≡ Menu

Early Interstellar Missions and Energy

The International Astronautical Congress is in full swing in Prague today, with regular updates flowing over #IAC2010 on Twitter and the first session of interstellar import now in progress as I write this. It’s a session on interstellar precursor missions that includes, in addition to Ralph McNutt (JHU/APL) on the impact of the Voyager and IBEX missions, a series of papers from the Project Icarus team ranging from helium-3 mining to communications via the gravitational lens of both the Sun and the target star (no specific target has yet been chosen for Icarus).

Claudio Maccone will be summarizing where we stand with the FOCAL mission, envisioned as the first attempt to exploit gravitational lensing for astronomical observations. But I’ll turn today to Marc Millis, who will wrap up the precursor session with a discussion of the first interstellar missions and their dependence on things we can measure, such as energy. The notion here is to look at the energy required for an interstellar mission and to weigh this against predictions of when those energy levels will be accessible and available to be used for space purposes.

Energy Needs Drive Spaceflight

How to get a handle on energy growth trends? Millis uses annual data on the world’s energy production from 1980 to 2007, calculating the ratio of each year’s energy production to the preceding year, then finding the average and standard deviation of all 27 of these years. How soon until Earth becomes a Kardashev Type I civilization — one capable of mastering all the energy reaching the Earth from the Sun? Acknowledging the wide span of uncertainty in the result, Millis pegs the earliest year this could occur as 2209, with a nominal date of 2390 and a latest date of 6498. A constant growth rate is assumed, which balances depletion of natural resources against unforeseen advances in new energy sources, leaving growth rates relatively stable.

Fascinating as they are in their own right, I won’t go through all the numbers (I’ll link to the paper when it becomes available online). But note the key factors here, which are the total amount of energy produced by our species and the proportion of that energy devoted to spaceflight. For the latter, Millis compares the annual Space Shuttle launch rate against the total annual energy consumed by the United States, finding that the maximum ratio of Shuttle propulsion energy to total US energy consumed occurred in the year 1985, equaling 1.3 x 10-6. The average ratio over the years 1981 to 2007 is 5.5 x 10-7. Millis then takes the maximum ratio (over an order of magnitude greater than the average ratio) to calculate the earliest opportunity for future missions. What he calls the Space Devotion Ratio is thus 1.3 x 10-6.

The Alpha Centauri Calculation

When could we launch a 104 kilogram interstellar probe to Alpha Centauri based on these calculations? Assume 75 years as the maximum travel time that might be acceptable to mission scientists and assume a rendezvous rather than a flyby mission, acknowledging the need to acquire substantial amounts of data at the destination. Millis extrapolates from existing deep space probes to arrive at a putative mass, adding the needed margins to ensure survival over a 75-year transit and the substantial communications overheard to relay information to Earth.

As to propulsion options, Millis works with two possibilities, the first being an ideal case that assumes 100% conversion of stored energy into kinetic energy of the vehicle (think ‘idealized beam propulsion’ or even some kind of space drive), the second being an advanced rocket with an exhaust velocity of 0.03c. We thus wind up with two sets of figures, again based on energy availability. Millis then converts the propulsion energy figures into equivalent world energy values, using the Space Devotion Ratio he first calculated earlier for US space involvement.

The result: The earliest launch for a 75-year probe is 2247, with a nominal date of 2463. This assumes idealized propulsion; i.e., a breakthrough technology like a space drive. Fall back on advanced rocket concepts and the energy requirements are much higher, with the nominal launch date of the probe now becoming 2566, the earliest possible date being 2301.

Strategies for Interstellar Research

The play in the numbers is huge, the uncertainty in the results caused by the wide span in possible energy production growth rates. Interestingly, Millis’ finding that the earliest interstellar mission will not be possible for two centuries coincides with earlier estimates from Bryce Cassenti and Freeman Dyson based on economic and technological projections. We can, obviously, adjust the numbers based on our projections of technological growth, and as with any projection, sudden changes to world economic patterns would be a substantial wild card.

But Millis argues that in the absence of a single technological solution, it would be premature to focus on specific propulsion options to the exclusion of other, more theoretical alternatives. For that matter, it would be foolish to be inhibited by the ‘incessant obsolescence’ postulate (a term that Millis himself coined), noting that earlier missions may well be overtaken by faster ones launched at a later date. Instead, what he calls ‘cycles of short-term, affordable investigations’ targeting key questions whose answers we can hope to find today are the best way to proceed. And that means continuing our investigations of everything from the already operational solar sails to technologies that today seem impossible, such as travel faster than the speed of light.


