A spaceship sails in space, and navigation is a problem. Because the GPS navigation system formed by the earth orbit satellite network is useless in places far away from the earth. If you take just one wrong turn en route to a distant planet, your chances of getting back on track are slim. You need some form of interplanetary navigation system to help you see where you are.
For this task, astronomers have turned to some of the most extraordinary objects billions of light-years away: quasars. Quasars, the brightest class of objects in the universe, not only guide us with precision to distant stars, but also help us learn more about Earth itself.
Neither the earth nor the stars are the best frames of reference
As we all know, to determine the position of an object, we need to establish a coordinate system as a reference. The Earth has its own coordinate system, which is a grid of lines of longitude and latitude. It can pinpoint the position of any object on the surface relative to the Greenwich meridian and the equator. If the distance from the center of the earth is used as the third coordinate, the aircraft, clouds and satellites above the surface can also be precisely positioned.
It is certainly possible to extend this frame of reference further into cosmic space (just like a ray, any frame of reference can be extended arbitrarily). But there is a problem, because this reference system rotates and revolves together with the earth, so the celestial bodies leaving the earth system are changing their latitude and longitude every moment. For example, assuming that a celestial body is above 20° north latitude and 110° east longitude of our earth, that is, about Haikou City, Hainan Province, if the earth does not revolve and rotate, after 6 hours, the celestial body should move westward according to its own speed. The sky above 20° north latitude and 95° east longitude, that is, about the sky above Nay Pyi Taw, the capital of Myanmar. But during this time, the earth rotated 90° eastward (regardless of the earth’s revolution, but only considering the earth’s rotation), then this celestial body will actually appear in the sky above 20° north latitude and 5° east longitude, that is, approximately in Algeria, Africa over the southernmost tip. Calculated in this way, the speed of this celestial body is 7 times the original speed. So if you calculate their moving speed in this frame of reference according to this, you will get very absurd results: for example, the closest star to the solar system, Proxima Centauri (Alpha Centauri), is flying at a speed thousands of times faster than light!
Why such absurd results? Because the velocity of the celestial body relative to us = the angular velocity of the celestial body relative to the earth × the distance between the celestial body and us. Now, due to the Earth’s rotation, the farther away it is from us, the faster the object will move relative to us. However, distant celestial bodies in the universe are often more than a few light-years away from the earth, so it is very common to calculate “superluminal speed”. But the result calculated in this way obviously cannot reflect the real motion of the object.
Obviously, in interstellar navigation, we need a relatively stationary frame of reference. What is relatively immobile in the sky? Yes, distant stars! Until the end of the 20th century, astronomers established their frame of reference to a few distant stars, using the positions of those stars as points of reference. An observer standing on Earth can measure the angle between, say, a comet and a reference star, giving the comet’s coordinates in the sky. This frame of reference, which has been in use until recently, uses more than 1,500 stars to mark the sky.
A quasar, the brightest type of celestial body in the universe, has a huge black hole hidden in its center.
But even that has drawbacks. The positions of the stars are not perfectly fixed, they are moving slowly across the sky. Although this movement generally has little effect, it still causes an error of about a few millionths of a degree in the angle.
Based on the distance between the earth and the moon, the error is a few meters. The impact on the execution of lunar missions is not too great. Based on the distance from Earth to Mars, the error is about 1 kilometer. This has a certain impact on the execution of Mars missions. The longer the distance, the greater the effect.
Ideal reference point – quasars
Now is the time for a radical overhaul! Since the 1990s, astronomers have moved away from relying on stars hundreds of light-years away and looking beyond billions of light-years. Because the farther away an object is, the more fixed its position on the sky appears to be, making them ideal points of reference.
But they have to be bright enough to be clearly visible from Earth. And what is the brightest thing in the universe? Quasars! Quasars are places where matter emits radiation due to intense friction before it falls into a supermassive black hole. Another advantage of using quasars for marking is that supermassive black holes are often at the center of galaxies, and their masses are often billions of times that of the sun. They are generally stable and will not easily leave their place due to accidental collisions.
Although quasars can emit visible light, as far as current human technology is concerned, it seems that using the radio waves they emit can be used for more accurate positioning. That’s because pinpointing the quasar’s position requires the cooperation of multiple telescopes distributed around the globe. Visible light is more likely to be blocked by obstacles in space due to its short wavelength, so it is difficult for visible light from quasars to be received by multiple telescopes around the world at the same time, while radio waves do not have this problem due to their long wavelength.
