because its rotation has become locked to its orbit over billions of years.

Why the game uses an Earth-centered view

For anything happening near Earth, Earth's gravity dominates, so it makes sense to describe motion relative to Earth. On the first routes, the camera holds Earth fixed at the centre while the Moon and an orbiting station called Gateway circle it at different speeds. The station is closer in, so it laps Earth faster than the Moon does, which is exactly the rule that applies everywhere in the solar system: the closer the orbit, the shorter the period.

Free-return trajectories

A free-return trajectory is a path from Earth that swings around the far side of the Moon and comes back toward Earth without any major engine burn. Viewed from above, it often looks like a stretched figure eight. Its great advantage is safety: if the main engine fails after leaving Earth, the crew still comes home, guided by gravity alone. Crewed lunar missions have used this type of trajectory for exactly that reason.

A free return takes several days for the round trip. The second route in the game is named after this idea, with longer arcs that loop around the far side of the Moon before heading back to the station.

Stations in lunar orbit

Several space agencies have studied a small crewed station orbiting the Moon, which could serve as a staging point between Earth and the lunar surface. One favoured design uses a long, stretched orbit that passes close over one lunar pole and swings far out over the other. Such an orbit is relatively stable, keeps the station in almost constant contact with Earth, and is cheap to reach from both Earth and the lunar surface. The game's Gateway is a simplified nod to this kind of outpost, drawn on a neat circular orbit.

Earth–Moon cyclers

Cyclers have been proposed for Earth–Moon traffic too. A vessel on a repeating loop could pass close to Earth and to the Moon on a regular schedule, with small shuttles meeting it at each end. Because the Earth–Moon system is small, these loops repeat in days or weeks rather than years, but they need regular corrections, since the Sun's gravity constantly nudges anything travelling that far from Earth.

Useful numbers

• Average Earth–Moon distance: about 384,400 km.
• Orbital period relative to the stars: about 27.3 days.
• Time between full moons: about 29.5 days.
• Moon's diameter: about 3,475 km, a little over a quarter of Earth's.
• Light travel time from the Moon to Earth: a bit over one second.

Next stop on the tour: Venus. Or go back to the science overview.