Some thoughts on Mars Direct
Reconsider the direct to Mars approach
For good reasons, the plan argues against the need to construct a ship in space. Building an orbiting construction facility first and then launching each of the parts of the ship separately and assembling them into a functioning whole would be expensive, risky and very time consuming. The International Space Station is slowly being constructed in orbit out of multiple parts and it is still not at the core complete stage five years after it’s first section was launched.
The Apollo missions, on the other hand, took advantage of the direct route. Everything that the mission needed was launched aboard one Saturn 5 rocket that took the astronauts from the Earth to the Moon without stopping at a space station along the way. The Mars Direct plan advocates using a similar method for having a single heavy lift launcher send the crew from the Earth to Mars. I think a compromise exists between these two options that would avoid the time and expense of assembling a craft in orbit but might also reduce the risk inherent in a direct shot to Mars.
The Apollo 13 mission ran into some problems on the way to the Moon but was able to make it back home. If a similar accident occurred on the way to Mars, there may be an option to abort the mission and head back home but the crew might not be able to survive long enough in a damaged vehicle. The risk of this happening might be reduced if the vehicle was parked in orbit for long enough to perform a complete set of tests before heading on to Mars. Everything will certainly be thoroughly tested on the ground, but launching will put enormous stress on the vehicle and it seems only prudent to check things out before heading on a journey of over a hundred million miles. The crucial issue for this is if on-orbit diagnostics would reduce risk more than extra time in low Earth orbit would increase the risk of damage from space junk.
Another possible advantage of this parked approach would allow the crew to take along more supplies and equipment than could be launched in a single shot. Once the vehicle is in orbit and while tests are being done, one or two supply ships could be launched that could each bring several tons of additional material. Russia’s existing Progress vehicle can be used for this and so can two vehicles currently under development: the European Space Agency’s Automated Transfer Vehicle (ATV) and Japan’s H-2 Transfer Vehicle (HTV). This would give the advantage of increasing the amount of mass that can be sent to Mars without needing to assemble anything in orbit. Automated docking and the transfer of supplies is a regular part of the ISS program and the same methods could be used for this mission. If the US were to conduct a manned mission on their own this would not be an option because there is no similar cargo vehicle available or in development.
Reconsider the artificial gravity system
To combat the effects of weightlessness during the voyage, the Mars Direct plan proposes to create a ship that rotates to create artificial gravity for the crew. This system would be created by tethering the ship to the spent upper stage and then spinning these two parts around their center of gravity. By doing this it is possible to simulate Martian gravity for the crew on the way to Mars and back. Minimizing the amount of weightlessness the crew needs to experience would certainly help them adapt to the situation more easily when they reach their destination.
There is little doubt that a system like this could be built, but is it worth adding a brand new and untested system to the mission to offset the effects of weightlessness? Both the trip to Mars and the trip back take 180 days with the orbits used in the Mars Direct plan. This is almost the same time each expedition crew spends on board the ISS. Station crews are able to readapt to Earth’s gravity after their stay in space, so after a trip of a similar duration it can be assumed the crew will be able to adapt to gravity on Mars.
The International Space Station was set up in part to study the effects of long duration space voyages. Because of the experience gained on the ISS and previous space stations, the risk of 6 months of weightlessness is well known to us. The risks involved in relying on a never before used artificial gravity system are unknown and are potentially much greater. Although it might not be ideal to abandon the idea of artificial gravity for the voyage, I think it’s essential to bring the level of risk within reasonable limits. Once we are ready to start taking trips beyond Mars then there will be a definite need for artificial gravity systems.
On to Mars
The Mars Direct plan was born out of the desire to continue our exploration of the solar system and the frustration of our country’s post-Apollo lack of direction. I can certainly understand these emotions. I was born several months after Apollo 17, the last manned mission to the Moon, returned to Earth. The Moon missions are history for me and I hope that at some point in my lifetime I will witness our next step beyond Earth orbit. Although I have a few small suggestions for how the plan might be updated, Mars Direct is the most likely way that we will be able to send humanity from one world to another.