Wang Yaping: Space is deep black! Revealing the original appearance of space, is it very different from NASA photos?

The long-awaited first space lesson on the China Space Station has arrived as scheduled. The three Shenzhou 13 astronauts Wang Yaping, Zhai Zhigang and Ye Guangfu gave everyone on earth a lively and interesting space lesson. class. This is Wang Yaping’s first space teaching activity since Shenzhou 10’s first space teaching eight years ago.

In this Tiangong class, astronauts showed us various strange phenomena in a weightless environment, such as cells that emit fluorescence, table tennis balls that do not float in water cups in space, astronauts Turn around in space, make water balloons in space and add effervescent tablets to them.

In these two space lectures, students asked astronauts similar questions: "What kind of scene will you see in space? What color is the sky?" In this regard, Wang Yaping said that in space The sky seen in is a deep black, not the blue we see on Earth.

If you have seen a rainbow, you will know that sunlight is not monochromatic, but composed of 7 colors. The difference between different colors of light is their wavelength, which are collectively called visible light. The shorter wavelength visible light in sunlight is blue-violet, and the longer wavelength visible light is red-orange.

When sunlight hits the Earth's atmosphere, not all wavelengths of light can travel in straight lines to reach the ground. There are gas molecules such as nitrogen, oxygen, and carbon dioxide in the atmosphere. When sunlight shines on it, light with shorter wavelengths is prone to Rayleigh scattering, because the scattering intensity is inversely proportional to the fourth power of the wavelength. Blue light is scattered by the atmosphere in all directions, causing it to fill the sky. As a result, no matter which direction we look at the sky, we will receive blue light, so the sky becomes blue.

There are only an average of 6 atoms per cubic meter of space in space, which is almost a vacuum. The light emitted by the sun will propagate along straight lines in space without scattering, so when viewed from space It looks black. There is no atmospheric pollution or light pollution in space, which makes space appear particularly deep.

During Wang Yaping’s previous spacewalk, the space we saw was indeed completely black, and we could only see the suspended blue earth and the dazzling sun. So, why does NASA's Hubble Space Telescope show that the universe is colorful? Why is there such a big difference?

When astronauts are walking in space, no stars can be seen in the space in the picture. This is because starlight is much dimmer than the earth, the space station illuminated by the sun, and the sun itself. In order to be able to photograph the space station and the earth, the amount of light entering must be kept as small as possible, which results in the dim starlight not being displayed.

When my country's Yutu 2 was on the moon without an atmosphere, the lunar sky photographed was completely black, with no stars visible. The reason was the same. But astronauts can see stars with their own eyes in space, as long as they avoid areas with strong light and let their eyes adapt to the darkness. In fact, the sky in space is clearer and brighter, and the stars appear brighter and do not blink due to atmospheric interference like they do on Earth.

But no matter what, the starry sky that astronauts see in space will not be like what Hubble photographed. The nebulae, star clusters, and galaxies photographed by Hubble are all very far away from the earth. The light they emit is already very dim when it reaches the earth, and many of them are invisible to the naked eye.

In order to capture those dim deep sky objects, it is necessary to maximize the amount of light and extend the exposure time. In fact, through this method, colorful photos of the universe can also be taken on earth. The following picture is the Orion Nebula (M42) photographed by the author using a 15-centimeter astronomical telescope + frozen CMOS after a cumulative exposure of 4 hours:

The following picture is a triangular galaxy with a cumulative exposure of 8 hours (M33):

Whether it is the Orion Nebula (1,300 light-years away) or the Triangulum Galaxy (3 million light-years away), they are all visible to the naked eye from the Earth, but what is visible to the naked eye is It's a white cloud. Only through long-term exposure through astronomical telescopes and gathering a large number of photons can their original appearance and color be revealed.

Finally, what we are even more proud of is that this time the space live broadcast was clear and smooth, and the whole process was quite perfect. This was due to the put into use of my country's Tianlian 1 and 2 relay satellite systems, allowing The signal downlink rate of the space station is as high as 1.2 Gbit/s. Because of this, our country's astronauts can also use mobile phones and computers to access the Internet on the space station, and can also make video calls with the ground, which makes the astronauts' space life richer.