From launch to land could a person travel in space with only mechanical devices like a sextant? With enough experience what would be the minimum devices needed? Which vehicle currently would be easier to fly this way?

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    $\begingroup$ Just a sextant? No rocket? Then no! :). Joking aside, i think it depends on how much accuracy you need. I don’t think sextants really help much in take-off or landing, and may only be useful in orbit. In addition to the sextant, a really accurate clock would be extremely helpful! $\endgroup$ – Paul Nov 13 '18 at 21:37
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    $\begingroup$ I think the answer is yes, but the lack of the precision would result an elevated fuel usage. $\endgroup$ – peterh - Reinstate Monica Nov 14 '18 at 0:59
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    $\begingroup$ I think your question is fine without the new last sentence, which is really a different question. Your question is about navigation, instrumentation, and human piloting. Asking about the vehicle would require a different answer, so it should be done separately. $\endgroup$ – uhoh Nov 14 '18 at 1:56
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    $\begingroup$ @uhoh orbit is different than interplanetary travel. What I meant by "on Earth" I meant within its spherical coordinate system which orbit is still part of. But in space that relative coordinate system becomes lost. $\endgroup$ – anon Nov 14 '18 at 3:16
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    $\begingroup$ The navigation on the oceans of Earth required a very precise clock together with the sextant. The determination of longitude was not possible without a precise clock. Therefore a sextant and a clock should be used in space. $\endgroup$ – Uwe Nov 14 '18 at 10:45

Depends on that do we call a "computer". Would a slide rule count? This extremely simple, hand held device can do multiplication, division, and also functions such as exponents, roots, logarithms and trigonometry. Some included specialized features for aviators. Or may I bring my Curta into spacecraft? It also has been highly favored by commercial and general-aviation pilots. Such simple, mechanical devices would make work of human much easier and, I think, possible.

Where the time is critical, it may often be possible to use prepared plans and pre-computed tables for adjustment. Where errors are critical, three or more humans can work in parallel, picking the result that coincides.

Apart the speed of calculations, there is fundamentally nothing in the math that computer can do and human cannot.

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    $\begingroup$ A computer as a calculator is faster than a human, but no more precise. A computer as a pilot, however, can be far more precise in the timing and duration of course corrections. A computer-controlled craft could make maneuvers that a human would have great difficulty replicating, even with perfect instruction laid out. $\endgroup$ – Nuclear Hoagie Nov 16 '18 at 14:33
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    $\begingroup$ Surely, some kind of watch/chronometer is also required and for more accuracy it could be connected to the controls of spacecraft to fire engines at the needed time and duration. But I suspect that a second accuracy human can easily achieve may be enough for actions with no feedback. Where there is a feedback like in keeping the course or orientation humans are not slow. $\endgroup$ – h22 Nov 16 '18 at 18:33

You can travel in space without any navigation devices or computers at all, establishing suborbital flight and orbital flight is done by the rocket, it's accuracy there that matters and early manned rockets had no computers. De-orbiting is a matter of setting up the spacecraft to the right attitude for retro fire, which can be done by eye if need be, early spacecraft used gyros for that with a manual backup.

If you want to get anywhere specific in space you have to be accurate. A spacecraft must be held in a specific position and engines must fire at exactly the right time and for the right amount of time. Gyroscopes and timers can give you a fair amount of accuracy, so you don't need a computer for that (although it really helps).

The biggest challenge without a computer would be the mathematical calculations, which can be done by hand as long as you have a room full of human computers to back you up. Just by yourself it will take you longer perform some calculations than they are worth - by the time you have the answer you're past the point you needed them.

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    $\begingroup$ If you use a sextant, a chronometer and the nautical tables to determine the position of a sailing ship, your position may have changed by about a nautical mile when you are finished with calculations. But a spaceship is much faster, about 3000 to 10000 times. $\endgroup$ – Uwe Nov 14 '18 at 16:02
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    $\begingroup$ @Uwe, Re, "...your position may have changed..." I don't know about this modern sat-nav era, but back in the day, navigators were keenly aware of that fact. The hour-by-hour navigation of the ship always was done by dead reckoning. And that was occasionally corrected by obtaining "fixes" (star sights, radio direction finding, etc.). If you think you're following a certain course line on the chart, but you compute that ten minutes ago you passed through a point not on that line, you draw a parallel line through the "fix" point, and move your estimated position to the new line. $\endgroup$ – Solomon Slow Nov 14 '18 at 16:50

Yes, as long as you don't care where you end up

Early spacecraft used electrical controls, but had no computers:

  • Six Vostok each carried one person to orbit. The first flight carried the first man and the last flight carried the first woman into space.
  • Six Mercury flights each carried one American astronaut into space, four of these into orbit. Although the vehicles had no computers, they were supported by IBM 701 computers on the ground (offline and not real-time control).
  • Voskhod flew two unmanned and two manned flights. The first manned flight carried three cosmonauts. The second manned flight carried two cosmonauts for the first spacewalk.

