In a recent talk with Warren Redlich, I made mention of Mars Cyclers. A cycler is an orbit that travels between Earth and Mars, and each time it encounters one of these planets uses a gravity assist to realign itself. Cyclers can have periods any whole number of synodic cycles, but the one that has a period of a single synodic cycle was first suggested by Buzz Aldrin and thus is referred to as an Aldrin Cycler. The word “cycler” can refer both to the orbit such a station is on, and to the station itself. The recent Netflix film Stowaway features a cycler being used to travel to Mars - but that fictional cycler is much smaller than one that would be used in real life. To get the benefit of a cycler, you have to make it big, as I will explain below.

Cyclers offer a way to vastly improve on the cost and safety of travelling to Mars with Starship, but are only likely to be available to do this once Starship has established a regular route to and from Mars.

Below is a rough diagram of the orbit a Aldrin cycler follows (green) between Earth (blue) and Mars (red)

Beginning with an encounter at Earth at (1), the cycler then meets Mars around 5 months later at (2). After another 21 months it encounters Earth again at (3) in a different position (because a non whole number of Earth years have passed). At this stage, Earth and Mars are in the same position relative to each other, but the cycler orbit is no longer correctly aligned for another encounter. At this point, the cycler performs a gravity assist at Earth. Gravity assists add velocity to a spacecraft as it passes a planet, and are normally used to speed up or slow down in order to save propellant. In this case, the velocity change is used to rotate the orbit so that it once again it is correctly aligned relative to Earth and Mars. Only minimal station keeping is required, and the cycler does not require a large boost each time it travels to Mars.

Requirements of a Cycler

To launch from LEO to an Aldrin Cycler requires about 6 km/s of delta-V. Mars trajectories can be closer to 4 km/s during the right part of a favourable window, but they can take a year to arrive. More reasonable travel times require delta-V comparable to that required for the cycler. A 100 tonne Starship payload, with a 120 tonne dry mass and 370s specific impulse Raptor engines, would require around 9 tanker refills to achieve the cycler orbit; a more typical Starship trajectory would take around 7. At the projected $2m cost and including the ship itself, this would mean $20m per cycler launch versus $16 million per regular launch.

The cycler can pass close to Earth; in fact, the closer it is the less propellant it needs to augment the gravity assist. If it passed just outside geostationary orbit, it would not interfere with satellites and a journey to meet the cycler would only take a few hours. Over such a timescale, a Starship can be packed quite densely without discomfort for passengers. This is where the savings come in. In the table below, I have calculated how many passengers must be put into a cycler-bound Starship for it to reach $32,000 - 10 times cheaper than a conventional Starship with 50 passengers and 7 tanker refills. I have chosen 50 passengers for the baseline because many people have questioned if the originally stated number of 100 passengers can comfortably live in a Starship for months. If more than 50 can be squeezed into a direct Starship flight, then this also saves money compared to the baseline case, but at the cost of much reduced comfort and safety.

The column on the left adds 0-10 extra Starship launches required to maintain the supplies of the cycler station. There would be multiple passenger flights to the cycler station each trip, and so the cost of cargo flights, including tankers, would be split between them. Because of the substantial difference caused by how many cargo flights are needed, the economics of a cycler scheme depend on the closure of its life support system i.e. what fraction of its supplies can be sourced from its recycled waste. No budget is included for cargo to support passengers on Mars, as it is assumed by the time this is done there will be a well functioning city at the other end to support them, and we are just working out the cost of getting there and not keeping the colony going.

Elon Musk has stated that 1000 people could fit into Starship for Earth-to-Earth operations, so even taking this with a pinch of salt it looks like 10x improvement is possible here as long as the resupply of the cycler station does not take too many flights. I’ll be covering more of the economics of the cycler in future articles; to receive these, please hit the subscribe button.

The Radiation Issue

One thing a cycler gets you is not having to reboost any mass that is reused between missions; and so this mass can be really high without costing extra. The ideal candidate for this is radiation shielding.

At a solar maximum, there is around 500mSv/yr of GCR. Radiation worker limits in the US are 50mSv/yr, which is acceptable adults but not for children. For an entire 9 month pregnancy, 5mSv is considered the safe limit, based on experience with medical scans of pregnant women. NASA recently announced a 600mSv limit for professional astronauts - a bit conservative for Mars missions, but probably doable if radiation is limited on the surface. 

The idea is that you begin Mars missions with a low level of shielding and carefully selected crew. You then add shielding as you go, broadening the number of people who can safely cross. At the same time, you increase the shielding at the Mars base. Eventually, this should be low enough as to be unambiguously safe for the entire population.

One way to build up shielding is through using waste. Current space stations use their discarded cargo vessels to get rid of waste that cannot be recycled, leaving it to burn up in the Earth’s atmosphere. What a cycler could do is package it in some way that it could be fixed to the outside of the station. Thus each trip delivers more and more shielding, without any extra mass cost because the supplies would’ve had to have been taken along anyway.

Once shielding on the cycler station becomes substantial, one of the main sources of exposure would be passage through the van Allen radiation belts. This site estimates the dose during the Apollo mission, inside the spacecraft, was around 20mSv. This is fine for the general population but too high for children and pregnant women. If we really want to open up the frontier for colonisation, these are obviously two important groups. The solution is either to bring the heavily shielded cycler inside the van Allen belts, or having a sufficiently shielded transfer vehicle perhaps modified from a Starship.

Cargo for the Mars base which is not radiation sensitive can be sent on lower energy trajectories if needed; although most likely cargo will be needed by the crew on arrival so should travel with them, and the cycler will not lack storage space for it. 

Inside the Cycler

In addition to shielding, the high potential mass of a cycler allows it to be a large, rotating structure. As such, it could remove the problems of weightlessness in addition to the problems of radiation. As stated above, to spread the cost of running the cycler it should be large enough to accommodate several starships worth of people. It should be easy to see why scale is your friend here, as I have argued for space stations in general.

The cycler station would be big enough to be its own community, and perhaps have a permanent population who remain on board during the majority of the orbit outside of the Mars transfer. Life on board could perhaps have the atmosphere more like a university campus than a cruise ship, where passengers are trained for new lives and roles on Mars, and when the students are away, research is conducted.

Building a cycler on its own orbit would be hard; better to build it somewhere near Earth in pieces, and then ship the completed station on to its cycler orbit. Expansion can be carried out by adding other large pre-assembled pieces as demand dictates. A fully fuelled Starship, with 12 tanker refills, could carry 165 tonnes to the cycler orbit. If multiple Starships could be set up to tow a large station between them, its mass could be quite substantial and the station would not have to fit inside the payload fairing of Starship. This and the assembly operations would require a lot of Starship launches, the cost of which would have to be recouped during operations and would add to the ticket prices. Hopefully this cost could be amortised among a large number of passengers so it would not make that much difference.

Opening the Frontier

Dropping the prices of a Mars ticket from $320,000 to $32,000 would make a substantial difference in the demographics of who could go; the latter price is in the range of what people who aren’t considered rich by western standards pay for a car. However, it still wouldn’t quite get to a full democratisation of space flight. It should be clear that in the above calculation, launching extra propellant is a large part of the cost. As such, I believe the next 10x would have to come from sourcing propellant in space. I will discuss this in a future article.