Back to the space age

39,693 km/hYesterday, Artemis II launched four people on a flyby of the Moon, the first such crewed mission in 56 years. I have dim memories of the Apollo program, not so much the missions themselves -- I don't remember whether I heard Neil Armstrong's "One small step" live or later, for example -- but I do remember details like drinking Tang, because astronauts drank it (and still do), and a print on the back of a cereal box (I think?) that you could cut out and fold up into a model of the lunar lander.

The original Apollo program was a truly remarkable engineering feat, particularly considering how much progress there has been since then in fields like materials science and, of course computing. Today, we build massively powerful datacenters (at least, they seem massive now). At the start of the Apollo program in 1961, computers were much, much smaller and the field was so new that the word software had only been coined three years before.

It would be tempting to say that Artemis is just a retread of 50-year-old technology. In the years since the Apollo missions, space flight has become routine. There were 330 orbital launches in 2025, 317 of them successful. The ISS has been in continuous operation for 25 years. A dozen countries have launched satellites into orbit. Spacecraft have gone to all eight planets, Pluto, the Kuiper Belt object Arrokoth and to within about 6 million kilometers of the Sun (harder than it might sound). There were even two lunar landings last year, not to mention ongoing missions on Mars.

Except ...

The vast bulk of space activity has been launches to Low Earth Orbit (LEO for short). An orbital launch is not nothing. It means accelerating whatever you're launching to about 8 kilometers per second (17,500 mph) and handling all the details of tracking exactly where the launch vehicle is at all times, deploying the actual satellite and plenty of stuff I'm leaving out because I don't know any better. Nonetheless, as far as space travel is concerned, LEO is easy mode.

Everything else in the last 50+ years has been uncrewed. No human has been past LEO since Apollo 17 splashed down in 1972.

There are several reasons for this, not all of them technical, but the technical obstacles are considerable. For one thing, crewed missions are much heavier. Besides the mass of the people themselves, you need a life support system, food and water, equipment for a cabin and so on. More mass means a bigger rocket. 

Missions beyond LEO need a significantly higher delta-v budget, which is the total of all speed changes for the maneuvers the mission needs to do. LEO needs about 8 km/s of delta-v. Artemis II will use around 13km/s, about 1.6 times as much. Since it takes more fuel to lift more fuel, that means significantly more than 1.6 times as much fuel. The Space Launch System (SLS) that launched Artemis II was the most powerful rocket that NASA has every launched. The Saturn V rockets used in Apollo are not too far behind.

The stakes are also higher. 13 of the 330 launches in 2025, or about 4%, failed. If you leave out LEO launches (crewed or uncrewed), that number is much higher. There were two successful lunar landings last year, but also at least three failures. Of the two successes, one landed on its side. This sort of thing is OK if it's just expensive equipment getting destroyed, so you can afford to take more risks. For a crewed mission, nothing major can go wrong, and even minor problems like toilet malfunctions require serious attention.

Even with the need to reduce risk, Artemis II is still pushing the envelope a bit, and not only in the power of the SLS. When Artemis II splashes down (assuming everything goes well up to then), it will be traveling at around 11,000 m/s, breaking the previous record held by Apollo 10  [As it turned out, Apollo 10 still holds the record at 11,082 m/s (24,791 mph) vs. 11,026 m/s (24,664 mph) for Artemis II  -- D.H 6 Jul 2026]. It will also go a bit further from Earth than the Apollo missions, so the Artemis crew will be further from Earth than anyone has ever been before [That one panned out. Artemis II reached 406,773 km (252,757 miles) from Earth, vs. 400,171 km (248,655 miles) for Apollo 13].

From a strictly economic perspective, crewed missions make very little sense. The real reason to send people around to the Moon is that we want to send people to the Moon, either for its own sake, or so that we can establish a presence there and eventually send people to Mars and beyond. Whether that's a worthwhile goal is a matter for debate and I'm not going to take a position on it here.

My point here is that a crewed mission to the Moon, or anywhere beyond LEO, wasn't just a major engineering feat 50 years ago. It's still a major engineering feat now. Practically all of the progress in the past 50 years ago has been aimed at solving different problems: Getting equipment and people to LEO, and getting equipment beyond LEO. Crewed missions like the ISS have told us a lot about what happens to people in space, and the Artemis mission reflects that, but not much about how to get them there that we didn't already know.