None of these flights had a precise target location, either in space (no rendezvous with a space station, lunar module, or satellite) or at re-entry (no runway or landing pad). As long as you landed somewhere in Siberia or the Pacific, an entire army or fleet of ships would find you.

The first computer in space was for Project Gemini. Its flight computer was able to perform four functions:

Ascent - serves as a backup guidance system. The switchover is manually controlled by the astronauts.

Orbital flight - provides a navigation capability to the astronauts to determine the time of retrofire and to select the landing site for safe reentry in the case of an emergency. (on extended missions ground data may become unavailable when ground data network rotates out of the orbital plane.)

Rendezvous - serves as primary reference by providing guidance information to the astronauts. The orbit parameters are determined by the ground tracking which are then sent to the spacecraft; the guidance computer was responsible for processing the information along with sensed spacecraft attitude. The information was presented to the astronauts in terms of spacecraft coordinates.

Reentry - feeds commands directly to the reentry control system for automatic reentry or provides the guidance information to the astronauts for manual reentry.


Most of these weren't strictly needed for the Gemini missions, but rather testing the technology so it could be used for Apollo. However,

  • Gemini had a radar system, which essentially needs a computer to be useful. This was used to practice rendezvous and docking, which would be needed in the Apollo missions between the CSM and the LEM. Apollo also used radar to measure the distance to the lunar surface; otherwise it would have likely crashed.
  • Rendezvous is nearly impossible without a computer. Nearly every manned mission now uses rendezvous with another spacecraft in some way.
  • If you want to land in a precise area (e.g. shuttle runway, SpaceX landing pad), you need computer control.

One of the other answers is wrong about Apollo. The CSM and LEM each had a guidance computer, the LEM had an abort computer, and the Saturn booster had another computer.

So, if you don't care where the spacecraft goes, you don't need a computer.

  • $\begingroup$ Gemini should test the rendezvous maneuvers nececessary for the Apollo mission using the LM. Therefore Gemini had a rendezvous radar. $\endgroup$ – Uwe Nov 14 '18 at 12:05
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    $\begingroup$ Mercury did have specific targets for re-entry; Earth-side tracking and computers were used to pick the right time to perform the retrofire maneuvers. The final two crewed Mercury flights both landed within 10km of their targets -- remarkably good considering they had no steering capability once they hit atmosphere. (The less said about MA-7 the better, though.) $\endgroup$ – Russell Borogove Nov 14 '18 at 16:36
  • $\begingroup$ -1 I disagree with your as long as you don't care where you end up banner-sized heading. This implies that if one does care, it would not be possible, but you don't support that conclusion at all in your answer. I think it is unsupported, vote-attracting eye-candy. Would it be possible to add some support to that conclusion? Or perhaps adjust the wording to match what your answer does support? $\endgroup$ – uhoh Nov 16 '18 at 4:29
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    $\begingroup$ Three answers point out that this probably is not impossible (1, 2, 3) and so I'll be asking a follow-up question on this shortly. $\endgroup$ – uhoh Nov 16 '18 at 4:34
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    $\begingroup$ Using a sextant won't help you dock with another spacecraft; or land on the moon, a runway, or a landing pad. I stand by my assertion. $\endgroup$ – DrSheldon Nov 16 '18 at 4:39

Is it possible? Certainly, and it has been done as stated.

It just takes a lot more time to calculate your burns and trajectories by hand than it does using computers, and performing them by hand rather than relegating things to a computer controlled automated flight control system requires a very steady hand and excellent timing.

Mind that the calculations aren't rocket science, and could probably be taught to anyone with a decent aptitude for mathematics and spatial insight. But they're not trivial either, having some preprinted worksheets to fill in your numbers and guide you through the calculations would speed things up enormously.

That said, using computers it's just a lot more convenient and reliable.


Robert A. Heinlein is a Science Fiction author well known for the use of extremely accurate calculations in launching and flying spacecraft. His novel The Rolling Stones is particularly noted for the use of a slide rule as the means of calculating space navigation.

Now weirdly, this future was already obsolete by 1977. Calculators had been invented, just about. But I didn’t notice that. I just took a sliderule for granted as a futuristic whatnot. Source; Pass the slide rule: Robert Heinlein’s The Rolling Stones

Fictional work or not, if Heinlein says you can calculate space navigation with a slide rule, then you can.

Related sister site question Heinlein slide rule calculations


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