At 8:31 today (20th), the Queqiao-2 relay satellite of the fourth phase of the lunar exploration project was successfully launched by the Long March-8 Yao-3 carrier rocket from the Wenchang Space Launch Site in China. As a "key link" of the follow-up project of the fourth phase of lunar exploration, the Queqiao-2 relay satellite will build a new "Queqiao" to provide relay communication between the Earth and the Moon for Chang'e-4 and Chang'e-6 missions.
What is a relay satellite? Simply put, it's a microphone. If we want to talk to a probe on the moon, we have to tell the relay star first, and then the relay star will tell the probe.
Why bother? Because the relationship between the moon and the earth is very special, the moon revolves around the earth, but the moon always has only one side facing the earth when it revolves around the earth, which leads to the fact that the moon is actually divided into positive and negative sides, and what we can see directly on the earth is always the front.
Historically, the United States and the Soviet Union have been landing on the moon, both unmanned and manned, and the far side of the moon has only been circumnavigated by human probes, and has never officially landed, let alone returned with the soil on the back of the moon.
Queqiao-1 and Chang'e-4
The first probe to actually land on the far side of the moon was the mainland's Chang'e-4, which landed on the moon on January 3, 2019. We can't see the far side of the moon, and the probes that fall on the far side can't see it, and the fact that we can't see it means that the radio signal can't be transmitted directly. Although today's probes can do all kinds of control operations on their own, it is impossible to get them to take a picture and send it back to Earth.
Comparison of the front and back of the moon
So, in 2018, we launched a relay satellite, that is, Queqiao-1. Where did Queqiao-1 fly? There is a point in the space on the far side of the moon, called the Earth-Moon Lagrange two points, at which the satellite can revolve around the earth along with the moon, and the Queqiao can also orbit around this point in a small area. In this way, the probe can see the satellite on the far side of the moon, and we can see the satellite from Earth. It can be used as a microphone.
Queqiao-2 and Chang'e-6
This year, the continental probe will land on the far side of the moon to "rub its back" and send the "mud" back for the first time in human history. Therefore, it is necessary for relay satellites to play their role again.
However, the "shelf life" of Queqiao-1 is five years, it was launched in May 2018 and has now been overdue for a year. Although Queqiao-1 is still easy to use, but in the spirit of foolproof, we will launch another relay satellite, these two will cooperate, the communication situation will be better, in case of Queqiao-1 problems, we can also use Queqiao-2 alone for relay.
However, the location of Queqiao No. 2 and Queqiao No. 1 is different. This time, Queqiao-2 orbited the moon and became a satellite of the moon. In this way, Queqiao-2 will be closer to the surface of the moon, and cooperate with Queqiao-1, one close and one far, not only the communication effect is better, but also can do some other exploration projects while communicating. In fact, Queqiao-2 itself carries three scientific payloads, which can be used for scientific research while relaying.
Queqiao-2 relay satellite. Source: CCTV News
However, many friends may see that if you revolve around the moon, not every moment the satellite can see the earth and the far side of the moon at the same time. This means that the communication between the Moon and the Earth may not be continuous, and it is even possible that the relay satellite will receive the signal on the far side of the Moon and wait until it is transferred to the front before returning to the Earth.
There is no problem with this, because we do not need real-time remote control for lunar exploration, the probe can complete the action by itself, as long as the detection data is transmitted back, not to mention the cooperation of Queqiao-1, so the Chang'e-6 mission will be more secure in the near future.
After Chang'e-6, Queqiao-2 will also adjust its orbit to escort future lunar exploration missions.