It’s a long-term conundrum in interstellar studies: When do you launch a mission, knowing that faster methods may make your spacecraft obsolete? We might think about this problem again in light of Mike Gruntman’s paper on a precursor interstellar mission to the local interstellar medium (LISM). As we saw on Friday, Gruntman (USC) has examined a probe to 400 AU, a region well outside the heliosphere where interstellar space is thought to be unperturbed by the Sun’s influence.
Keeping to technologies that are close to the required readiness level (he considers solar sails and nuclear electric propulsion), Gruntman works out a nominal escape velocity of 75 kilometers per second. To those who argue that a twenty-year mission to the 400 AU target is sure to be superseded by faster spacecraft, the counter-argument is clear: If we wait for a breakthrough, how do we know its timing? What if, Apollo-style, political decisions slow the development of sound alternatives? Incremental missions like these take us the needed next step beyond Voyager and New Horizons to create the first dedicated interstellar spacecraft, from which the learning opportunities will be priceless.
Innovative Interstellar Explorer is the current project incorporating these concepts. Gruntman works on its team, and would be the first to note that the idea of IIE has changed since the 2004 paper we’ve been considering (the IIE site has mission details, including a shorter-range target of 200 AU, and an interesting new take on propulsion). But the instrumentation question is still a lively one. Get a probe into true interstellar space and you want to learn such things as the composition of interstellar matter in gaseous and dust forms, the nature of the interstellar magnetic field, the status of low-energy cosmic rays that cannot reach the inner heliosphere, and the characterization of any organic matter that may exist in this medium.
Image: Concept study for a mission beyond the heliopause. Credit: NASA/Johns Hopkins University Applied Physics Laboratory.
That’s just the beginning, of course, because we also have much work to do while crossing the interface between heliosphere and the LISM, not to mention a whole series of remote observations related to the density of neutral hydrogen in the nearby interstellar environment, and study of the distribution of objects in the Edgeworth/Kuiper belt. What gets tricky here is not just the number of needed instruments but the high speed of the spacecraft. With the probe moving in the ‘upwind’ direction (in relation to the interstellar wind), the relative velocity of interstellar matter with respect to the spacecraft approaches 100 kilometers per second.
New instrumentation concepts are called for. From the paper:
It is not clear, for example, what is the best way of analyzing complex organic molecules in interstellar gas and plasma. The high energy of such molecules, 52 eV/nucleon, would likely destroy their molecular bonds (complicating identiﬁcation) when captured on a surface for a subsequent analysis. On the other hand, the molecule velocity and energy would not be sufficient for analysis in conventional thin foil-based time-of-ﬂight instruments….Interpretation of dust grain measurements and search for traces of organic matter in the grains would also be complicated by an exceptionally high speed of grains with respect to the spacecraft. The grain velocities would be even higher than those at the Comet Halley ﬂybys by the Giotto and Vega spacecraft.
So we need to start thinking in non-traditional ways, trying to tease out basic information about plasma, dust and fields. Dust particles bombarding the spacecraft, for example, can be characterized by measuring the effects of hot plasma produced by the impacts — the whole spacecraft, in other words, becomes the detector. Moreover, the new instruments developed for the mission, heavy on miniaturization and autonomy, will require low energy solid state particle detectors.
But the opportunities are vast, including interesting analysis of physical effects. Gruntman proposes using a precursor interstellar mission as a testbed for the Pioneer effect, the anomalous acceleration of both Pioneer spacecraft whose origin is still unknown. And how about the ‘look back’ effect:
As our ﬁrst interstellar spacecraft leaves the solar system, a ‘‘look back’’ would provide us with an unusual view of our home stellar system, a view from the outside. A view back will provide a unique opportunity for a global study of the heliosphere, a vast essentially 3-D region governed by the sun. This view back would also be a glimpse of what a truly interstellar mission of the distant future would encounter in approaching a target star. A combination of obtaining images from two vantage points, one from the outside of a stellar system and one from inside, would allow the characterization of an astrosphere.
Image: Looking back, a long way from home. Credit: NASA/Johns Hopkins University Applied Physics Laboratory.
Keep your eye on Innovative Interstellar Explorer as this mission concept continues to evolve. We’re asking basic questions about the nature of space outside the heliosphere, what the IIE team calls the ‘undiscovered country’ through which future, much faster missions will one day journey. With a penchant for Latin mottoes, I like IIE’s: “Si requiritis futurum nostrum, spectate astra!” Translation: ‘If you seek our future, look to the stars!’ IIE would be the first mission to do that free of the Sun’s effective influence. Learning how to design it is part of the incremental process of making our way, step by step, toward a true star-faring civilization.