Sails have to be rigid? Have you looked at a sailing boat lately?

Reflectivity would double the thrust over simply absorbing the photons, but any metal coating except maybe lithium or beryllium would more than double the mass, right? So I'll take naked graphene, with some kind of rip-stop reinforcement of course.

The Darth Vader coins.

Yes ... but it took millions of years for something as massive as the sun to condense. These craft will have be lighter and have somewhat less gravity than the cloud of gas that gave birth to the Sun. By the time they form a compact ball, it's even possible that the US debt limit will have been raised.

SpaceX, for one, has several contracted launches (announced on their web site) that aren't from NASA. The PR verbiage suggests that they won some of these on price. They'll charge NASA more than an ordinary customer, of course.

The big difference is that, under cost-plus, the contractor gets more profit by the simple and undemanding expedient of making the vehicle more expensive than it needs to be. That's how we got to this place where only megamillionaires can afford a ride to orbit. With fixed bids, the contractors get more profit by making the vehicles as cheap as they can be, consistent with safety and reliability (and no, I don't think any of those CEOs want their names to be coupled in the history books with dead astronauts). That has the potential to lead to a place where I could afford to take the trip. Not probable, but better than no hope at all.

Oh yes, and with four or five suspects, real competition is more likely to emerge than from a cosy duopoly.

This update points out that dark forces within NASA are urging the return of cost-plus contracting for crew transport (scroll down to "Commercial Crew" section therein). This will get us straight back into the traditional world of missed schedules and massive overruns, if it's allowed to happen. USAan readers please hold yourselves in readiness to contact your elected lords and, um, representatives.

Good point indeed. Bear in mind that satellites in orbits very close to each other would be unstable: the orbits would change in a time much shorter than it would take life to evolve. IIRC the closest pair of Galilean satellites are in a 3:2 resonance ... lemme see ... actually 2:1 by http://en.wikipedia.org/wiki/Galilean_moons and their closest approach is 250,000 km, which makes for tides about four times as strong as on Earth (tidal force is inversely proportional to the cube of distance). With bigger satellites the tides would be stronger in proportion to their masses. OK, so a system with two Earth-sized satellites might have very big tides.

1. The moon would be tidelocked, if it were close enough to have such huge tides, no question.

2. Depends on the orbital period; three of Jupiter's Galilean satellites have periods of a week or less, and a quick calculation based on the diameter of Jupiter versus the diameters of their orbits suggests that none of them is in total eclipse for more than a few hours ... better numbers here. Since I routinely survive a twelve-hour night with no ill effects, the eclipse seems to be a minor problem. A tidelocked planet would have a day equal to its month, though, which might be a problem if the month were more than two or three days, but a lot would depend on the presence of oceans, which are huge reservoirs of heat, and on wind patterns.

"Those who refuse to do arithmetic are doomed to talk bollocks."

And what does the gov't produce that a teenager would want to listen to?

Under some circumstances it makes sense. I flew from the UK to the US one day after the original liquid explosive plot was uncovered back in 2006. The airport was stiff with armed guards -- putting a bullet through a bottle of liquid explosive does not seem to me to make anyone safer, but that's just a detail -- and my wife, kids, and self got both the standard security treatment and a pat-down on the jetway. You see, some of the plotters had been found with airport employee clothing. Suppose there had been other plotters who got away, and were working the security detail that day, and passing their friends through. A second search meant they had to infiltrate another spot, which they had probably not planned for, and so it cut that avenue of attack, or at least narrowed it a lot.

I don't suppose there are many al-Qaeda sympathisers in Korea, but it's entirely possible there are some in China, and we know that corruption is endemic there. I'm perfectly sure that Hu Jintao doesn't want any incidents on flights out of China, but things can happen without his knowledge. Defence in depth: it really can help.

1. Launch costs will have to come down by a hefty factor before it becomes economic to launch entire factories and bring raw materials from far away. Once launch costs have come down that far (and I'm not holding my breath), the value of the raw materials that are in orbit today will seem slight. Meanwhile, even one more collision between derelict satellites will make the orbital environment more dangerous and harder to clean up.

2. The raw materials that are in orbit today are in a wide variety of orbits, by both altitude and inclination. If your factory is in equatorial orbit, the delta-V needed to collect a given mass from a polar or near-polar orbit (which spysats tend to use) is more than the delta-V needed to launch it from Earth, and far more than the delta-V needed to launch it from an asteroid etc.