Suborbital crewed missions like Virgin Galactic's and Blue Origin's are pretty much irrelevant to all this.

As I write this, Artemis has successfully executed all but the last major maneuver in its mission. Before long, if all goes well, it will do the trans-lunar injection burn that will put it on a path to swing by the Moon and return directly to Earth. At that point, the crew needs to survive a bit more than a week in space and splash down safely. They also have a long list of mission goals to accomplish, of course.

I'm a bit surprised by my feelings about this. I've studied enough about the Apollo missions and spaceflight in general to know how much can go wrong, even with the most careful planning. NASA itself has lost crew members on multiple occasions. So while I'm excited for the crew and the many people on the ground, I'm also more nervous about it than I expected to be.

Beyond that, though, is a strange feeling of being in two timelines at once: a young kid in the late 1960s curious about all the moonshot stuff going on, and an adult watching nearly the same things happen 50 years later, almost as though for the first time.

Wicked gravity

Every once in a while in my news feed I run across an article about colonizing other planets, Mars in particular.  The most recent one was about an idea that might make it possible to raise the surface temperature by 10C (18F) in a matter of months.  That would be enough to melt water in some places, which would be important to those of us who need liquid water to drink and to irrigate crops.

All you have to do is mine the right raw materials and synthesize about two Empire State Buildings worth of a particular form of aerosol particle, and blast it into the atmosphere.  You'd have to keep doing this, at some rate, indefinitely since the particles will eventually settle out.

The authors of this idea don't claim that this would make Mars inhabitable, only that it would be a first step.  This is fortunate, since there are a few other practical obstacles, even if the particle-blasting part could be made to work:

• The mean surface temperature of Mars is -47C (-53F) as opposed to 14C (52F) for Earth.  The resulting -37C (-35F) would not exactly be balmy.
• Atmospheric pressure at the lowest point on Mars is around 14 mbar, compared to about 310 mbar at the top of Mount Everest.  Even if the atmosphere of Mars were 100% oxygen, the partial pressure would still be around 20% of what it is atop Everest, and there's a reason they call that the Death Zone.  In practice, you'd at least want some water vapor in the mix.
• But of course, the atmosphere on Mars is not 100% oxygen (and even if it were, it wouldn't be for long, since oxygen is highly reactive -- exactly why we need it to breathe).  It's actually 0.1% oxygen.  There is oxygen in the atmosphere, but it's locked up in carbon dioxide, which makes up about 95% of the atmosphere.

It's at least technically feasible to build small, sealed outposts on the surface of Mars with adequate oxygen and liquid water, at a temperature where people could walk around comfortably, all using local materials.  Terraforming the whole planet is Not ... Going ... To ... Happen.

But let's assume it does.  Somehow, we figure out how to crack oxygen out of surface rocks (there's plenty of iron oxide around; again, there's carbon dioxide in the atmosphere, but nowhere near enough of it) and pump it into the atmosphere at a truly massive scale, far beyond any industrial process that's ever happened on Earth.  Mars's atmosphere has a mass of about 2.5x10¹⁶ kg, and that would need to increase by a factor of at least five, essentially all of it oxygen, for even the deepest point in Mars to have the same breathability as the peak of Everest.

By comparison, total emissions of carbon dioxide since 1850 are around  2.4×10¹⁵ kg and current emissions are around 4×10¹³ kg per year.  In other words, if we could pump oxygen into Mars's atmosphere at the same rate we're pumping carbon dioxide into Earth's atmosphere, it would take about three centuries before the lowest point on Mars had breathable air -- assuming all that oxygen stayed put instead of, say, recombining with the iron (or whatever) it had been split off from or escaping into space.

This is just scratching the surface of the practical difficulties involved in trying to terraform a planet.  Planets are big, yo^{[citation needed]}.

But then, not always big enough.  Broadly speaking, there's a reason that there's lots of hydrogen in Jupiter's atmosphere (about 85%, another 14% helium), while Mars's is mostly carbon dioxide and the Moon has essentially no atmosphere.  Jupiter's gravity is strong enough to keep light molecules like molecular hydrogen from escaping on their own or being carried away by the solar wind.  Mars's isn't.  It can hold onto heavier molecules like carbon dioxide OK, though still with some loss over time, but lighter molecules aren't going to stick around.

