LETSGO: A spacecraft-based mission to accurately measure the solar angular momentum with frame-dragging

LETSGO (LEnse-Thirring Sun-Geo Orbiter) is a proposed space-based mission involving the use of a spacecraft moving along a highly eccentric heliocentric orbit perpendicular to the ecliptic. It aims to accurately measure some important physical properties of the Sun and to test some post-Newtonian features of its gravitational field by continuously monitoring the Earth-probe range. Preliminary sensitivity analyses show that, by assuming a cm-level accuracy in ranging to the spacecraft, it would be possible to detect, in principle, the Lense-Thirring effect on it at a 10^-3-10^-4 level over a timescale of 2 yr, while the larger Schwarzschild component of the solar gravitational field may be sensed with a relative accuracy of about 10^-8-10^-9 during the same temporal interval. The competing range perturbation due to the non-sphericity of the Sun would be a source of systematic error, but it turns out that all the three dynamical features of motion examined affect the Earth-probe range in different ways, allowing for a separation in data analyses. The high eccentricity would help in reducing the impact of the non-gravitational perturbations whose impact would certainly be severe when LETSGO would approach the Sun at just a few solar radii. Further studies should be devoted to investigate both the consequences of the non-conservative forces and the actual measurability of the effects of interest by means of extensive numerical data simulations, parameter estimations and covariance analyses. Also an alternative, fly-by configuration is worth of consideration.

💡 Research Summary

The paper proposes LETSGO (LEnse‑Thirring Sun‑Geo Orbiter), a novel space mission designed to measure the Sun’s angular momentum by directly detecting its gravitomagnetic (Lense‑Thirring) field. The spacecraft would follow a highly eccentric heliocentric orbit (semi‑major axis ≈ 0.51 AU, eccentricity ≈ 0.92) whose plane is perpendicular to the ecliptic, allowing the probe to pass within a few solar radii of the Sun. Continuous Earth‑probe ranging with centimeter‑level laser ranging accuracy is the primary observable.

Numerical integrations of the Earth‑probe distance with and without the Lense‑Thirring term show that, over a two‑year data span, the LT‑induced range shift Δρ_LT reaches several centimeters, while the larger Schwarzschild (1PN) contribution Δρ_Schwarzschild reaches meters. The solar quadrupole moment J₂ produces a range perturbation of tens of centimeters. Assuming cm‑level ranging, the LT effect could be detected at the 10⁻³–10⁻⁴ level, and the Schwarzschild term at 10⁻⁸–10⁻⁹ relative accuracy.

The dominant systematic error is the uncertainty in J₂ (≈ 10 %); however, its temporal signature differs from LT and can be separated in a multi‑parameter fit. Non‑gravitational forces (solar radiation pressure, thermal recoil, electromagnetic torques) become significant near perihelion; the authors argue that a drag‑free system with acceleration noise of 10⁻⁸–10⁻⁹ m s⁻² Hz⁻¹⁄² at ≈ 10⁻⁷ Hz, comparable to LISA Pathfinder performance, would mitigate these disturbances.

Data analysis would involve simultaneous estimation of the LT, Schwarzschild, J₂, and non‑conservative force parameters using least‑squares fitting and covariance analysis, thereby reducing parameter correlations and achieving the target precision. The paper also discusses mission implementation issues such as launch Δv, orbital insertion, fuel budget, and an alternative fly‑by configuration.

In summary, LETSGO leverages a carefully chosen high‑eccentricity, high‑inclination orbit and state‑of‑the‑art ranging to provide a realistic pathway for a high‑precision test of solar gravitomagnetism and a direct measurement of the Sun’s spin angular momentum, complementing existing indirect helioseismic estimates. Further work is required on detailed non‑gravitational modeling, long‑term simulations, and engineering design to bring the concept to fruition.