How to cure "universe-level directionlessness" in one go? Scientists have come up with this method...

Who are you? Traveler...

Where are you? Outside...

Where are you going? Far away...

If a person gives these answers to the three soul-searching questions, it's no big deal. Everyone has a period of confusion and rebellion. But if a spacecraft gives these answers, the scientists on the ground will go crazy. If you want to explore the sea of ​​stars, it is a basic task to always know your position. However, this is easier said than done. Let's talk about it briefly.

Traveler. Image source: NASA

How to find "North" in space?

Imagine this experiment: In your living room, draw the curtains tightly, so that you can't see your hand in front of you. Then, a host wearing night vision goggles holds your hand and asks you to walk a few steps to the left and right in the room and turn a few circles. In short, it is a random movement to ensure that you are completely dizzy. At this time, you are asked to tell where you are and point out the direction of the door. Can you still do it? Otherwise, how can we say "completely dizzy and unable to find the north"?

At this time, the host put a small fluorescent ball on the table with very weak light, which could only illuminate a small area, and said: "This is your dining table." Can we immediately point out the direction of the door? I am afraid that we still cannot know our location based on this mark alone.

Now the host takes out another fluorescent ball and says, "The little sofa you like to sit on is here." Now, our navigation skills will be activated immediately, and we can easily point out the location of each furnishing in the house. Using these two small lights as a reference, we can even walk backwards to the door. This is because for a place like a room that can be simplified into a flat map, there are two clear reference points that can determine our location.

So the question is, how does a probe traveling in the aimless space know its position and direction? Who am I? Where am I? Where am I going? The spacecraft determines its position in a similar way to us in a small dark room, except that it is more difficult to locate in the vast three-dimensional space. In order to accurately reach its destination, it must be given enough and clear reference points for it to determine its position, attitude and flight direction.

Only by looking towards home can we run towards the distance

Take the famous Voyager 2 probe as an example. It is equipped with a solar sensor and a Canopus tracker, which keeps track of the positions of the sun and the second brightest star in the sky, Canopus. With these two stars as reference, Voyager can "go backwards" to explore the solar system and the vast space.

You may ask: Why do we track the second brightest star? Why not Sirius, which ranks first? Because Sirius is too close to the ecliptic, the light path is easily disturbed by the glare from the sun. Canopus is far enough away from the sun, so it is an ideal reference for the direction.

In the days when Voyager was being developed, every program and every bit of memory was very valuable. Its method of determining that "Canopus is now appearing in the tracker" was still very primitive, that is, measuring the brightness of the star and transmitting it back to Earth for confirmation: "Yes, that's it, keep watching it."

The thoughtful reader will stop here: Wait a minute! You said that Voyager sends brightness data back to Earth for confirmation? But since the star that appears in the tracker may not be Canopus, and the probe's antenna may not be pointing toward Earth, how can you guarantee that Earth can receive the data?

Scientists have a very careful idea. They let the travelers use low-gain antennas with divergent beams to communicate with the earth instead of high-gain antennas with directional transmission during the first 80 days of the mission. At this time, the probe has not flown far, so even if it is not completely facing the earth, there is no problem for the two sides to communicate.

Today, when memory is not valuable, people store the spectral data of many bright stars in the detector, allowing it to make its own judgment based on brightness and spectrum.

Some star tracker manufacturers even put the angular distances between bright stars into a database. Since the positions of bright stars are very random, each distance data is unique and very reliable. For example, if the tracker sees two bright stars 27.1045° apart, it can immediately determine that they are Sirius and Betelgeuse by checking the database. After quickly locking the identities of both, it can measure the spectrum or find another star to compare, and then it can identify which is Sirius and which is Betelgeuse.

Voyager 2, it really lost...

So, what happens if a spacecraft suddenly loses track of where it is while flying? One possibility is that it deviates from its orbit, drifts farther and farther away until it is lost, and some spacecraft can be rescued.

For example, not long ago, the legendary probe Voyager 2, which had been flying in space for 46 years, was almost "lost". On July 21, NASA sent some instructions to Voyager 2, but there was a bug in it, which caused its antenna, which had been pointing to the earth, to deflect by 2°. What does 2° mean?

If you raise your arms horizontally for a while, your arms will certainly shake when you get tired. With the shoulder as the axis, the arm will deviate up and down by 1°~2°. At this time, the fingertips will deviate by only one or two centimeters, because the arm of an adult is only half a meter long. However, Voyager 2 has already flown 20 billion kilometers away. This small 2° angle deviation will cause its signal beam center to deviate by 700 million kilometers from the earth - you know, the earth is only 150 million kilometers away from the sun! The saying "a small error can lead to a big mistake" is very suitable for the universe. As a result, Voyager 2 lost contact.