Earth is somewhere in the middle.  We don't have any loose hydrogen to speak of because it reacts with oxygen (because life), but we also don't have much loose helium because it escapes.

Blasting oxygen into Mars's atmosphere would work for a while.  Probably for a long while, in human terms (to be fair, atmospheric escape on Mars is measured in kg per second, or thousands of tons per year, much smaller than the in-blasting rate would be).  In the end, though, trying to terraform Mars means taking oxygen out of surface minerals and sending it into space, with a stopover in the atmosphere.

But there's another wildcard when it comes to establishing a long-term presence on a planet like Mars.  Let's put aside the idea of terraforming the atmosphere and stick to enclosed, radiation-shielded, heated spaces with artificially dense air.

The surface gravity of Mars is about 40% of that on Earth.  What does that mean?  We have no idea.  We have some idea of how microgravity (also known as zero-g) affects people.  Though fewer than a thousand people have ever been to space, some have spent long enough to study the effects.  They're not great.  They include loss of muscle and bone, a weakened immune system, decreased production of red blood cells and lots of other, less serious issues.

Obviously, none of this is fatal, there are ways to mitigate most of the effects, and some of them, like decreased muscle mass, may not matter if you're going to spend your whole life in space rather than coming back to earth after a few months (no one has ever spent more than about 14 months in space).  But then, that's a problem, too.  No one has spent years in microgravity.  No one has ever been born in microgravity or grown up in it.  We can guess what might happen, but it's a guess.

No one, ever, has spent any significant time in 40% of Earth gravity.  The closest is that two dozen people have been to the Moon (16% of Earth gravity), staying at most just over three days.  We know even less about the effects of Mars gravity on humans than we do about microgravity, which is only a little bit.

Maybe people would be just fine.  Maybe 40% is enough to trigger the same responses as happen normally under full Earth gravity.  Maybe it leads to a slow, miserable death as organ systems gradually shut down.  Maybe babies can be born and grow to adulthood just as well with 40% gravity as 100%.  Leaving aside the ethics of finding that out, maybe it just won't work.  Maybe a child raised under 40% gravity is subject to a host of barely-manageable ailments.  Maybe they do just great and enjoy a childhood of truly epic dunks at the 4-meter basketball hoop on the dome's playground.

Whatever the answer is, there's absolutely nothing a hypothetical Mars colony could do about it.  You can corral a bit of atmosphere into a sealed space and adjust it to be breathable.  You can heat a small corner of the new world to human-friendly temperatures.  You can separate usable soil out of the salty, toxic surfaces and grow food in the reduced light (the Sun is about 43% as bright on Mars).  You can project scenes of a lush, green landscape on the walls.

No matter what you do, the gravity is going to be what it is, and whoever's living there will have to live with it however they can.

When is a space elevator not a space elevator?

Canadian space and defense company Thoth Technology has recently announced plans for a "Space Elevator".  The idea is to use pressurized Kevlar to build a tower 15 kilometers high (nearly twice as high as Mount Everest), complete with hotel and observation decks, and to launch and land space planes using a rooftop runway.  It's an ambitious project, to say the least, but I wouldn't call it a space elevator -- thus the they-said-it-I-didn't quotation marks.

The term space elevator usually refers to a structure built around a very long cable with one end attached to a counterweight somewhere beyond about 36,000 kilometers above the surface of the Earth.  The cable remains taut due to centrifugal force*.  Vehicles called climbers move up and down the structure, delivering payloads into space.

We're not even close to being able to build such a thing.  The main reason is that the cable would be under far more tension than any commercially available material can withstand (it's not theoretically impossible, just not practical with current technology).  Assuming we clear that hurdle, actually building the thing would still be a major piece of engineering.

How would a space elevator work in practice?  Since a space elevator cable is pulled taut by its counterweight, and it's rotating in sync with the Earth's rotation, the higher up you go, the faster that bit of the elevator is moving relative to the ground, since the higher you go the more distance you have to cover every 24 hours.  As a climber goes up the elevator, it will feel a slight force pushing it in the direction the cable is moving (technically a Coriolis force).  This is the force of the cable keeping the climber moving with it, ever so slightly faster for each meter you move farther out from Earth.

