Shijian 1 was the second mission in the Dong Fang Hong 1 (DFH-1) project, intended for the testing of the solar cell power technology and probing space environment in low Earth orbit. It used the backup satellite originally built for the DFH-1 mission, added with technologies and scientific mission payload that could not be fitted on DFH-1 due to time constraint.
The first step of China’s satellite development plan was to develop and launch scientific experimental satellites to demonstrate the technology and exploring the space environment in orbit. However, to meet the tight timeframe for the first satellite mission, the programme manager decided to simplify the satellite’s design by removing its solar power cells and mission payload. As a result, the DFH-1 satellite launched in April 1970 was largely a showcase with no function for scientific experiment.
China Academy of Space Technology (CAST) proposed in 1968 to use the backup satellite for the DFH-1 mission for a scientific experiment mission using the systems stripped off from the original design, including solar cells, nickel-cadmium batteries, active thermal control, and mission payload. The satellite was originally designated Dong Fang Hong 1A, but was later renamed Shijian 1 (Shijian = “Practice”). The project was approved by the NDSTC in May 1970, shortly after the successful launch of DFH-1.
Like its predecessor, Shijian 1 was also a symmetrical 72-faced polyhedron covered with solar cells. The satellite was 1 m in diameter but significantly heavier than its predecessor, with an orbital mass of 225 kg (compared with 173 kg of DFH-1). The satellite consisted of six sub-systems: space-frame, antenna, tracking, telemetry, thermal control, and power.
Shijian 1 featured eight new technologies:
- Long-duration power supply consisting of solar cells and nickel-cadmium batteries;
- Long-duration telemetry radio beacon;
- Active thermal control system;
- Geiger- Müller counter;
- X-ray detector;
- MOS memory circuit;
- External temperature sensor.
Onboard equipment included those for both short-duration operation (15 days) and long-duration operation. The short-duration equipment, powered by silver-oxide batteries, was not very different to that onboard DFH-1, including 4 high-frequency (20 MHz) radio signal antennas in the middle of the satellite, a single 40 cm long VHF radio beacon antenna at the top of the satellite, and 5 cm radio transponder and 10 cm UHF radio beacon antennas. The long-duration equipment consisted of solar cells / nickel-cadmium batteries power supply, active thermal-control, and long-duration telemetry.
Concept definition of the satellite was carried out between August 1968 and December 1969. The prototype development was between October 1969 and May 1970. The operational satellite development was between March 1970 and January 1971. The completed satellite was delivered to the Jiuquan launch site on 15 January 1971.
During the checkout examination at the launch centre, technicians noticed that the satellite had been accidently switched to internal power supply for some time, and the two nickel-cadmium battery cells were almost flat. They managed to recharge the battery cells so that they could still work for another three months.
The launch of Shijian 1 was approved by Primer Zhou Enlai on 2 March 1971. At 20:00 local time on 3 March, the CZ-1 launch vehicle blasted off from Pad 5020 at the Jiuquan launch site. The satellite was successfully placed into a 266 km by 1,826 km elliptic orbit inclined at 69.9 degrees, with a period of 106 minutes.
However, the ground stations were unable to track the signals from the satellite after orbit insertion. On 4 March, the tracking station in Shandong briefly received some extremely weak signals from the satellite, with only 1% of the expected strength.
The tracking signals from the satellite were supposed to be transmitted via the four VHF antennas located symmetrically on the middle section of the satellite. These antennas were folded during launch and deployed by the centrifugal force of the satellite spin once in orbit. Satellite engineers believed that the satellite didn’t separate from the launch vehicle, resulting in the four radio antennas failing to deploy. However, unsurprisingly the theory was challenged by the engineers who designed the launch vehicle. They were convinced that the payload had separated from the rocket, based on the telemetry signals from the rocket showing that the four explosive bolts attaching the satellite to the third-stage had all been detonated. They believed that the four antennas were detached from the satellite during the separation by centrifugal force.
By analysing the weak signals received by the Shandong tracking station, CAST engineers noticed that the satellite was still spinning at a speed of 3 rounds per seconds. Even if one of the four antennas had been deployed, the spin speed would reduce significantly. They also noticed that the internal temperature of the satellite was dropping fast, suggesting that it was still attached to a large radiating object. Finally, the weak telemetry signals of the satellite were almost overwhelmed by the background noise, something also detected during the ground tests while the satellite was attached to the launch vehicle. They concluded that the satellite was still not separated.
This conclusion was partially confirmed when the optical station in Yunnan reported that only one object could be observed after orbit insertion. Initially it was thought that this may have been caused by the third-stage flying closely behind the satellite. In the following days, the optical station was unable to observe the satellite due to unfavourable weather conditions.
On day 8 of the mission (11 March), the Shandong tracking station suddenly received strong and clear telemetry signals from the satellite. On 20 March, the Yunnan optical tracking station reported detecting two objects in the orbit. Later analysis suggested that the increased orbital mass of Shijian 1 resulted in the mechanical separation mechanism, which was essentially a spring, unable to push the satellite away from the rocket. The two flew attached for eight days until the stuck satellite was separated under the centrifugal force.
Shijian 1 remained operational in the following eight years, until it de-orbited on 17 June 1979. During this time, the satellite continued sending telemetry signals with no apparent drop in output power. The output of the solar cells and nickel-cadmium batteries remained stable despite tens of thousands of discharging and recharging in their lifetime.