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Update on Innovative Interstellar Explorer

by Ralph McNutt

Because of the interest that the Innovative Interstellar Explorer mission generates whenever I write about it, I was pleased to receive Ralph McNutt’s latest update on IIE. This was written in response to a recent article in these pages on the Voyager missions and refers to several of the comments in that thread. I first talked to Dr. McNutt about interstellar precursors back in 2003, when researching my Centauri Dreams book. Now at Johns Hopkins University Applied Physics Laboratory, the physicist’s space experience is comprehensive. He is Project Scientist and a Co-Investigator on NASA’s MESSENGER mission to Mercury, Co-Investigator on NASA’s Solar Probe Plus mission to the solar corona, Principal Investigator on the PEPSSI investigation on the New Horizons mission to Pluto, a Co-Investigator for the Voyager PLS and LECP instruments, and a Member of the Ion Neutral Mass Spectrometer Team on the Cassini Orbiter spacecraft. He has published over 150 science and engineering papers and over 250 scientific and engineering abstracts. I’m also pleased to say that he is an active consultant on the Project Icarus interstellar design in addition to continuing the push for an interstellar precursor mission.

Nice article on Voyager on 15 December. With the questions posted about a followup mission, I thought that an update might prove helpful. We are, of course, not currently funded for any work, so this effort remains a loose consortium of interested scientists, engineers, and others. We did present a poster at the recent American Geophysical Union meeting in San Francisco (attached) to update the summary of where all of this is. The current real question is what will be said about the idea of an interstellar probe in the Heliophysics Decadal Survey which should be out in the next few months. NASA science missions have always required prioritization and the current economic climate has made that an even more pressing issue. The fact that restarting Pu-238 production in the U.S. – required for any real mission past Jupiter, to Europa, and for some closer to home as well – has been so difficult is a reflection of the those economic challenges.

One of the recent things we did was to take a quick look at what Falcon Heavy might enable (see poster). SpaceX continues to work hard to bring that – and other products – to market, so the performance is yet to be known in the same detail as that of a Delta IVH. Our look suggests that the advantage may be in the the cost but not the performance. At this time the Delta IVH looks more capable for high-energy Earth escape trajectories, but that could change. The two are certainly in the same class of performance.

To set the record straight, as with Ulysses, we would not have a camera for imaging Jupiter. The Juno and follow-on missions to the Jupiter system would be better for that task. What will be the significant issue with Interstellar probe will be asymptotic speed away from the gravitational pull from the Sun. The key to that performance will be a mass-optimized payload for the primary science of the mission along with a performance optimized propulsion system. Energetic neutral atom imager(s) to look at the interaction of the solar wind and interplanetary medium as well as IR imager(s) to look at the dust environment may be in the cards, but visible imagers will not be – the mass is simply not there.

You are correct that the 2014 window will not be used. That is also related to the prime propulsion question. What is still “on the table” is some combination of large launch vehicle, plus a Jupiter gravity assist, plus (hopefully!) radioisotope propulsion (REP) for 10 to 15 years or a solar sail. Both approaches need additional real engineering study (i.e. study $$s) to pin down better. On paper, a solar sail looks like a potentially better approach: small launch vehicle, high speed out of the solar system, and no worry about (because no advantage from) a Jupiter flyby.

All of that said, I am not at the moment optimistic about using a sail for implementing an interstellar probe. While IKAROS and NanoSail-D have demonstrated sail deployment – a significant technical step – they are a very long way from the characteristics of a sail required for an interstellar probe. The basic requirements can be found here, as those have not changed. A good comparison is Pioneer 10/11 (Pioneer 11 had an additional magnetometer): 258 kg mass (including the instrument payload), 165 W of electrical power (nominal), and a 2.74-m diameter high gain antenna (cf. http://nssdc.gsfc.nasa.gov/nmc/spacecraftDisplay.do?id=1972-012A). Even if payload electronics can be miniaturized to “zero mass” (which they can’t) spacecraft sensor-and-subsystem mass can only be taken so low due to real physical constraints.

