What are the most difficult challenges SpaceX will face getting astronauts to Mars by about 2025?

Question

SpaceX has been announcing very ambitious goals such as starting to send manned missions to Mars in order to (eventually) create a colony, and recently there has been a substantial release of more technical details about this plan (see links at the the end). As quoted in Spaceflight Now's article SpaceX’s Elon Musk announces vision for colonizing Mars:

SpaceX plans to launch its first mission to Mars, a robotic test flight with a modified Dragon capsule, as soon as May 2018. After that “Red Dragon” flight, Musk said SpaceX’s goal is to send at least one spacecraft to Mars during every interplanetary launch opportunity, which come every 26 months or so.

The concept detailed Tuesday features a huge rocket standing 400 feet (122 meters) tall, and a fleet of passenger-carrying spaceships and refueling tankers.

Musk’s long-term vision is to build a self-sustaining civilization on Mars, complete with “iron foundries and pizza joints.” Eventually, it might have a million residents or more.

“When will we reach that million person threshold? It’s probably between 20 and 50 total Mars rendezvouses,” Musk said, counting Mars launch windows occurring every other year. “It’s probably anywhere from 40 to 100 years to fully achieve a self-sustaining civilization on Mars.”

He counts those numbers from the time of the first crewed flight, which might some as soon as the 2020s.

“We aspire to launch in late 2024 with an arrival in early 2025,” Musk told reporters after his presentation at the International Astronautical Congress in Guadalajara, Mexico. “That’s optimistic, so I would describe that as an aspiration and within the realm of possibility, but a lot of things need to go right. That said, I don’t think it would be significantly beyond that if it did go later.” (emphasis added)

This seems really soon. Has technology advanced enough for this type of mission to be fulfilled - transporting astronauts all the way to Mars, and (presumably) returning them safely home again? And what about the radiation problem in the trip?

Question: Based on the more detailed information available by SpaceX recently, what are the most difficult challenges that SpaceX faces accomplishing this goal by 2025, or not "significantly beyond that"? Are there any obvious technology or logistic "show-stoppers" that could make this goal convincingly untenable?

More about the plan can be found on the SpaceX page http://www.spacex.com/mars and a link to a PDF of Elon Musk's position paper Making Humans a Multi-Planetary Species can be found at http://online.liebertpub.com/doi/full/10.1089/space.2017.29009.emu

Answer 1

Since there have been no answers in over a week, I'll venture a partial answer.

There are of course several challenges. Among them in no particular order:

• Crew selection and training

  Putting together, evaluating, and training the crew would be the first challenge. They need to train together on accurate replicas of the systems to be used in the future. Just as a comparison: astronauts for the ISS train for almost two years. The apollo astronauts trained for at least five years.

• Landing site

  Landing site selection is a difficult process. See for example a summary of the third Mars 2020 Landing Site Workshop held in Monrovia, CA, from February 8-10, 2017 for an example from NASA. And thats just for rovers, not a site that allows for the relaunch of a large space craft.

• Space suits reliable for EVAs and Mars-walks

  What kind of space suit are they going to use? The one they are building now is the "indoor" space suit for use inside the space craft.

• Dust, toxic perchlorates

  On the moon missions, regolith caused a few problems. It stuck to the suits, entered the living space, and it caused a lot of wear and tear. See Air and Space's article Stronger Than Dirt; Lunar explorers will have to battle an insidious enemy—dust. for more. Martian regolith is also known to contain a substantial amount of perchlorate, a nasty, toxic combination of chlorine and oxygen. The Space.com article Toxic Mars: Astronauts Must Deal with Perchlorate on the Red Planet covers this nicely, with a good overview of the challenges.

Devilishly dangerous

The high levels of perchlorate found on Mars would be toxic to humans, Smith said.

"Anybody who is saying they want to go live on the surface of Mars better think about the interaction of perchlorate with the human body," he warned. "At one-half percent, that's a huge amount. Very small amounts are considered toxic. So you'd better have a plan to deal with the poisons on the surface."

The value of one-half percent is mentioned in the abstract of the 2013 paper Perchlorate on Mars: a chemical hazard and a resource for humans, while a lower but significant value of 0.6 g/kg is found in the abstract of the 2017 paper The nitrate/(per)chlorate relationship on Mars at a different location.

Proposed solutions include leaving EVA suits docked on the outside of the spacecraft/habitat, though it's not clear how practical this could be, another proposed there is a water wash down in the airlock.

• Refueling station

  While in theory, the Sabatier process will work, they have to research and develop it at first for a practical and reliable implementation on mars, because nobody currently knows how to really do that. An often overlooked aspect in this regard is, that you are going to need mining equipment to bring the reactants into a proper state for processing. NASA does have a research project on that. See for example the SBIR summary In-Situ Resource Utilization - Mars Atmosphere/Gas Chemical Processing:

State of the art (SOA) technologies for these ISRU processes either do not exist or are too complex, heavy, inefficient, or consume too much power.

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The Mars Exploration Program Analysis Group seems to be a good source for further reading. There should be a risk analysis for a mars mission there, but I was unable to locate it.

As an aside; Buzz Aldrin thinks that the SpaceX plan is unfeasible. More on this can be found in Purdue University's course introduction page Project Aldrin-Purdue, Mission to Mars. The page also links to the nicely produced video Project Aldrin-Purdue, Mission to Mars which includes Aldrin's discussion.

I hope that I was able to at least provide a starting point and encourage more people to answer. This is a very interesting and developing topic and there should be a place to collect current information.

