Aug 09 2021

Going to Mars

This past weekend we hosted the second all-digital NECSS, which ended with two talks from NASA scientists, Sarah Noble who spoke about going to the Moon and Julie Robinson who spoke about sending people to Mars. The two projects are intimately connected, really part of one overall goal, as the lessons learns on the Moon will be invaluable to going to Mars. And both will benefit from ongoing research and experience aboard the ISS. Their talks reinforced the impression I had that NASA is thinking in terms of a long term overall strategy for expanding human activity into space. About 40 years ago I first read an article by Isaac Asimov in which he argued strongly that establishing a permanent presence on the Moon will be a critical stepping stone for getting to Mars, and it seems NASA agrees.

There area few technical points I found interesting, and they reinforce the notion that going to Mars is going to be very challenging. Right now NASA hopes to send people to Mars in the 2030s, but after listening to both talks and having the opportunity to ask some questions, my sense is that this is a bit optimistic. Here are the specific issues that struck me:


Every time we discuss going to Mars on the SGU the challenge of shielding from radiation comes up, and Julie Robinson not only reinforced this but painted a more pessimistic picture than I already had. Out in space, away from the Earth’s atmosphere and magnetic field, humans would be exposed to dangerous levels of radiation. There are three ways to mitigate this danger. The first and best method is to shield astronauts from the radiation so they are not exposed to it in the first place. The second is to decrease the amount of time that astronauts are in space. The third is to use medical interventions that reduce the biological effects of radiation. When going to Mars, none of these options are currently adequate.

There are two types of radiation in space to be concerned about. The first is solar radiation, ionized particles from the sun. These are actually the easier of the two to deal with, and can be shielded with standard material. Water, in fact, is a great shield for solar radiation, and so storing water, or even waste, in the hull of a ship could be sufficient shielding for even long missions in space. This type of shielding is employed on the ISS.

Solar radiation, however, can come in storms, which can present a hazard. If a ship, station, or Moon base were hit without warning, the amount of radiation could overwhelm the shielding. Astronauts will therefore need an early warning system (probably from observers on Earth) so that they can get into  highly shield locations for the duration of any solar storms. Again, this is a risk, but manageable with current technology.

The second kind of radiation is high-energy cosmic galactic rays. These are much higher energy particles than solar radiation, and current shielding technology will not block them. Cosmic rays do not storm, but they are present in a steady flow that will cause biological damage over long missions. This is ionizing radiation capable of damaging DNA molecules. Shielding, in fact, may be counter productive. Robinson related a story from the Apollo era when NASA protected film in lead containers, only to find that the film was completely exposed by cosmic rays. The problem was, whenever a particle did get through the lead it would bounce around inside the shielding, causing far more damage then if it were allowed to simply pass through. Shielding from solar radiation, therefore, may make cosmic ray damage worse, unless it is carefully calibrated. Effective shielding against cosmic rays would simply be too heavy for a Mars transit vessel, and so is not feasible with current technology.

This means we are left with the last two options – minimize time in space, and research treatments to mitigate biological damage. Any Mars mission with current technology will likely take three years at a minimum. The only way to really reduce this time is to build faster ships, and that will require nuclear thermal or other advance propulsion. We are pretty much at the limit of chemical rocket technology, with only incremental advances possible. If we want to reduce travel time by half, we will need nuclear thermal propulsion, at least in the near term. More advanced ships, such as light sail ships, are also possible but probably farther in the future for something as large as a Mars transit vehicle.

So even if all the other pieces to the puzzle are in place, we may not be able to go to Mars until we have something beyond chemical rocket technology. This likely pushes a Mars mission out by a decade or two, at least.

Artificial Gravity

Astronauts on a long journey to Mars will also have to make the trip in microgravity. Again, NASA has concluded that it is easier to mitigate the negative effects of prolonged microgravity than to solve the engineering problem of producing artificial gravity. This is where ISS research comes in, with longer and longer missions and techniques to minimize bone and muscle loss, for example. But why is artificial gravity so hard?

The only plausible mechanism for artificial gravity on a long Mars journey with any near-term technology is rotation. You can rotate a torus or a cylinder, with centrifugal force taking the place of gravity. While this could work, there are several technical limitations. One is that the spinning vessel would have to be enormous, kilometers wide. Smaller spinning vessels would produce vertigo, which is a significant problem on a long journey when everyone will have to be functioning well. Also, changing orientation will have a significant effect – just turning your head could produce disorientation and vertigo.

This is not unsolvable. You cold, for example, have two capsules that are connected by a long cable which then spin around each other. But still, the engineering challenges, such as the strength of the cable and all connections, are non-trivial. So much could go wrong, and any glitch would spell disaster. Further, any ship or compartment that then have to be rigged for both microgravity and rotation, with all equipment engineered for function properly in both environments.

For now NASA considers using rotation for artificial gravity to be out of range for a transit vehicle. This would work for a large space station, however, just not a ship.


All the other challenges of getting people to Mars seem to be well in hand, or at least have highly plausible solutions that do not require any technological breakthroughs. We just need to develop the specific technology and the infrastructure. Radiation seems to be the most significant barrier at this time. Low gravity is also a challenge, but they seem to be dealing with that well aboard the ISS. If our goal is to get people to Mars, or even establish a permanent presence there, it seems we will need nuclear thermal rockets or better, and we will need to build underground shelters that are full shielded.

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