A Moonwalker Invents a Mars Cycler

Before he traveled to the moon, Dr. Buzz Aldrin completed a PhD thesis exploring how to dock spaceships in the event of instrument failure. Since returning from the moon, Dr. Aldrin has been working to get humans to Mars...

Our future?
Source: NASA.

There are many, many, many obstacles to establishing a human colony on Mars. One huge challenge is the cost of ferrying people and the supplies they'll need between the two planets. Another challenge is the length of the trip to Mars. It took four days for Dr. Aldrin to travel from the Earth to the moon; it took the Curiosity Rover nine months to travel from the Earth to Mars.

Mars Science Laboratory, on its way to the red planet.
Source: Wikipedia.

Dr. Aldrin's "Mars cycler" plan comes in handy in addressing both of these challenges. Dr. Aldrin formulated this plan in the mid-1980s. He called for the establishment of a permanent human base on Mars, supplied by a fleet of of uniquely tasked spaceships. Some of these spacecraft would be used to ferry people and supplies between the surface of Earth and Earth orbit; some would transport people and supplies between the surface of Mars and Mars orbit. Meanwhile, traveling between Mars and Earth there would be a continuous cycle of interplanetary spacecraft: "cyclers."

These cyclers would essentially be space stations orbiting a path that would take them between Earth and Mars every few months. They'd be similar to the international space station, but with heavy-duty rockets attached, more radiation shielding, and maybe a big centrifuge creating artificial gravity. You could have two of these cyclers, with one always going towards Earth and one away. Or you could launch even more cyclers, allowing for more frequent trips between the two planets.

A Mars cycler approaches Mars.
Source: Scientific American, March 2000.

One upside to the cycler is that it makes a faster trip to Mars than traditional spaceships. In contrast to Curiosity's nine month trip to Mars, the cyclers could make the same trip in just five months. Cyclers are faster because they take advantage of a gravity assist. Meaning, they are aimed for a close encounter with Earth and then Mars, hurtling around each planet before shooting out back towards the direction they came from, picking up a bit of the planet's momentum as they go. Gravity assisted spacecraft (like the Voyager spacecraft) can build up much higher speeds than just firing a rocket.

Voyager 1 gained the momentum needed to escape the Sun's gravity via a gravity assist from Jupiter and Saturn.
Source: Wikipedia

The cyclers have other advantages. You don't have to pay for the fuel to repeatedly accelerate or decelerate the spacecraft when they reach Earth or at Mars, and you aren't constantly building giant spaceships capable of leaving Earth's atmosphere and landing on Mars. In these multi-stage spacecraft, almost every stage gets discarded after accelerating and decelerating between Earth and Mars.

The little bitty command module, that I've circled in red, is the only bit of Apollo 11 that made it home to Earth.
Source: Universe Today.

Are there downsides to the Mars cycler? Yes... maintaining the Mars-Earth orbit requires more than just the occasional course correction boost that the International Space Station gets to maintain its Earth orbit. As Dr. Aldrin acknowledges, "moderately large" maneuvers are required at irregular intervals to keep cyclers from smashing into a planet or zipping out of orbit into empty space. But, that said, the cycler is still essentially an orbiting space station: it is not having to expend massive amount of propellant to escape Earth or Mars gravity every time it flies to those planets.

Another problem is actually reaching the cycler from vehicles launching from Earth or Mars. The launch craft must catch up as the cyclers make their once-every-five-months pass by the Earth or Mars. The cycler could be travelling as fast as 27,000 miles per hour as it encounters Mars. That's close to the fastest speeds that the Apollo spacecraft ever traveled. So, a rocket leaving Mars attempting to rendezvous with the cycler would expend a great deal of energy. Or, alternatively, you could significantly slow down the cycler when it reaches Mars (by aerobraking- dipping into and out of the Martian atmosphere, with the friction of Martian air slowing the craft down). Then, it would be easy for a spaceship leaving Mars to rendezvous with the slowed cycler... though the cycler would need a big rocket boost to speed up and travel back to Earth.

An illustration of Mars Reconnaissance Orbiter aerobraking on arrival to Mars.
Source: Wikipedia.

Will humans ever hitch a trip to Mars on a cycling interplanetary space station? Nearly thirty years after he first proposed it, Dr. Aldrin's cycler idea still seems like a doable approach to establishing a long-term human presence on Mars. So, whether Mars Cyclers come to pass probably depends on our dedication to exploring the solar system.

Sources: Next Big Future; March 2000 Scientific American; buzzaldrin.com; NASA; D.V. Burnes, J.M. Longuski; B. Aldrin, Cycler Orbit Between Earth and Mars, Journal of Spacecraft and Rockets (1993); Buzz Aldrin et al.; Evolutionary Space Transportation Plan for Mars Cycling Concepts.