Re: Burning Cosmic Commons (was: ... Fermi's Paradox?)

David Blenkinsop (
Fri, 5 Mar 1999 19:58:27 -0600

Billy Brown wrote:

>David Blenkinsop wrote:
>> Compared to such grand space-trashing ideas, I still wonder if a
>> relatively mundane idea might be worth thinking about, namely the chance
>> that intelligence might survive preferentially on quite "large"
>> planets as compared
>> to the Earth. If a solid planet with the Earth's density were
>> twice as large
>> in overall dimensions, it would then have eight times the
>> mass of the Earth..

>There is some question about whether such planets exist (unless you are
>including gas giants in your analysis, which is a completely different

I've been reading the web site for the proposed Kepler planet search mission at . There, it says that planets between two and ten Earth masses are considered "Large Terrestrial Planets" -- this is in contrast to even larger planets, like Uranus and Neptune, which are referred to as "Giant Cores".

> But even granting that, there is no serious barrier to space travel
> in the long term.
>Certainly, a species on such a planet isn't going to get very far with the
>kinds of rockets we have now. However, there are two easy ways out of this
>1) Even primitive nanotech will allow you to build rockets that can easily
>reach orbit. All you need is the ability to synthesize diamondoid materials
>with a very high level of quality. This improves the lift-to-weight ratio
>so much that even a 3G liftoff through dense atmosphere would be feasible.

Well, it may not be so easy -- see my response to Hal Finney, below.

>2) Nuclear power can also be used for rockets. We don't build them because
>too many people are afraid of the word "nuclear", but the technical barriers
>are modest. Again, 3Gs isn't enough to keep such vehicles from making

One nuclear powered item that I've read a little bit about was something that I understand a relative of nanotechnologist Ralph Merkle was involved in many years ago, something called "Project Proteus", I believe it was, a project to build a long range nuke aircraft or cruise missile of some kind. This had *real*, *big*, problems with safety, understandably so given that you can't afford to carry much shielding on a flying vehicle. Nanotech mightn't be all that much help on the safety front when dealing with the rational concerns that any intelligent "folks" might be expected to have about such things. This sort of thing is dangerous *way* beyond someone worrying about the Cassini probe's radioisotope power supply, or something like that, if you really want nukes to propel your rocket from ground to orbit.

>On a more fundamental level, let me point out that both of these are
>short-term solutions. Do we really think that any possible planetary
>environment could keep a technological civilization stranded for a million
>years? How about ten million? A hundred million?

Well, if getting off the planet is really quite tough, it becomes hard to say for sure. I'd tend to agree that sophisticated technology building would have to arrive at molecular factories and nanotech sooner or later, but what if that technology is both expensive to use for building really "large" rockets, plus it's unacceptably dangerous for existing populations on this large planet? If launching things via rocket is really *tough* for them, maybe their funding always gets cancelled, or civilization falls, or just retrenches somewhat, while they're trying to plan a space launch. Maybe things are just tough enough that they're out of the space business repeatedly, a *lot* of time could get eaten up that way, until they get discouraged or decide "this really isn't a good idea". You never know with ET's, but certainly a large physics barrier to getting to orbit would be no help.

Hal <> wrote:

>I'm not too knowledgable about rocketry, but increasing the energy
>requirements by a factor of two shouldn't by itself be that great
>a barrier. The energy to reach orbit is around half the energy for
>escape velocity. We were able to reach orbit in the late 1950s, and reach
>escape velocity in the mid to late 1960s. So it only took an extra ten
>years or so. If energy requirements were doubled because we lived on a
>bigger planet, we could reach orbit with late 1960s technology. I would
>think that you could then go from orbit to escape velocity by assembling
>a rocket in orbit, if nothing else. While space exploration would be
>more costly, it would seem to only slow things down by a decade or two.

Well, this is the difficulty, with chemical rockets you have the thrust needed to get away from a planet, but for the rocket to carry enough fuel to get into a stable, low, orbit, *that's* another thing. I understand that we've reached a fundamental limit for chemical rocket fuels with current efforts to build a "Single Stage to Orbit" vehicle to get into near-Earth space economically. Basically, fuels like hydrogen burning with oxygen have only enough energy content to propel a small fraction of a rocket's propellant mass into orbit. So, for a one-stage craft, what goes into orbit must only be about 10% of the ship's total weight, loaded with fuel as it sits on the ground. Note that the 10% "mass remainder" that gets into low orbit must include both the payload and the entire hull of the rocket. In a single stage design, this way of doing things may not even be successful launching from *our* planet, since, if the technology falls just a bit short you can end up launching "all rocket and no payload". With real nanotech, the chances of something like this working should be much better, but a chemical rocket still can't get away from that 10% limitation, maybe only half that 10% would be payload of some sort, even with fairly advanced nano-construction?

Now, the kicker on this is that for the planet size that I'm postulating, it's 2X the Earth's measurements, 4X the Earth's energy well, which means that the low orbit speed will be *twice* that of Earth, with escape speeds a factor of 1.41X the respective low orbit speeds. With low orbit speed twice that of Earth you'd need to take two nanotech "Single Stage to Orbit" craft, one 20 times bigger than the other, and stack the little one on top of the big one, to launch a payload only 1/400th of the initial spacecraft weight into low orbit! For comparison, the current space shuttle, being non-nanotech, but with a much easier job, has a payload about 1/70th of the vehicle's launch mass. For our ET's, launching one part in 400 might be *just* manageable, unless they have other problems, other priorities at that point? For instance, there they are, they've developed nanotech now, but the planet's heavily populated, too many folks downrange, too expensive to launch those big tech items and still comply with environmental regs? Depending on their exact situation, "climbing the well" *could* be a real showstopper -- maybe technology as such can't solve everything that Nature puts in the way?

David Blenkinsop <>