So one is likely looking at a 250 kg spacecraft sans sail, a ~400m diameter sail, and still ~1 g/m^2. But to get the required performance, the system will have to tack its way in to ~0.25 AU and then reflect the full solar pressure of the Sun at that distance. The problem is that – as with a probe applying a large delta-V at a close perihelion pass (cf. http://interstellarexplorer.jhuapl.edu/mission/niac_study.html although the link to the NIAC studies is now gone) – there is no way of actually testing this critical part of the mission without actually doing the mission. That is a real problem, which is not going to go away.

Image: An early Innovative Interstellar Explorer design concept. Credit: Ralph McNutt/APL).

The Voyagers will be going “off line” ~2025 as the Pu-238 that powers the subsystems continues to decay. Even for Voyager 1 – the fastest and furthest spacecraft from the Sun (New Horizons will be slower as it continues to climb out of the Sun’s gravity well) – 200 AU will not be reached. It is also worth recalling the cancellation of the initial “Grand Tour” mission due to cost driven by the mission requirement of a 12-year lifetime, replaced by the Voyagers with a 5-year lifetime (and they were almost turned off in 1983-4 because that requirement had been met!).

So 200 AU may be “pretty uninspiring” – but it is still really difficult. Hence, many of us believe that (1) the next step past Voyager needs to be taken and that scientific case can be made, (2) speed is important, and (3) one has to be realistic about what can – and cannot – be accomplished with that next step. A large launch vehicle, upper stage, Jupiter gravity assist, and REP continues – at least to me – to look like the current best bet, but I am always open to practical suggestions. To get to the “interesting” region of the sky as seen by Cassini MIMI and IBEX instruments in the last couple of years, the next window for a Jupiter gravity assist opens in ~2024 – and that could be done.

I hope that this helps to clarify where we are and some of the current thinking. No one ever said any of this is easy.

For further information, see McNutt et al., “Enabling interstellar probe,” Acta Astronautica 68 (2011) 790–801 and McNutt et al., “Interstellar Probe: Impact of the Voyager and IBEX results on science and strategy,” Acta Astronautica 69 (2011) 767–776. Dr. McNutt also sends a link to this video showing the original interstellar precursor concept as developed in studies for NASA’s Institute for Advanced Concepts.


Comments on this entry are closed.

  • Scott G December 20, 2011, 15:19

    I’m all for a pure physics-based or technology testbed mission, but without putting a camera on the thing, it’s going to be rather difficult to garner substantial public interest (and therefore dollars) for the IIE. Personally, I’d rather see a flyby of Sedna (similar to New Horizon’s planned flyby of Pluto) as part of an interstellar precursor mission. Besides, with an Aphelion of 937 AU, Sedna has already been further than any near-term IIE mission will ever be able to go. It may even be a member of the inner Oort cloud or a captured object from a passing star – which makes it an even more interesting destination.

  • Istvan December 20, 2011, 19:34

    @ Scott G – I can’t argue with the extreme value of a camera for outreach, but fundamentally, an interstellar precursor is not going to have anything much to photograph. The real science value on such a mission is from magnetic field and ion sensors for examination of the heliopause and interstellar medium. As McNutt points out, anything else is unwanted mass.

    If we want an outreach-friendly mission, then New Horizons itself, or your Sedna-flyby, is much better. However, their trajectories are probably not compatible with investigating the region of interest in the heliopause already identified by other missions… and the extra mass required in a Kuiper object mission profile, again, is unwanted in something we want to cost (and speed) optimize for an interstellar precursor.

    I guess that’s the core issue: no one wants to (or can) drop the funding needed to do a “cool” flagship-style interstellar precursor that will also fly by something interesting on the way out. Too slow, too heavy, too expensive.

    It’s a disappointment. I myself would like to see a serious reactor powered, ion-engine propelled testbed thrown as hard as we can, with a kick from the Sun and/or Jupiter on the way out, with the goal being to test propulsion technologies, encounter something large in the Kuiper Belt, cross the heliopause, characterize the interstellar medium, and lastly identify an Inner Oort object and alter course to encounter it as well. That’d be great, right? Anyone familiar with mission constraint management is doubtless convulsed with hilarity about now. You don’t combine tech testing, lofty goals, and long mission lifetimes in a single very costly and high-risk package.

    At least. NASA doesn’t.

