Actually, we are 99%+ certain we can do substantially better than
that.
So much better that the proper measurement is not the fraction of C,
but rather, the fraction of G.
.01 G is probably achievable without propellant or power, at least in
an area from Venus orbit to Saturn orbit. By diving toward the sun
first and then using gravity whips, speeds in excess of .01 C are
probably achievable and we might manage .1 C.
Up to that speed, relativity effects are ignorable for the sort of
back-of-the-envelope calculations worth doing now.
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