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    Re: Relativistic interstellar navigation (or how to avoid getting Lost in Space!)
    From: Frank Reed
    Date: 2021 Jun 3, 13:09 -0700

    Yes, I saw this a few months ago, apparently an earlier version. There's some interesting statistics in this article, but apart from that the author is really making a simple problem sound much more interesting than it is. The "relativistic" part is just plain old aberration of starlight. Yes, you have to do the math right for higher speeds, but this is "Physics 101" material today. 

    What about the basic navigation? This is fundamentally an ordinary piloting problem writ large.

    Suppose I have a couple of buoys in a harbor, each a few hundred meters/yards away from my vessel, and I can see them at certain angles relative to distant landmarks. Buoy A is 8° to the right of the radio tower on Mount McMountainous, 40 miles away, and Buoy B is 12° to the left of the spire on top of the bank building in downtown Navigopolis, also many miles in the distance. It's a basic piloting problem, and you can get your position easily. Deep space navigation can be done much the same way. You divide the "universe" (your collection of charted objects) into two categories: "nearby stuff" and "really far away stuff". The nearby stuff is equivalent to the local buoys in the harbor. The "really far away stuff" is equivalent to the distant radio tower and the building in the harbor example. You look at the positions of the nearby stuff relative to the far off stuff, and you have your fix. 

    Within the Solar System, one can do this form of navigation with nearby minor planets (asteroids). There are hundreds of thousands of them with well-determined orbits. For background objects, we can use the true stars. Then it's just a piloting problem. The nearby asteroids will change their positions relative to the background stars as you travel through the Solar System, and from their relative positions you navigate, just as you navigate in a harbor. It's three-dimensional, but that's not a big deal.

    Outside the Solar System, it's not entirely necessary to make a firm divide between nearby and distant objects, but it's also not difficult, and it does make life a little easier. For imaginary travel within a few dozen lightyears of the Earth, we can make a nice list of the nearest few hundred stars, and we can also make a list of stars further away than 1000 lightyears to serve as a nearly-fixed background. After that, once again, it's a piloting problem --also known as "simple triangulation". As you travel through local interstellar space, the positions of the nearby stars will shift relative to your background stars.

    Publishing this analysis in a scientific journal is a bit weird. It's science fiction (*). But it's only the Journal of the Astronomical Society of the Pacific, so... yeah. I can see why they would want it. Look at the viral publicity they're getting from this title. Everybody needs publicity to survive.

    Frank Reed

    *Speaking of science fiction, I can use the techniques discussed above to prove that Star Trek: the Next Generation never left near-Earth space. Turns out those starships may be fake! All that money... swallowed up by graft and corruption. Starfleet is a damn lie.

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