Without humans, 20% is achievable in the very near future. Payloads would be measured in grams, though. That's what Breakthrough Starshot [1] is all about. Giv

Without humans, 20% is achievable in the very near future. Payloads would be measured in grams, though. That's what Breakthrough Starshot [1] is all about. Given suitable infrastructure (e.g. lasers stationed across the solar system), even 1/3 c is possible for machines. Crewed missions would be seriously limited given near future technologies - the craft would simply be too heavy for using solar sails and even nuclear thermal propulsion only gets to a tiny fraction of c. Also keep in mind that you need to slow down at the target, too, if your mission is crewed as fly-bys like Breakthrough Starshot aren't desiderable for crewed missions. The cold hard truth is that your ΔV = 9.8 * ISP * ln(MR), where MR is the mass-ratio and MR = (Mass[Propellant] + Mass[Spacecraft]) / Mass[Spacecraft]. ISP and MR for known near-term propulsion systems are as follows: • Chemical rockets: ISP <500s MR ~10 => ΔV < 11 km/s • Ion engines: ISP <5000s MR ~2 => ΔV < 34km/s • Nuclear Thermal rockets: ISP <15000s MR ~2 => ΔV < 102 km/s • Multi-stage ion engines: ISP <25000s MR ~2 => ΔV < 170 km/s So basically the fasted currently known option that doesn't straight up use nukes + pusher plate can get you to 0.5% c give or take. [1] https://en.wikipedia.org/wiki/Breakthrough_Starshot https://en.wikipedia.org/wiki/Breakthrough_Starshot