Elsternbrücke (relay satellite)

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Elsternbrücke (relay satellite)

NSSDC ID 2018-045A
Mission goal Earth moon
Client CNSA
Launcher Changzheng 4C
construction
Takeoff mass 425 kg
Course of the mission
Start date May 20, 2018, 21:28 UTC
launch pad Xichang LC-3 Cosmodrome
 
 20th May 2018 Start from Queqiao
 
 May 25, 2018 Arrival near the moon
 
 29 May 2018 Arrival at L 2 in the earth-moon system
 
 June 14, 2018 Halo orbit around L 2
 
 Jan. 3, 2019 Landing of Chang'e-4, communication to the back of the moon since then
 
 2023 planned service life
 
 ? Mission end

Magpie Bridge ( Chinese  鵲橋  /  鹊桥 , Pinyin Queqiao , English Magpie Bridge  is a - "Magpie Bridge") relay - satellite of the Chinese space agency (CNSA). The relay satellite operates at the Earth-Moon Lagrange point L 2 and enables communication to the remote side of the moon.

construction

The 425 kg satellite with a planned lifespan of five years was built by the " Dong Fang Hong Satellite GmbH" ( 航天 东方 红 卫星 有限公司 , Hángtiān Dōngfāng Hóng Wèixīng Yǒuxiàn Gōngsī ) according to the plans of the Chinese Academy for Space Technology , which the project ends Publicly presented in 2016. It serves as a communication relay for the lander and rover Jadehase 2 ( Yutu 2 ) of the Chinese moon mission Chang'e-4, which started on December 7, 2018 , as it landed on the back of the moon and therefore cannot communicate directly with the earth. For this purpose, the satellite has a fold-out parabolic antenna with a diameter of 4.2 m, through which it can communicate with the earth at a speed of 2 MB / second. Communication with the lander and rover on the lunar surface takes place at a speed of 100 KB / second.

Start and run to the finish

Elsternbrücke was launched on May 21, 2018 at 05:28 local time with a Changzheng 4C launcher from the Xichang Cosmodrome . It was positioned in the Halo orbit around the Earth-Moon Lagrange point L 2 behind the moon. Its name is derived from the story of the shepherd boy and the weaver girl from Chinese mythology .

25 minutes after take-off on May 20, 2018 at 21:28 UTC, the satellite swiveled into a transfer orbit to the moon with a perigee of 200 km and an apogee of 400,000 km, the solar cell panels and the parabolic antenna were deployed.

At a distance of 100 km from the moon, the technicians at the Beijing Space Control Center briefly ignited the engines on May 25, 2018 at 13:32 UTC and placed the satellite on a lunar L 2 transfer orbit. Four days later the satellite reached the L 2 point 65,000 km behind the moon. During the approach as well as during operation, the satellite must not come into the radio shadow of the moon. In a complicated maneuver with several burning and correction phases, Elsterbrücke swiveled into a halo orbit until June 14, 2018, from which it can safely communicate with the earth past the moon.

The ground stations Giyamusi and Kashgar of the Chinese deep space network were interconnected to form a 3000-kilometer VLBI antenna for the maneuver under the command of the Xi'an satellite control center , and the Delta-DOR method was used for precise orbit determination. Giyamusi and Kashgar served not only to determine the orbit, but also to receive the telemetry signals and to send the control signals. The People's Liberation Army's deep-space stations were designed in 2009 for just such multitasking . In order not to endanger the safety of the probe, a modified Delta-DOR procedure was chosen for the critical phase of the Halo orbit placement, in which the synchronization error of the station clocks can be detected during the relatively quiet approach between May 21 and June 2 and determined the refraction of the radio waves in the atmosphere in order to get an impression of the regularities of the change over several hours. In the critical phase, only a quasar measurement took place at the beginning of the working day, after which the tracking by Giyamusi and Kashgar was not interrupted for several hours. On May 27, the Beijing and Urumqi stations had carried out conventional Delta-DOR orbit tracking with a similar baseline and confirmed the validity of the makeshift method. In the end, it was actually possible to determine the position of the Elsternbrücke during the entire process to within 40 cm or 3 nanoseconds.

