Can we track a satellite that is out of control? 30 years ago today, Chinese scientists did it!

Satellite launches are not uncommon, but sometimes some satellites go out of control for one reason or another. But 30 years ago today, on December 25, 1993, Chinese scientists successfully tracked and captured a recoverable satellite that had gone out of control more than two months ago, creating a miracle in my country's aerospace technology. Can a satellite be tracked even if it goes out of control? To answer this question, we must first understand recoverable satellites.

1

Recoverable satellite : the " executor " of space missions that goes out and comes back

A recoverable satellite, as the name implies, is a satellite that can partially re-enter the atmosphere and return to the ground after completing its space mission . Satellite recovery technology is a pioneer in manned space flight. The United States successfully recovered the Discoverer 13 recoverable satellite for the first time in 1960, and China successfully launched and recovered the Pioneer 1 remote sensing satellite in 1975, becoming the third country after the United States and the Soviet Union to master satellite recovery technology .

Discoverer 13 was launched from Vandenberg Air Force Base in the United States on October 8, 1960 (Source: Wikipedia)

On November 26, 1975, China's first recoverable satellite was successfully launched (Source: CCTV News video screenshot)

The emergence of recoverable satellites was originally to meet the needs of military reconnaissance and national census . At that time, photography technology required the use of negative film, and in order to send photos taken in space back to the ground for development and analysis, recoverable satellites came into being.

Nowadays, with the rapid development of digital photography, data transmission and other disciplines, the image data captured by satellites can be directly transmitted from space to the ground, and recoverable satellites are increasingly used for space science and technology experiments . For example, recoverable satellites can carry microgravity science laboratories and return to the ground after completing the experiment, thereby realizing the recovery of test samples. These test results have been widely used in the research and development of new materials, new medicines, new crops and other products.

2

Why did the recoverable satellite “ lose control ”?

The return of a satellite usually requires the separation of cabins, deorbiting, transition, reentry and landing . Satellites traveling in space occasionally disobey commands, which may be caused by a combination of factors such as mechanical failure, software failure, communication interruption, and power supply problems. For example, France's SPOT-3 satellite once lost control due to a gyro failure, and the solar panels could not receive sunlight and could not generate the electricity required for the normal operation of the satellite; Japan's "Chrysanthemum" No. 6 satellite also deviated from its intended orbit due to a failure of the jet propulsion device. In addition, the space environment can also affect the normal operation of satellites. For example, solar storms and cosmic rays are very likely to interfere with satellite communication functions and the power system on the satellite , causing the satellite to lose control and rush out of orbit.

Take the first recoverable satellite tracked by my country as an example. On October 8, 1993, my country launched the 15th recoverable satellite. However, when the satellite was returning, the pitch infrared channel of the operating phase failed and the attitude failed to reach the predetermined angle. Therefore, after the ground issued a return command, the satellite received the command with a wrong attitude, thus deviating from the predetermined orbit and causing the satellite to fail to "return home".

On December 25, 1993, Chinese scientists successfully captured a recoverable satellite that had lost control more than two months ago. The picture shows the scene when the Long March 2C rocket from the Jiuquan Launch Center launched the satellite into space on October 8, 1993 (Source: Screenshot of the paper)

In order to prevent the returnable satellite from losing control, aerospace engineers have designed a set of standardized processes. First, higher standards are set for the design and manufacture of returnable satellites to comprehensively improve the reliability of software and hardware. Secondly, technical optimization is carried out on the satellite's communication and monitoring systems to timely grasp the satellite's health status and operation status and respond quickly. In recent years, people have also introduced advanced technologies such as machine learning and artificial intelligence to achieve intelligent control of satellites.

3

How to track a recovering satellite after it loses control ?

Trying to track down an out-of-control satellite in the vast space is like looking for a needle in a haystack. How to find these lost satellites is still one of the difficult problems that aerospace engineers need to solve.

First, we can try to restore radio contact with the satellite . Through technologies such as the Doppler effect, phase difference measurement, and code tracking , we can obtain key information such as the satellite's position, speed, and direction of movement.

In addition, in recent years, with the development of technologies such as artificial intelligence and big data, people have been able to achieve both high efficiency and accuracy in continuous and automatic tracking of satellites.

Using radar detection technology to search for satellites is also one of the most reliable and effective ways to track satellites. For example, in 2017, NASA used advanced land-based radar to discover the Indian Chandrayaan-1 probe that had been missing for eight years.

India's Chandrayaan-1 probe (Source: Wikipedia)

In addition, intelligent hardware and software can be installed on the satellite to enable it to carry out self-rescue in a timely manner and actively restore communication with the ground when it loses control.

4

If tracking fails, where will the out-of-control satellite go ?

If an out-of-control recoverable satellite cannot be tracked, it may fall into the Earth's atmosphere as its energy runs out, or it may remain in space for a long time and become space junk. Since recoverable satellites often carry various test devices, these test devices are likely to carry harmful chemicals or radioactive substances. Therefore, once they enter the Earth's atmosphere or the ground, they will inevitably pollute the air, soil and water resources, causing immeasurable harm to human health and the ecological environment . In addition, out-of-control satellites may be controlled and used by other countries, thus threatening international security and regional stability . The United States destroyed an out-of-control reconnaissance satellite in 2008 to prevent the advanced imaging sensor equipment it carried from leaking technical secrets.

Schematic diagram of space junk (Source: Wikipedia)

5

If the tracking is successful , how does the out-of-control satellite return home ?

Once a satellite is out of control, we can try to regain control of it through some technical means, the most important of which is remote control, that is, remote control of the satellite through the ground control center to achieve control of its attitude and orbit. However, if you want to get the satellite back to the ground safely, you still need to overcome many obstacles.

The first step is to adjust the attitude, that is, to accurately adjust the satellite's attitude to the return attitude and keep it stable .

The second is braking, which is to accurately ignite the braking rocket so that the satellite can escape from the original erroneous orbit and enter the predetermined return orbit.

However, due to the limited power of the returning satellite, it is difficult to achieve the above process. However, even if it is still stranded in space, if the satellite communication is normal, it can still send its own operation data and test monitoring data back to the ground and continue to "make use of its residual heat".

Author: Zhu Lei, PhD in Aerospace Propulsion Theory and Engineering, Nanjing University of Aeronautics and Astronautics

Reviewer: Deng Xiaotao, Senior Engineer, AECC Hunan Power Machinery Research Institute

Produced by: Science Popularization China

Produced by: China Science and Technology Press Co., Ltd., China Science and Technology Publishing House (Beijing) Digital Media Co., Ltd.