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Saturday, June 19, 2010

Interstellar Tour


“Earth is mankind's cradle. But nobody stays in the cradle forever.”
Konstantin Tsiolkovsky (1857-1935)

“Reason not the need!”
King Lear

Project Icarus, proposed by the Tau Zero Foundation and the British Interplanetary Society, is a plan to send an unmanned space probe to a nearby star. It is a sequel to the 1978 Project Daedalus, also by the BIS. Although neither project has ever had a chance of being funded any time soon, because of the admittedly “astronomical” cost, it is still fun to speculate on the possibilities, and try to predict how long it will be before these things are feasible. See Ian Crawford’s article, making the case for an interstellar mission.

Any realistic mission to even the closest stars must involve attaining a speed that is a substantial fraction of the speed of light. We will want to get results back within the lifetimes of the people who will design and build and pay for the mission. Chemical propulsion systems, solar sails, ion drives, etc, will never do. So the first order of business is that we need a new propulsion system. But for now, I’m interested in the easier question of choosing a destination, so let’s assume that we have a ship that can attain, let’s say, 10% of the speed of light. We still have to imagine whether our vehicle can accelerate indefinitely at some constant rate, or whether it can thrust for a few hours or days, get up to relativistic speed, and then coast until the engines are needed again. I’m putting that on the back burner, for now. Just imagine that this “minor engineering problem” has somehow been overcome!

The obvious first choice for a destination would be the Centauri System. At 4.37 light years, it is the closest star system. One star, Alpha Centauri, is just a bit bigger and brighter than Sol, our Sun. The second, Beta Centauri, is somewhat smaller and dimmer, and somewhat orange. Alpha and Beta orbit their common center of mass, with a distance (between them) that varies from 11.2 AU to 35.6 AU. In other words, the orbits of both stars fit easily into a space the size of our Solar System. Besides Alpha and Beta, there is also a red dwarf star, Proxima Centauri, so called because it is currently the closest to us, at 4.22 light years. It is not known for sure whether Proxima is gravitationally bound to Alpha and Beta, or whether it is destined to continue on its own trajectory through the galaxy. At any rate, Proxima is so small and distant that it won’t affect the orbits of Alpha and Beta. However, it may be less likely to find stable orbits for planets in an essentially binary system like Centauri than in a single star system. (It used to be thought almost impossible for a planet to have a stable orbit in a binary system. Now that we understand more about resonant orbits, it seems more possible.)

Another popular candidate is Epsilon Eridani, at 10.5 light years. Read Winchell Chung’s blog about it. This is a single star, not too different from Sol, and it is the closest star known to have a planet. Although much younger and more active than Sol, Epsilon Eridani is a little smaller, dimmer and less massive.

About 80% of the nearby stars are red dwarfs. There may even be more red and brown dwarfs near us, that we have not even discovered yet, because they are so hard to detect. (See my previous blog about these small, dim stars.) There is much controversy as to whether life is more likely or less likely to be found on planets orbiting red dwarfs than on planets orbiting other stars. Be that as it may, it is becoming clear that most red dwarf stars - even the smaller, cooler, quieter ones - are capable of very large and unpredictable flares. This would seem to render the vicinity of a red dwarf star hostile to human colonies or passing spaceships. If we’re looking for a good spot for a permanent colony, we should probably concentrate on sun-like stars.

The purpose of a mission like Icarus would be to investigate nearby interstellar space, including one or more nearby stars, and possible planets. We might search for signs of life, or scout possible locations for future colonization. The probe would send pictures and scientific data back to Earth, by radio.

Given a target star or star system, we can either rendezvous or flyby. With a flyby mission, we can basically keep accelerating for the entire journey. This gives us maximum speed when we get there, and also gets us there as quickly as possible. The drawback is that we will go past the target so fast that we won’t be able to observe much.

With a rendezvous mission, the idea is that, when the vehicle gets to the halfway point, it turns around, with thrusters pointed toward the destination, and reverses all the velocity it picked up in the first half of the trip. Then it gets to the destination with low enough speed that it can go into orbit around the target star, and take measurements for years instead of a few minutes. A compromise might be made between rendezvous and flyby, where the vehicle slows down enough to get some good measurements, but not so slow as to prolong the mission unacceptably.

For example, the Pioneer and Voyager missions were flybys. They flew by several planets, taking pictures and measurements, and each eventually flew on their way out of the Solar System. They each made a kind of tour of the Solar System, using gravity assists to climb around from planet to planet. In contrast, the Galileo mission to Jupiter, and the Cassini mission to Saturn, went into orbit around their targets, and were able to send back a large amount of very detailed data.

What I would like to suggest is a kind of tour of nearby interstellar space. Given a sequence of target stars, the vehicle would slow down enough to take some good pictures, but also use the gravity of the star to deflect it toward the next target in the list.

Unfortunately, the stars near us are moving very slowly compared with the substantial fraction of the speed of light that our mission will require. So we won’t get much of a “gravity assist” in the sense of an increase in speed. The propulsion system will have to provide any increase or decrease in speed. However, we can get a change in direction from passing close to a massive object like a star. How much deflection depends on the vehicle’s speed, the mass of the star, and on how close we get. Of course the radius of the star puts a hard lower limit on how close we can get. But since the vehicle is unmanned, we might be able to shield it well enough so that it can pass within maybe 2 or 3 times the star’s radius.

I’m planning to work out the geometry of these interstellar tours. I’ll blog about it, soon. Before that I probably need to write up a blog explain the special relativistic aspects of interstellar missions. I’ve got one half-written, around here somewhere. Stay tuned! And please comment.


  1. Interesting blog. I'm a scifi writer and currently writing a novel that deals with traveling from the Centauri system to Earth. I have a PhD in molecular biology and found this stuff not of my turf. I found what you said very useful, as I've spent time calculating the years it would take to travel the distance at a given percentage of lightspeed. All my characters will have to do it in "hyersleep."

  2. Thanks Tina! I read your short story "Europa" a week or two ago, after hearing about it on the Writer's Coffeehouse list. Very interesting story!

    I'm not keeping up this blog any more. For some reason, I moved to Posterous.

    I have tried social networking all over the place, trying to see what will attract readers.


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