A study of the trip to Mars shows that even with a nuclear thermal engine, you still need a lot of fuel to get to Mars and back. How can you lessen that requirement? A thought occurred to me to go to Ceres for water, then send the hydrogen on to Mars. Instead of a manned mission to Ceres, use a robotic mission instead. It will go there, grab a lot of hydrogen, then run to Mars and deliver it for the crewed expedition.
The hydrogen will be used for the trip down. Presumably, a way could be found to get hydrogen back up from Mars so that you won't need return supplies from Earth. A plan like this will minimize the amount of mass needed to send a crew to Mars. There's plenty of water on Mars, so you need to land near a water source.
How to get the water from Ceres? The orbital velocity of Ceres is about the speed of an airliner. Now, if preliminary reports are correct, Ceres has water vapor in its atmosphere, near the North Pole. Set up a polar orbit, and then collect the water vapor over a period of time. Crack the water vapor into hydrogen and oxygen and release the oxygen. Fill up your hydrogen tank, and then go on to Mars.
It would require about 5k of delta-v for the trip to Mars from Ceres. From Earth to Ceres? Over 17k to land! Landing doesn't need that much delta-v, but we save a little by not landing. But at least the trip over will be light. The trip from Mars will be heavy with hydrogen.
To get to Ceres from Earth does not necessarily mean lifting all the fuel off the Earth. You can lift enough to get to L2 where the asteroid is going to be processed for water. The hydrogen from the water there will power you on to Ceres.
Why not go to Mars directly? There's just so much water in that asteroid. You won't have enough for a crewed mission. You may have enough for an unmanned mission to Ceres and maybe some manned missions to the surface of the moon. You could possibly set up a refueling station at L2 and use Ceres as another refueling station for Mars. You won't be called upon to get all your fuel from the moon or the asteroid at L2.
This wouldn't be an optimal mission perhaps, but it may work. The attraction is an unlimited amount of hydrogen for any number of trips to Mars. Sending an empty tank back to Ceres for more refills takes less delta-v than sending another tank from Earth to Mars or to Ceres for that matter.
Update:
NASA is sending a spacecraft there to study Ceres. Perhaps it will find some water vapor in its atmosphere that could support such an idea. As for the asteroid at L2, a mission is planned sometime in the future for an asteroid to be brought back to L2.
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