Answer 2

While Mike's answer is very well researched and written. I disagree with its challenges (though I don't disagree with his conclusions).

The biggest challenges that SpaceX will need to solve in my opinion are.

1. Re-entry heat shielding and Earth landings. It is still working on mastering the landing flip without losing fuel flow. But it also has to show that orbital re-entry can be done safely and repeatedly without excessive heat stress degrading or destroying the Starship. Without both you don't have a fully re-usable launch system, and without full re-usability you can never have the cadence and low costs to support cheap interplanetary travel.

2. Refueling in space It's never been done on the scale they need with the fuels they will be using. If they can't master refueling in space Starship can never take anyone beyond low earth orbit.

3. Long Term Life Support Systems Crew Dragon shows they can make a safe life support system for space travel. But they have to show they can make a system that will last years without failing, and is easy to maintain and repair when millions of miles from earth.

These first three should be solvable problems, the question is whether they can be solved by 2025 and that seems very speculative. #1 and #2 have to be solved for the Human Landing System that SpaceX will be providing NASA to land on the moon. I expect them to master landings this year (2021), and start testing orbital re-entries by year end or in 2022. That gives them two years to master refueling for Lunar expeditions in 2024.

I think the real challenge to landing humans in 2025 is the need for prep/test missions.

4) Mars Prep Missions Finding the right landing site isn't a real problem, there are a lot of great candidates. But SpaceX will need to send Robotic Starships to test landing on them, ensure that they aren't littered with boulders, or too uneven. But orbital mechanics (the relative position of Mars and Earth at any one time) mean we can only efficiently travel to Mars every two years. This means SpaceX needs to launch manned missions in late 2024 to get to Mars in 2025, which requires launching robotic missions in mid-late 2022. That seems like a very high order.

But beyond testing landing sites, the SpaceX plan is to land a good number of Cargo Starships packed with supplies and equipment before sending their first astronauts. The Cargo Starships would also require launching in 2022 to support a 2025 human landing, which would require combining test/cargo missions in same launches. That's super risky, if the first sites have bad characteristics and cause many of that first wave fail to land successfully there won't be enough supplies and equipment to safely send humans in 2025.

As for other challenges people may think of, here are the reasons I don't rate them as high a risk.

5. Radiation NASA studies have estimated that a two year round trip to Mars only produces a 4% higher lifetime risk of cancer. As long as Starship has a solar storm shelter for the crew to avoid the most dangerous radiation periods, risk should be minimal.

6. Regolith The risk of Regolith on the moon is that it's razor sharp dust because it's never had any environmental weathering. Thats not true of Mars, it's risk is Perchlorates in the soil and like radiation this is an overdone risk. It's not at a level that's directly poisonous, but if astronauts continually track perchlorate dust into their habitats eventually they'd suffer from thyroid issues. The simple solution is water, which drops the perchlorate into a safe solution. Spray the astronauts with before returning through the airlock, and spray the suits again before storing in sealed closets.

Mars is lousy with water at every latitude. If the tools they are sent with prove inadequate for digging up water or melting ice, they will certainly have tens if not hundreds of tons of distilled water in the Cached Cargo Starships to cover losses outside the life support systems.

7. Crew Training Starships will be highly automated, the main training needs to be on the tools and equipment to be used on Mars. Apollo needed years of training because for the first time ever they were going to land on the surface of another world in a tiny lander with very little margin for error. Every potential risk had to be trained for and have a checklist for. Unlike Apollo, Starship missions will be equipped with a great deal of redundancies, supplies and equipment to deal with unforseen situations.

8. In Situ Fuel Generation Again the cached Cargo Starships will have plenty of equipment and tools for producing methane and LOX for return trips. If they don't work, they are only two years away from another round of Cargo Starships with more supplies and updated tools and equipment designed directly to solve any problems that stymied the first attempts. Ultimately if the problem can't be solved, the solution is sending Starship Tankers to refuel on Mars itself. This plan will only be a problem if it causes SpaceX to struggle to recruit qualified crew for the missions, and I don't anticipate that being true.

I couldn't understand Buzz Aldrin's objections, other than Elon's cost estimates are targeted to decades away when launch cadence is at least hundreds per Synod, and its possible for Crew Starships to hold 100 passengers on Mars expeditions. They are also highly aspirational and not necessary to enable the first manned exploration missions.

Starship launches should cost roughly \$20M each when fully re-usable (based on the partially re-usable Falcon 9 costs), which would mean each Mars sent Starship costs \$160M for launches (assuming 7 tanker launches). A complete Falcon 9 costs less than \$60M to make, the similarly sized Cargo Starships (with 4 fewer engines) should be significantly cheaper to build using Stainless Steel construction. Crew Starships will be more expensive with their sophisticated life support systems, but SpaceX should only be sending a handful per Synod at first.

Conclusion: These first four challenges are solvable problems, the question is whether they can be solved by 2025 and that seems impossible. I expect SpaceX to master landings in 2021, and start testing orbital re-entries by year end or in 2022. That gives them two years to master refueling for Lunar expeditions in 2024. But the first expeditions can't be manned. So the real challenge to landing humans in 2025 is the need for Site Prep/Cargo Cache missions.

The net result is that landing humans in 2025 looks impossible today. I think it's far more feasible to land robotic Starships on Mars to site test in 2025, land cache Cargo Starships in 2027, and then land humans in 2029. And each of these dates to slip two years if anything goes wrong.