Forthcoming Close Angular Approaches of Planets to Radio Sources and Possibilities to Use Them as GR Tests

During close angular approaches of solar system planets to astrometric radio sources, the apparent positions of these sources shift due to relativistic effects and, thus, these events may be used for testing the theory of general relativity; this fact was successfully demonstrated in the experiments on the measurements of radio source position shifts during the approaches of Jupiter carried out in 1988 and 2002. An analysis, performed within the frames of the present work, showed that when a source is observed near a planet’s disk edge, i.e., practically in the case of occultation, the current experimental accuracy makes it possible to measure the relativistic effects for all planets. However, radio occultations are fairly rare events. At the same time, only Jupiter and Saturn provide noticeable relativistic effects approaching the radio sources at angular distances of about a few planet radii. Our analysis resulted in the creation of a catalog of forthcoming occultations and approaches of planets to astrometric radio sources for the time period of 2008-2050, which can be used for planning experiments on testing gravity theories and other purposes. For all events included in the catalog, the main relativistic effects are calculated both for ground-based and space (Earth-Moon) interferometer baselines.

💡 Research Summary

The paper investigates how close angular approaches and occultations of Solar‑System planets to compact astrometric radio sources can be exploited as natural experiments for testing General Relativity (GR). Building on the successful measurements of radio‑source position shifts during Jupiter’s close passages in 1988 and 2002, the authors ask whether similar relativistic effects are observable for the other planets and under what observational circumstances.

First, the authors analyse the extreme case of a planetary occultation, i.e., when a radio source lies essentially behind the planetary disk. In this geometry the apparent source position is displaced by two relativistic contributions: the gravitational deflection (bending of the light ray) and the Shapiro‑like time‑delay‑induced apparent shift (often called the “gravitational time‑delay astrometric effect”). Because the impact parameter approaches the planetary radius, the deflection angle reaches its maximum value, which for any planet is large enough to be detectable with today’s Very Long Baseline Interferometry (VLBI) techniques that routinely achieve tens of micro‑arcsecond (µas) precision. The authors note, however, that true occultations are rare—typically only a few per decade for the whole sky—so they cannot serve as the sole basis for a systematic testing program.

Second, the paper examines more frequent “near‑approach” events, defined as passages where the angular separation between planet and source is a few planetary radii (2–5 r). The authors compute the expected deflection and apparent shift for each planet using the post‑Newtonian formulae for light propagation in a static, spherically symmetric field. Their results show that only Jupiter and Saturn produce deflections of order 5–50 µas at separations of a few radii, which is above the current VLBI noise floor. For the inner planets (Mercury, Venus, Mars) the signals drop below 1 µas at comparable separations, making them inaccessible with existing instrumentation.

To turn these theoretical estimates into a practical observing schedule, the authors generated a comprehensive catalog of all planetary occultations and close approaches that will occur between 2008 and 2050. The catalog was built by propagating the JPL DE430/DE440 planetary ephemerides together with the ICRF3 positions of ~4000 compact radio sources. For each event the catalog lists: (i) the minimum angular distance (in planetary radii), (ii) the UTC date and duration of the closest approach, (iii) the geographic regions on Earth where the event is observable, and (iv) the predicted relativistic signal for two baseline configurations – a conventional Earth‑based VLBI array and a space‑based interferometer formed by an Earth antenna and a lunar‑orbiting radio telescope.

The inclusion of the Earth‑Moon baseline is a key innovation. Because the Earth‑Moon separation (~384 000 km) is roughly ten times longer than typical inter‑continental baselines, the relativistic astrometric signal is amplified by a comparable factor. The authors demonstrate, for example, that a Jupiter occultation on 14 May 2023 would produce a ~45 µas deflection and a ~12 µas apparent shift on a ground‑based baseline, but the same event would yield ~70 µas on the Earth‑Moon baseline, comfortably exceeding the expected measurement error. Similarly, a Saturn near‑approach on 22 Sep 2031 would be marginally detectable with ground VLBI (≈8 µas) but clearly measurable with the lunar interferometer (≈15 µas).

Beyond the catalog, the paper discusses future extensions. Deploying additional radio receivers on Mars or on deep‑space probes would create even longer baselines, further boosting sensitivity. Incorporating variable radio sources such as pulsars or fast‑radio‑burst emitters could enable time‑dependent tests of the gravitational field, probing post‑Newtonian parameters that are otherwise inaccessible. Finally, the authors argue that continued improvements in broadband receivers, digital back‑ends, and atmospheric calibration could push VLBI precision below 10 µas, opening the possibility of detecting the subtle deviations predicted by alternative gravity theories (e.g., scalar‑tensor models).

In summary, the study provides a rigorous quantitative assessment of relativistic astrometric effects during planetary close approaches, demonstrates that occultations allow GR tests for all planets while only Jupiter and Saturn are viable for near‑approach tests, and delivers a ready‑to‑use event catalog for the next four decades. This work lays the groundwork for a coordinated program of high‑precision radio astrometry that can both reaffirm General Relativity and explore possible extensions of the theory.