Feasibility

Do we know how to do this?

Whether the mechanism exists today, needs to be invented, or depends on unsolved research.

The scale

1. 1Depends on unsolved research
2. 2Plausible, unproven
3. 3Demonstrated at small scale
4. 4Proven elsewhere, needs porting
5. 5Off the shelf today

Five is the good end on every criterion, so no row reads backwards.