All posts by Tom James

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Propellantless propulsion

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Tether-satellite-NASAFollowing on from solar sails we have a discussion of that other science fictional bastion of propellantless propulsion – the space elevator – it turns out that space elevators and space tethers can be used for more than just getting into orbit:

A series of bolo tethers, each tether passing a spacecraft onto the next, could be used to achieve even larger orbit changes than a single system. For example, one tether system could catch a spacecraft from a very low orbit and swing it into a somewhat higher orbit. Another bolo picks it up from there and puts the satellite into a geosynchronous transfer orbit (GTO). A third tether catches the load again and imparts sufficient velocity to it so that it reaches escape velocity. A satellite initially orbiting just above the atmosphere could thus be slung all the way into an interplanetary orbit around the Sun, and all this without using any rocket propulsion and propellant…

This is in the context of a review by Centauri Dreams of Space Tethers and Space Elevators by Michel van Pelt, which explores tethers and space elevator concepts in some detail.

[from Centauri Dreams][image from Wikimedia and NASA]

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Thrown off course by relativity

Cosmos1-2006-2A preview of space-flight issues of the future: how do you account for the effects of relativity when travelling long distances? A solar sail launched from close to the Sun would have to account for relativistic effects when navigating to the edges of the solar system:

And even though those effects are relatively minor to start with, they have a significant effect over long distances.

The calculations carried out by Kezerashvili and Vazquez-Poritz show that the effects of general relativity could push a solar sail off course by as much as a million kilometers by the time it reaches the Oort Cloud

The promise of solar sails as a propulsion mechanism is impressive:

By one calculation, a solar sail with a radius of about a kilometer and a mass of 300 kg (including 150 kg of payload) would have a peak acceleration of about 0.6 g if released on a parabolic trajectory about 0.1 astronomical unit (AU) from the sun (where the radiation pressure is higher).

That kind of acceleration would take it beyond the Kuiper belt to the heliopause, the boundary between the solar system and interstellar space (and a distance of 200 AU), in only 2.5 years.

In 30 years, a solar sail could travel 2,500 AU, far enough to explore the Oort Cloud.

Of course we need to actually build one of these things first.

[from Technology Review, via Technovelgy][image from Wikimedia]

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A spoonful of friendly bacteria helps the medicine go down

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pillsGenetically engineered bacteria have been used to deliver therapies for bowel disorders like inflammatory bowel disease:

The bacterium is able to deliver the protein, a human growth factor called KGF-2, directly to the damaged cells that line the gut, unlike other treatments which can cause unwanted side effects. Also unlike other treatments, it is envisaged that patients will be able to control the medication themselves by ingesting xylan, perhaps in the form of a drink.

I am not 1 of the 400 Britons who suffers from IBD but it is wonderful to see that genetic engineering has such excellent medical applications.

[from Science Daily][image from Deco Fernandez on flickr]

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The ethics of autonomous devices

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heart_surgeonThe Royal Academy of Engineering in the UK says that the imminent rise of autonomous and semi-autonomous cars, robotic surgeons, planes, war machines, software agents, and public transport systems raises important ethical and legal questions:

Professor Stewart and report co-author Chris Elliott remain convinced that autonomous systems will prove, on average, to be better surgeons and better lorry drivers than humans are.

But when they are not, it could lead to a legal morass, they said.

“If a robot surgeon is actually better than a human one, most times you’re going to be better off with a robot surgeon,” Dr Elliott said. “But occasionally it might do something that a human being would never be so stupid as to do.”

Professor Stewart concluded: “It is fundamentally a big issue that we think the public ought to think through before we start trying to imprison a truck.”

And when and if true AI or artificial general human-level intelligences show up, will they commit crimes, and if so, who will be responsible?

[from the BBC][image from Wonderlane on flickr]

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Charlie Stross on the future of nuclear power

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power_plantCharles Stross has made an interesting point on the view that there is only a very short supply of useable nuclear fuel:

firstly, the supply of known uranium deposits will only last 80-100 years if we don’t recycle it and start burning MOX. I’d like to note that today’s light water reactors are horribly inefficient — they only extract 3% of the available energy from their fuel before it is considered “spent” and reclassified as waste. If we use high burn-up reactors such as the EPR, we can get a whole load more energy out of the same amount of fuel. And if we use fast breeders and run a plutonium cycle we can convert U238 into Pu239 and burn that instead of U235: there’s 500 times as much U238 lying around.
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Secondly, we haven’t even tried to build a thorium reactor yet, although we’ve got good reason to believe it would work — and thorium is considerably more abundant than uranium.

As I have mentioned before, nuclear really should be part of the future energy mix of any industrialised country. Renewables can provide a large chunk (depending on local availability) of our energy needs but that still leaves a gap that needs to be plugged with something reliable and non-carbon-dioxide emitting.

David JC MacKay has more on nuclear power in his excellent free online textbook Sustainable Energy – Without the Hot Air.

[image from christian.senger on flickr]