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Manned Missions to the Outer System

Ralph McNutt’s contributions to interstellar mission studies are long-term and ongoing. We’ve looked at the Innovative Interstellar Explorer concept he has been studying at the Applied Physics Laboratory (Johns Hopkins), but IIE itself rose out of earlier design studies for a spacecraft that would penetrate the heliopause to reach true interstellar space. One possibility for that earlier probe was a ‘Sun-diver’ maneuver, a close pass by the Sun to gain a gravitational slingshot effect, followed by an additional kick from an onboard booster.

The thinking a few years back was to reach 1000 AU in less than fifty years, but Innovative Interstellar Explorer has lost the Sun-diver maneuver and focuses on a more realistic 200 AU, as part of a NASA Vision Mission study that contemplates a gravitational assist at Jupiter and the use of radioisotope electric propulsion. IIE is subject to the same funding constraints as any other mission of this nature but it’s well worth perusing its specs on the site, for McNutt is both scientist and visionary, a man who looks beyond the ‘lifetime of a researcher’ limit for mission duration.

That has taken him into interesting intellectual terrain, writing in a study for NASA’s now defunct Institute for Advanced Concepts of a future technology that could reach speeds of 200 AU per year. That’s fast enough to get you to Epsilon Eridani in 3500 years, approximately the lifetime of the Egyptian empire. Writes McNutt:

“A more robust propulsion system that enabled a similar trajectory toward higher declination stars such as Alpha Centauri could make the corresponding shorter crossing in a correspondingly shorter time of ~1400 years, the time that some buildings have been maintained, e.g., Hagia Sophia in Constantinople and the Pantheon in Rome. Though far from ideal, the stars would be within our reach.”

Human Expeditions to the Gas Giants and Beyond

Given these musings, where does McNutt stand on human exploration of the Solar System itself? We learn the answer in an interesting piece that has just appeared in the Johns Hopkins APL Technical Digest where, writing with Jerry Horsewood and Douglas Fiehler, he notes the sharp constraint that radiation exposure places upon mission designers. We know we can reach the outer Solar System — our unmanned probes continue to demonstrate the capability — but humans in deep space have to cope with solar energetic particles from the Sun (SEPs) and galactic cosmic rays (GCRs). That means getting to the destination quickly.

The article looks at optimized trajectories to Callisto, Enceladus, Miranda, Triton and Pluto, five expeditions that each demand one-way flight times of no more than two years, with a total mission time of five years. Solar energetic particles can be shielded against, but running the numbers on galactic cosmic rays shows they would require a huge mass penalty for shielding. To approximate the shielding effect of the Earth’s atmosphere would involve a shield massing thousands of tons. Limiting flight times seems the only solution.

To make this happen, McNutt envisions a nuclear electric propulsion system with an overall power level of 100 MWe, with the electricity generated by the nuclear reactor being used to power up the plasma stream that propels the vehicle. The Neptune mission, targeted for a 2075 launch, would achieve 197.5 kilometers per second with a thrust time of 1.2 years — compare that to the 16.2 kilometers per second New Horizons is currently managing on its trajectory to Pluto/Charon. And the trajectories of these five fast missions are themselves interesting:

The striking point for all of these trajectories, and especially for the three outermost targets, is the lack of curvature. To date, planetary transfer trajectories make use of near-Hohmann-transfer orbits (minimum-energy solutions), albeit sometimes with intermediate planetary gravity assists. Propulsive maneuvers typically are used for gravitational capture at the target, rather than slowing down from faster-than-required transfer orbits. The “straight” trajectories are driven by the requirement of a fixed transit time; without the interplanetary deceleration period before reaching the target planet, the spacecraft in each case would escape from the solar system.

Demands of the Journey

It’s the radiation constraint that pushes our propulsion technologies well past current capabilities, shortening acceptable trip times and demanding speeds that in our current context are almost surreal. Back in 1968, Clarke and Kubrick’s 2001: A Space Odyssey sent the ‘Discovery 1’ mission to Jupiter without evident regard for radiation shielding, and young optimists like me in the audience assumed that the outer planets would be within reach some time in the early 21st Century. Now we’re talking about putting together a set of missions that vaguely resemble Clarke and Kubrick’s a century later than the film had supposed.

