The TianQin project: current progress on science and technology (2008.10332v1)

Abstract: TianQin is a planned space-based gravitational wave (GW) observatory consisting of three earth orbiting satellites with an orbital radius of about $10^5~{\rm km}$. The satellites will form a equilateral triangle constellation the plane of which is nearly perpendicular to the ecliptic plane. TianQin aims to detect GWs between $10^{-4}~{\rm Hz}$ and $1~{\rm Hz}$ that can be generated by a wide variety of important astrophysical and cosmological sources, including the inspiral of Galactic ultra-compact binaries, the inspiral of stellar-mass black hole binaries, extreme mass ratio inspirals, the merger of massive black hole binaries, and possibly the energetic processes in the very early universe or exotic sources such as cosmic strings. In order to start science operations around 2035, a roadmap called the 0123 plan is being used to bring the key technologies of TianQin to maturity, supported by the construction of a series of research facilities on the ground. Two major projects of the 0123 plan are being carried out. In this process, the team has created a new generation $17~{\rm cm}$ single-body hollow corner-cube retro-reflector which has been launched with the QueQiao satellite on 21 May 2018; a new laser ranging station equipped with a $1.2~{\rm m}$ telescope has been constructed and the station has successfully ranged to all the five retro-reflectors on the Moon; and the TianQin-1 experimental satellite has been launched on 20 December 2019 and the first round result shows that the satellite has exceeded all of its mission requirements.

• The paper demonstrates the TianQin project’s phased roadmap and technological breakthroughs in precision laser interferometry and drag-free satellite systems.
• The study reports successful initial satellite tests with superior residual acceleration noise and thrust resolution metrics.
• The research highlights TianQin's capability to detect diverse gravitational wave sources, including GCBs, MBHBs, and EMRIs, to probe cosmic phenomena.

Overview of the TianQin Project: Progress in Science and Technology

The paper provides a detailed examination of the TianQin project, a planned space-based gravitational wave (GW) observatory with a focus on developing critical scientific and technological capabilities. The TianQin observatory aims to commence operations around 2035, leveraging a constellation of three geocentric satellites designed to detect gravitational waves with frequencies ranging from $10^{-4}$ Hz to 1 Hz. This effort aligns with the broader scientific objective of exploring a variety of astrophysical and cosmological GW sources, such as galactic ultra-compact binaries (GCBs), massive black hole binaries (MBHBs), and potential exotic sources.

Technical Framework and Milestones

TianQin's design features a geocentric orbit with the satellites forming an equilateral triangle, optimally oriented nearly perpendicular to the ecliptic plane. Key technological initiatives include precision laser interferometry and impeccable drag-free environments within these satellites, essential for isolating minute gravitational wave signals from other disturbances.

The project's execution follows the "0123 plan," a structured roadmap progressing through various development stages:

• Step 0 emphasizes acquiring accurate orbital data through laser ranging, already demonstrating success via lunar laser reflection.
• Step 1 involves single satellite trials such as the launch and operation of the TianQin-1 satellite, with results surpassing expectations in aspects like residual acceleration noise measurements and thrust resolution testing.
• Step 2 will involve two-satellite missions to validate inter-satellite laser interferometry.
• Step 3 culminates with the full deployment of the three-satellite array.

Scientific Prospects

Galactic Ultra-Compact Binaries (GCBs): TianQin expects to resolve around 8,700 GCBs, utilizing these sources to test theories about star formation, galactic dynamics, and fundamental gravity. TianQin's sensitivity will allow calibration using verification binaries (VBs) such as HM Cancri, detectable with an SNR of 5 within days, affirming its potential to refine the characteristics of the detected systems.

Stellar-Mass Black Hole Binaries (SBHBs): The dual-band detection potential of SBHBs positions TianQin as a crucial tool in exploring intermediate GW frequencies, complementing existing detectors such as LIGO. Under optimistic conditions, detections may provide insights into cosmological parameters and constraints on modified gravity.

Massive Black Hole Binaries (MBHBs): The detection of MBHBs at high redshifts offers vital data on black hole formation and galaxy evolution. With detections projecting SNRs exceeding 20 even at redshifts of 15, TianQin stands poised to enhance understanding of cosmic expansion history and black hole demographics.

Extreme Mass Ratio Inspirals (EMRIs): EMRIs, characterized by numerous orbital cycles, are excellent probes for testing General Relativity (GR) and mapping strong gravitational fields around black holes. TianQin's ability to measure these with high precision could enable detailed tests of GR and explorations of exotic astrophysical objects like boson stars.

Cosmological Probes: TianQin's observations could illuminate the nature of stochastic gravitational wave backgrounds (SGWB) from cosmic processes like first-order phase transitions or the formation of topological defects like cosmic strings. It may also access energy scales beyond those achievable by current particle accelerators.

Technological Advancements and Collaboration

Through initiatives such as the TianQin-1 satellite's successful demonstration of spacecraft technologies, significant progress has been achieved in key technical areas necessary for the observatory's operation. Furthermore, international collaboration and dedicated research facilities are underscored as pivotal for meeting TianQin's complex technical demands.

Conclusion and Future Directions

The TianQin project signifies a vital development in gravitational wave astronomy, promising a wide array of scientific outputs, from astrophysical discoveries to theoretical physics probes. By executing its phased development roadmap and achieving technological milestones, TianQin is strategically positioned to contribute substantially to multi-messenger astronomy and fundamental science. Future developments might leverage these detections to refine models of cosmic phenomena and scrutinize the fundamental laws of physics.