In order to put something into orbit it's not enough simply to lift it out of the atmosphere.  If you took a payload up a space elevator to an altitude of 200km, a typical distance for a low Earth orbit, and let it go, it would fall back to the ground.  At that height, the payload would start with a forward motion relative to the ground of around 50 km/h -- a typical speed limit for residential streets -- and lose most or all of it to wind resistance on the way down.  To go into orbit, it would need a forward motion of more like 28,000 km/h.  As Randall Munroe says, getting to space is easy. The problem is staying there.

To go faster you have to go higher, but fortunately you also need less speed to go into orbit as you get further from the Earth -- the Moon orbits at less than 4000 km/h, for example.  By the time a climber got to about 36,000 kilometers, not much less distance than going around the world, it would finally have enough speed (about 11,000 km/h at that point) that a payload would stay in orbit if released.

Orbits at this distance are geosynchronous (geosync for short), meaning the orbiting object is in sync with the Earth's rotation.  If they're also at the equator, which a space elevator would have to be, they're geostationary, meaning they always stay over the same point on the equator.  Otherwise they will appear to move north and south over the course of the day, but stay at roughly the same longitude.

From the point of view of someone on the elevator at geosync, the payload would just appear to stay where it was, at least for a bit.  In real life, it would tend to drift away over time due to factors like the gravity of the Moon and the Earth's not being a perfect sphere.  For basically the same reason, an object on the cable at this point would experience zero g (again neglecting secondary effects).  Points below would experience a pull toward Earth, however slight, and points above would experience a pull away from Earth.

Where does that leave us with Thoth's structure?

A 15km tower is nowhere near high enough to function as a space elevator.  The advantage to launching from there is not the difference in speed between the ground and the top, which would amount to a moderate walking pace, but being above about 90% of the atmosphere.  That's certainly helpful, but not enough to neglect the atmosphere entirely.  To do that, you would need to be somewhere above the Kármán line, somewhat arbitrarily defined as 100km, though the Wikipedia article asserts that 160km is the lowest point at which you can complete an orbit without further propulsion.

In other words, Thoth's structure doesn't even reach into space, or even a tenth of the way for the purposes of launching things into orbit.

While Thoth's structure might be still useful in launching thanks to bypassing much of the atmosphere, landing, on the other hand, seems a bit of a stretch.  If you're leaving low Earth orbit, you'll have to get rid of that 28,000 km/h somehow.  You could use rockets, the same as you used to get into orbit, but that means more fuel -- not just twice as much but a bunch more, because the extra fuel is just dead weight on the way up and you'll need more fuel to compensate for that.  And more fuel for that extra fuel, and so forth.

This is why actual reentry uses the Earth's atmosphere to turn kinetic energy (energy of motion) into heat.  If you're going to do that, you might as well go all the way to the ground, where you can have a nice big runway, safety crews and other amenities if you're a spaceplane, or at least your choice of an ocean or a large expanse of open ground to aim for if you're not.

Of course, if you have an actual space elevator, you can re-attach to the elevator at geosync and let the climber spill your kinetic energy gradually on the way down, neglecting the not-so-small matter of moving from your current orbit back to the elevator, matching speed, not just location.  But that's not what Thoth is proposing.

So the whole "space elevator" thing is marketing hype.  Is there anything left after you account for that?  Well, maybe.  It depends on the numbers.

The Really Tall Tower idea still has some value, I think.  Actually building it would turn up all kinds of interesting issues in building tall structures, from how to build a stable structure about 20 times taller than the current record holder to the logistics of supporting a crew well above the Everest death zone to even just getting materials to a construction site 15km up in the air.  At the end, though, you'd at least have a unique hotel property and, depending on how much load that inflated Kevlar can take, potentially a whole lot of residential and office space, though most of it will need to be pressurized.

Do you have a space elevator?  No.

Do you have a compelling value proposition for someone wanting to put things in orbit in a world where ground-launched rockets are pretty much the only game in town?  I really don't know enough to say, but my guess is that it would be better if the business model didn't depend on that happening.



(*) If you prefer, feel free to recast this in terms of centripetal force.  You'll get the same vectors, just with different names.