Interestingly, by McNutt’s calculations, these expeditions would be mounted in a vehicle offering a habitable volume about twice that of the spaceship in 2001 if we assume a crew of ten (a crew of six is also considered in the paper). And if 2001 didn’t concern itself with enroute radiation, another thing it didn’t dwell on was the method for constructing the interplanetary craft. To build such a vehicle, we’ll need something like the extremely heavy lift launch vehicles (EHLLVs), or ‘Supernovas,’ that were originally studied in the 1960s. McNutt discusses lifting a thousand tons to low-Earth orbit with each launch for assembly of the outer system spacecraft in space. The study envisions 30 Supernova launches for the five expeditions.

Costs of an International Venture

All of this adds up to huge costs, some $4 trillion, which compares to a US GDP of $13 trillion in 2006 and a world GDP in the same year of $48 trillion. The five expeditions to the outer planets would clearly demand an international initiative, one that would cost 1.5 times the U.S. cost of World War II in 2006 dollars. From the study’s summary:

A 5-year round-trip mission will require ~10 t per person of expendable supplies with a likely crew of at least six people and an extremely reliable vehicle with an extremely dedicated and stable crew. Infrastructure capable of putting tens of thousands of metric tons of materials into LEO will be required as well. Such a project is potentially achievable at the cost of at least 10% of the current world GDP. With current investment in human space activity in the United States, even with growth projected on the basis of the growth of the overall U.S. economy, a dedicated, international effort will likely be required if the entire solar system is to have an initial reconnaissance by human crews by the beginning of the 22nd century.

Getting a human presence to the outer planets by the end of the century is going to be tough even if we assume the propulsion advances that can achieve 200 kilometers per second — or in the case of Pluto/Charon, over 300 kilometers per second. But this is exactly the kind of study we need to place our current technology in context. We can’t assume anything about future breakthroughs. We can only define the problems we face so that in that context, future work may produce solutions that can lower travel times and costs to acceptable levels.

The report is McNutt, Horsewood and Fiehler, “Human Missions Throughout the Solar System: Requirements and Implementations,” available online. McNutt’s Phase I and II studies for the NASA Institute for Advanced Concepts are still available on the NIAC site.


Comments on this entry are closed.

  • Kenneth Harmon June 3, 2010, 11:07


    McNutt and company propose the hardest and most expensive solution circa 2075 to what is certainly a very challenging problem. There is a much easier way, live off the land. This would mean 1) Become asteroid experts through manned exploration 2) Invest in various types of advanced propulsion technology on a sustained basis 3) Establish a Moon base as an eventual “Asteroid Processing Area” 4) Start to mine Asteroids that fly near Earth 5) Start to attach new types of propulsion systems to near Earth/Moon Asteroids to learn how to guide and fly them. 6) Capture a few of the Asteroids and place them in easy access orbits near the Moon and away from Earth. Hollow a few of them out, attach propulsion engines and begin to use Asteroids as ships to fly through the Solar System in protected fashion and perhaps with luck even to the nearest star systems such as Alpha Centuari. 7) Generate the wealth required to support all of this activity by developing circa 2075 a full scale “Asteroid Economy” since this is where some serious money can be made from the minerals and rare Gases found in Asteroids.

    The path laid out above seems to be the only cost-effective way to explore our Solar System by 2075, unless of course there is a huge breakthrough in advanced propulsion technology based on currently unknown physics. The real key to Space exploration in the 21st Century are the Asteroids since they are something worth investing in while also learning how to protect the Earth from them. And if we develop advanced propulsion based on new physics by 2075 which I believe we will, then it simply makes the future “Asteroid Economy” that much better and more viable. In fact, it could even mean that a manned mission to Alpha Centauri (if there is anything there) could be launched as early as 2075 in a hollowed out “Asteroid Ship” that would protect the “Human Colony Crew” from virtually everything that Interstellar Space can throw at it out to about a 5 LY radius. Dust, Comic Ray’s, etc, you name it Humans are protected against it if located deep in the Asteroid. The key will be how to accelerate and deaccelerate such a large body, but atleast we would only be down to one major technology miracle, advanced propulsion and perhaps artificial gravity enhancemet to make this work. Furthermore, in the mean time we get the Minerals in the Asteroids.

  • James M. Essig June 3, 2010, 14:41

    Reaching Epsilon Eridani in 3,500 years would seem plausible if some sort of nano-technology based near freezing hibernation is ever developed.

    Also interesting to consider is some form of technology that slows non-relativistic reference frame time down such as the reference frame of home based human civilizations. This would enable the crew and the space craft and home base to perhaps experience space craft travel at speeds effectively greater than C.

    How we would live in a slow temporal dimension is any one’s guess, and perhaps there may exist more than one temporal dimension that we could access wherein said temporal dimension is not of the “Many Worlds Interpretation” of quantum theory, such as in the meaning of parallel histories.