  • Eniac December 20, 2011, 23:24

    My cellphone has a camera in it the mass of which has to be measured in milligrams. Sure, not an “imager” of any quality, but surely a place could be found for it somewhere without altering mission parameters too much, and we could see pictures of objects we fly by, and the Earth and sun looking back.

  • Nick December 21, 2011, 2:19

    Ralph McNutt: “spacecraft sensor-and-subsystem mass can only be taken so low due to real physical constraints. ”

    What are these? Why can’t the minimum spacecraft mass go far below 250 kg? It is now becoming common to launch satellites that mass 1 kg or less. A 1 kg tablet computer contains several radio transmitters and receivers and a wide variety of sensors (accelerometers, camera(s), etc.) on top of electronics that required a large room 50 years ago.

    Sure, the space environment is harsher, but spacecraft component minitiarization in geosynchronous orbit also continues apace. Perhaps ahead of the eye-catching propulsion technologies, the advances we need the most for in situ interstellar science involve “porting” our increasingly advanced electronics and communications devices to work in interplanetary and interstellar environments.

  • William December 21, 2011, 12:42

    This “Innovative Interstellar Probe” would reach about 1/1400 of the way to Alpha Centauri, if it was going to be pointing in that direction. It should have been called the Innovative Interstellar-Medium Probe instead, or more honestly still, the Innovative Outer-Solar-System Probe.

    Anyway, if I were in charge of NASA I would not have any more missions to the outer solar system, instead: 1) Concentrate on developing the VASIMIR to make it flexible — and relatively cheap — which can then be used for anything from boosting the orbit of a space station (being developed for that right now for the ISS) to a manned mission to Mars to a probe to the outer system. 2) Develop large, but highly lightweight solar photovoltaic panels for deployment in space. This would avoid the potential bogeyman of using nuclear power. These too would have versatile uses in space, for everything from space stations and lunar outposts, to (in the context of the article) powering the propulsion system for a probe to the outer solar system. (Granted, a probe to the outer solar system would require these to be *really* large and *really* lightweight for a given area.)

  • Istvan December 21, 2011, 12:46

    @ Nick – For a time in the late 90’s I understand some very limited space missions were designed entirely as technology testbeds. The goal was to move away from NASA’s continuing use of “tested / off the shelf” technology dating from the 60’s and 70’s, and test 80’s and 90’s electronics in space environments. Perhaps someone got tired of the press ragging on the need for an astronaut to flip a cassette so the shuttle computer could read its landing program (I’m pretty sure that was finally changed with the “glass cockpit” refurbishment series).

    I know nothing about the results or actual state of this program, however, which may or may not have been related to the Deep Space 2-4 mission plans which likewise never happened after Deep Space 1’s outstanding success. The program seems to have drifted out of existence during the Bush administration, probably due to funding being absorbed by the Shuttle program or the Mars mission cost overruns that convinced Dr. Alan Stern to resign his NASA position in disgust/outrage.

    This is particularly awful, because it’s absolutely true that our modern handheld technologies are far more miniaturized than what we launch as “space-rated” electronics.

  • ljk December 21, 2011, 13:11

    Often when a relatively slow interstellar mission plan is discussed, someone usually chimes in with “Why don’t we wait until humanity builds a faster starship? The slow vessel will just be overtaken by it anyway.”

    The IIE is a prime example of why we cannot afford to hope and wait for a presumed future humanity that will be overall wiser, more technologically sophisticated than us, and have a strong(er) desire and need to explore the galaxy. Already the main power source for this probe is in danger of become scarce or non-existent, due – ironically enough – to our reduction of nuclear weapons in this post-Cold War era. Add in all the factors of a global economy that has been playing with another Depression since 2008 and a space agency that just keeps escaping budget cuts (for now), and the future of all interstellar missions (plus many much closer to home ones) are in jeopardy.

    If you asked most people during the Apollo 11 mission in July of 1969 if regular flights to the Moon and manned landings on Mars were going to happen by The Year 2000 A.D., they would have responded in the positive. After all, a manned mission to Jupiter aboard a nuclear-powered spaceship with a thinking computer at the helm as depicted in 2001: A Space Odyssey was looked upon as almost a given in that era. Then everything fell apart in just a few short years. No Hilton hotel in space, no Clavius Base on the Moon, and the USS Discovery and HAL 9000 remain fantasies.

