Interstellar Travel – Part Three

Posted on 2012-11-06 by arjaybe

Between the StarsSee also parts one, two and four.

Last time we looked at some ways of traveling to nearby stars using currently understood technology. They were methods that we could use now if we had to, such as robots, light sails and generation ships. And we would be prepared for missions measured in decades and centuries. This time we’ll look at some possibilities that we don’t understand yet, but which might come within our grasp soon. These are things that we could not do now if we wanted to, but which are not so different from what we can do already.

Starting with propulsion, first up is nuclear fusion. This is about ten times as efficient as nuclear fission, but it still only converts about one percent of its fuel into energy. And you can’t just shoot energy out the back of the rocket, you have to use the energy to shoot matter out, or you don’t get any push. The biggest problem is that we can’t do nuclear fusion yet. We’re sure we will be able to soon, but there are no guarantees.

More efficient than nuclear power is antimatter. Antimatter is just like normal matter, only completely opposite. For example, antimatter hydrogen is exactly like normal hydrogen – one proton, one electron and so on – except everything is the mirror opposite. The interesting thing for the purpose of propulsion is that when matter and antimatter come in contact, they convert one hundred percent of their mass into energy. The bad thing is that it’s hard to keep them from doing that prematurely. And even though we know how to make antimatter now, it’s beyond us to make enough for interstellar travel.

So let’s find the fuel on the way. That requires using something called a Bussard Ramjet, proposed in 1960 by Robert Bussard. It’s based on fusion power but it collects its hydrogen fuel using an enormous magnetic scoop at the front and blasts the resulting helium out the back as thrust. Even interstellar space holds one or two hydrogen atoms per cubic centimeter, so the fuel is available. Unfortunately, the drag caused by the big scoop might exceed the resulting thrust.

The other side of the problem – the long trip times – would be handled in three ways, none of which we can do now. We could send frozen embryos and have robots rear the children at the destination. We could develop an effective form of hibernation and sleep en route. Finally, we could extend our lifespan and stop worrying about it.

We can do none of these things yet, but maybe one day.

rjb

Posted in Uncategorized | Tagged | 2 Comments

The Artists of Autumn – Reprise

Posted on 2012-11-03 by arjaybe

Autumn Leaves

Note: It seems I was a bit premature when I posted this in September. Autumn was still a long way off in many places then, and it wasn’t even very advanced here. It’s looking a lot more autumny now, so here it is again. rjb

Leaves use the same technique as artists do to mix their colors. It comes down to what colors of light are absorbed and which are reflected. When most of the pigment reflects green light, we see green. When enough green pigment goes away, we get to see the other ones. There are also yellows, reds, blues and browns in most leaves, their amounts depending on the species of plant and local conditions.

When Fall comes and deciduous trees drop their leaves there’s an orchestration of events going on. The tree draws moisture and sugars out of the leaf, for storage in the roots. At the same time it seals off the branch and weakens the base of the leaf’s stem. Soon the leaf is holding on by just the fibrous veins that used to flow with the tree’s juices.

While all this is going on the chemistry in the leaf is changing. Chlorophyl, the green pigment that spent the summer converting water, carbon dioxide and sunlight into sugar, is no longer being replenished and is breaking down and fading away. Now the yellow pigments can be seen. They are carotenoids, which have been converting some of the green light not used by chlorophyl, but which reflect light in the yellow range.

The red color seen in some leaves is due to another pigment which is actually produced as the leaves die. Stray sugars combine with colorless flavonols and sunlight to produce a pigment that ranges from red to blue. The amount of red seen in leaves is dependent on the species of tree and the weather. Cool nights and sunny days stimulate the production of the blue-red anthocyanin pigments, and thus encourage the reddest leaves.

The yellows and reds also decay and fade away, leaving tans and browns. The most common brown pigment is tannin, and by the time the leaves hit the ground they are almost exclusively brown.

The color in autumn leaves depends on what pigments the plant produces, and in what ratios. Then there’s the different rates at which the various pigments decay. And finally there’s the effect of weather on the production and decay of the pigments. Different types of trees turn different colors, and trees of the same type can differ, depending on their local conditions.

Nature has been mixing pigments all summer. It’s time for the show.

rjb

Posted in Uncategorized | 3 Comments

Interstellar Travel – Part Two

Posted on 2012-11-02 by arjaybe

Too Close
See also parts one, three and four.

How will we get to the stars? With extreme difficulty. We have enough trouble getting to Mars and it’s right next door. The time it takes light to get from Earth to Mars is measured in minutes. The closest extrasolar star is over four light years away. A ship with people on board will take months to get to Mars using current technology. At that rate it would take over a hundred thousand years to reach the nearest star. Obviously we’re not going to get there in chemical rockets, even forgetting about the impossibility of carrying enough fuel to do it.

As any engineer worth their salt would say, there must be a better way. And there are a few. We’ll look at some that are theoretically possible using currently known technology.

Nuclear power is more efficient than chemical power. It still only releases about one tenth of one percent of its fuel as energy, though. It could be used to force something, steam maybe, through rocket nozzles to drive the ship in a traditional way. Another option is to detonate a series of atomic bombs behind the ship to push it along. These would require carrying less fuel than chemical rockets, but they’re still pretty bad.

The best method of propulsion that we could use now, with physics that we understand and technology that we can build, is the light sail. Four hundred years ago Johannes Kepler, an astronomer, noticed that something was blowing long tails off comets. He speculated that someday we might be able to use that wind to sail in space. These days Freeman Dyson, the legendary scientist, is saying the same thing. It’s been tried and it works. Light really does push on a reflective surface. One could simply use sunlight, but it gets weaker very rapidly with distance. A better way is to use a powerful laser, whose beam doesn’t scatter too quickly and which can provide plenty of power to the sail as it speeds away. Calculations show that such a ship could reach one tenth of the speed of light, and wouldn’t have to carry all that heavy fuel.

At that speed it would still take about five decades to reach the nearest stars, so it would be best to send robots first. With enough intelligence built in they could explore, set up a base, extract resources to build more robots, look for suitable destinations and prepare the way for us. Fifty years is still too long, though. The original crew would grow old on the way. The current best solution for that is the generation ship, carrying a clan of families to continually replenish the crew. Analyses show that a minimum of eighty people is needed for sufficient genetic diversity, but 150 would be a better number.

Next time we’ll look at methods that aren’t possible now, but could be soon.

rjb

Posted in Uncategorized | Tagged , | 8 Comments