Who are you? Traveler... where are you? Outside...where are you going? Go to a distant place... Faced with the three questions of the soul, if the person who gives these answers is an individual, then it is no big deal. Who hasn't been confused and rebellious? But if it were a spacecraft that answered the question like this... the scientists on the ground would go crazy on the spot.
If you want to explore the stars and the sea, knowing your position at all times is a basic topic. However, this is easier said than done. Let’s talk briefly about it next.
Traveler. Source: NASA
How to find "North" in space?
Let’s imagine an experiment like this: In your living room, draw the curtains tightly so that you can’t really see your fingers. Then, the host wearing night vision goggles takes your hand and takes a few steps to the left, a few steps to the right, and then turns around a few times in the room. In short, it is a random exercise to ensure that you are completely stunned.
At this time, if you are asked to tell your location and point to the direction of the door, can you still do it? How about saying "completely dizzy and unable to find the north"?
At this time, I saw the host put a fluorescent ball with very weak light that could only illuminate a small area on the table, and said: "This is your dining table." Can we immediately point to the direction of the door? I'm afraid it still won't work, because with this mark alone, we still have no way of knowing where we are.
Now the host takes out a fluorescent ball and says: "The little sofa you like to sit on is here." At this time, our navigation skills will be activated immediately and we can point out the location of every furnishing in the house.
Using these two small lights as a guide, we could even walk backwards to the door. This is because for a place like a room that can be simplified into a flat map, we can determine our position with two clear reference objects.
So the question is, how does a detector traveling in the untouched space know its position and orientation——who I am? Where am I? Where am I going?
When a spacecraft determines its position, it is similar to the way we do in a small dark room, except that it is more difficult to locate in a vast three-dimensional space. If it wants to accurately reach its destination, it must be given enough and clear reference objects for it to judge its position, attitude and flight direction.
Only by staring at the direction of home can you run towards the distance
The famous Voyager 2 probe is an example. It is equipped with a solar sensor and a Canopus tracker, and it always controls the position of the sun and Canopus, the second brightest star in the sky. With these two stars as reference, travelers can move forward "all the way backward" to explore the solar system and the vastness of space.
You may ask: Why should we track the second brightest star? Why not choose the number one Sirius? Because Sirius is too close to the ecliptic, the light path is easily disturbed by glare from the direction of the sun. Canopus is far enough apart from the sun that it is an ideal orientation reference.
In the era of developing Voyager, every program and every piece of memory was precious. Its method of determining "what is appearing in the tracker is Canopus" was still very primitive, which was to measure the brightness of the star and send it back to the earth to confirm: "Well, that's it, keep staring at it."
Thoughtful readers will say stop here: Wait a minute! You said the travelers sent brightness data back to Earth for confirmation? But since what appears in the tracker is not necessarily Canopus, and the detector's antenna may not be pointed at the earth, how can you ensure that the earth can receive the data?
The scientists are also very careful in their thinking. They asked the travelers to use divergent low-gain antennas to communicate with the earth instead of high-gain antennas for 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 in communication between the two parties.
Today, when memory is not valuable, people store the spectral data of many bright stars in the detector and let it make its own judgment based on the brightness and spectrum.
Some star tracker manufacturers even put the angular distances between pairs of bright stars into databases. Since the positions of bright stars are very random, each distance data is unique and very reliable.
For example, the tracker sees two bright stars separated by 27.1045°. When you check them in the library, you can immediately determine that they are Sirius and Betelgeuse. After quickly locking the identities of the two parties, you can then measure the spectrum or find another star to compare, and you can identify who is Sirius and who is Betelgeuse.
Voyager 2, it was really lost...
So, what would happen if the spacecraft is flying and suddenly loses track of where it is? One possibility is that they veered off course, drifting away until they were lost, and some spacecraft could be salvaged.
For example, not long ago, the legendary probe Voyager 2, which had been flying in space for 46 years, almost got lost. On July 21, NASA sent some instructions to Voyager 2, but there was a bug in it that deflected its antenna that had been pointing at the Earth by 2°. What is the concept of 2°?
If you hold your arms flat for a while, your arms will definitely shake when you get tired. Using your shoulders as the axis, your arms will shift up and down by 1° to 2°. At this time, the fingertips will only shift by a centimeter or two. This is because adults' arms are only half a meter long.