Because the time that the client spends looking at designs is also a cost to the client. Your proposal will work for the less demanding clients, but eventually the more demanding ones will say, "Screw this. We need someone we can sit down with."

Also, after the first three or four rejections, many wannabe designers will say, "Screw this. Maybe I know someone who has a birthday coming up."

So in the brave new world, the client will get 100 designs, none of which will be quite what they want. So they'll go back to the author of the one that came closest and ask for a meeting. At which point that designer can say, "I charge $X hundred for a design but $X thousand for a meeting."

Alternatively, the established firms can post pseudo contests, and if anyone produces anything that looks as good as their own team could do, tell that person, "We can pay you $X thousand instead of $X hundred. Talk to us before you join any more of these contests."

I've just looked up the latent heat of vaporization of nitrogen and it's 200 kJ/kg [wikipedia]; its specific heat as a gas is around 1.1 kJ/kg/K, so to boil it and heat it to 1000K takes roughly 1.2 MJ/kg. The kinetic energy of an orbiting spacecraft is roughly 30 Mj/kg and even a spacecraft in a vertical trajectory that reaches 200 km has an energy of roughly 2 MJ/kg. So unless the spacecraft consists almost entirely of nitrogen tank, most of the heat of re-entry will have to go elsewhere. I propose that a better way to think about this cooling scheme is that the nitrogen is being ablated as a way to protect the ceramic tiles.

Does this mean it's a bad idea? Noooo! Replacing the ablated nitrogen is as simply as putting a hose in the tank after the craft lands, while inspecting and replacing ablated ceramic is one of the reasons why the Shuttle takes months to turn around (true fact: the most Shuttle missions NASA ever flew in one year was 10, in a year when they had four birds to fly, i.e. 48 bird-months, or 4.8 months per flight). Also, it seems likely that you can adjust the flow of nitrogen to get the temperature you want (within limits) instead of having to design tiles that can take whatever temperature Nature hands you. I wish these guys the best of German luck.

Yes, I read Janis' article too. Search for "The Internet Debacle" to find it. She now sells CDs direct from her web site, and tours.

Fifteen years ago I lived upstairs from a guy who managed a jazz orchestra (and played drums). He put it in a nutshell for me. "There's a minimal price people will pay for just good music. If you want to make more than that, you have to be famous." He knew the big labels had the power to make his band famous, and that there were other bands out there who could play good music too. But he had more of a business head on his shoulders than 99% of musicians, so he didn't sell his band down the river in the hope of being made famous. And I learned that a band that doesn't have a big contract and isn't famous can sound just as good as one that has and is.

The fundamental problem was pointed out two or three years ago by some big dude from Yahoo!. As he put it to a room full of RIAA suits, the physics have changed. Disintermediation can no longer be prevented. Bands can get famous on YouTube. The artificial scarcity that RIAA exploited no longer exists, because it was a scarcity of information: there were ten thousand bands out there and the only way for me to learn which ones I would like was via some channel that RIAA controlled. Now there are more channels for information than anyone can control, this side of Beijing.

All the more reason for RIAA to screw even more out of the few artists they still have a legal clamp on. They now try to get artists to sign a so-called 360 contract, where the company takes the revenues from touring and gives the artist a few crumbs of those. And of course some artists fall for it.

What's left for the RIAA? People who don't care whether the music they're listening to is good music as long it's famous, as long as it's what the people around them are listening to. In a word: teenagers.

Some of this may come from '60s environmentalism and fellow-travelers thereof. When the public started to believe that they were being harmed by pesticides, nuclear power, and other technologies that owed a lot to recent scientific advances, they started to believe that scientists were doing them harm ... which meant that scientists were evil, or at least some of them were ... and everyone knows that you can't trust evil people to tell you the truth. So they stopped trusting the conclusions they heard from scientists; instead, they wanted to hear the reasoning, see the data, and draw their own conclusions. They weren't equipped to do this, and scientists weren't equipped to help them. Science teachers might have been able to help them, if said teachers had known any significant amount of science themselves, but only a few did. And bad/scary news sells much faster than good/reassuring news, so journalists were unwitting (well, sometimes unwitting) spreaders of the "science is evil" meme.

I'd love to attribute it all to head-in-the-sand religion, but that doesn't explain the decline in public trust for scientists by people outside the USA.