In 1985 a fascinating interstellar probe concept was published by Graeme Aston (of the JPL) in the Journal of the British Interplanetary Society , worth comparing to the SGL Mission I covered previously. Using a Rotating Bed fission reactor with a Brayton Cycle thermodynamic converter, and a Moving Belt Radiator to dumping waste heat, the overall system efficiency of 40% produces 1 GW of electrical power (2.5 GW Thermal power) to power a megavolt ion drive system with an effective exhaust velocity of 4,000 km/s.
|Specific impulse (sec)||400,000|
|Engine input power (MW)||1,000|
|Ion beam current (A)||60.1|
|Ion beam energy (MeV)||16.7|
|Initial vehicle mass (tonne)||427|
|Burn time (yr)||64.9|
|Terminal velocity (km/sec)||3,660|
|Terminal velocity (c)||0.0122|
Propellant fraction was 0.6, including 69 tonnes of Uranium 233 fuel, which was vapourised and added to the reaction mass as depleted. Payload, as well as Command/Control, Communications and Navigation masses five tonnes. The Rotating Bed Reactor massed 9.2 tonnes, while the rest of the systems were assumed to scale according to the power and propulsion required. Mercury was the assumed propellant – it and Caesium were the ion drive propellants of choice before the modern obsession with Xenon. Of course an updated version might choose Iodine, since Xenon has supply limitations and Mercury/Caesium are toxic heavy metals.
The continuous low acceleration burn covers impressive distances in a reasonable time. At burn-out the probe attains a third of a light year, but would fly-by Pluto in 3.3 years, the Heliopause in 5 and 1,000 AU in 15. It would fly-by Proxima Centauri at ~4.3 light-years in 389 years, so it is a long duration mission quite unlike anything that the JPL or NASA could hope to seriously fund. Yet the mission while still under power for the first 65 years would achieve multiple science goals as an Interstellar Precursor.
To make it an interstellar rendezvous mission would be considerably more ambitious, as the survival of the probe’s systems after centuries of cruising is a bigger problem. In theory a staged system would be capable of the Power-to-Mass ratio required, using a fresh fission reactor for power for the deceleration stage. Alternatively a light-sail, using ultra-light materials, should be able to brake from 0.0122 c to orbital velocity. What payload would make a four century mission worthwhile?
 G. Aston, Electric propulsion: A far reaching technology, JBIS, 39, 503 (1986).
 A. R. Martin, A. Bond and R. A. Bond. “ULTIMATE PERFORMANCE LIMITS AND MISSION CAPABILITIES OF ADVANCED ION THRUSTERS”, 88-084. https://electricrocket.org/IEPC/IEPC1988-084.pdf