Communication with the remote side of the moon with lander and rover via the relay satellite Elsternbrücke ( Queqiao ) (scheme)

Although it is relatively difficult to operate a satellite in a Halo orbit around the L 2 point - 74 orbit correction maneuvers were required in the first two years up to May 21, 2020 - this is the most cost-efficient method, a connection with the the far side of the moon. A satellite in stationary orbit over the equator of the moon - analogous to geostationary satellites - would need an orbit altitude of 88,000 km to keep up with the lunar rotation of 27.3 days. But since the inner Lagrange point L 1 is only 58,000 km away from the moon, this means that the satellite would fly in the earth's gravitational field and would be pulled out of its stationary orbit by this.

Shepherd boy and weaver girl on the Elstern Bridge of the Milky Way in the colonnade of the New Summer Palace in Beijing

Another possibility would have been to build a network of communication satellites circling the moon in low orbits, similar to the Global Positioning System on Earth. This is actually being considered for a later lunar station that will remain manned for the long term. However, with an unmanned probe like Chang'e-4, with a limited mission time, this method was found to be too expensive.

After Chang'e-4 reached the lunar orbit on December 12th, the connection with the relay satellite was tested first. Since the lunar probe initiated the landing on January 3, 2019, the telemetry and the commands of the ground station in Beijing have only run over the Elstern Bridge, as has the transmission of the images from the lander camera.

Mission end

The fuel that the relay satellite needs for its orbit correction maneuvers is expected to be used up in 2023, after which the communication of the lander and rover with the earth is no longer guaranteed. During the Chang'e-7 mission to the south pole of the moon, planned for 2024 , a new relay satellite will be carried, which will maintain the connection to the earth in cooperation with an orbiter that is also carried - whose main task is to measure the lunar surface and magnetosphere. The life expectancy of the new relay satellite, like that of the orbiter, should be at least eight years, i.e. sufficient until the manned landing planned for 2030.

Individual evidence

  1. ↑ a b c Steed: 鹊桥 号 启程 ， 为 嫦娥 四号 登陆 月球 背面 架设 通信 桥梁. In: guokr.com. May 20, 2018, accessed January 5, 2019 (Chinese).
  2. ↑ a b 付毅飞: 嫦娥 四号 中继 星 发射 成功 人类 迈出 航天 器 月 背 登陆 第一步. In: news.china.com.cn. May 22, 2018, accessed January 5, 2019 (Chinese).
  3. ↑ a b Luyuan Xu: How China's lunar relay satellite arrived in its final orbit. In: planetary.org. June 15, 2018, accessed on August 17, 2020 .
  4. ↑ 冯华 、 蔡金 曼: “鹊桥” 中继 星 成功 实施 近 月 制动. In: paper.people.com.cn. May 26, 2018, accessed January 7, 2019 (Chinese).
  5. ↑ Wang Mei et al .: Application of Deep Space VLBI System in Queqiao Mission. In: jdse.bit.edu.cn. Retrieved May 9, 2019 .
  6. ↑ 王 美 et al .: 深 空 测控 网 干涉 测量 系统 在 “鹊桥” 任务 中 的 应用 分析. In: jdse.bit.edu.cn. Retrieved May 9, 2019 (Chinese).
  7. ↑ 2 年前 的 今天 ： 嫦娥 四号 任务 “鹊桥” 中继 星 发射 成功. In: clep.org.cn. May 21, 2020, accessed May 22, 2020 (Chinese).
  8. ↑ Ye Peijian et al .: An overview of the mission and technical characteristics of Change'4 Lunar Probe. In: engine.scichina.com. April 17, 2017, accessed January 8, 2019 .
  9. ↑ Zou Yongliao et al .: Overview of China's Upcoming Chang'E Series and the Scientific Objectives and Payloads for Chang'E 7 Mission. In: hou.usra.edu. March 17, 2020, accessed on August 17, 2020 .
  10. ↑ 宋 猗 巍: 关于 开展 探 月 工程 四期 嫦娥 七号 任务 载荷 竞争 择优 的 通知. In: clep.org.cn. August 27, 2020, accessed August 29, 2020 (Chinese).
  11. ↑ 巅峰 高地: 比 美国 还 多 出 三分之一！ 我国 航天 员 规模 骤增 ， 天宫 空间站 只是 开局. In: mbd.baidu.com. January 11, 2020, accessed August 29, 2020 (Chinese).

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