The engineering requirements for all interstellar precursor missions studied can be summed up as "reach the interstellar medium as rapidly as possible." Reaching 200 AU in 15 years is an average speed of ~13 AU/yr or 63 km/s, just more than twice Earth’s orbital speed. Such large velocities cannot be practically achieved with any chemical rocket or even nuclear thermal propulsion (NTP). Previous studies have focused on ballistic missions using powered near-Sun gravity assists, nuclear electric propulsion (NEP), and solar sail propulsion. The first approach (ballistic) has significant thermal issues and still requires a non-conventional propulsion system near the Sun to reach the desired speeds. As an example to reach a solar system escape speed of 15 AU/yr with a maneuver applied at 4 solar radii (measured from the Sun’s center), the implied delta V is ~8.2 km/s. The second approach (NEP) provides inherently large systems that require power system mass-to-power systems of less than ~30 kg/kW to have the required capability. The third approach (solar sails) requires large (~400-m dia) high-performance (~1g/m²) sails and small (~250 kg) spacecraft and science payloads. This is the first detailed study of radioisotope electric propulsion (REP) for such a mission.

For the top-level requirements for the mission we adopt the following:

• Asymptotic trajectory within a 20° cone of the "heliospheric nose" (+7°, 252° Earth ecliptic coordinates)
• Provide data from 10 AU to 200 AU
• Arrive at 200 AU "as fast as possible"
• Consider all possible missions that launch between 2010 and 2050
• Use existing launch hardware
• No "in-space" assembly
• Launch to escape velocity
• Keep new hardware and technology to a minimum
• Provide accepted "adequate" margins

These requirements leave open the transit time to 200 AU as an optimization variable while minimizing new required technology and infrastructure.