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Precise Doppler tracking of the Juno spacecraft in its polar orbit around Jupiter is used to determine the planet’s gravity harmonics, showing north–south asymmetry caused by atmospheric and interior flows. Probing the depths of Jupiter The Juno mission set out to probe the hidden properties of Jupiter, such as its gravitational field, the depth of its atmospheric jets and its composition beneath the clouds. A collection of papers in this week's issue report some of the mission's key findings. Jupiter's gravitational field varies from pole to pole, but the cause of this asymmetry is unknown. Rotating planets that are squashed at the poles like Jupiter can have a gravity field that is characterized by a solid-body component, plus components that arise from motions in the atmosphere. Luciano Iess and colleagues use Juno's Doppler tracking data to determine Jupiter's gravity harmonics. They find that the north–south asymmetry arises from atmospheric and interior wind flows. To determine the depths of these flows, Yohai Kaspi and colleagues analyse the odd gravitational harmonics and find that the J 3 , J 5 , J 7 and J 9 harmonics are consistent with the jets extending deep into the atmosphere, perhaps as far as 3,000 kilometres. They conclude that the mass of Jupiter's dynamical atmosphere is about one per cent of Jupiter's total mass. The composition of Jupiter beneath its turbulent atmosphere remains a mystery. If different parts of a spinning object rotate at different rates, then the object probably has a fluid composition. Tristan Guillot and colleagues study the even gravitational harmonics and find that, below a depth of about 3,000 kilometres, Jupiter is rotating almost as a solid body. The atmospheric zonal flows extend downwards by more than 2,000 kilometres, but not beyond 3,500 kilometres, as is also the case with the jets. The gravity harmonics of a fluid, rotating planet can be decomposed into static components arising from solid-body rotation and dynamic components arising from flows. In the absence of internal dynamics, the gravity field is axially and hemispherically symmetric and is dominated by even zonal gravity harmonics J 2 n that are approximately proportional to q n , where q is the ratio between centrifugal acceleration and gravity at the planet’s equator 1 . Any asymmetry in the gravity field is attributed to differential rotation and deep atmospheric flows. The odd harmonics, J 3 , J 5 , J 7 , J 9 and higher, are a measure of the depth of the winds in the different zones of the atmosphere 2 , 3 . Here we report measurements of Jupiter’s gravity harmonics (both even and odd) through precise Doppler tracking of the Juno spacecraft in its polar orbit around Jupiter. We find a north–south asymmetry, which is a signature of atmospheric and interior flows. Analysis of the harmonics, described in two accompanying papers 4 , 5 , provides the vertical profile of the winds and precise constraints for the depth of Jupiter’s dynamical atmosphere.

With the continued acquisition of high-precision tracking data by the Juno spacecraft, significant progress has been made in accurately determining Jupiter’s dynamical parameters. In this study, we utilize the orbit determination and gravity field recovery software SPOT, developed by Wuhan University, to process all available two-way Doppler tracking data of Juno’s perijove passes between 2016 and 2024. Incorporating 19 additional perijoves (PJ39–PJ68) beyond the 26 arcs used in the previous study, a joint estimation of Jupiter’s 40 degree zonal gravity harmonics, four tesseral degree-2 terms, spin-axis orientation parameters, and tidal Love number is determined. The results indicate that, compared with previously published Juno-based gravity field solutions, the accuracy of coefficients J2–J5 has improved by more than a factor of 2, while the J13–J36 terms exhibit an average improvement of about 30%. A stochastic force was introduced to absorb unmodeled small perturbations near perijoves, but its rapidly varying orientation does not point to an identifiable physical origin. The uncertainty of Jupiter’s spin-axis rotation parameter is improved to about 1 × 10–7 rad, indicating no significant deviation between the principal axis of inertia and the rotation axis. The estimated accuracy of the static tidal Love numbers is improved by roughly a factor of 2 compared with earlier Juno-based tidal analyses. Due to limitations in orbital geometry, the satellite-dependent tidal Love numbers cannot be determined with sufficient accuracy to reveal potential dynamical tidal effects. This work provides improved dynamical parameters for constraining Jupiter’s interior structure.

Planetary Radio Interferometry and Doppler Experiment (PRIDE) is a multi-purpose experimental technique aimed at enhancing the science return of planetary missions. The technique exploits the science payload and spacecraft service systems without requiring a dedicated onboard instrumentation or imposing on the existing instrumentation any special for PRIDE requirements. PRIDE is based on the near-field phase-referencing Very Long Baseline Interferometry (VLBI) and evaluation of the Doppler shift of the radio signal transmitted by spacecraft by observing it with multiple Earth-based radio telescopes. The methodology of PRIDE has been developed initially at the Joint Institute for VLBI ERIC (JIVE) for tracking the ESA’s Huygens Probe during its descent in the atmosphere of Titan in 2005. From that point on, the technique has been demonstrated for various planetary and other space science missions. The estimates of lateral position of the target spacecraft are done using the phase-referencing VLBI technique. Together with radial Doppler estimates, these observables can be used for a variety of applications, including improving the knowledge of the spacecraft state vector. The PRIDE measurements can be applied to a broad scope of research fields including studies of atmospheres through the use of radio occultations, the improvement of planetary and satellite ephemerides, as well as gravity field parameters and other geodetic properties of interest, and estimations of interplanetary plasma properties. This paper presents the implementation of PRIDE as a component of the ESA’s Jupiter Icy Moons Explorer (JUICE) mission.

