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In this perspective, we outline that a space borne gravitational wave detector network combining LISA and Taiji can be used to measure the Hubble constant with an uncertainty less than 0.5% in ten years, compared with the network of the ground based gravitational wave detectors which can measure the Hubble constant within a 2% uncertainty in the next five years by the standard siren method. Taiji is a Chinese space borne gravitational wave detection mission planned for launch in the early 2030 s. The pilot satellite mission Taiji-1 has been launched in August 2019 to verify the feasibility of Taiji. The results of a few technologies tested on Taiji-1 are presented in this paper. Gravitational wave astronomy has opened the door to test general relativity and the effect of gravity in the Universe. The authors present the capabilities of an overlap between space gravitational wave detectors LISA and Taiji to constrain the Hubble constant to 0.5%, in 10 years, and what can be learned from the satellite pilot Taiji-1 launched in 2019.

Taiji-1 satellite was successfully launched on 31 August 2019, and it has been operating normally in orbit until now. A series of in-orbit experiments were carried out with the inertial sensor, which included the micro-thrust test, drag-free control test and laser interferometer test. Comprehensive performance simulations and tests of the inertial sensor were also carried out prior to the launch of Taiji-1, including the calibration and drop-tower tests. These tests were one of the preconditions for the success of these experiments. The calibration experiments were conducted in a cave-lab using the gravity-inclination method and the scale factors of the inertial sensor along Y- and Z-axis were measured. In addition, 20 drop-tower tests were carried out in the National Microgravity Laboratory of China (NMLC) drop tower and the control stability of all the axes was tested and optimized. A simulation model was used before each test, and the results showed that an accurate simulation prior to each experiment had an important role in ensuring the efficiency and accuracy of the experiment. The circuit-gain switch was realized for the first time during the drop-tower tests. The test results indicated that the microgravity level of the NMLC drop tower could reach about 13 [Formula: see text]g0 along the horizontal axes, offering an important reference for researchers planning to conduct microgravity experiments in the NMLC drop tower.