Getting off the ground

Not long after I published the previous post about the Drake Equation, a couple of headlines surfaced about a paper by Michael Hippke with the admirably straightforward title Spaceflight from Super-Earths is difficult.  The paper is actually a light rewrite of what was originally an April Fool's joke, but the analysis is real, even if the author originally considered the topic frivolous.

The term Super-Earth itself is fairly loosely defined.  For concreteness, Hippke chooses Kepler-20b, with a radius of about 1.87R_⊕ (Earth radii) and a mass of about 9.7 M_⊕ (Earth masses).  Since gravity is proportional to mass and inversely proportional to the square of distance, the surface gravity of this planet would be about 2.8g (Earth gravity).  This is assuming that the measured radius is actually the radius of the surface.  There's a good chance that Kepler-20b is actually a "Mini-Neptune" with an extensive atmosphere rather than a Super-Earth with a rocky surface, but let's assume the Earth-like scenario here.

Hippke argues that it would be impractical for a civilization on such a planet to build rockets because the amount of fuel you need to reach escape velocity* increases exponentially in relation to that velocity.  This is exponential in the literal sense that doubling the velocity of a rocket means squaring the ratio of fuel to mass, not in the colloquial sense of "a lot".  Escape velocity in turn increases as the square root of the surface gravity.  For example, four times the surface gravity means twice the escape velocity, so square the ratio of fuel to dry mass.  Taking the square root doesn't make a lot of difference in the big picture.  The exponential part still dominates everything else.

In short, a somewhat bigger planet doesn't mean somewhat more fuel to get to escape velocity.  It can mean a lot more.

On Earth, a chemical rocket which magically had a weightless engine, fuel tank etc. would need to have 26 times as much fuel as payload in order to reach Earth's escape velocity of about 11 km/s**.  In real life that ratio is more like 50 or even 83 since the engine and so forth actually do weigh something.

Escape velocity for Kepler-20b would be about 2.3 times Earth's escape velocity, or around 25 km/s.  Hippke calculates that for a typical chemical rocket, that 26:1 ideal mass ratio is more like 2700:1 and the more realistic ratio of 83:1 would correspond to something like 9000:1.  To send a 1-ton payload out of the planet's gravity well would take 9000 tons of fuel.  By contrast, the Saturn V -- the largest rocket actually put into service so far -- had a mass of around 3000 tons, not all of which was fuel.

All this is fine, and surely more than enough for something that started out thoroughly tongue-in-cheek.  So let's take it at face value and try to poke holes in it anyway.

First, the calculations are for a single-stage rocket, though the real-life rockets used for comparison purposes are multi-stage.  In a multi-stage rocket you use a rocket with plenty of thrust (the first stage) to boost another rocket (the second stage) through the atmosphere quickly and then jettison that first stage.  At that point you no longer have to worry about the mass of the first stage and you consequently get more acceleration out of your remaining fuel.  You don't have to stop there.  The Saturn V, for example, was a three-stage rocket.  Five-stage rockets have been successfully launched.

This doesn't just make a difference in that a multi-stage rocket allows you get more acceleration out of the same mass ratio.  It also means that you don't have to use chemical rockets for all stages.  You could, for example, use an ion drive, which has a much higher effective velocity and therefore a much lower mass ratio, for the final stage and use chemical rockets to get it into orbit.  Ion drives produce very low thrust, far too little to launch from the ground, but they can do it for a very long time using very little fuel, eventually reaching much higher speeds than chemical rockets.  Once in orbit, a modestly-sized ion-driven vehicle could easily escape even Kepler 20b's gravity well.

In other words, getting to escape velocity in a single stage is a red herring.  You really just have to get a reasonable mass to orbital velocity, and you can use multiple stages if that helps.  At a given distance from the planet's center of mass, the orbital velocity is smaller than the escape velocity at the same distance by a factor of the square root of two.  In real life the orbit is -- of course -- further from the center of mass than the surface is.  If escape velocity at the surface is 25 km/s, a more reasonable orbital velocity would be 17 km/s, depending on how high up you have to go to get out of the atmosphere.  That would mean a mass ratio of more like 150 for an ideal rocket and 500 for a more realistic one.

