The Innovative Interstellar Explorer mission discussed recently in these pages has received new support in a study of alternative propulsion concepts. IIE, you may remember, would use radioisotope electric propulsion (REP), tapping xenon as propellant. The mission’s goal is to deliver a scientific payload to 150-200 AU within a 15 to 20 year time frame; the concept thus tracks earlier mission concepts built around solar sails and allows useful comparisons beween the various propulsion methods that have been proposed for such deep space work.

In a paper to be published as a chapter in a book on NASA ‘Vision’ missions this summer, Thomas Zurbuchen (University of Michigan) and a team of researchers discuss the specifics of powering such a probe by nuclear methods and find them wanting. The paper is so rich that I want to discuss several issues from it in coming weeks. For now, though, let’s consider the propulsion dilemma as seen by scientists running the numbers using existing technologies.

A solar sail gets you to the interstellar medium more quickly than the kind of chemical propulsion with gravity assists used by Voyager, but even so the task is daunting, requiring the probe’s escape velocity to be a factor of 3 greater than Voyager 1’s. And existing sail designs deliver speed but at a cost in payload weight.

NASA’s now defunct Prometheus project would have created a spacecraft too heavy and slow for a mission to the nearby interstellar medium — the Zurbuchen paper centers around a nuclear power source in conjunction with electric propulsion fine-tuned for the mission. For a variety of reasons, the best compromise between the various proposals seems to be the radioisotope electric propulsion advocated by Ralph McNutt and the IIE team.

One reason, of course, is the end of Prometheus funding, which affects all thinking on nuclear systems for an interstellar probe (ISP). From the paper:

This chapter details the results of one of two highly complementary technical approaches for an ISP that were funded under NASA’s Vision Mission initiative. The study described here focuses on an ISP that utilizes nuclear reactor technology that is not currently available for use in space. The study explores the utility of nuclear technology, its challenges, and its effects on scientific instruments. When this study was initiated, NASA was aggressively pursuing such technologies, but those efforts have been reined in. The second study, led by McNutt et al., seeks to address this challenge using an entirely different and perhaps more promising approach given the current political and technological conditions. These two reports should be read, in conjunction with a previous study exploring solar sailing technologies discussed later in detail, as attempts of an enthusiastic and unified science community to find a way to make ISP a reality in our lifetimes.

“Leaving the Heliosphere: A Nuclear-Powered Interstellar Probe” points out this key fact: nuclear electric methods deliver substantially more power, but at a considerable cost in technical complexity. The study used an active nuclear reactor as power source and envisions a primary spacecraft with two ‘daughter’ probes. The payload is 1500 kg, accounting for these probes as well as the suite of scientific instruments aboard the mothercraft. Thrusting begins after launch and continues for 7.5 years, after which the spacecraft coasts; a Jupiter flyby allows the team to reduce propellant and time of flight.

Some problems:

None of which is to say that nuclear options are forever discounted in deep space work. But it is to say that the best we can do with existing technologies seems to favor the Innovative Interstellar Explorer’s radioisotope electric propulsion system. If, that is, we are intent on getting a dedicated mission into the interstellar medium, which Centauri Dreams argues is the essential next step to follow up the extraordinary discoveries of the Pioneers and Voyagers that have gone ahead. As Zurbuchen and team sum it up:

The task at hand remains exciting and the motivation of the scientific community has not diminished. We are more committed than ever to determine how we can be involved, during our lifetimes, in one of the most historic missions of exploration that will ever happened: Interstellar Probe. Exploration is not easy—reports of the perils and the challenges faced by those who do it fill libraries. Sometimes the reason for failure to explore can be tracked to human weaknesses in the political arena. But always, humanity overcomes these obstacles, and finally, there is the day when the ship leaves the harbor and moves into a new, unexplored world. Now, the anchor lifts and the eyes are firmly set at the horizon, where surely the most exciting worlds are going to emerge.

The paper is Zurbuchen, Patel and Fisk, et al., “Leaving the Heliosphere: A Nuclear-Powered Interstellar Probe,” to be published this summer under the auspices of the AIAA.