This is more of a hypothetical question.

Say space travel at near light speed was possible, and I wanted to travel in my spaceship to some distant star many light years away. At the time and location of my departure, looking out at space, I could measure that star's coordinates and adjust my spaceship's initial direction of motion accordingly.

However, because of the star's large distance, there would be a deviation between the coordinates I measured, and its "actual coordinates" (because of the amount of time it would take light to travel from the star to me). Even more so, in the time it would take me to reach the coordinates I have measured, the star would have moved even more from its original coordinates (supposing it has some velocity).

This means my spaceship would just miss the star. The greater the distance of the star, the greater the deviation.

Another factor to consider: because the spaceship is travelling at near light speed, it will have no way of receiving information from outside its frame of reference (relying on light for receiving information), so the spaceship could not adjust its direction of motion so as to not miss the star.

In this case, near light-speed space travel to faraway stars would not be very practical. Is there something I am missing? Are there ways of solving the problems presented?

  • 3
    $\begingroup$ You can still see the star you're heading towards. Its light will be severely blue-shifted, though. $\endgroup$
    – PM 2Ring
    Aug 13, 2022 at 1:35
  • 7
    $\begingroup$ Why would you travel towards the apparent position of the star instead of taking account of the relative velocities in the first place? $\endgroup$
    – ProfRob
    Aug 13, 2022 at 7:11

1 Answer 1


The most precise way to measure the distance of a star is by parallax, measuring the angles to the star from two points in the Earth's orbit that are separated by two Astronomical Units (AU) six months apart. The distance to the star can be calculated from the tiny difference in angles.

Astronomers have been inventing more and more precise techniques for measuring smaller and smaller angles over the last few centuries. And they will continue to do so for all the decades, centuries, or millennia it will take to invent spaceships capable of travelling at almost the speed of light.

So when a space ship starts for a distant star, it will know fairly exactly how far away it was when the light reaching Earth at the time was emitted. And thus they will know fairly exactly how long that light was emitted and how long a time there has been for the relative positions of the two stars to change.

In shooting there is technique called "leading the target", not aiming in the present direction to the target, but to where the target will be when the bullet or cannonshell arrives.

By noting the shift in spectral lines in the spectrum of the star, astronomers measure how fast the star is getting closer to or farther from the Earth.

By measuring changes in the direction to the star over time, astronomers will know how fast the star will be travelling sideways compared to Earth.

And computer programs can easily calculate the past and future positions of stars compared to Earth, once enough data has been secured.

Here is a link to a table of calculated past and future close passes between the Sun and other stars within a few million years of the present time.


And if a future society has spaceships which can travel almost as fast as light, they will send manned or unmanned observatories outside of he solar system to make parallax observations using a much wider baseline than the Earth's orbit, which has a maximum width of 2 AU.

If a star is observed from two positions, each position 1 light year to the side of the line between Earth and the star, the two positions will be 63,241.077 times as many AU apart as in Earth based observations, so parallaxes taken with equally precise techniques would result in distances 63,2141.077 times as precise.

If a star is observed from two positions, each position 1 parsec to the side of the line between Earth and the star, the two positions will be 648,000 times as many AU apart as in Earth based observations, so parallaxes taken with equally precise techniques would result in distances 206,264.81 times as precise.

Thus it will be simple to aim the spaceship ahead of the star's current direction so that the spaceship will arrive at the future position of the star instead of the present position of the star.

Furthermore, the velocity of a star is likely to be less than 1,000 kilometers per second relative to the Sun. Suppose the voyage takes 1,000 years. By definition, a light year is the distance travelled by light in 365.25 Earth days, so there are 31,557,600 light seconds in a light years. Thus at 1,000 kilometers per second, the star would move 31,557,600,000 kilometers in one year, or 0.00333564 of a light year. In 1,000 years the star would move at most 3.335640952 of a light year. If the starship has the ability to accelerate to almost the speed of light and then decelerate again, the ability to travel three more light years wouldn't be much of a problem, even if no adjustments were made for the velocity of the star before leaving Earth.

In a much shorter voyage to star much nearer Earth, and travelling much slower relative to the Sun, the position error would be much smaller.

And of course the course calculations for the voyage would take the future movements of the star into account, as I wrote above, instead of pointing the ship at the present position of the star.

Furthermore the starship should be able to see the target star for most or all of the journey, and account for various relativistic effects to keep track of its position. Thus they should be able to make course corrections when and if needed.


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