Scientists on Earth slapped their thighs in regret while trying to retrieve it. On August 1, they found that the Deep Space Network that was communicating with Voyager could still smell a trace of the "I'm still alive" carrier signal. On August 3, scientists used the Deep Space Network's 100-kilowatt S-band uplink in Canberra to "yell" in the direction of Voyager 2: "Turn your head around~"

The Deep Space Network antenna in Canberra. Image source: NASA

Although the signal sent by Voyager 2 deviated from the Earth, the Earth would not mistake its position, and the roar hit it squarely. Although it tilted its head, it still heard it. 37 hours after the command was issued, the Earth received the normal signal of Voyager 2 again, and people really found it back.

If the call didn't work, would Voyager 2 be lost forever? In fact, the possibility of finding it is still quite high, because every once in a while, it will correct its posture and re-point its antenna towards the earth. October 15th, which just passed, was such a planned day, but it's better not to lose it...

Fine-tuning is essential

It is important for a spacecraft to know where it is, and it is also important to know and be able to adjust its attitude . If a satellite used to photograph the earth's surface is turned upside down and still doesn't know it, then all the work will be in vain. Fortunately, with the advancement of technology, we have no shortage of space positioning and attitude perception technology.

For example, if the heading, attitude or speed of a spacecraft changes in a short period of time, gyroscopes and accelerometers can be used to detect it. Gyroscopes use the principle of conservation of angular momentum to sense changes in direction, while accelerometers sense changes in speed. Just like the genius boy who was kidnapped by robbers in the movie, he can tell how many turns the car has turned (gyroscope) and how many lights it has waited for (accelerometer) even with his eyes blindfolded, and can even lead the police straight to the robbers' lair afterwards.

The star positions mentioned many times before can not only let the spacecraft know where it is, but also let it know its current posture. Just like when we are in our own room, even without referring to gravity, we can see that we are lying flat when we see the ceiling in front of us, our feet facing the wall, and our heads against another wall. After knowing our posture, the spacecraft can conduct observations wherever it points.

For example, the Hubble Deep Field is a composite of 342 images taken of a 2.6 arc minutes area in the Ursa Major constellation, while the Kepler telescope locks its sights between Cygnus and Lyra.

The observation area of ​​the Kepler telescope. Image source: NASA

For spacecraft such as communication satellites and meteorological satellites flying near the Earth, which need to face the Earth at all times, they also have to turn over every time they orbit the Earth. In addition to tracking stars or using gyroscopes to obtain attitude, there are some low-cost and reliable methods. For example, an infrared horizon instrument can quickly sense the circular outline of the Earth by comparing the infrared radiation of the Earth's atmosphere with the cold space, with the center of the circle being the Earth directly below the spacecraft.

The infrared horizon instrument obtains the contour of the earth by observing the sharp rise and fall of infrared radiation, and judges its own posture. The satellite is flying over Xi'an. The author made a schematic diagram

You may still have questions about star tracking: stars are distributed in three-dimensional space, not fixed on a sphere. Even if they are on a sphere, how can the position of stars remain unchanged as the spacecraft speeds through space? How can we put them in a database for reference?

This is because stars are too far away. Even the nearest star, Proxima Centauri, is 4.22 light years away. Voyager 2 has been flying for 46 years and has only reached 1/2000 of the distance to Proxima Centauri! This is like putting us in the center of a circle with a radius of two meters, and asking us to move one millimeter in 46 years, and asking us if we feel any changes. In the eyes of the spacecraft, except for the sun, the positions of stars have hardly moved.

But if our spacecraft lives forever, or if we simply have a "Wandering Earth" and keep flying and watching, the positions of stars in our eyes will gradually change as we travel between the stars, and the familiar constellations will also become distorted, and the existing attitude perception methods will become ineffective.

Of course, there are two solutions. One is to use more distant galaxies as references. They are tens of millions of light years away from us, larger in scale, and therefore more stable. The other is to use more information about stars, not only their positions, but also their distances, proper motion, etc., so that the spacecraft can calculate how the position of the reference star will change when it flies to a certain place. In order to do this, we have to measure the distance to the stars very accurately.

Summarize

It is important for a spacecraft to know its position and attitude, which requires a reference object, and the most commonly used reference object is the stars. As humans continue to move towards the sea of ​​stars, our star maps will become more and more accurate and larger, helping more spacecraft fly to the distance.

Planning and production

Author: Qu Jiong, popular science creator

Review丨Liu Yong, Researcher, National Space Science Center, Chinese Academy of Sciences

Planning丨Ding Zong

Editor: Bai Li