And ironically, it’s XKCD What-if that is wrong. Well, not wrong by commission, but wrong by omission.
First off, I’m a huge fan of both XKCD (well the 98% of their comic strips that are SFJ) and XKCD-What If. If I was 1/20th as funny as Randall Munroe, I’d probably be writing witty webcomics instead of trying to build spacecraft robot arms and rocket ships…
But in this particular XKCD-What If, he was trying to answer the question “Is there enough energy to move the entire current human population off-planet?” The answer started off reasonably enough, talking about the energy needed to lift a mass to escape velocity, Tsiolkovsky’s rocket equation and such. The problem is that XKCD in effect only compared three options for transporting people to escape velocity: LOX/Gasoline Single-Stage-to-Escape-Velocity rockets, Space Elevators, and Orion Nuclear Pulse Propulsion.
No discussion of staging, of higher energy upper stages (like say LOX/LH2), of high Isp in-space propulsion (like Solar Electric Propulsion), of rotating tethers made using existing materials, and most offensively of all, no mention of propellant depots or tapping offworld resources for that last several km/s of delta-V!
If humanity had enough time to reconstruct an Orion, it could certainly afford to finish proving out propellant depots and tapping propellants from the Moon, NEOs, or even Phobos/Deimos if necessary. Which would be far less environmentally damaging than Orion or launching billions of LOX/Gasoline SSTEVs!
So, while it is still true that moving all of humanity off planet would be a very challenging enterprise, it’s not quite as bad as XKCD makes it sound.
[As a friend pointed out, I didn’t exactly disprove XKCD, because I didn’t use math. Ok, I’m lazy. Would people be interested in me doing the math to show my work?]
Jonathan Goff
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Yes.
Stuff like this is why I quit XCKD years ago. A large disconnect exists between what the guy thinks he knows and what he actually knows, in particular about physics. The straw that broke this camel back was the strip where he’s pondering the optimal path that traverses a lawn boundary. And it’s obvious to anyone who’s actually studied physics that it’s an application of Snell’s law and can be computed even without any variationals. Not to the author though.
Jon,
All the What-If’s have areas of disagreement, hence the activity in the forum for each What-If post.
http://forums.xkcd.com/viewtopic.php?f=7&t=88529
Pete,
It’s a freakin’ webcomic dude.
I stand corrected. The commenters for that topic are below xkcd’s usual standards.
Thanks for the discussion and the links.
Bob Clark