    People and governments are thinking about basic survival more than ever these days. A starship of any sort will only become a reality if it is required to save humanity, but by then it will probably be too late. Other than Orion, we do not have any real starship concepts ready to go in the near future, and that has the perennial hassle of a propulsion method that wigs most people out – at least in the West. IIE may not be able to get to Alpha Centauri, but if we do not support it now, the probe mission won’t be going 200 feet, let alone 200 AU.

    The general public and some professionals see the future depicted as Star Trek as a given. However, that world only came into being after humanity went through a devastating nuclear war about fifty years from our time. In short order after World War Three, we developed warp drive and encountered our first ETI (the Vulcans), who just happened to be passing by Earth as our first warp-powered starship was being taken out for its test drive. Anyone else find this just a wee bit implausible and overtly optimistic?

    The way things are going at present, we may not need a nuclear holocaust to bring our civilization and species to its knees or worse. Just let the economy keep stagnating while the politicians and bankers continue their turf battles. At the same time, the public becomes more disenfranchised and paranoid, easy prey to those who are in or who want power.

    Just look at Germany after World War One when their economy was in the toilet and the citizens were suffering from serious self-esteem issues. Oh sure, they had plenty of V-2 rockets in just a few decades, but at a terrible cost and with missions that had goals nowhere near space. In fact, Wernher von Braun was briefly arrested by the Nazis when he suggested that some of the V-2s could be used to explore space instead of exploding bombs on Allied cities.

    Obviously I want a bright, happy future for humanity, especially the kind I was sold on as a kid. But it will not happen if we do not work and fight for it now, as there are plenty of power-hungry sociopaths out there who would love to keep us all down literally and socially, and sadly there are plenty of people who would eagerly let them.

  • Bob Steinke December 21, 2011, 17:31


    If the reason to have an imager is public outreach then I think a non-science-quality cell phone camera is a good idea.


    “Why can’t the minimum spacecraft mass go far below 250 kg?”

    Primarily it’s the big radio dish you need to communicate from 200AU.

  • Tom W December 21, 2011, 19:49

    You need a camera. Can’t see some politician signing off $1 billion for a spacecraft which cant take a photo. Would like to see international collaboration on this project, much like the ISS has helped out all party’s that have been involved. I would include China in that to, by the earliest Jupiter flyby window (2024) they could be on the moon. Would seem silly to cut them out on the old argument that they steal everyone’s tech. The tech needed to do an interstellar mission of sorts has a long way to be developed so its hard to steal something if it doesn’t already exist. I also think there will be a commercial Super Heavy launch in operation by 2024 which could do some damage to the current journey times. Moores law be reaching physical limits so no reason not to have the the latest electronics on board as they will be the best you can do with silicon.

  • Tom W December 21, 2011, 20:08


    Totally agree on Vasimr but you couldn’t run it off solar panels. The aim of the game is to get as far away from the sun as you can can and hence almost negligible solar energy. You could do a fry-by, dive towards the sun (lots of energy for propulsion) and slingshot out but after that you would be stuffed. Think nuclear fission/fusion/hot/cold is the only way you could get a decent amount of energy density to do anything past Jupiter.

  • ljk December 22, 2011, 10:02

    The US robot probes Mariner 2 and Mariner 5 did not have cameras, yet their other instruments revealed the true nature of Venus, whose thick cloud cover had left astronomers making all kinds of wild guesses about that planet for centuries. When Giotto lost its imaging capabilities after being sprayed by particles from Comet Halley in 1986, that did not stop the ESA probe from successfully examining another comet a few years later.

    Unless IIE is going to fly by another world on its way to 200 AU, or unless it is equipped with the equivalent of a decent astronomical telescope, there will be little point in having any imaging system on the probe that I can think of. Except maybe a camera to show Earth and Luna receding into the void as IIE zips away towards its destiny. Then as it passes Pluto, the probe can look back at the Sol system and do its version of the famous Pale Blue Dot image:


    For those who want a big telescope in the outer regions of our Sol system, that task could go to this recently touted concept:


    And whenever IIE is launched, it and all other deep space probes should carry information about itself and who built and launched it for whoever finds them one day. The information package could be a vast technological improvement over the Voyager Interstellar Records and Pioneer Plaques, good as they were, perhaps able to carry just about every bit of information on humanity and our world possible. If our species ever fails, at least there will be a record of who and what we were preserved for billions of years in space.