The Martian gravity field is a key data set for studying the internal structure of Mars. For this purpose, we processed all the two-way Doppler tracking data of the Tianwen-1 orbiter from 2021 June to 2024 October and determined a new degree and order 80 Martian static gravity field model, TW80. The Tianwen-1 orbiter tracking data show high accuracy, with approximately 90% of arcs achieving an rms of residuals of less than 0.1 mm s–1. We evaluated this model in terms of gravity anomaly errors, gravity/topography correlation, and orbit determination performance. The TW80 gravity field model reaches a global resolution of degree and order 74, corresponding to a spatial resolution of approximately 140 km. The rms of the gravity anomaly errors is approximately 35.8 mGal, with notably smaller errors near the south pole region in the southern hemisphere and larger errors in mid- and low-latitude regions. The TW80 model shows a high correlation with topography and is consistent with the MRO120D gravity field model up to degree 40. Due to the highly elliptical orbit geometry, the orbit determination performance of the TW80 model for the Tianwen-1 orbiter is comparable to that of the MRO120D model, with radial differences less than 100 m and total position differences less than 500 m. Further improvements in orbit determination accuracy will focus on precise modeling of orbital maneuvers and the solar radiation pressure model, as well as extending the tracking duration of individual arcs.

The Juno spacecraft has been collecting data to shed light on the planet’s origin and characterize its interior structure. The onboard gravity science experiment based on X-band and Ka-band dual-frequency Doppler tracking precisely measured Jupiter’s zonal gravitational field. Here, we analyze 22 Juno’s gravity passes to investigate the gravity field. Our analysis provides evidence of new gravity field features, which perturb its otherwise axially symmetric structure with a time-variable component. We show that normal modes of the planet could explain the anomalous signatures present in the Doppler data better than other alternative explanations, such as localized density anomalies and non-axisymmetric components of the static gravity field. We explain Juno data by p-modes having an amplitude spectrum with a peak radial velocity of 10–50 cm/s at 900–1200 μHz (compatible with ground-based observations) and provide upper bounds on lower frequency f-modes (radial velocity smaller than 1 cm/s). The new Juno results could open the possibility of exploring the interior structure of the gas giants through measurements of the time-variable gravity or with onboard instrumentation devoted to the observation of normal modes, which could drive spacecraft operations of future missions. Juno spacecraft experienced unknown accelerations near the closest approach to Jupiter. Here, the authors show that Jupiter’s axially symmetric, north-south asymmetric gravity field measured by Juno is perturbed by a time-variable component, associated to internal oscillations.

The microhertz frequency band of gravitational waves probes the merger of supermassive black holes, as well as many other gravitational-wave phenomena. However, space-interferometry methods that use test masses would require further development of test-mass isolation systems to detect anticipated astrophysical events. We propose an approach that avoids onboard inertial test masses by situating spacecraft in the low-acceleration environment of the outer solar system. We show that, for Earth–spacecraft and interspacecraft distances of ≳10 au, the accelerations on the spacecraft would be sufficiently small to potentially achieve gravitational-wave sensitivities determined by stochastic gravitational-wave backgrounds. We further argue, for arm lengths of 10−30 au and ∼10 W transmissions, that stable phase locks could be achieved with 20 cm mirrors or 5 m radio dishes. We discuss designs that send both laser beams and radio waves between the spacecraft, finding that, despite the ∼ 104× longer wavelengths, even a design with radio transmissions could reach stochastic background-limited sensitivities at ≲0.3 × 10−4 Hz. Operating in the radio significantly reduces many spacecraft design tolerances. Our baseline concepts require two arms to do interferometry. However, if one spacecraft carries a clock with Allan deviations at 104 s of 10−17, a comparable sensitivity could be achieved with a single arm. Finally, we discuss the feasibility of achieving similar gravitational-wave sensitivities in a “Doppler tracking” configuration where the single arm is anchored to Earth.

The navigation of future interplanetary spacecraft will require an increasing degree of autonomy to enhance space system performance. A real-time trajectory determination is of paramount importance to reduce the risks of operations devoted to the exploration of celestial bodies in the solar system and to reduce the dependence and the loading on the ground systems. We present a technique for a sequential estimation of spacecraft orbits through the processing of line-of-sight relative velocity measurements that are acquired by the novel inter-satellite tracking system. This estimation scheme is based on the extended Kalman filter and is tested and validated in a realistic Mars mission scenario. Our numerical simulations suggest that the proposed navigation system can provide accuracies of a few meters in position and a few millimeters per second in velocity.