That's still considerably more expensive than here on earth, but not nearly as discouraging as the 9000 figure in the paper.  A 500 ton rocket could put a ton in orbit, and you wouldn't even need to do that to get out of the gravity well.  Japan's ion-driven Hayabusa craft had a mass of about half a ton.  It was able to get to an asteroid, grab a sample and bring it back to Earth -- a pretty impressive piece of engineering if you ask me.  Our counterparts on Kepler 20b could do that with something like a 250 ton rocket.

The rocket that launched Sputnik was 267 tons (the rocket that actually launched Haybusa was around 140 tons, for a mass ratio of around 280).  Sputnik itself was only 84 kg, for a mass ratio of somewhat over 3000.  Small payloads generally mean higher mass ratios because it's not practical to shrink the launch system proportionately.

Leaving all that aside, you could also do multiple launches and assemble the final craft in orbit, if your robotics were good enough.  If you can launch half a ton with a reasonable-sized rocket, you can launch five tons with ten such rockets, and so forth.

Which brings up another point.  In the early stages of space exploration, before Kepler 20b puts its ion drive into orbit, they'll want to start small, using relatively big rockets to put relatively small things in orbit, and before that, to blast relatively small objects -- on Earth, that mainly meant weapons -- across large portions of the planet.

There doesn't seem to be any reason intelligent beings on Kepler 20b couldn't do that, assuming they're there.  Start with toy rockets, then weather rockets to explore the upper atmosphere, work up to ICBM-style systems, then orbit, then out of the gravity well, just as we did.  As far as I can tell, the difference on Kepler 20b would mainly be a matter of time, not a night-and-day difference between plausible and clearly impractical.  The benchmark of putting a ton or more directly on an escape trajectory doesn't seem particularly relevant to the question of whether or not this could happen, though, being concrete and understandable, it's still useful to think about.

There's another way to bring down the mass ratio: faster rocket fuel.  Hippke's calculations use an effective velocity of 3430 m/s, but hydrogen/oxygen delivers more like 4400.  That brings our ideal mass ratio down closer to 50 as opposed to 150.  As I understand it we only use hydrogen/oxygen in specific situations, due to various engineering considerations, but the tradeoffs will be different on Kepler 20b.  It might make sense to find ways to make the faster fuel work in more situations.

Even if chemical rockets weren't a practical way of getting into orbit, there are plenty of other options, some more speculative than others, for doing so.  Space elevators ... mass drivers ... blast wave accelerators ... space fountains.  Some of these require materials we don't know how to make yet or other not-so-proven technologies, but to some extent this is all a matter of economics.  Rockets are easy and cheap enough for us, so we use rockets.

Finally, it's probably worth pointing out that escaping a planet's gravity well is necessary for sending an interstellar mission, but hardly sufficient.  Kepler 20 is 950 light-years away.  To get here from there in, say, less than 10,000 years, you'll need to be going about a tenth the speed of light, or 30,000 km/s.  If you can do that, getting into orbit or even to escape velocity doesn't seem like a major problem.  Conversely, the most likely reason not to receive a visit from Kepler 20b is that it's just too far, not that it's too hard to get off the ground.





* I suppose I should acknowledge that "velocity" here actually means "speed" since it's a magnitude with no particular direction.  But everyone says "velocity" anyway.

** In real life you also have to deal with gravity losses until you reach orbital velocity.  For example, for every second you spend going straight up against Earth's surface gravity, you lose 9.8 m/s.  For Kepler 20b, that's more like 28 m/s.  If your initial stages take 180 seconds (three minutes), that's an extra 4000 m/s or so, except it's not really that simple since you don't spend all your time going straight up, particularly if the goal is to reach orbit.  I'm handwaving that, though it's quite a bit to handwave, just to keep the comparison with the ideal mass ratio of 26.  Part of the reason real rockets, even with multiple stages, needed a higher mass ratio than just the change in speed would suggest was to deal with gravity loss.

The value of the thing ...

... is what it will bring.

I've now seen several headlines along the lines of "NASA to explore  $10,000 quadrillion metal asteroid"

What does this even mean?

Two things, really:

• NASA is planning a mission to the nickel-iron asteroid 16 Psyche, which is true
• That asteroid contains $10 quintillion worth of metal, which is, um ...

