A new proposal attempts to keep travel in the interstellar depths of the Milky Way as safe as possible: using pairs of stars to provide a frame of reference for the galaxy, and within our solar system, interplanetary spacecraft rely on terrestrial navigation systems.
When we send a radio signal to a spacecraft and it is received, the time delay of the response can be used to calculate the distance, and the spacecraft can also be monitored in the sky, and by combining all that information (location in the sky and distance from Earth), It can locate the spacecraft in the solar system and provide this information to the spacecraft itself.
The Doppler effect of those radio waves can also be used to estimate the speed at which a spacecraft is moving away from Earth. Also, by using dishes spread all over our planet, the delay from a spacecraft's signal arriving at one dish versus another can be measured.
And when we combine this data with the position information, we have a complete six-dimensional stabilization of the spacecraft: its three dimensions of position and its three dimensions of velocity.
This method relies on a network of ground-based radar systems, all of which are in constant communication with the spacecraft, and the technology works with spacecraft within the solar system and, hardly, NASA's twin Voyager sensors.
But any interstellar missions will need a new approach, and they will have to navigate independently. In principle, these spacecraft can use onboard systems, such as clocks and gyroscopes, but interstellar missions will last for at least decades, and small errors and doubts will result in those existing systems Onboard there is no doubt that the vehicles deviated from their course.
There is the option of using pulsars, which appear to flash at regular intervals, and since each pulsar has a unique rotation period, these objects can serve as reliable beacons for deep space missions, but this only works in a relatively small bubble near our solar system, Because rotational period measurements can be contaminated with interstellar dust, once you lose track of any pulsar, you're lost.
So interstellar spacecraft need a simple and reliable way to estimate their position within the galaxy, and a new paper recently published on arXiv.org offers just such a solution: the stars themselves.
This technique is based on a very old concept: parallax, if you stick your finger in front of your nose and alternately close your eyes, your finger will appear wobbly, the change in its apparent position comes from a new point of view when you move from one eye to another, and if you do the same exercise while looking at something Far away, this object will appear to vibrate less.
By looking, scientists were first able to measure the distance to the stars, and through the parallax, the spacecraft that roams away from home can get its directions, and before launching, scientists load the spacecraft with an accurate map of all the known stars in our vicinity of the galaxy, and then, As the spacecraft moves away from the solar system, it measures the relative distances between multiple pairs of stars.
As it moves, the stars closest to the spacecraft appear to shift significantly, while the distant stars remain relatively stationary. By measuring multiple pairs of stars and comparing the measurements with the original Earth-based catalog, the spacecraft can tell which stars are and how far away from those stars are, giving the spacecraft a precise three-dimensional position in the galaxy.
Getting the speed of a spacecraft is a little trickier, and it relies on a strange advantage of special relativity, given the finiteness of the speed of light, if you're moving fast enough, things can appear in different locations than they really are. Specifically, the position of the object will appear to have Offset in the direction of your motion, this is called the diffraction effect, and it's measurable from Earth: As our planet orbits the sun, the stars seem to swing gently back and forth in the sky.
As long as the spacecraft is moving fast enough (if an interstellar mission is to last for decades, not millennia, it must), the onboard systems will be able to measure this deviation. By noting stars that have been shifted away from their expected positions and magnitude, the spacecraft can determine its speed in 3D.
By taking parallax measurements, the spacecraft can retrieve its full hexagonal coordinates within the galaxy, it knows where it is and where it is heading.
How accurate is this technique?
According to the research paper, if the spacecraft could measure the positions of just 20 stars within 1 arcsecond of accuracy (an arcsecond is 1/60 of an arc minute, which is the same as 1/60°), it could determine their position within the galaxy with an accuracy of 3 AU. (AU) and its speed is within 2 kilometers per second, 1 AU is equal to the average distance between the Earth and the Sun, about 150 million km, so 3 AU is equal to about 450 million km.