Worried about getting lost in space? A new method can help you find your way home, and it's really good

New interstellar navigation system

Artist's impression of the Voyager probe heading for interstellar space (Image courtesy of NASA/JPL)

The universe is vast and boundless. If humans want to explore the depths of the Milky Way, an accurate and reliable navigation system is essential. Someone proposed a new idea to achieve interstellar navigation with the simplest possible technology, that is, to use pairs of stars to establish a galaxy-level reference system.

Interplanetary navigation within the solar system relies on a navigation system based on the Earth. The ground sends radio waves to the spacecraft, which receives and sends back echoes, and then calculates the distance between the spacecraft and the Earth through the delay time between the two signals; at the same time, our real-time monitoring of the spacecraft can obtain its position in the air. Combining these two pieces of information (the position of the spacecraft in the air and its distance from the Earth) we can obtain the precise positioning of the spacecraft in the solar system and transmit this data to the spacecraft.

The Doppler effect of radio waves is used to estimate the speed of a spacecraft. There are signal receivers scattered around the earth, and the echoes from the spacecraft arrive at these receivers at different times. By measuring the data from different receivers and combining them with the position parameters of the spacecraft, we will get a complete set of six-dimensional data, including the three-dimensional coordinates and three-dimensional speed of the spacecraft.

This method works only because of the radar network on Earth that constantly sends and receives signals. But it can only provide navigation for spacecraft within the solar system, and barely cope with the Voyager twin probes that have already flown out of the solar system.

Spacecraft traveling between stars need new autonomous navigation systems. In theory, the spacecraft's own clock pulses and gyroscopes can do this. But for interstellar travel that takes decades, a tiny miscalculation or an unsure decision will cause the spacecraft to deviate from its course.

Another option is to use pulsars for positioning. Pulsars rotate continuously, causing them to flash or pulsate in a rhythmic manner. Since each pulsar has a different rotation period, they act like lighthouses in deep space, guiding the direction of spacecraft. Unfortunately, this method only works in a small area outside the solar system, because interstellar dust will interfere with our measurement of the rotation period of pulsars. Once the measurement errors confuse these stars, we will be lost among the countless stars.

Peter Pan positioning method: the second star from the right

As mentioned above, we need an accurate, reliable and simple method to help interstellar spacecraft determine their positions. A preprint article published in the journal Science proposes such a solution: direct reference to star navigation.

The technique is based on the ancient principle of parallax. Hold a finger in front of your nose, close one eye and look at your finger with the other eye, and you will see that it seems to sway back and forth. This is parallax caused by changing the point of view. If you observe a distant object in the same way, you will find that the sway is much smaller.

It is based on the parallax principle that scientists can measure the distance of each star from the earth. Similarly, drifting spacecraft can also use this principle to locate themselves. Before launching a spacecraft, we can pre-load the position information of all known stars in nearby galaxies. As the spacecraft leaves the solar system at high speed, the system calculates the relative positions of the stars pair by pair. At this time, the stars closer to the spacecraft seem to have a large position shift, while the positions of stars farther away are relatively fixed.

By comparing the real-time measured data with the original data measured on Earth, the spacecraft can identify these stars and calculate the distance between the spacecraft and each star, thereby obtaining accurate three-dimensional coordinates.

Determining ship speed: relativistic effects

Calculating the speed of a spacecraft is a little more complicated, and is based on a strange distortion effect mentioned in the theory of special relativity. When you move fast enough, because the speed of light is constant, the objects you see are not actually where they actually are. Rather, they appear to be located forward in the direction of your movement. This is the aberration effect, and we can observe it here on Earth. As the Earth orbits the Sun, we see the positions of the stars wobble slightly back and forth.

The spacecraft must travel at high speeds, otherwise interstellar travel would take thousands of years instead of decades. As long as the speed is fast enough, the spacecraft can measure the aberration effect and know which star is not where it should be in theory and how much its position is offset, and from this it can calculate the three-dimensional speed of the spacecraft.

Using parallax measurement, the spacecraft gets real-time six-dimensional coordinates, telling it where it is and where it is going. The question is how accurate are the coordinates obtained by this method? According to the article, assuming that the number of stars that the spacecraft can measure is 20, and the measurement accuracy is 1 arc second (a unit of measurement, 1 arc second = 1/60 arc minute, 1 arc minute = 1/60 degree), then the error range of the coordinates is within 3 astronomical units, and the error range of the speed is less than 2 kilometers per second (that is, 1.2 miles per second).

One AU is the average distance from the sun to the Earth, or 93 million miles (150 million kilometers), so an error of three AU is about 279 million miles (450 million kilometers). That may sound like a lot, but compared to the thousands of AUs between stars, the error is negligible.

What's more, the stars we can accurately locate are far more than 20. We have loaded data of hundreds of millions of stars into the spacecraft system. It seems that the positioning of the spacecraft can achieve considerable accuracy.

All that remains is to build a spaceship that can travel among the stars.

BY:Paul Sutter

FY: Renee

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