I mean, on the one hand it's a simple calculation: Psyche contains X tons of nickel at $Y/ton, and likewise for iron.  Total value: $10 quintillion or, for whatever reason, $10,000 quadrillion.

Except that's about 100,000 times the world's GDP, so maybe we're missing something?

Suppose we could magically bring all the nickel and iron in Psyche to earth.  That's a ball about 200km across, so we'd have to be a bit careful, but say we break it down into a few million 1km heaps distributed strategically around the world.  How much is that really worth?

You might think "Yay, free iron and nickel!" but that's not quite right.  Even scrap iron, which has already been refined and packaged into usable pieces, costs something to buy, something to transport and something more to put to use, unless it happens to be in just the form you need.  More realistically, it would mean no more iron mining, which is great unless you happen to be in the iron/nickel mining business.  That's not nothing -- world iron production looks to be around $300 billion and nickel maybe more like $20 billion.  But it's not a trillion dollars, much less a quadrillion or quintillion.

Or look at it another way: We've got a rock out in space that's worth as much as the entire world economy would produce in 100,000 years at current rates.  The total budget of NASA is around $20 billion, with ESA JAXA and the Russian space agency accounting for a few billion more.  Surely it would be worth it to throw the world's entire space budget into mining that rock.

Except, the question isn't whether there's a bunch of valuable metal to be mined. The question is whether it's worth mining.  It currently costs about $20,000 per kilogram to get a payload to low earth orbit.  It's anybody's guess what it would cost to actually mine a given amount of metal in the asteroid belt and bring it back to Earth safely -- though if you're transporting a hunk of metal I suppose you just have to make sure that it doesn't hit anything on the way in.  But bulk nickel from Earth runs more like $10/kg and iron is cheaper yet, so ... maybe not.

I don't really want to pick on NASA for trying to drum up a little interest in its latest mission -- though it's probably worth mentioning that the past couple of decades of unmanned missions by NASA and the other space agencies have been spectacularly successful in exploring the solar system and in an ideal world that would speak for itself.  If there's a point here, it's that it's a good idea not to take numbers, especially eye-catching dollar amounts, at face value without asking what they actually mean.

Fermi and the revenge of the machines

Previously, I argued that the odds of our actually directly detecting an intelligent species from another star system are very low, partly because of the vast distances involved, but also because of timing.  However, there is another possibility besides detecting such a species on their home world: maybe they'll come to us.

"Or maybe we'll come to them", you might counter, and maybe, eventually.  I'm going to leave that aside for now because, while we have some interesting theoretical ideas of how we might reach another star system, at this point they're just that, and the more realistic ones involve moving maybe a ton of payload to a nearby star in somewhere around a human lifetime, and score around a seven on the scale of difficulty I gave earlier.

The amount of energy involved in doing even that is staggering.  By comparison, the New Horizons probe that's currently nearing the Pluto system as I write this was launched on one of the larger rockets ever produced, received a significant gravitational boost from Jupiter, and has been traveling for around ten years.  To get to Proxima Centauri in a small number of decades, you'd need to be traveling around 2000 times as fast.

The energy required goes as the square of the velocity (until you get to relativistic speeds, where it gets much, much worse), so we're looking at 4 million of the rocket that launched New Horizons.  Put another way, the kinetic energy of a one-ton object moving one-tenth the speed of light is comparable to

• The energy released by the largest nuclear bomb ever detonated
• The energy released by the eruption of Krakatoa
• The electricity consumed by Norway in a year

Actually accelerating such an object to that speed would take considerably more energy.

However, one of the main points of the previous post was that some significant portion of intelligent species would have arisen millions or hundreds of millions of years ago, and another was that there were likely a great many such species scattered throughout the galaxy, not to mention other galaxies.  Maybe billions in our galaxy alone, using not-totally-implausible guesses. Put that together, and there might be a large number of species which have had plenty of time to reach us, and plenty of time to develop the technology to do it.  The timing that undermined our prospects of spotting other life directly actually works in our favor now.

So let's say that some technological species arose ten million years ago and 50,000 light-years away.  If they'd built a craft capable of traveling 1/200 the speed of light, or about 100 times faster than New Horizons, that craft could have reached us by now.  And maybe even have stopped.

Or, it could have reached any of a few hundred billion other stars.  Even if it had launched, say, ten craft a year for a million years, each equipped with a means of navigating to a distant star and then slowing down, the odds would still be about 10,000 to one against one reaching us.


