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Interstellar Instrumentation and Its Uses

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 To those who argue that a twenty-year mission to the 400 AU target is sure to be superseded by faster spacecraft, the counter-argument is clear: If we wait for a breakthrough, how do we know its timing?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.

Basic spacecraft design

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.

Artist's concept of IIE

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.

Comments on this entry are closed.

  • david January 14, 2008, 8:41

    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?

    Suppose the probe does takes 20 years to get to the region of space we want to study. If we were to launch another probe 10 years later it would have to travel twice as fast as the earlier probe in order to get there at the same time. I’m not so sure we are likely to be able to maintain such a rate of increase.

    For true interstellar journeys that might take 100 years it would be an issue, but not for journeys of 20 years.

    Even if such a breakthrough did occur then nothing would be lost. We let the earlier probe continue on its way and build the new probe to study another region of space we are interested in. Surely sampling the interstellar medium in more than one place would be of benefit if only for the sake of comparison.

    The voyager crafts have yet to be superseded in their explorations and its been how long since they were launched?

  • johnF January 14, 2008, 10:53

    Or to put it another way; A bird in the hand is worth two in the bush!

  • Starfleet commander January 14, 2008, 11:15

    Re: Measuring devices and instruments:

    I would have thought crafts built for really high speeds need to be very compact without too many exposed appendages. But perhaps one way of testing interstellar dust is with some sort of small impact platform made of super strong materials, which has little holes in it so dust or gas that gets whipped against it can be analysed from some sort of internal and safely concealed apparatus. So what you are measuring are the tiny impacts, and the spectra of their residue left on the sieve like screen.

    This same screen could collects samples of the vaccum gases and more subtle measurements. Only thing is it would be a very dirty screen after a while and would be hard seperating new molecular or chemical readings from old material. Or maybe the little screen pops out takes a sample and slides back into probe for testing. Then when its done it chemically? sterilises the screen and out it goes for the next sample etc…

  • dad2059 January 14, 2008, 11:59

    If things keep going the way they are, I don’t think we’ll make the first launch window.

    But if we’re able to, I’ll take the ion drive and Jupiter gravity assist. Constant acceleration is the key here.

    Some interstellar flight is better than none at all.

  • Hans Bausewein January 14, 2008, 13:37

    ‘‘look back’’: would we be able to find a habitable planet from that distance?

  • Administrator January 14, 2008, 16:36

    Starfleet, you can only get so economical in terms of appendages for a mission of this kind. To handle the needed measurements of the magnetic field in the LISM (expected to be extremely low), you have to have sensors on booms deployed more than 30 meters. Gruntman also points out that measuring plasma and radio waves requires a dipole antenna exceeding 100 meters from tip to tip.

  • Administrator January 14, 2008, 16:38

    Interesting question, Hans, re whether with current or near-term technologies (which is to say, the technologies assumed for the IIE mission) we could detect a habitable world from 400 AU. That look back at Earth could be helpful in making that determination, another good reason for IIE to be built.

  • Administrator January 14, 2008, 16:39

    I’m with David on this. The Voyagers have been out there since 1977 and are still doing cutting edge science. I’m also quite intrigued by what IIE might be able to tell us about the density of the Edgeworth/Kuiper belt.

  • occam's comic January 14, 2008, 18:40

    How about using a magnetic sail to provide propulsion on the way out of the solar system and then as a detector in interstellar space. You could detect magnetic fields and funnel plasma to detectors.

  • Administrator January 15, 2008, 14:46

    occam, I’m keenly interested in the magsail concept, the only drawback in this case being that it’s nowhere near the readiness level that would make it feasible for the IIE mission. But yes, go beyond that and you’re looking at a propulsion system with, in my opinion, real possibilities for getting around the Solar System much faster than before. And with the particle beam option, you may even be talking interstellar missions.

  • Starfleet commander January 16, 2008, 6:45


    Okay thanks for the answer. Ya i guess appendages are necessary to some extent :-)

  • ljk February 27, 2008, 14:34

    Interstellar Dust Inside and Outside the Heliosphere

    Authors: Harald Krueger, Eberhard Gruen

    (Submitted on 26 Feb 2008)

    Abstract: In the early 1990s, after its Jupiter flyby, the Ulysses spacecraft identified interstellar dust in the solar system. Since then the in-situ dust detector on board Ulysses continuously monitored interstellar grains with masses up to 10e-13 kg, penetrating deep into the solar system. While Ulysses measured the interstellar dust stream at high ecliptic latitudes between 3 and 5 AU, interstellar impactors were also measured with the in-situ dust detectors on board Cassini, Galileo and Helios, covering a heliocentric distance range between 0.3 and 3 AU in the ecliptic plane.

