What? Why are satellites scheduled for morning and afternoon shifts?

On August 3, our Fengyun-3 satellite 06 was successfully launched into space. In many related news reports, we may see that it is called a "sun-synchronous morning orbit satellite." What does a "morning" orbit satellite mean? Are there afternoon satellites, evening satellites, etc.?

These are actually mainly related to the orbit and function of the satellite. Let’s talk about them briefly today.

What is the difference between morning and afternoon?

Satellite has early morning shift

Since the satellite launched this time is a meteorological satellite, let's take the meteorological satellite as an example. According to the business characteristics of meteorological satellites, they generally have two orbits: one is the geostationary orbit, and the other is the near-polar sun-synchronous orbit, also called the polar orbit.

We are most concerned about our own weather, so we need a satellite to keep an eye on China all the time. So from the ground, this satellite is always hanging above us without moving. We also know that the earth is rotating, so this satellite "stationary" above our country is actually rotating synchronously with the earth's rotation cycle. The double-numbered Fengyun satellites (such as No. 2 and No. 4) are all such stationary satellites, and their satellite cloud images always show the side of the earth where China is located.

The world is one family, and China's weather is not isolated, so we also need polar orbit satellites to scan the world. Polar orbit satellites shuttle between the North and South Poles, not only can they see the other side of the earth, but also can make up for the shortcomings of geostationary satellites that cannot observe the poles.

The polar orbit of a meteorological satellite is called a "near polar sun-synchronous orbit", which means that no matter where the Earth moves to the sun, the angle between the satellite orbit and the day-night boundary is always fixed. In this way, every time the satellite moves to the day side, the ground below it is always at the same time point, and the illumination angle is always the same, which can ensure the consistency of the observation data. Our Fengyun single numbers (No. 1 and No. 3) are all such near polar sun-synchronous orbit satellites.

A sun-synchronous polar orbit seen from the North Pole "looking down". Image source: author

So why do we have "morning stars" and "afternoon stars"? Because the weather is often closely related to the morning and afternoon.

Let's take a practical example: Clouds on the Qinghai-Tibet Plateau often "go to get off work" like humans, rising from the ground in the morning, forming thick clouds at noon, and falling back to the ground in the afternoon and evening. Even on the Great Plains, everyone will notice that severe convective weather generally occurs in the afternoon and evening, and rarely in the morning or morning. Therefore, polar-orbiting meteorological satellites are divided into morning satellites and afternoon satellites, which can be used to observe weather phenomena in different time periods of the morning and afternoon. When the morning satellite appears on the day side of the earth, the area directly below it is always in the local morning, while the afternoon satellite is in the afternoon.
China's Fengyun-3E satellite is somewhat special. It is the world's first civilian "morning and evening star", also known as the "dawn star". It orbits above the day-night boundary (also known as the morning-evening line), filling the gap that international civilian meteorological satellites only have morning and afternoon orbit satellites. Moreover, its orbit is always in the sun. In addition to observing the earth, it can also continuously observe solar weather, which is of great significance for the monitoring and early warning of solar activities such as flares and coronal holes.

What track to choose depends on the function

After reading this, everyone must have realized that the orbital deployment of satellites must correspond to their functions.

Indeed, it is. For example, China's Xihe solar observation satellite operates in a sun-synchronous orbit above the terminator (a bit like the meteorological terminator) to continuously study the dynamics of the solar atmosphere. The Webb Space Telescope in the United States floats at the second Lagrange point away from the sun and the earth to avoid interference from infrared rays. It also has a five-layer sunshade as big as a tennis court to block the radiation from the sun and the earth.

Another example is the space station. Let's think about it together. If we were asked to design the orbit of the space station, where should we put it? We have to consider that the space station is a place where people live, so the space conditions around it cannot have too strong ionizing radiation. We are usually on the ground, protected by the earth's magnetic field and atmosphere, and always think that space is just desolate. Extravehicular activities only need to keep warm and keep pressure.

In fact, there is a circle of Van Allen radiation belts at an altitude of 640 to 58,000 kilometers above the ground, which is full of high-energy charged particles captured by the Earth's magnetosphere, and each particle is full of malice. Therefore, the orbit of the space station must avoid this area. The altitude of the Chinese space station and the international space station is only about 400 kilometers, which is the straight-line distance from Xi'an to Zhengzhou.

The range within which humans can stay in space for the medium to long term.

It can also be seen from the cross-section of the Van Allen radiation belt that in the high latitudes of the earth, the radiation belt is closer to the ground. We all know that the aurora is formed by the solar wind bombarding the earth's atmosphere. Since the space environment near the earth's poles is very bad, the space station must be low in both altitude and latitude. The Chinese space station only operates between 42° north and south latitude. Since the International Space Station has to load people and cargo from the Baikonur launch site in Russia, it has to choose a larger inclination of 52°.

Another example is navigation satellites, such as Beidou and GPS. We need to see at least three or four satellites at any time in all parts of the world, including the poles, for positioning and navigation. Therefore, most of them must be in motion, but they cannot fly close to the ground like space stations. Their ideal position is in the "medium earth orbit" about 20,000 kilometers above the ground.

What the satellite does and what the space environment is like determine how it flies. If we extend this idea, we can also understand the seemingly strange orbits of some deep space probes sent to other planets. For example, Juno, which is doing atmospheric research near Jupiter, should fly close to the face of Jupiter since it is studying the atmosphere. But Juno's orbit is actually a very flat ellipse. When it is close, it is only 4,200 kilometers away from the surface of Jupiter, but when it is far away, it has to fly 8 million kilometers away. Why? Because Jupiter has an extremely powerful and terrifying magnetosphere. If you stay in it all the time, you will die. So Juno can only adopt this "touch and run" strategy, each time it flies quickly, and then quickly escapes to a far distance, in a safe place, and slowly organizes the data and sends it back to Earth.

The extremely elongated orbit of the Juno spacecraft at Jupiter.

Satellite orbits, so distinguish

At the beginning of the article, we said that meteorological satellites have two orbits, one is geostationary orbit and the other is polar orbit. Now let's learn a little skill: when we see a cloud picture, how can we identify which orbit the meteorological satellite took it?

Don't worry, there are no formulas or theorems, and you don't need to use a calculator. You just need to look at the field of view and determine the height.

For example, the global cloud map below. We know that geostationary satellites rotate synchronously with the earth, and the images above China can only see the Chinese hemisphere. Therefore, since the whole world is seen, it must not be the work of geostationary satellites, but a mosaic of images scanned by polar satellites. In fact, we can see particularly obvious and regular "scanning" traces (fuzzy parallel white marks) above the ocean, which are the reflections of the ocean surface captured by polar orbit satellites when they fly over the ocean.

Global cloud map taken by Fengyun-3D.

Another example is the cloud map of the Earth below. The orbit of a geostationary satellite is very high (about 36,000 kilometers above the ground), and from its position, only such a sphere can be seen, which is about 1/3 of the entire Earth, also known as a "disc map." In contrast, satellites operating in polar orbits can only see a very small part of the Earth.

Cloud map of the Asia-Pacific region taken by Fengyun-4A.

To sum up, if you see a sphere, it was taken by a geostationary satellite. If it looks like a world map hanging on the wall, it was taken by a polar satellite!

Author: Qu Jiong, popular science creator

Review丨Yang Lei, Deputy Chief Engineer of Fengyun-4, National Satellite Meteorological Center