Interstellar Tour

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“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.

Stealth in Space

“Fixed fortifications are monuments to the stupidity of man.”

General George S. Patton

Stealth has been an important aspect of military campaigns since prehistoric times. Two trends have continued since then. First, military assets have become easier to hide. Second, targets with known position have become easier to destroy.

Weapons have become incredibly accurate and reliable. If your enemy knows where you are, you’re dead.

Populations are especially vulnerable to modern weapons, because they are hard to hide. Hence the era of strategic nuclear warfare. Generally, the term “tactical warfare” refers to the battlefield, or limited theater of war, in which military units try to destroy each other, and “strategic warfare” involves each side using its military to destroy the population and civilian assets of the other. If you think the Cold War was MAD, just wait until you see it in space!


As we have seen with various missile defense programs, defending a fixed position is much more difficult than offense. You have to be able to outspend your enemy at least 100 to 1, to have any chance of defending a population. In space, that factor of 100 may increase to 1000 or more.


Future populations will have to become stealthy, in order to survive. That means no colonies on planets or moons, or anything too massive to be moved out of a predictable orbit. It means living in hollowed-out asteroids, or ships painted black, or ships disguised to look like natural objects. Above all, it means radio silence.

Detecting a threat at a distance of several millions of kilometers would be virtually impossible. To put this in perspective, consider our efforts at detecting near-Earth asteroids. A similar object large enough to carry thousands of H-bombs would go undetected until it entered our atmosphere. As another example, consider Voyager 1. It’s not even trying to be stealthy – in fact, it periodically sends out radio signals to Earth. But trying to find it, if you didn’t know where to look, would be very difficult with a radio telescope, and impossible with visual or infrared. Now imagine it is coming toward us at 170 km/sec, instead of away from us at 17 km/sec, and imagine that instead of 722 kg of scientific instruments, it carries 7000 kg of H-bombs. And it’s radio silent. We would never know what hit us!

A military ship, or any spaceship, would be more visible in the optical and infrared, when thrusting. More visible from the stern (more precisely, the direction of thrust) than the bow. The visibility depends on the type of engine used, but as long as it isn’t emitting a lot of radio energy, it won’t be detectable at great distance. As an example to illustrate this, recall that the volcanic activity on Io, one of Jupiter’s moons, was not discovered until Voyager 1 flew by it in March, 1979. I don’t know if they have been detected since then by any Earth-based or LEO-based optical or infrared telescopes, but the point is that they are very difficult to detect from the typical Earth-Jupiter distance. And they are very hot, and much bigger than any rocket engine yet proposed.

Radar may not play as important a role in space warfare as it has in Earth warfare. Radar returns are attenuated by a factor of 1/r^4. In other words, it takes 16 times as much power to detect something twice as far away. So you quickly reach a distance beyond which you can’t detect something as large as a ship. For modern military radars, that maximum range is classified, but a ballpark figure might be a few hundred, or maybe a thousand, kilometers. You might imagine a much more powerful radar, with 16 times the power, able to reach out to 2000 km, or with 256 times the power, at 4000 km. You can see that you are going to need an “astronomical” amout of power to get out to a few million km. That’s why radar is used in astronomy only for near Earth objects, or by sensors that get close to their targets.

If warfare becomes large-scale enough that light delay becomes a factor, then radar pays another penalty, for relying on a there-and-back light time.

From the stealth point of view, the bad thing about radio waves is that they travel great distances, through atmospheres and interplanetary dust, needing very little power. But on the positive side, they are hard to pin down, in terms of direction. It's difficult to tell exactly where a radio source is located. There are many ways around this, involving advanced antenna design, multiple sensors, etc, but the fundamental difficulty remains. The offense will probably have to send a missile in the general direction of a radio source, and then use radar or optical/infrared sensors mounted on the missile, when it gets close.

A population center might also deploy decoys (imagine a decoy city in space!) or other countermeasures. But the best policy is to turn off any source of radio. And hide under a very dark or camoflaged covering.

Even though radar won’t be as important in space warfare (at least at long range), the passive detection of radio signals will be crucial. The military will use radio telescopes to identify targets, and populations must avoid any radio emissions. Developing a civilization without radio emissions will be difficult, but the alternative is extinction.