The Difficulties of Interstellar Flight

It’s been our trendsetting exoplanet week at the RBC. After the heavy stuff from Mike on tenured stargropers with too much immunity and from me on Renaissance cranks with not enough, here is something lighter for a rainy Sunday.

Fan image of Niven's puppeteer

Fan image of Niven’s puppeteer

We love Bruno-style speculations about intelligent aliens fron other stars. Thrint, tnuctipun, puppeteere, kzin, kdatlyno, grogs, pak, moties, and outsiders – and that’s just from Larry Niven’s imagination. But they all come up against Enrico Fermi’s famous question, made in a Chicago cafeteria in 1950:

Where are they?

I had a nice explanation for the non-appearance of intelligent aliens. No, not the one that intelligent life does not exist on earth either. Human history indicates that contact between societies at different levels of culture and technology is normally disastrous for the weaker. Any ethical civilization would minimize it, except to prevent disaster. Now any civilization capable of building interstellar ships must be ethical, or it would have destroyed itself by ecocide or the genocidal warfare enabled by the weapons available well before that technological level. Ergo, any alien civilization surviving long enough to launch interstellar ships would refrain from doing so on ethical grounds. QED.

Cute, isn’t it. The weakness is the disaster cop-out. Iain Bank’s SF novels about the very ethical and advanced Culture depend on this loophole, in the adventures of agents of the embarrassing spook agency Special Circumstances. This is set up to carry out deniable interventions in murderous lesser civilizations, for their own good of course. Closer to home, when European colonialists reached Easter Island in 1722, weren’t they justified in intervening to halt the self-destruction of the Easter Islanders described by Jared Diamond? Their population had crashed 80% to at most 3,000. It might have been – only in fact the colonialists made the situation much worse by disease and slave-raiding, reducing the native population to 111 by 1877.  Non-interference looks the better course.

I now have a much more prosaic explanation. Space is full of junk.

Oort cloud

Astronomers believe (based on robust inference) that the solar system is surrounded by a vast spherical cloud of small to midsize rocks called the Oort cloud. A very few of them get knocked loose and become comets.


The Oort cloud … is a theoretical spherical cloud of predominantly icy planetesimals believed to surround the Sun at a distance of up to around 100,000 AU (2 light years). This places it at almost half of the distance to Proxima Centauri, the nearest star to the Sun…The outer Oort cloud may have trillions of objects larger than 1 km.

Since Sol is an unremarkable, middle-of-the road, Mom-Dad-John-Jane-and-Rover sort of star, it is reasonable to assume that Proxima Centauri and other stars have similar clouds. Here’s an estimate of the mean separation between stars in the galactic arm of 5 light-years. Chances are the whole neighbourhood is loosely packed with Oort clouds, like untended suburban gardens.

Unfortunately the next-door neighbours do not have any potentially habitable planets. The nearest known exoplanet in the habitable zone of its star is Kapteyn-b  at 13 light-years, though it’s far too massive to be livable for us. We can take this as the minimum distance for a worthwhile interstellar voyage. The nearest exoplanet habitable for humans is likely to be much further away.

Remember the scene in Star Wars where Han Solo, pursued by Empire fighters, escapes through a belt of whirling asteroids, missing them by inches? Interstellar travel is like that only worse.

(C3PO’s probability estimate is 2:05 minutes in)

Suppose our Millennium Falcon is only travelling at a footling 1% of the speed of light, or 3,000 km per second. Hitting a pea-sized rock at that relative velocity will be instant annihilation. [Update: see technical discussion in comments whether a pea will do the job or if you need a 20-kg basketball rock, of which there are still plenty in your way.] To survive, we need radar capable of detecting the peas [or basketballs] infallibly at 10,000 km – the distance from Barcelona to Los Angeles. We also need very powerful and responsive software and manoeuvering rockets to miss them with 3 seconds warning. Just conceivable – but at that speed we will still take 1,300 years to Kapteyn’s Star.

Let’s try to shorten this by speeding up to half the speed of light, 150,000 km/sec. The trip will only take 26 years plus a year or so for acceleration and deceleration, just doable. The radar signal is only travelling twice as fast as the ship, so whatever the range of the radar, the pulse gets back to the ship at the same moment as it hits the rock. Not good. We have therefore to go slower, much slower. At 25% of the speed of light, the trip takes 52 years plus. Sticking to the 3-second warning, our radar needs to detect the pea [or basketball] at 450,000 km, more than the distance from the Earth to the Moon. The Star Wars missile defence problem is trivial in comparison. I can’t believe it can be done reliably.

Quadrillions of rocks and pebbles are not the only hazard. There are plenty of full-sized rogue planets out there, floating free of any star, and undetectable from a distance in the dark. We aren’t likely to hit one, as the captain of the Titanic said, and size makes no difference to the catastrophe.

We can’t however avoid interstellar dust.  The density is estimated at 10−6 × dust grain/m3. Assume our spaceship has a cross-section of 100 m2. We will hit one grain every kilometre. En route to Kapteyn-b, we will hit 1.23 x × 1014 grains. That’s only 10 grammes in total, similar to our killer pea. But at high velocities, they will together destroy the ship, and radar will not help.

Interstellar travel therefore looks suicidally dangerous or impractically slow. To get round these problems you have to posit unknown and completely new technology like force fields: magic it used to be called. This is not the way to bet. If an alien civilisation were prudent enough to have survived – we don’t need virtue here – it would not go in for aliened interstellar spaceflight. This solves the Fermi paradox. QED again.

There is a wrinkle though. The strictures don’t apply to robot spacecraft, since losing one is only money, provided it isn’t sentient. You can either send them fast, accepting a high attrition rate, or safely slow. “Slow” means tens of thousands of years. It could look prudent to keep an eye on possible threats in the neighbourhood, or just satisfy curiosity about life elsewhere. So where are the alien robot spycraft?

Puppeteer robot spycraft will naturally be extremely well protected: stealthy to the point of invisibility,  with gnat-sized microdrones. As backup, they will have psywar capabilities including memory manipulation. You don’t see them and if you do you will forget. But once in a millennium, both systems fail. That’s why there are persistent reports that one has been captured and concealed in a secret government base in … [uutyfd ffgvg vxu n88c pn c c uun c90nn cn likjf 3e984 318n99  nv4n9vn[n The RBC apologises for the temporary loss of service, due to a small glitch in a scheduled software upgrade. Normal service will be resumed shortly.]

The argument above (not the robots or Roswell) is adapted freely from The Straight Dope.


Remember manned space exploration? Your children don’t. The last men on the moon were the American astronauts Eugene Cernan and Harrison Schmitt, who left it on December 14 1972. Most of the people alive today have not witnessed a moon flight. The Mars mission keeps being put off. The exploration that actually takes place is done by robots. Robots get more capable and cheaper all the time, humans stay expensively the same.

The last man on the Moon: Eugene Cernan

The last man on the Moon: Eugene Cernan