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I was watching this video essay discussion about space elevators, and on a bit of a tangent, the physicist talks about difficulties with interstellar missions, specifically comments about navigation like

...galactic navigation is an unsolved problem...

and

...we don't know where the Earth will be in, like, 8000 years. We can't do that math, it's non-linear dynamics, we cannot accurately predict where other stars would be in the time that it would take for us to get to them...

Basically, is this true?

I was under the impression that, yes, on long time scales the movements of celestial bodies become rather unpredictable because error builds up, however on cosmic time scales, 8000 years is rather "short" and still well within our ability to predict, especially if only considering our local stellar neighborhood. Given that we have several "realistic" proposals that could send probes on interstellar missions with "today's" technology including nuclear proposals or externally propelled ones like Breakthrough Starshot that could do the trip in, lets say, less than 1000 years, are our current navigational/mathematical/predictive skills good enough to actually reach the target? Granted, Starshot has additional difficulty because it can't course correct at all, but still.

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  • $\begingroup$ Related: en.wikipedia.org/wiki/… $\endgroup$
    – PM 2Ring
    Nov 30, 2023 at 14:57
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    $\begingroup$ I wonder if the problem is more with making course corrections even if you had accurate prediction data. You can do your best to make your trans stellar injection burn accurate, and even tidy it up a bit after obtaining tracking data after the burn. But any tiny error will be greatly magnified as the spacecraft travels for dozens of trillions of miles. By the time you realize that your spacecraft is going to miss the target star by say 10 billion miles, there may not be enough fuel onboard to make the needed correction. $\endgroup$ Nov 30, 2023 at 17:37
  • $\begingroup$ @StevePemberton I guess there are two variants of the question here: one more relevant to Starshot which is "unguided" so we need to get it right on day one and an actual powered probe that has some sort of long acceleration and deceleration phase which it can theoretically use to correct out error detected through nearer spatial or chronological observation of the target. $\endgroup$
    – Dragongeek
    Nov 30, 2023 at 18:02
  • $\begingroup$ Dragongeek - I would think that the quoted physicist was assuming that interstellar spacecraft will have some method of maneuvering, and having now watched the clip yes she does seem to be referring to a current inability to accurately predict planetary and stellar position over time to the level needed for interstellar navigation, and even communication. However one YouTube content creator, even one with a PhD is just one data point, which I realize is why you are asking the question. It would be interesting to get more information about the potential challenges. $\endgroup$ Nov 30, 2023 at 18:46
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    $\begingroup$ Navigation is the least of the problems Starshot would have to overcome. $\endgroup$ Nov 30, 2023 at 20:25

1 Answer 1

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*Question: Using today’s technology, is interstellar navigation possible?

Answer: Yes. It is even possible with yesterday's technology

Navigation: "the … activity of accurately ascertaining one's position and planning and following a route." Oxford Dictionary

Interstellar navigation in our neighborhood (say, within 10 parsecs) is not an insurmountable problem.

Despite the claim in the internet video, the peculiar velocity (absolute motion) of the Sun and neighboring stars has been measured and is expected to remain unchanged for centuries. Stars do move, but when in low density (like our neighborhood) and over short periods (centuries) they move in straight lines at constant velocity. So their relative positions are predictable with high accuracy over the duration of a few centuries.

There are several methods a spacecraft can use to ascertain its position during the voyage.

1) Dead reckoning using inertial navigation. This is established technology. Unfortunately, errors accumulate rapidly between fixes so its usefulness would be very limited on a long voyage.

2) X-ray Pulsar-based navigation gives accurate spacecraft position (relative to the mission starting point). Current technology gives positions to within 5km without cumulative errors. It is the equivalent of “galactic GPS”. It has been demonstrated by XPNAV-1 spacecraft and SEXTANT project on ISS. https://en.wikipedia.org/wiki/Pulsar-based_navigation

3) Celestial Navigation. The relative position of “neighborhood” stars is well known. As an interstellar voyage progresses, the angles between these stars and their position against the celestial sphere will change. Sextant angles and trigonometry would give the spacecraft’s position, only limited by the accuracy of the measurements. Traditional sextants are accurate to within 1/600th of a degree (0.1 mile on the Earth’s surface). If stars are, on average, 4 light years apart, that would correspond to positional accuracy of a few astronomic units during an interstellar voyage.

4) Piloting (The process of navigating using fixed points of reference) using the Constant Bearing Method. In marine and aviation navigation, if an approaching object (stationary or moving) maintains a constant bearing (apparent angle from the vessel’s heading), you are on a collision course. This applies to a lighthouse, a log drifting in the current, or another vessel under way. It also applies to stars during an interstellar voyage. Just point the nose of the ship at the destination star and keep it there. You will hit it. If the target star is drifting against the celestial sphere, you’re off course. “Second star to the right, and straight on till morning”

5) Target Magnitude. Light intensity falls with the square of the distance. Your target star will increase in apparent magnitude for the entire voyage (assuming it is not a variable star), An accurate measure of the destination star's brightness will give its range.

If we put Buck Rogers in his ship with just a slide rule, photometer, sextant and ephemeris, he could find his way to Barnard’s Star

enter image description here

Buzz Aldrin gearing up for interstellar navigation

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    $\begingroup$ Do you have any source on the 10 parsecs range, or is this just an estimate? $\endgroup$
    – Dragongeek
    Dec 1, 2023 at 14:15
  • $\begingroup$ The linked article says it can be accurate to within 5 km, which of course on the surface seems good enough. But it seems like it could create a problem calculating current trajectory. Trajectory to a star dozens of trillions of miles away requires very high precision to avoid excessive fuel use making course corrections. I wonder if 5 km position accuracy is good enough to keep a spacecraft on that very fine line that it has to follow in order to not use up all of its fuel with nearly endless minor course corrections during a trip of say 40 trillion miles. $\endgroup$ Dec 1, 2023 at 18:01
  • $\begingroup$ @StevePemberton ... Since XPAV position is available during the entire voyage, trajectory control would not require "endless" minor corrections. If the first correction burn corrected the trajectory with a delta V accurate to within 1%, the next correction burn would be expected after 100X the distance, and the 3rd after 10,000X the distance, ect. All to maintain a track within 5 km of the plotted course. $\endgroup$
    – Woody
    Dec 1, 2023 at 19:34
  • $\begingroup$ @Dragongeek ... I chose "10 parsecs" as an arbitrary distance since "neighborhood" is not a recognized astronomic unit. There are about 50 known stars within this distance, and travel time to them is within 300 years at 0.1c $\endgroup$
    – Woody
    Dec 1, 2023 at 19:39
  • $\begingroup$ From my understanding of pulsars, the number of pulsars that we can observe on or near Earth is only a very small fraction of the total amount: we only see those pulsars where the pulsar's "beam sweep paths" just happen to intersect the Solar System. If we move a couple light years, will we still be within the "beam path" of the navigational pulsars we see here on Earth (or in the Solar System)? $\endgroup$
    – Dragongeek
    Dec 2, 2023 at 19:07

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