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A Practical Mission to the Interstellar Medium

The first true interstellar mission may be on the drawing board right now. Yes, Voyager 1 has already crossed the termination shock 94 AU out and is still returning data, but we’ve never had a mission targeted from day one at interstellar space. Yet that region just beyond the influence of the Sun — the Very Local Interstellar Medium — is crucial; it will tell us much about the interface between the solar wind and deep space. Probing it will create new data on everything from gravitational waves to anomalous forces like those that may be acting on the Pioneer spacecraft, not to mention setting the stage for future missions.

Now dubbed the Innovative Interstellar Explorer, the concept is for a robotic mission beyond the heliopause, and as refined through studies led by Ralph McNutt (Johns Hopkins University Applied Physics Lab) for NASA’s Institute for Advanced Concepts, and now through continuing development as a NASA mission study, the IIE would take a 1000-kg payload on the first journey specifically designed to explore interstellar regions.

Mission to the interstellar medium

Image: An earlier concept called Realistic Interstellar Explorer, a key step in refining the design for the current mission concept. Credit: NASA/Johns Hopkins University Applied Physics Laboratory.

The genesis of Innovative Interstellar Explorer can be found in a 1976 conference called ‘Missions Beyond the Solar System,’ held at the Jet Propulsion Laboratory, which discussed the engineering challenges of such a journey. A subsequent JPL project examined nuclear electric propulsion for what was dubbed the Thousand Astronomical Unit (TAU) mission. Later, a precursor to IIE called the Realistic Interstellar Explorer was designed around a powered solar gravity assist and used solar thermal propulsion.

As the mission has evolved, one hope remains: to achieve a flyout time of between 15 and 25 years, representing roughly half the professional lifetime of the scientists and engineers who would build the craft. A set of links on the IIE site provides excellent background. The concept continues to develop in the direction of the practical, creating a mission that can be flown with existing technology.

The problems faced by all who have tackled interstellar mission work are immense: to reach 200 AU in 15 years, for example, requires speeds of 63 kilometers per second, about twice the orbital speed of the Earth (IIE currently targets not 15 years but a more realistic ‘as fast as possible’ travel time). A powered gravity assist closing near to the Sun presents huge thermal issues and still demands an advanced propulsion system. Nuclear electric propulsion places heavy demands on spacecraft power systems, while solar sails require high performance sails in the area of 400 meters in diameter coupled to small spacecraft. And none of these technologies are ready for a mission of this complexity.

To find a practical solution, the team is now examining radioisotope electric propulsion (REP), using xenon as propellant in an engine that would draw electricity from a radioisotope power source. A Jupiter gravity assist would provide an additional boost. Interestingly, the IIE design would use existing launch hardware and involve no in-space assembly. In other words, this is meant to be a practical mission that requires no revolutionary breakthroughs, one that could be built, if funding can be found, and flown by the next launch window.

That window appears on the missions’ Web site, where a countdown clock ticks off the days, hours and seconds to 12 Noon Eastern Daylight time on October 22, 2014. It’s a bold digital statement — given NASA’s funding woes, we’re unlikely to meet such a deadline — but McNutt and team are convinced that the launch day for such a mission will come. Si requiritis futurum nostrum, spectate astra, reads the mission’s motto: ‘If you seek our future, look to the stars.’ That’s a future that will arrive whether IIE lifts off in 2014 or 2050, and it represents a commitment to pushing beyond safe boundaries and taking the first tentative steps toward Alpha Centauri and other nearby stars.

Comments on this entry are closed.

  • Timothy J Mayes March 24, 2007, 15:37

    Radioistope decay powered electric generators ” RTGs ” can deliver 400 watts
    of electric power for each pound of weight of the generator. They convert heat directly into electric energy using the thermionic conversion method.
    A large RTG massing 10,000 kg for example and fueled by 8000 kg PU 238 can deliver 6 megawatts of electric power to an electric propulsion system for up to 100 years 24/7 . If Pu-239 were used as the fuel for an RTG it can deliver electric power to an electric propulsion system for 10s of thousands of years of steady thrusting . It would have to consist of lead boxes with each box containing no more then 10-15 kg of Pu239 per RTG cell however in order to avoid assembling a critical mass of PU 239 in the RTG system.

  • Adam March 24, 2007, 23:42

    Hi Tim

    I just checked and Pu 238 puts out 5.5 MeV per decay, which means with a half-life of 88 years the stuff puts out 400 W/kg of heat. Just how you got the figure of 400 We/lb of RTG I’d like to know. From a reactor maybe, but not an RTG. An RTG uses thermoelectric conversion to turn heat into electrical power, typically with about 5-8% efficiency. Some thermoelectric converters being developed might hit 20-40% efficiency which would be good news, but the power-density of an RTG is still low because of the heat source.

    So do you want to reconsider your figures?

  • Timothy J Mayes June 25, 2007, 20:10

    The figures came from nasa information sources, and Pu 238 is not the only
    fuel rtgs can use.. I gave my RTG fuel figures in kilograms . 1 kilogram
    is 2.2 pounds . So lets do the math to check this . 8000 kg * 2.2 = 17600 pounds . 17600 pounds * 400 = 7,040,000 watts. 7,040,000 * .4 =
    2,816,000 watts of electric power .Other radiaoactive materials may
    produce more watts of heat per pound if theuy are used in rtg .
    Also Pu-238 is a fifferent istotope of plutonium then is plutonium
    239 that has some different nuclear properties from it

  • Adam June 26, 2007, 4:46

    Hi Tim

    You made a boo-boo – I quoted the Watts per kg figure. Thus 8,000 kg produces 3.2 MW of heat. Assume a perfect heat-sink then a standard RTG with 5% conversion gets 160 kWe – but heat sinks are rarely perfect. With a density of 19,500 kg/m^3 8,000 kg Pu238 would be a sphere 0.921 m across with a surface area of 3.4 sq.m. Its surface would radiate at an equilibrium temperature of 2,000 K – just a bit too hot to handle, but assuming we can the maximum Carnot efficiency for a system with radiator panels at 600 K is about (1,400/2,000) = 0.7. Thus 0.4 of 0.7 is 0.28 – you get about 900 kWe at best. A lot less in practice. Plus a big mass of radiators. Liquid drop radiators would be lighter, but I think they need a higher radiator temperature.

    As for other radioisotopes the energies produced are roughly the same, but Pu 238 is the easiest to make.


  • Adam June 26, 2007, 4:58

    BTW you wouldn’t use Pu239 in an RTG as the half-life is 24,110 years, thus its decay heat output is less than 0.4% of Pu 238. You could use it in a reactor with a very high burn-up fraction, though that’d add considerably to the mass due to the reprocessing machinery.

  • Louis Varricchio, M.Sc. January 19, 2010, 12:00

    Whatever became of the 1980s study done by JPL for the proposed TAU (Thousand Astronomical Units) mission? It’s rarely mentioned today; it was an ambitious precursor interstellar mission plan, which also included a Pluto orbiter. Too bad it was shelved. It would have traversed far beyond IIE’s planned “end point”, to 1,000 A.U.s. FYI: There isn’t much public material that I can find online about TAU. Anyone have any details? But IIE is bold and better than nothing. (I trust the IIE mission will provide data to resolve the “Pioneer Anomaly”.)

  • Administrator January 19, 2010, 14:19

    The TAU mission was indeed interesting but did not get far within NASA for a variety of reasons, and you’re right about the lack of public material, but TAU wasn’t around as a concept under discussion long enough to generate a great deal of material. Your comment reminds me that I’ve been planning on an article on TAU for some time now — I’ll put that into the agenda for the near future.