    The interstellar dust stream in the inner solar system is altered by the solar radiation pressure force, gravitational focussing and interaction of charged grains with the time varying interplanetary magnetic field. The grains act as tracers of the physical conditions in the local interstellar cloud (LIC). Our in-situ measurements imply the existence of a population of ‘big’ interstellar grains (up to 10e-13 kg) and a gas-to-dust-mass ratio in the LIC which is a factor of greater than 2 larger than the one derived from astronomical observations, indicating a concentration of interstellar dust in the very local interstellar medium.

    Until 2004, the interstellar dust flow direction measured by Ulysses was close to the mean apex of the Sun’s motion through the LIC, while in 2005, the data showed a 30 deg shift, the reason of which is presently unknown. We review the results from spacecraft-based in-situ interstellar dust measurements in the solar system and their implications for the physical and chemical state of the LIC.

    Comments: 10 pages, 2 b/w figures, 1 colour figure; submitted to Space Science Reviews

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0802.3787v1 [astro-ph]

    Submission history

    From: Harald Krueger [view email]

    [v1] Tue, 26 Feb 2008 11:00:16 GMT (225kb,D)


  • ljk March 20, 2008, 13:12

    Quantum Physics Exploring Gravity in the Outer Solar System: The Sagas Project

    Authors: P. Wolf, Ch. J. Bordé, A. Clairon, L. Duchayne, A. Landragin, P. Lemonde, G. Santarelli, W. Ertmer, E. Rasel, F.S. Cataliotti, M. Inguscio, G.M. Tino, P. Gill, H. Klein, S. Reynaud, C. Salomon, E. Peik, O. Bertolami, P. Gil, J. Páramos, C. Jentsch, U. Johann, A. Rathke, P. Bouyer, L. Cacciapuoti, D. Izzo, P. De Natale, B. Christophe, P. Touboul, S.G. Turyshev, J.D. Anderson, M.E. Tobar, F. Schmidt-Kaler, J. Vigué, A. Madej, L. Marmet, M-C. Angonin, P. Delva, P. Tourrenc, G. Metris, H. Müller, R. Walsworth, Z.H. Lu, L. Wang, K. Bongs, A. Toncelli, M. Tonelli, H. Dittus, C. Lämmerzahl, G. Galzerano, P. Laporta, J. Laskar, A. Fienga, F. Roques, K. Sengstock

    (Submitted on 2 Nov 2007 (v1), last revised 19 Mar 2008 (this version, v3))

    Abstract: We summarise the scientific and technological aspects of the SAGAS (Search for Anomalous Gravitation using Atomic Sensors) project, submitted to ESA in June 2007 in response to the Cosmic Vision 2015-2025 call for proposals. The proposed mission aims at flying highly sensitive atomic sensors (optical clock, cold atom accelerometer, optical link) on a Solar System escape trajectory in the 2020 to 2030 time-frame.

    SAGAS has numerous science objectives in fundamental physics and Solar System science, for example numerous tests of general relativity and the exploration of the Kuiper belt. The combination of highly sensitive atomic sensors and of the laser link well adapted for large distances will allow measurements with unprecedented accuracy and on scales never reached before. We present the proposed mission in some detail, with particular emphasis on the science goals and associated measurements.

    Comments: 39 pages. Submitted in abridged version to Experimental Astronomy

    Subjects: General Relativity and Quantum Cosmology (gr-qc); Astrophysics (astro-ph); Quantum Physics (quant-ph)

    Cite as: arXiv:0711.0304v3 [gr-qc]

    Submission history

    From: Peter Wolf [view email]

    [v1] Fri, 2 Nov 2007 13:23:10 GMT (657kb)

    [v2] Fri, 30 Nov 2007 18:23:35 GMT (791kb)

    [v3] Wed, 19 Mar 2008 09:14:39 GMT (792kb)