Comments on this entry are closed.

  • jim henson October 2, 2010, 21:41

    Let’s not be in a hurry to send anything to another star until we can approach near relativistic speeds, and defend the ship from far more intense lethal destructive radiations, like cosmic rays supernova swarms that wouldn’t be deflected by our suns magnetic field that saves the solar system inside the heliosphere ribbon where ENAs exchange charge

  • Kevin S. October 3, 2010, 11:48

    I am a professional physicist. In my opinion, any time FTL propulsion is mentioned in an otherwise serious scientific discussion about interstellar travel, its credibility drops by a factor of e. One might as well substitute the words “magic” or “time travel”. (The latter is mathematically equivalent to FTL, by the way: you can’t speculate about the one without embracing the far more profound implications of the other.)

    And spare us any talk of “but the laws of physics may be wrong”. Yes, we know. But shouldn’t that also affect your most basic engineering assumptions? As long as you’re speculating, why not start by pitching that pesky F=ma? You can whittle your fuel requirements all the way down to zero that way.

    Also, let’s put aside our hope that the Vinge singularity will bestow magical technologies upon us within our lifetimes. I believe that may actually happen, and that it may even make interstellar travel technically possible, but as it does, it will at the same time destroy any hope of it. The time and distance scales relevant to any post-singularity intelligence grow ever shorter. In effect, the stars are flung away to infinity.

    We all look at the quantitatively gigantic difficulty of interstellar travel and say, “Cripes, I hope that’s wrong,” and yearn for the remembered ease of Sagan’s “spaceship of the imagination”. But if we want a real-world solution to the problem, we must shelve that next to our propeller beanies and work honestly and exclusively with the equations mankind worked so hard to discover.

  • Ronald October 3, 2010, 19:05

    Question in relation to FTL etc.:

    What is the speed of gravity? This may sound silly at first, but I have not been able to find an uambiguous answer. The answers vary from ‘light speed’ to ‘infinite’.

    In other words: if a source of gravity, a large object, suddenly originated somewhere in the universe, or any kind of gravity waves generated in any possible way, how fast would its effect be noticed at a certain distance?

    I know some gravity wave research is being carried out underground in Italy and the USA, also to test relativity, but I do not know whether the issue of gravity velocity is also being researched.

  • Duncan Ivry October 3, 2010, 21:26


    All four kinds of physical interactions, i.e. gravity, electromagnetism, strong and weak nuclear force, propagate their effects with the speed of light.

  • Eniac October 3, 2010, 22:57

    Also, let’s put aside our hope that the Vinge singularity will bestow magical technologies upon us within our lifetimes. I believe that may actually happen, and that it may even make interstellar travel technically possible, but as it does, it will at the same time destroy any hope of it. The time and distance scales relevant to any post-singularity intelligence grow ever shorter. In effect, the stars are flung away to infinity.

    This is an excellent point and an interesting solution to the Fermi Paradox, right there.

  • Kevin S. October 4, 2010, 1:00

    Ronald, gravity is a field, and as such doesn’t have a speed. What you mean is the speed of gravitational waves, which are the undulations–ripples–in the field (just as light is an undulation in the electromagnetic field). For deep geometrical reasons, gravitational waves will move at the speed of light.

    There are a lot of crackpot notions and handwaving arguments floating around about how the speed of gravitational waves must be infinite. Somehow they always overlook the fact that by the very same line of reasoning, the speed of electromagnetic waves (such as light) would have to be infinte, too. It isn’t, of course.

  • Ronald October 4, 2010, 9:34

    Duncan, Kevin: thanks for enlightening me, makes a lot of sense indeed.

    I gained some knowledge, but lost an illusion ;-)

  • Kenneth Harmon October 4, 2010, 12:13

    Kevin S,

    Quick question, perhaps I am dense, but I did not understand your comment that the emergence of post Vinge Singularity Intelligence would fling the stars away towards Infinity. A post Vinge Singularity Intelligence would be capable of travel velocities approaching the speed of light since it would be energy based. In fact it may be that the only way for a Civilization to achieve Interstellar Travel is to first achieve a Vinge Singularity and modify itself enough to travel relatively easily between the stars. In essence, if you can tailor the “intelligent payload” for Interstellar Travel then all things are possible. This in turn gets at Jim Henson’s issue about how to protect things against high intensity Cosmic Rays and the other problems that Interstellar Travel creates

  • Greg October 4, 2010, 21:23

    @Kevin S.