So, astronomers are now using a technique called very long baseline interferometry (VLBI) to focus radio telescopes around the world on the same quasar radio source at the same time. These radio telescopes can all receive the same radio signal, but because they are located in different places, the angles relative to the quasar are also different when observing, so the received signals are sequential in time, which is called “time delay”. According to this time delay, the position of the quasar can be located. Astronomers can currently measure time delays down to 10 picoseconds (1 picosecond = 10-12 seconds).
The most accurate frame of reference for space navigation to date
In 1998, Patrick Charlotte of the University of Bordeaux in France and his team released a new frame of reference, the International Celestial Reference System (ICRF-1). After a major revision in 2009, it was upgraded to ICRF-2. In 2018, it was upgraded to ICRF-3 again.
The new version of ICRF-3 selected 303 quasars as reference points, and all of them were selected from the quasar sources with the most stable brightness and the most accurate position measurement so far. The angular positioning of these quasars is on average accurate to about 30 microarcseconds, or eight billionths of a degree. To give you an impression of this, let’s use an analogy: this accuracy is equivalent to being able to identify a bacterium from an airplane 10,000 meters away.
For convenience, the center of the ICRF-3 reference system is set at the barycenter of the solar system. All celestial bodies in the solar system, including planets, satellites and even the sun, move around this center of mass. The north-south direction of this frame of reference is chosen to be parallel to the earth’s rotation axis in order to be consistent with the meaning of north-south on the earth, but considering that the direction of the earth’s rotation axis will also change slightly over time, it is specifically designated as the earth’s rotation axis In the direction pointed on January 1, 2000.
In addition to the main 303 quasars, ICRF-3 also includes about 4000 other quasars distributed in the whole sky, which can also be used as reference points for navigation, although compared with the 303 quasars, their Reliability is slightly worse. In this way, ICRF-3 covers the entire sky area and can be used for all-round navigation.
With such a space navigation system, astronomers can detect and correct any deviation from the planned course in time by observing the angular position of the spacecraft relative to a nearby quasar.
Help us understand the planet itself
Even more surprisingly, the ICRF frame of reference is also being put into use here on Earth. Through it we can measure the Earth’s position in space very precisely. The rotation of the earth in 24 hours is not absolutely uniform, because various violent atmospheric and geological movements will also have a slight impact on the speed of the earth’s rotation. By measuring the Earth’s rotation rate relative to the ICRF, tracking changes in these rotation rates lets us know when we need to insert leap seconds into Universal Time to keep our clocks in sync with Earth’s rotation.
Precisely measuring changes in the Earth’s rotation can tell us a lot more. For example, based on intense atmospheric and geological movements, combined with subtle changes in the Earth’s rotation speed, it can be used to verify various climate models and Earth structure models.
In the very long baseline interferometry (VLBI) technique, radio telescopes around the globe focus on a radio source at the same time.
Changes in the Earth’s rotation also affect GPS and other satellite navigation systems. These systems are calibrated with extremely precise atomic clocks that keep time on all satellites in sync. But after the time synchronization is realized, new problems arise. What GPS wants to locate is an object that moves with the earth, and since atomic time is not affected by weather changes and the internal movement of the earth, as a result, there is a slight difference between the position that the earth should turn to and the actual position in the time displayed by the atomic clock. Therefore, the GPS positioning is often slightly different from the real longitude line on the earth. As long as a few days, the error can accumulate to a few centimeters. This error is fine for the average driver or hiker, but not for Earth scientists, so satellite navigation systems are recalibrated from time to time with new measurements of VLBI.
ICRF also helps us measure the slow motion of Earth’s plates. The quasar signals received by two telescopes on different continents will have a delay in time, and the time delay changes with the drift of the plate, so the change in the time delay can reflect the change of the plate drift.
The invisible black hole has actually created a precise navigation reference system in space. The black hole is so useful to us, which is probably beyond your imagination!
Very Long Baseline Interferometry
The mathematical problem involved in this technology is actually: a triangle △ABC, knowing the positions of the vertices B and C (where the two telescopes are located on the ground) and the length of BC, and knowing the difference between the other two sides (AC-AB), find The position of the third vertex A. This point A is not unique. In geometry, all points satisfying the condition form a half branch of a hyperbola.
Then there is another triangle △ABD, knowing the positions of the vertices B and D (the two telescopes on the ground are located) and the length of BD, and knowing the difference between the other two sides (AD-AB), find the position of the third vertex A . This point A is also not unique, and all points that satisfy the condition form a half branch of another hyperbola.