Ground-based observations of spacecraft signals have been used to study space weather. However, single spacecraft measurements observed from the Earth have limitations in studying the structure and evolution of solar plasma as they are unable to differentiate spatial and temporal variations. To overcome this limitation and improve our understanding of interplanetary scintillation, we simultaneously observed radio signals transmitted by two co-orbiting spacecraft: the ESA Mars Express (MEX) and the Chinese National Space Administration Tianwen-1 (TIW-1). We conducted the observations from April to November 2021 using the University of Tasmania’s VLBI radio telescopes at 8.4 GHz. We employed the Planetary Radio Interferometer and Doppler Experiment (PRIDE) technique to determine the topocentric Doppler measurements and residual phase of the carrier signal. These observables were used to quantify the phase fluctuations of the spacecraft signals caused by solar wind and hydrodynamic turbulence in the interplanetary medium. The measured phase fluctuations RMS from both spacecraft show small differences which are caused by factors such as the spacecraft’s motion, onboard electronics, and variations in the uplink signal path through Earth’s ionosphere. These fluctuations decrease with solar elongation and correlate with solar radio flux at 10.7 cm (2800 MHz), indicating solar activity. The estimated total electron contents along MEX and TIW-1’s radio lines of sight are similar, with higher values at lower solar elongations. Simultaneous multi-spacecraft observations also enable RFI characterization, frequent spacecraft performance comparisons, and investigation of solar activity effects on spacecraft performance and scientific outcomes.

Near-Earth Objects (NEO) are the topic of several research studies, with objects smaller than 1km in size posing the most threats and being the less understood of this scientific domain. The Asteroid Impact and Deflection Assessment (AIDA) mission involves NASA and ESA with the main mission goal to perform and analyze the asteroid deflection using the Kinetic Impactor technique. The mission target is Didymos-B, a moon of a binary asteroid called Didymos. NASA oversees the Double Asteroid Redirection Test (DART probe), and ESA is responsible for HERA probe, that will measure the Dydimos-B deflection caused by the impact. The Light Detection and Ranging (LIDAR), the Radar, the Satellite-to-Satellite Doppler tracking, the Seismometer, and the Gravimeter are instruments integrated into HERA spacecraft. Information synergy between the instruments allows the detailed characterization of the asteroid including internal structure. This experiment allows further understanding and will provide important information to improve the current NEO understanding and modelling. In this paper, scientific advances related to the LIDAR instrument are reported, including the innovative optomechanical design resulting from thermal and mechanical optimizations. The LIDAR has a compact design and needs to withstand extreme conditions, such as radiative and thermal conditions, without compromise its high accuracy measurements. The LIDAR is a time-of-flight altimeter instrument that will measure the distances from the HERA spacecraft to the target. It provides information for a 3D topographic mapping and calculates the asteroid reflectivity. The measurements are to be performed at a distance from 500 m to 14 km while operations such as fly byes or landings remain a possibility.

Purpose This paper aims to report the first iteration on the Light Detection and Ranging (LIDAR) Engineering Model altimeter named HELENA. HELENA is a Time of Flight (TOF) altimeter that provides time-tagged distances and velocity measurements. The LIDAR can be used for support near asteroid navigation and provides scientific information. The HELENA design comprises two types of technologies: a microchip laser and low noise sensor. The synergies between these two technologies enable developing a compact instrument for range measurements of up to 14 km. Thermal-mechanical and radiometric simulations of the HELENA telescope are reported in this paper. The design is subjected to vibrational, static and thermal conditions, and it was possible to conclude by the results that the telescope is compliant with the random vibration levels, the static load and the operating temperatures. Design/methodology/approach The Asteroid Impact & Deflection Assessment (AIDA) is a collaboration between the NASA DART mission and ESA Hera mission. The aim scope is to study the asteroid deflection through a kinetic collision. DART spacecraft will collide with Didymos-B, while ground stations monitor the orbit change. HERA spacecraft will study the post-impact scenario. The HERA spacecraft is composed by a main spacecraft and two small CubeSats. HERA will monitor the asteroid through cameras, radar, satellite-to-satellite doppler tracking, LIDAR, seismometry and gravimetry. Findings The HELENA design comprises two types of technologies: a microchip laser and low noise sensor. The synergies between these two technologies enable developing a compact instrument for range measurements of up to 14 km. Originality/value In this paper is reported the first iteration on the LIDAR Engineering Model altimeter named HELENA. HELENA is a TOF altimeter that provides time-tagged distances and velocity measurements. The LIDAR can be used for support near asteroid navigation and provides scientific information. The HELENA design comprises two types of technologies: a microchip laser and low noise sensor. The synergies between these two technologies enable developing a compact instrument for range measurements of up to 14 km.