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https://forum.nasaspaceflight.com/in...?topic=38524.0

It's an often discussed topic. Essentially you need nuclear powered
rocket engines (fusion would be best, but fission would work too) so you
can go out to the Ort Cloud, find suitable objects, then use some of
their mass as reaction mass for the drive to get it to Mars. We're
likely talking many thousands of comet like objects. The advantage of
getting them from the Ort Cloud is that their volatiles haven't been
boiling off after many passes around the sun (like an actual comet).

I suspect you mean the Kuiper belt or the scattered disc. It would be
difficult to get an object from the Oort cloud to hit Mars in less than
20,000 years using technologies resembling what is known today. Of
course, who knows what technologies will be available in 1000 years.
Kuiper belt and scattered disc objects could be suitable for this
purpose. But even with closer Kuiper belt and scattered disc objects, I
consider that to be a long term project.

You're right. I got the two mixed up. Kuiper belt is closer in and
according to Wikipedia it should contain an estimated "100,000 KBOs over
100 km (62 mi) in diameter". You start with some small ones (easier to
move in a "reasonable" timescale) then work your way up in size as the
reliability and size goes up on the tugs.

The nice thing is that the tugs would all get their reaction mass from
the Kuiper belt objects they harvest, so they're all self refueling.
They just keep bringing objects to Mars until something fatal breaks
down. Ideally, you'd make these things really, really big with their
own mineral and metal processing facilities and machine shops with all
the systems ideally being automated. The tugs would ideally manufacture
more engines, tanks, structure, and etc. so over time they could handle
bigger and bigger KBOs.

And yes, this is a very long term project. Likely many centuries at a
minimum.

Jeff
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