However, Voyager 2 has flown 20 billion kilometers away. This small angular deviation of 2° will cause the center of its signal beam to deviate from the earth by 700 million kilometers - the earth is only 150 million kilometers away from the sun! The so-called "a slightest mistake can make a thousand miles", this sentence is so suitable for the universe. As a result, Voyager 2 lost contact.
Scientists on Earth slapped their thighs in regret while trying to get it back.
On August 1, they discovered that the deep space exploration network that communicates with travelers can still smell a trace of the "I am still alive" carrier signal. On August 3, scientists used the 100-kilowatt S-band uplink of the Deep Space Exploration Network in Canberra to "yell" in the direction of Voyager 2:"You should turn your head around~"
Deep Space Network antenna in Canberra. Source: NASA
Although the signal sent by Voyager 2 deviated from the earth, the earth could not mistake its position, and the roar hit it without any bias. Even though it tilted its head, it still heard it. 37 hours after issuing the command, the earth received the normal signal from Voyager 2 again, and people really found it back.
If this cry doesn't work, will Voyager 2 be lost forever? In fact, the possibility of retrieval is still quite high, because every once in a while, it will correct its attitude and re-align its antenna with the earth. October 15th, which just passed, was such a day in the plan, but it was best not to lose it...
Fine adjustments are 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. Suppose a satellite used to photograph the earth's surface is turned upside down without knowing it, then everything will be in vain. Fortunately, with the advancement of science and technology, we have no shortage of space positioning and attitude sensing technology.
For example, short-term changes in the course, attitude or speed of the spacecraft can be detected using gyroscopes and accelerometers. Gyroscopes use the principle of conservation of angular momentum to sense changes in direction, and accelerometers sense changes in speed.
Just like the genius boy kidnapped by the robbers in the movie, he can tell how many turns the car took (gyroscope) and how many lights it waited for (accelerometer) while blindfolded. Afterwards, he can lead the police to the robber's lair.
The star orientation mentioned many times before not only allows the spacecraft to know where it is, but also allows the spacecraft to know its current posture.
Just like when we are in our own room, even without reference to gravity, when we look at the ceiling in front of us, with our feet facing the wall and our head against another wall, we know that we are lying flat. After understanding its own posture, the spacecraft can point where to conduct observations.
For example, the Hubble Deep Field was synthesized after taking 342 images of a sky area of only 2.6 arc minutes in the Ursa Major, while the Kepler telescope locked its sights between the constellations of Cygnus and Lyra.
Kepler telescope's observation area. Image source NASA
For spacecraft such as communication satellites and meteorological satellites flying near the earth that need to face the earth at all times, they also have to turn around every time they circle 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 can quickly perceive 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 outline of the earth by observing the steep rise and fall of infrared radiation, and determines its own attitude. The satellite is flying over Xi'an. Schematic diagram produced by the author
You may still have questions about star tracking: stars are distributed in three-dimensional space, rather than fixed on a sphere. Even on a spherical surface, as the spacecraft speeds through space, how can the position of the stars remain unchanged? How can I check it in the database?
This is because stars are so far away that even the closest ones to usProxima Centauri is 4.22 light-years away.
Voyager 2 has been flying hard for 46 years and has just reached one-2000th of the distance to Proxima Centauri! It's like putting us in the center of a circle with a radius of two meters, asking us to translate it one millimeter over 46 years, and asking if we feel any change. In the eyes of the spacecraft, except for the sun, the position of the stars has almost never moved.
But if our spacecraft has an eternal life, or we simply come to a "wandering earth" and keep flying and watching, as we travel among the stars, the positions of the stars in our eyes will gradually change, and the familiar constellations will also become out of shape, and the existing attitude sensing methods will be ineffective.
Of course, there are two solutions. One is to use more distant galaxies as reference. They are tens of millions of light years away from us, have larger scales, and are therefore more stable.
The second is to use more information about the stars, including not only the orientation, but also the distance, proper motion, etc., so that the spacecraft can calculate how the orientation of the reference star will change when it flies to where it flies. 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 a star. As humans continue to move toward the sea of stars, our star maps will surely become more and more accurate and larger, helping more spacecraft fly to distant places.
Planning and production
Author丨Qu Jiong Popular Science Creator
Review丨Liu Yong, researcher at the National Space Science Center, Chinese Academy of Sciences
Planning丨Ding Ao
Editor丨Bai Li