However, there's a scheme that's been floating around for quite a while that would dramatically increase the chances of something eventually getting to us: send self-replicating craft.  This is not something we could do right now but it's quite plausible that we could in the next century or so.  It doesn't seem too much to ask that some civilization, somewhere, with a million or more years of head start, could have done this.

It's an interesting scenario to contemplate.  Weight is critical at these speeds, so the craft will probably carry only the bare minimum it would need.  It would enter a new star system and find a solid planet or moon to set down on.  It would then start digging into the surface (assuming there's no one there to run across it and ask "Hey, what's this?") and gradually assemble raw materials.  From those it would assemble basic tools, use those to assemble more sophisticated tools, and eventually spacecraft parts, which it would then proceed to put together.  Some amount of time later it will have made a copy of itself, which would then take off for a nearby star system, while the original goes on building copies of itself ...


You'd want to be a little careful with this.  If you literally sent a copy to every nearby star system, that would include ones you've already visited (even the home world), and some of those would make their way back to where they started, and start making copies of themselves.  That is, you would get exponential growth, everywhere the probes visit.

The total number of craft might double every few centuries.  If every copy used a ton of raw materials, they would consume the mass of our Moon in about 70 generations of copying, or a few dozen millennia.  In a few million years, probes could eat every planet, planetoid and moon in the galaxy.  Or, at least, every unpopulated one.  Or, at least, the portion of the material of the unpopulated ones that was suited for making the craft.

The polite way to send such probes would be to remember which places you'd already visited and only send copies to new places.  In technical terms, this results in a breadth-first search of the galaxy.  Instead of exponential growth you get cubic, that is, a steadily expanding radius of exploration.  In our scenario of craft traveling 1/200 the speed of light (and not taking too long to build copies), this would cover the whole galaxy in around 20 million years, starting at the edge, or 10 million starting near the center.

In sum, if anywhere in the hundreds of billions of stars in the galaxy 20 million years ago someone had successfully launched a self-replicating interstellar probe, one copy (or more) could have made it to the solar system.  A heady thought, to say the least.


One question that always bothered me about this scenario was "why?"  Even if the various probes could relay information back to the home world (and this is totally handwaving how one would produce a signal strong enough), the finite speed of light becomes a problem.  It's going to take a thousand years to get information back from a system a thousand light years away, and 200,000 from one edge of the galaxy to the other.  Why bother?  I mean, who am I to say how long a lifespan, or attention span, an alien species might have, or what its motivations might be, but still ... it's hard to see the point.

However, you don't have to set out to cover the whole galaxy in order to end up doing it.  Suppose we just wanted to explore our neighborhood of a few thousand stars?  A self-replicating probe would be cheaper and easier than trying to send a probe to every star individually.  In which case, why stop?  If we're interested in data from a star 50 light-years away, why not 60, or 100?  The easiest approach is to just let the probes keep copying.

A bit unsettlingly, it's even easier not to bother screening out already visited systems, that is, letting exponential growth continue unchecked.

In a slightly less scary variant, the probes somehow send messages to each other ("heartbeats", effectively) and only send copies to stars that aren't sending anything.  This way if a probe goes dead, another will eventually take its place and the galaxy will remain completely covered indefinitely.  Among other things, this means that if you're actively trying to get rid of probes, that may turn out to be somewhat difficult -- it only encourages them.


Once we get into galaxy-spanning schemes, we're looking at hundreds of billions of copies.  There are bound to be a few mistakes here and there, and those mistakes may continue to propagate.  The ones that do will continue to make copies, and some of those copies will be imperfect.  "Mistake" here just means "different from the original".   Such a mistake might be innocuous, or it might result in a less worthy probe, or it might even result in an improvement.  One way or another, the population will evolve over time.

So we have something that can use energy in an organized way, reproduce and evolve.  We may as well call that "life".  It's most likely not the same life form as the one that first built it on the original home world, but that seems like a minor point.  It's quite possible that there is life spreading out through the galaxy, or already occupying every corner of it, even while its creators are still confined to a single world, or no longer around at all.  And that life might well have reached us, or even be here now.

This makes the question of "why haven't we noticed" a bit more interesting.