"See through!" Chinese scientists take first complete X-ray of the moon

"The bright moon rises over the sea, and people across the world share this moment." Since ancient times, people have mostly viewed the moon through visible light that can be perceived by the eyes. On the morning of the 17th, my country's "Einstein Probe" (EP) space X-ray observatory also sent back X-ray photos of the moon from space during the Mid-Autumn Festival, allowing people to enjoy a different moon in the X-ray band. These images were obtained by the "Fengxingtian" X-ray Telescope (FXT) on the EP satellite. This is also the first time that Chinese scientists have used their own space telescope to observe the X-ray image of the complete moon.

△ Lunar X-ray images taken by the Einstein Probe Satellite Fengxingtian X-ray Telescope

The EP satellite is one of the major missions of the space science satellite series approved and implemented by the Strategic Priority Research Program of Space Science (Phase II) of the Chinese Academy of Sciences. It is led by China, and the European Space Agency, the Max Planck Institute for Extraterrestrial Physics in Germany and the French Space Agency participate in the satellite development in the form of international cooperation. Among them, Fengxingtian is one of the two scientific payloads configured by the EP satellite, and is led by the Institute of High Energy Physics of the Chinese Academy of Sciences.

Chen Yong, chief designer of Fengxingtian and researcher at the Institute of High Energy Physics of the Chinese Academy of Sciences, said that the X-rays emitted by the sun can excite the lunar elements to produce X-ray fluorescence radiation. Since the characteristic energy of X-rays of different elements is different, the distribution of various elements on the lunar surface can be revealed by studying X-ray images of different energies on the lunar surface. However, these X-ray radiations cannot penetrate the Earth's atmosphere and therefore cannot be detected on the ground. For this reason, scientists usually use special X-ray satellites to observe outside the atmosphere.

This year's Mid-Autumn Festival coincides with the moon being near the perigee of the Earth-Moon orbit, only 357,400 kilometers from the Earth. Therefore, the moon is about 14% larger than an ordinary full moon and brighter, so it is called a "supermoon", which is also a good time for astronomical observation. In addition, this year is the peak year of solar activity, and solar flares erupt frequently. When a solar flare occurs, the sun's X-ray flux will increase sharply, the energy spectrum will become harder, and the X-ray radiation on the lunar surface will also increase accordingly.

Historically, only two satellites, ROSAT from Germany and Chandra from the United States, have successfully conducted imaging observations of the lunar surface. Fengxingtian conducted X-ray observations of the full moon during the special period of the Mid-Autumn Festival and successfully sent back clear X-ray photos of the lunar surface, sharing with everyone the joy of moon-watching during the Mid-Autumn Festival from another perspective. Compared with other X-ray satellites in orbit internationally, Fengxingtian has a large imaging field of view, which can capture the "supermoon" at a glance. At the same time, Fengxingtian has better X-ray energy resolution and a higher effective area, which can reveal the distribution of elements such as oxygen, iron, magnesium, aluminum and silicon on the lunar surface.

Yuan Weimin, chief scientist of the Einstein Probe satellite and director of the National Astronomical Observatory of the Chinese Academy of Sciences, said that although the core scientific goal of the EP satellite is to detect and study high-energy transient celestial bodies in the universe, its super-strong X-ray detection capability enables the EP satellite to have a wide range of detection targets and application prospects. The X-ray images of the lunar surface taken by Fengxingtian are of great value to conducting scientific research related to the moon.

(CCTV reporter Shuai Junquan and Chu Erjia)