    If you could please let me know some of your publications? Not that I don’t believe you, but I have heard several people say they’ve been “professional physicists” (as compared to the non-professional ones) before, that we’re not.
    Thanks in advanced.

  • Eniac October 5, 2010, 0:19


    I think you should judge Kevin by what he says, not what his job is or whether he writes papers or not. That is a common practice on on-line forums, and a good one, too, I might add. I, for one, think that all he has said makes a lot of sense, way more than the rule on even this excellent forum.

  • Greg October 5, 2010, 9:04

    @Kevin S.

    I’m surprised, as a scientist that you would even state something like Vinge Singularity, which has no scientific bases what-so ever. Sadly, That would make your credibility drop as e^2.

  • Greg October 5, 2010, 10:10


    Some of what he says does make sense, I do believe though that bold claims should be backed up. In the past I’ve worked in academics research pertaining to physics mostly, all though not a PhD student myself, as an Engineer I do work with several Physicists today. Good Physicists tend to be more conservative about there views and not given into anything that would in anyway harm there credibility.
    I cannot judge what he says unless I know where it is coming from and is valid as well. Anyone can say what Kevin S. has said, he hasn’t mentioned anything more than what an undergraduate already knows.
    Sincerely, I’m just trying to verify his claims and do not wish him anything else.

  • Eniac October 6, 2010, 14:09

    There is a big difference between FTL the Singularity. FTL is impossible according to the laws of physics as we know them. The singularity is a perfectly valid speculation that does not conflict with any physical laws, but cannot be proved or disproved because it is a prediction about the future. Regardless of what you or me may think about the Singularity, for the Good Physicist you mention, to say “I believe that may actually happen” is quite acceptable about the singularity, but not so about FTL.

    Note that for physicists, as opposed to religious fanatics, “I believe” is a very weak expression of conviction. It is akin to (and shorter than) “I think it is so, but I have no evidence whatsoever to back this up, and will right away change my mind if evidence to the contrary emerges.”

  • ProtoAvatar October 7, 2010, 8:56


    “FTL is impossible according to the laws of physics as we know them.”

    Is it? Alcubierre was the first to check if general relativity allows for FTL “warp drive”. He was by no means the last.
    What was found out?
    If one has astronomical amounts of energy (or an efficient method of generating gravity fields) and a way of producing negative energy, one can make an FTL warp drive or a wormhole.

    The same is true about time travel – there are papers by Hawking, Thorne, Novikov that show that general relativity allows for time travel – by flying in your ship around a rotating singularity, for example.
    In fact, Kurt Godel was the first to show that in a rotating universe (since then it was shown that a rotating singularity suffices), time travel is possible, as per general relativity.

    And frankly, assumptions like ‘a rotating singularity exists’, ‘one can fly with a ship around said singularity’ and ‘the equations of general relativity hold for this situation’ are FAR more down to earth (they’re actually supported by evidence) than what is, essentially, a religious affirmation: ‘the technological singularity is coming’.

  • Greg October 7, 2010, 9:32

    One, I never claimed FTL is possible, if you re-read what I said, “How about the “laws” of physics, as we understand them today, imply that Faster than Light travel is impossible”, which is true.
    No good physicist will ever, or ever should, discredit valid research, even if it is most likely proved to be wrong. We should always test our boundaries of knowledge and never be comfortable in staying within them, that’s how knowledge progress’s. We probably will never achieve FTL, but what other wonders will we uncover by pushing that boundary?
    And you may believe in the Vinge Singularity but please understand, it isn’t taken seriously within the mainstream scientific community. As a professional I would worry of any scientist who really did.

  • Eniac October 7, 2010, 18:33

    Greg, if you re-read what I said, you will find I never said I believed in the singularity. Just that I find it acceptable to do so.

    And it might not have been obvious, but I tried to poke fun at your presumption of telling us what good physicists do. I believe anyone is entitled to speculate, even wildly, even about FTL. Including good physicists. As long as you remain aware that you are really “out there” when you are doing it. Like saying “I believe it may…”, instead of stating it as a fact, or claiming that “it has been shown”. It is when you lose that perspective and start treating (or worse: selling) abstract thought experiments as practical solutions that you are on the way to crankdom.

  • Greg October 8, 2010, 9:20


    It’s sometimes hard to read intentions in written form. My apologies if I missed it. I was saying experience has shown a physicist worth his mettle will always challenge one to think, never to discourage them.
    But yes I do believe your correct in regards to ‘crankdom’, we should never try to push thought experiments as a practical solution, but it should lead you to one, and if it doesn’t then you throw it away, and move on.