    Nanotechnology will likely lead to interesting ways to do human hibernation states, and so it may enable journeys throughout our galaxy at high Newtonian velocities.

    I believe that we will at some point be zipping about at extreme gamma factors, and with breakthroughs, at superluminal velocities, or perhaps travel of distances mediated through higher spatial, or spatial temporal distances, but I will settle for high Newtonian ship velocities to reach Epsilon Eridani in 3,500 years as a starter even if such ships are passed in route by more capable craft.

  • Adam June 3, 2010, 16:38

    One thing puzzles me with their discussion of magnetic shielding against GCRs. They worry that the mission would fail if something happened to the coils, or what have you, needed by the shield – yet ignore the fact that the plasma drive and nuclear reactors probably both need superconducting coils to operate – and their failure really would be mission critical. What’s the difference? Seems like a reductio argument that wasn’t thought through.

  • Matt Robare June 3, 2010, 22:13

    @Adam I think the difference is that, for the nuclear power at least, the technology is better known and understood while magnetic sheilding is newer. It’s an experience level. Also, if the drive goes there might still be a chance to change course and head back to Earth and the possibility of rescue a la Apollo 13. If the shield goes I suspect you’d be dead even if you did turn around.

    I, for one, hope that as new discoveries and inventions are made, the price will become less prohibitive. Unless a probe discovers an extraterrestrial vehicle or something else utterly unexpected and world-changing I don’t think there’s much of a reason to send a manned massion at such a cost–how many hospitals, well-funded research institutes, schools and clean water supplies could you build for $4 trillion? How many people could you feed for a lifetime? You could probably build several power satelites and take care of a lot of energy problems.

  • David W June 4, 2010, 0:14

    I just read the Phase 1 Final Report for NIAC . It looks like Innovative Interstellar Explorer is an early 21st century trial run for this midcentury proposal . The current plan has a maximum speed of 100kps and the NIAC paper looks like 900.
    Would Icarus be the late century fusion plan for 9000kps?

    Also Isaac Asimov outlined a manned solar system plan aiming for Pluto by ……..2100! It was in the 1966 World Book Yearbook

  • Dreamer June 4, 2010, 1:43


    Investing in, maybe (hopefully Dawn will begin to confirm this next year!), but protection? I suspect we’re already doing too much of a good job with that one right here on Earth. Unless we discover an imminent threat on course for Earth (in months, years, or decades) then I suspect missions to experiment with nudging asteroids around will remain on the back burner. As it is, we’re rapidly eliminating all but the smallest NEOs as threats, and I expect the bulk of our energy will continue to be put into detecting what’s out there rather than active defense.

  • Dreamer June 4, 2010, 1:56

    I get the sinking feeling that it’s going to be a very long time before we’re going anywhere beyond the asteroid belt in person—perhaps even centuries down the road, and we could be talking about 500 years or more before the first interstellar mission is undertaken. Even with the best will in the world, the numbers are extremely daunting, and will be for many, many years to come.

    I think James is right, in that nanotechnology will be key to a lot of what happens, and unlike areas such as propulsion in space, it’s a field that will get heavy investment for earthly matters — namely for medical applications, and eventually for protection against cell damage (like vaccinations on steroids) and, even further down the road, for enhancement, including the extension of the human lifespan. The profit motive is all that’s needed to drive that work forward.

    But again, I don’t think we can underestimate the length of time this will take. I just had my hair cut today, and was chatting to the hairdresser about my receding hairline, and I remember how, 20 years ago I got in a panic because my hair was starting to fall out (I was only 25 so it was a big deal!) and started reading up on all the research going on back then trying to find a real cure for baldness (as opposed to hair transplants and scalp reductions). People were very optimistic about finding a treatment that would work within a few years, but here we are 20 years later, with nothing yet in sight, despite the fact that the first person to develop a cure that was safe and effective would essentially be a billionaire overnight, there is so much pent up demand for it. (BTW: my hair is still hanging on, so much fears that I was going to be entirely bald by the age of 30 were unfounded!)

  • Michael Spencer June 4, 2010, 7:15

    It’s a magical description! And, I admit that I am caught in the magic of 1000 tons lifters and huge accelerations, of a world united behind a great voyage to the unknown.

    On some level, though, there is the sense of simply adding more horses to the wagon in an attempt to race with an automobile.

    That’s my clumsy way of wondering about the current and proposed technology: are we being imaginative enough? Certainly nobody envisions a world that would support these expenses?

