https://en.wikipedia.org/wiki/Solar_gravitational_lens
Its quite brilliant... if you can get a detector out there and maneuver it. Especially if you can slow it down once it gets there.
Fission fragment rockets are also an old idea, and a good one: https://www.projectrho.com/public_html/rocket/enginelist2.ph...
> All of the other nuclear thermal rockets generate heat with nuclear fission, then transfer the heat to a working fluid which becomes the reaction mass. The transfer is always going to be plagued by inefficiency, thanks to the second law of thermodynamics. What if you could eliminate the middleman, and use the fission heat directly with no transfer?
> That what the fission fragment rocket does. It uses the hot split atoms as reaction mass. The down side is that due to the low mass flow, the thrust is minuscule. But the up side is that the exhaust velocity is 3% the speed of light! 9,810 kilometers per second, that's like a bat out of hell. With that much exhaust velocity, you could actually have a rocket where less than 50% of the total mass is propellant (i.e., a mass ratio below 2.0).