    At the least, such a gift to the galaxy should be better than the token effort made by New Horizons:


  • Duncan Ivry December 23, 2011, 0:44

    “… whenever IIE is launched, it and all other deep space probes should carry information about itself and who built and launched it … such a gift to the galaxy should be better than …”

    … and much better would be, as far as I’m concerned, to invest the money into something which is useful for *us* and not for some hypothetical alien beings in the far future. Money is a scarce resource. Whereas sentiment is not :-)

  • ljk December 23, 2011, 12:09

    Duncan Ivry said December 23, 2011 at 0:44

    [LJK said] “… whenever IIE is launched, it and all other deep space probes should carry information about itself and who built and launched it … such a gift to the galaxy should be better than …”

    “… and much better would be, as far as I’m concerned, to invest the money into something which is useful for *us* and not for some hypothetical alien beings in the far future. Money is a scarce resource. Whereas sentiment is not :-)”

    LJK replies:

    This information may also serve our spacefaring descendants, who will obviously have a higher chance of finding such probes than any ETI. I am sure they will appreciate the sentiment of having invaluable information about us and our era, probably much of which may not survive if left on Earth.

    Just ask any archaeologist or historian how valuable finding the equivalent of a library from an ancient civilization would be to the understanding of our ancestors. A prime example is the discovery of a library from the Roman Empire at Heraculaneum, which was only preserved (and just barely at that) by the whims of nature:


    As for your quite wrong implication that placing information packages aboard deep space probes has and would be expensive, such efforts cost a pittance and have so far been conducted by others outside the various space agencies at their own time and expense. New Horizons was probably the least expensive of all, largely due to the team members deciding not to focus on such a project and not bothering to ask for outside assitance.

    Perhaps a space probe devoid of any identifying information about itself or its makers will actually speak volumes about at least some members of the human species in this era of self-centeredness and economic mismanagement.

  • Duncan Ivry December 23, 2011, 18:38

    ljk, what you say may all be well and good, but how is sending (a) sentimental messages to (b) hypothetical alien beings (c) in the far future — this is where it started — *useful* for *us*? And let me add: here and now? This is my main point — and, very sorry to say, your answer missed it completely. My comment refered to what you said in your comment: something we should do and which will be a gift. I really would appreciate it very much, if you would come to the point.

  • Nick December 24, 2011, 18:35

    me: “Why can’t the minimum spacecraft mass go far below 250 kg?”

    Bob Steinke: “Primarily it’s the big radio dish you need to communicate from 200AU.”

    Is the main issue capturing some minimum necessary energy, or just the diameter of the dish for resolution purposes? Large diameters can be achieved with interferometry between separated small spacecraft — there’s no need for actual structure/mass to have a large diameter “dish”.

    OTOH if energy is the issue, what about optical communications? If transmitting power by laser across such distances could ever possibly be feasible, surely communicating by laser is far easier than that. If achieving some minimal energy is the main communications barrier, lasers can transmit this energy across such distances many orders of magnitude more efficiently than radio.

    I’m skeptical about the idea that this is some barrier permanently posed by the laws of nature. It sounds much more like a shortcoming of our current space-rated technology, one that we already have most or all of the pieces of technology from other efforts to solve. And solving it would give us far more bang for far fewer bucks than, e.g., a better rocket engine.

  • Eniac December 25, 2011, 1:08

    I would agree with Nick. Laser communications seems like it would allow far smaller deep space probes to be built by eliminating the need for a large antenna.

  • ljk November 16, 2012, 12:04

    To Duncan Ivry – and anyone else with his attitude: That is what continues to get the human race in trouble, asking what good something can do RIGHT NOW FOR US?

    Apparently you are the one who missed my point entirely, and the point of having an information package on all deep space missions. I could go into more detail, but this is the point you asked for.