A brief history: from early studies to Taiji-1 in-orbit demonstration

Wen-Rui Hu , Yue-Liang Wu, Natl. Sci. Rev. Vol 4 (2017)

Yue-Liang Wu et al., Commun. Phys. 4, 34 (2021)

2008

—   CAS initiated early studies on space-based gravitational-wave          detection.

2010

—   Early national proposal for space-based GW observation.

—   Officially named Taiji, and publicly introduced to the international community.

2016

—   Taiji-1 was launched as an in-orbit demonstrator for Taiji's technologies.

2018-2019

—   Overall mission design and development roadmap              were published.

2021

—   Current focus: engineering preparation, system-level verification, data challenges, and science-pipeline development.

2025-2026

Taiji has evolved from early conceptual studies to in-orbit technology verification and is now moving toward full-mission engineering and science-data readiness.

The millihertz window

HW, Minghui Du et al., Sci Sin-Phys Mech Astron 54, 270403 (2024)

Taiji targets a broad range of mHz sources: Galactic compact binaries, MBHB/IMBH mergers, EMRIs/IMRIs, stellar-mass BBHs, stochastic backgrounds, and unknown sources.

Galactic Compact Binaries

Verification binaries,
DWDs, and the
Milky Way foreground

MBHBs / IMBHBs

EMRIs / IMRIs

Stellar-Mass BBHs

Stochastic Backgrounds

Unknown Sources

MBHB mergers,
black-hole seeds, and
galaxy co-evolution

Precision probes of
massive BHs and
strong-field gravity

Long inspirals before
LVK; multiband GW astronomy

Astrophysical
foregrounds and
early-universe phase
transitions

Discovery space for
unmodeled and
unexpected signals

Baseline full-mission concept

• Three drag-free spacecraft forming a near-equilateral triangular constellation
• Arm length: \(3\times 10^6\) km
• Heliocentric orbit, leading/trailing the Earth by  ~\(20^\circ\)
• Laser displacement sensitivity: \(8\times 10^{-12}\mathrm{m} /\sqrt{\mathrm{Hz}}\)
• Test-mass acceleration noise: \(3 \times 10^{-15}\,\mathrm{m\,s^{-2}}/\sqrt{\mathrm{Hz}}\)

Taiji Full-Mission Concept

A heliocentric triangular constellation for mHz gravitational-wave astronomy

100-m underground interferometer

2008-2016

2018-2019

2020-202x

202x-203x

From Taiji-1 to full-mission preparation

Early studies and
mission formulation

Taiji-1 in-orbit
demonstration

Key technology
development

Full-mission
engineering
preparation

concept · proposal ·
public introduction

technology demo ·
launch · in-orbit
validation

drag-free · laser
interferometry ·
phasemeter ·
grav. reference sensor

 integration · verification · data challenges

10-m ground interferometer

Outline of Taiji Data Processing Pipeline

The analysis starts from raw telemetry and ends at scientific interpretations

L0

L0 - L1

L1 - L2

L2 - L3

scientific measurements

auxililary data channels

Ranging · Clock sync ·
· data quality · calibration · TDI

Low-latency search ·
global fitting

Catalogs · cosmology · astrophysics ·
fundamental physics

Raw
Telemetry

Preprocessing

Scientific
Analysis

Science
Products

Data processing pipeline: raw telemetry to scientific interpretation of the signals

Taiji Data Challenge

A typical criterion for unbiased parameter estimation:

The effect of realistic orbital motion (especially armlength variation ) in response:

1. Complexity in waveform and response modeling

difference between waveforms with / without the subdominant features: HMs, precession ...

Taiji Data Challenge

A typical criterion for unbiased parameter estimation:

1. Complexity in waveform and response modeling

The effect of realistic orbital motion (especially armlength variation ) in response:

sensitivity averaged over a year, with all the covariance among TDI channels considered

Taiji Data Challenge

Drag-free control simulation vs. mission requirement

2. The unknown and non-stationary noise properties (besides glitches and gaps)

The non-stationarity of T channel's sensitivity introduced by arm variation. Also, It fails to serve as a null channel in the unequal-arm regime.

Taiji Data Challenge

3. The coupling between pre-processing and scientific analysis stages

Tilt-to-noise coupling noise suppression with GW signals in the data

PD4L channel has much less zero points in the noise spectrum than Michelson channel (much less divergence in the evaluation of likelihood)

Proposed by Gang Wang in 2502.03983, verified via TDC simulation code.

Michelson-X

PD4L-1

Taiji Data Challenge

signal and noise overlap in both time and frequency domains, 10^4 resolvable signals, 10^5 undetermined parameters

Taiji Data Challenge

Open datasets and analysis tools: https://github.com/TriangleDataCenter

• Release date: May 2025 for blind set, Nov 2025 for training set
• Aim: building Taiji's data simulation & analysis pipelines
• Triangle: open-source toolkit for TDC

Triangle-Simulator

Time-domain prototype simulator for signals, raw measurements and TDI

Triangle-BBH

Fast frequency-domain MBHB modeling, search and parameter estimation

Triangle-GB

Fast frequency-domain GB modeling, search and parameter estimation

• Activities: training schools, invited talks, workshops. Participation and suggestion are encouraged!

EMRI ?

verification binary data: warm up

global-fit data

pre-processing data: not detrended, unsynchronized

Minghui Du et al., 2505.16500

Taiji Data Challenge

Triangle-Simulator

Triangle-BBH

The reconstruction of MBHB waveforms

The reconstruction of GB waveforms

The TDC global fit data

The collaboration is working on the multiple-source analysis of TDC: an iterative search-subtraction approach

Taiji Data Challenge

Triangle-Simulator

Triangle-BBH

Decreased GB foreground noise

GB location in different stages

The TDC global fit data

MBHB parameter posteriors in different stages

Taiji Data Challenge

Triangle for space detector network

• Enhancing sensitivity -> better constraints on parameters
• Breaking  sky location degeneracy -> unique and more precise
• Separating SGWB & GB foreground & noise
• Cross-validation in global fit (open question)
• EMRIs？

Complementary orbital configurations of Taiji, TianQin, and LISA

Adapted from Ruan et al., Nature Astronomy (2020).

Network sensitivity

Network location of MBHB

mode-by-mode heterodyned likelihood with network-covariance will be released in Triangle-BBH

From mission readiness to open collaboration

Taiji as part of a shared millihertz gravitational-wave ecosystem

Open interfaces for collaboration

Simulation interoperability · calibration and TDI · data challenges · global fitting · source populations · detector-network science
 

Leadership coordination within China’s space-based GW community · 2025

Prof. J. Luo

TianQin Chief Scientist

Prof. Y.-L. Wu

Taiji Chief Scientist

Thank you