Thursday, July 23, 2015

Starship Detection – Nosecone Heating

It would be nice if starships were like a US Space Shuttle entering the atmosphere, glowing bright and hot, and easy to see with infrared scopes. Unfortunately, the galaxy is not made that way.

A starship traveling through the galaxy from an origin star to a destination star will impact the interstellar medium, which is not uniform entirely, but largely composed of tenuous gas, mostly hydrogen atoms, at cool temperatures. The number density of these atoms is low, less than one per cc. Even flying at a tenth of the speed of light, heating is very small, and any heat could easily be dissipated by conduction through the ship’s outer skin and radiation away at low temperatures. The heating effect of this atomic impact would not be detectable.

The galaxy does have much denser regions, composed of molecular hydrogen with a smattering of other elements, and the number densities of these regions range from a hundred to a million atoms per cc. It is very fortunate for us that these clouds of dense gas exist, as they are what forms stars. These clouds are dense enough so that self-gravitation can overcome the diffusion effects, turbulence, shear, and so on, and gradually get more and more dense, leading to a star and a solar system being formed. We live in one of these.

The dense regions are here and there in the galaxy, but mostly in the center of spiral arms. The highest density clouds are what lead to the formation of the hot stars, O, B, and A, and these are what gives off the light in galaxies that we detect. When we look at another spiral galaxy in a telescope, what we see are these hot stars, as they produce the majority of the light, especially in the visible regions. They form the characteristic arms of the spiral. Since they don’t live long, the gas that formed them is still nearby, and therefore can be assumed to be in spiral form as well. The gas we observe here in the Milky Way also has this form, and is one of the reasons we have deduced we live in a spiral galaxy as well. Nobody from Andromeda Galaxy is sending snapshots.

There are other dense gas clouds in the galaxy. Not all stars are formed in the spiral arms, and especially the other stars, below A stars, can be formed outside. This means there are gas clouds elsewhere, and these are detectable, but it is a difficult problem. We see the gas because of the absorption of the light that passes through it, but that only gives us a two-dimensional view of the gas, and figuring out the three-dimensional view involves some clever deductions. These clouds are thought to be free-floating, self-gravitating blobs of dense gas, that may someday become a star, or if there is not enough gravitational force generated by it, to eventually dissipate. Other ones can form, so there is a change with time, but only with time measured in galactic time scales.

When a starship goes through one of the densest of these clouds at a tenth of the speed of light, it will experience heating from the impact of the hydrogen at very high speed on the forward cross-section of the ship. These clouds are of the size of tens of light years, so a starship entering one of the clouds will be there a long time, and will grow hotter. Perhaps a very large ship would be detectable, if there ever was one there.

Why wouldn’t the starship avoid such clouds? The answer is that it would. A more advanced civilization would have determined accurate maps of the gas in the galaxy in which it lived, and would know where are the high-density clouds and would plan to avoid them, if they could. A solar system located in one cannot, so an alien civilization living there can either run at slow speed until they exit the cloud, which could be a very long time, or not go at all. Stars and clouds have a random component to their velocity in the galaxy, so having a star run through a cloud is simply a matter of random chance. These clouds only occupy less than 1% of the galactic volume, are mostly but not all in the plane of the galaxy, and so few alien civilizations would find themselves living in one. The only exceptions might be those on the lower end of G class stars, formed in the very large clouds along the spiral arms. An alien civilization on a planet around one of these stars has the bad luck to find itself on a star with a short lifetime, necessitating emigration sooner or later, and then they notice they are inside a huge gas cloud, making the trip more difficult.

If instead, an alien civilization outside the core of the spiral arms found itself sitting near a small cloud, it could restrict its voyaging to directions that do not intersect the cloud. This should be easily done, unless the galaxy happens to be pretty much occupied, and there is little choice they have. Perhaps a solar system in the middle of a high density gas cloud is a sweet spot world, just perfect for emigration. Then their destination is perforce inside a cloud, and they have to figure out how to build their ship to tolerate it.

Note also that a ship going a tenth of the speed of light does not maneuver well. Once you get the starship going at that speed, turning it enough to avoid a gas cloud, which is huge compared to a solar system, is likely impossible. So knowledge of where they are and planning to avoid them is the key to star-faring.

This means that detection of nosecone heating, or heating of whatever sits on the front end of a starship, is not going to happen. Only in some very unusual cases might it ever happen, and the probability of that happening during even a very long observational time is virtually nil. We will know better and better where the nearest clouds to us are, but monitoring them will likely be a waste of time. It will be necessary to devise some other signature to search for if we want to detect emigrant trains or nearby probes or any other type of vessels that we can deduce alien civilizations might want to produce.

There also does not seem to be any apparent reason why an alien civilization would voluntarily choose to send a starship into a small galactic cloud, either before it condensed to a star, or after the condensation had progressed, to ignition or any other time. After ignition, the star’s output, unless it were very cool and weak, would serve to expel the gas from the area. Once a small gas cloud has served its purpose of forming a star, it is of no further use and might as well dissipate.

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