    There are huge technology challenges out there, and technology breakthroughs that we simply cannot imagine-yet.

    The magic remains, though.

  • david lewis June 4, 2010, 10:32


    If the aim is to launch those 5 missions by the beginning of the 22nd century (year 2100) then those costs can be applied over a 90 year period – 45 billion a year on average. As a species we’re willing to toss away more than a trillion a year to kill each other but 45 billion a year is too much to explore the solar system?

    No wonder ET doesn’t pay us a visit or call…. ET probably doesn’t recognize us as an intelligence species. I think maybe they are right.

    The one question I would ask is do we get more value if we stick with robot missions for now? For that 4 trillion we could do 4-5 THOUSAND unmanned missions. That is a lot of science.

  • Zachariah McNaughton June 4, 2010, 11:23

    The last paragraph talks about the high cost, namely 4 trillion dollars. However, the figure 4 trillion dollars is nonsense. Money, the idea of money, only makes sense in the context of a society exchanging goods with one another and using a currency to facilitate the trades. However when we are talking about sending manned, or unmanned, missions into the outer solar system, the price would much more accurately be stated in terms of mass, energy, human learning/expertise, and time.

    Money assumes individual, or corporate, perogative. In an international effort to visit other planets however, the notion is moot. It doesn’t apply. Certainly I think the exploration would be an investment, only that money is not the correct unit to measure either the expenditure, or the outcome of such an investment.

  • Bounty June 4, 2010, 13:58

    @ Matt


    Nature has an interesting sense of justice. With well-funded research institutes, hospitals and plenty of water, a lifetime may mean forever. Well at least until the sun boils off the oceans, then you’ll wish we had colonies on Titan. :> Speaking of colonies, how far would 4 trillion go towards a permanent non-earth (Mars, Moon, Asteroid or ship) colony?

  • Ron S June 4, 2010, 14:48

    It isn’t true that if some large dollar amount is spent on space that important earthly priorities are excluded. It’s a matter of priorities, and recognizing that societies can do more than one thing at a time. It is possible to build missions to the stars while also killing large numbers of people and feeding others.

    Regarding horses racing against cars (Michael Spencer’s comment), actually this is perfectly feasible. Over a short course, say no more than 100 meters, a race horse should almost always win. Longer races are a problem since there is a large weight penalty to give a horse team a transmission to achieve higher speeds, and a horse has a lower W/kg output than a motor vehicle.

    (What more can I say except that it’s Friday…)

  • NS June 4, 2010, 15:30

    As I’ve said before, if the currently proposed Innovative Interstellar Explorer mission launches on schedule, I’d be 88 years old by the time it gets out to 200 AU, so I’m hoping a faster mission (e.g. sundiver solar sail) becomes possible in the next 20+ years…

  • David W. June 4, 2010, 17:05

    Am I right is IIEs speed 100kps?
    Sundriver closer to 1000?
    Icarus 10,000 to 30,000?
    Just an opinion but if IIE goes well we could build on it to get to sundrivers
    I would be pleased to see Icarus start construction when I am 100. I like to be optimistic that we can get underway on a real intersteallar craft by the latter half of the century and that I will still be around!

  • T_U_T June 4, 2010, 21:27

    the spaceflight is in decline and you dream about manned missions to outer planets. I am sorry to disappoint you, but highest achievement in the next few decades to centuries would be that people don’t forget that we did fly to the orbit, and don’t start to dismiss the very idea of past, and possible future spaceflight as just another crazy myth.

  • bigdan201 June 4, 2010, 23:12

    As far as radiation shielding, could there be a more efficient solution? Perhaps a material that’s very effective at blocking radiation. Or what if you had suits that would block radiation when the crew was outside their shielded habitation module? Just putting ideas out there.

    Although manned missions are attractive, we can often accomplish nearly as much with robotic probes. The thought of a human planting a flag on Titan is truly great, but realistically, manned missions should be the last step. Robotic missions should try to get as much done as possible before sending humans out, which requires much more resources as well as putting lives at stake.

    I do believe that our destiny is to colonize the solar system and eventually the stars. It has already started with scientific exploration. The next step is economics, which will include mining (asteroids and gas giants) and commerce (space tourism). Eventually, people will colonize space and establish permanent settlements off-earth… which is something that must happen eventually for a number of reasons.

    But it will be eventual. Building up space infrastructure will take a long time – hopefully there will be breakthroughs along the way.

  • James M. Essig June 5, 2010, 13:37

    Hi Dreamer;

    One cool example of suspended animation was the freezing of Han Solo in Carbonite in the Empire Strikes back. It might get to be the case that nanotech hibernation or frozen states of the human body can be just as thermodynamically and statistical mechanically still as Han Solo in the Carbonite.

    Nanotechnology will give us much more control of macroscopic matter systems, as well as at the cellular, sub-cellular and even DNA levels. This could come in handy for manned missions, even long duration manned missions within the outer reaches of our solar system at Keplerian velocities as well as to provide ways to correct biological damage resulting from cosmic radiation.

  • Tibor June 6, 2010, 9:40


    Reminds me to recall ourInterstellar Bet.

  • Duncan Ivry June 6, 2010, 12:17

    The study by McNutt, Horsewood, and Fiehler is a really interesting piece of sound scientific work. I’m especially impressed by the authors taking a lot of relevant and important aspects into consideration, incuding economic ones.

    If I take just the economic requirements of human missions throughout the outer solar system — “literally a monumental undertaking” –, then I think, these requirements will be prohibitive for a very long time. The authors state: “Such a project is potentially achievable at the cost of at least 10% of the current world GDP”; and they continue: “by the beginning of the 22nd century”. Well, I don’t think so.

    For me, it looks like the economic considerations of the study don’t involve the current economic crisis. Because of this “at least 10% of the current world GDP” (gross domestic product, i.e. total market value of all final goods and services produced in a given year) is not valid any more. The USA, the EU, and some space-oriented allies
    – lost between 20% and 30% of it’s wealth,
    – are on an economic path substantially below the one they would be on without the crisis,
    – will, if nothing else happens, be back to normality only after more than one decade (hint: Japan’s 1990s crisis lasted much more than one decade).
    Regarding “if nothing else happens”: politicians, managers, and nature will not hesitate …

    Currently, and for several decades in the future, the cost of human missions throughout the outer solar system will be *much* higher than 10% of the world GDP. As far as I’m concerned, this *is* prohibitive. And, yes, I know, there is uncertainty in statements about the future. But, because so many people, especially taxpayers and their representatives, don’t like uncertainty, what does this imply?

  • AcesHigh June 9, 2010, 23:11

    I think this whole article is very short-sighted, specially when it comes to costs.

    With the advent of fusion power, there are some good solutions ahead that will make space travel much cheaper. Take for example Inertial Electrostatic Fusion Propulsion… we are talking about SSTO crafts with a cost of $25/kg to LEO, 20% of payload at 30 days to Mars


  • Tobias Holbrook June 10, 2010, 5:39

    We’d be even less intelligent if we went with the plan outlined in the article.

    How we focus on colonies in the inner solar system, and building up an infrastructure? Once we have crude shipbuilding facilities on Luna (basic hulls, tanking, plumbing and the like) along with with a magnetic rail to launch them, we can outfit the ships with the payload of a single EELV (mostly electronics and computer systems, plus a powerplant). Heck, we don’t even need the Lunar shipyards – every burntout upper stage is a potential hull, Skylab style, that comes fitted with tanking and plumbing as well as rocket engines.

    Once we have colonies established throughout the inner system, market forces should drive the evolution of the transportation system and shielding, until we can reach Jupiter in a couple of years without having to worry about radiation. If you don’t demand that they crew return, and make it a colonization mission… they can do the same with Saturn later, then Uranus, then Neptune. By which time they’ll find Terran’s already there.

  • Robotbeat June 17, 2010, 1:47

    Cosmic rays aren’t so bad that you need that kind of Battlestar Galactica ship. Stick a person in a MRI machine, and they will be shielded from the vast majority of galactic radiation. That’s an engineering problem we can solve with current technology and even current budgets. Heck, with a large enough magnetic field gradient, you even have artificial gravity! Two birds with one stone.

    Still, I think radiation is a little bit overblown, and a lot of it could be fixed with drug treatments. That’s not to say we shouldn’t invest in shielding, but we certainly don’t need 1000-ton-to-LEO launchers for this.

    And Jupiter and Saturn provide a whole ton of Moons to explore before we get to Neptune or Uranus…. I mean, colonization efforts on Mars are probably a lot more important.

    As far as interstellar (or near-interstellar) exploration, the best technology I can think of (that doesn’t require harnessing a whole civilization worth of energy) is the fission fragment rocket with exhaust velocities ~5% of the speed of light. Combine this with perhaps some short of braking (maybe magnetic field braking of some sort), and trips to Alpha Centauri within fifty years are possible, though likely not in our lifetimes. One-way, of course! Pandora, here we come!