Paul Hartogh

The Submillimetre Wave Instrument (SWI) onboard the Jupiter Icy Moons Explorer (Juice) spacecraft consists of a 29 cm off-axis paraboloid antenna and two double-sideband heterodyne receivers operating in the frequency ranges of 530–638 and 1066–1286 GHz. This instrument is designed to study Jupiter's atmosphere and the exospheres of the Galilean moons. We used the Lunar-Earth-Gravity-Assist (LEGA) data from August 2024 to characterise this antenna using several SWI observations pointing towards the Earth. The data analysis of the LEGA observations dedicated to the antenna beam allows us to characterise the antenna beam Full Width at Half Maximum (FWHM), sidelobes, absolute pointing and receiver coalignement. Moreover, an estimate of the calibration efficiency factor has been obtained from an optical model simulation.

On 19 and 20 August 2024 the Jupiter Icy Moons Explorer (Juice) executed during its cruise phase towards Jupiter a combined Lunar Earth Gravity Assist (LEGA) maneuver. These close flybys of the Moon and the Earth provided so far the best opportunity to test the behavior, performance, and calibration of the Submillimetre Wave Instrument (SWI) onboard Juice. This paper shows typical data taken during the Earth Gravity Assist and the following few days. Data quality and problems resulting from unexpected behavior of the hardware are discussed.

The Jupiter Icy moons Explorer (Juice) was the first spacecraft ever that performed a combined gravity assist using both the Moon and Earth in succession. The double flyby required highly precise navigation to succeed. The LEGA allowed Juice to make a shortcut through the inner solar system on its way to Jupiter, using less fuel than would have been otherwise required. On 19 August 2024, Juice had its closest approach to the Moon. This first part of the manoeuvre accelerated the spacecraft by approximately 0.9 km s ⁻¹ relative to the Sun. On 20 August 2024, the spacecraft swung past Earth. This second part of the manoeuvre reduced the spacecraft's speed by 4.8 km s ⁻¹ relative to the Sun. This was a unique opportunity for its payloads to observe the Moon and Earth from a close distance as both calibration and science targets. The Submillimetre Wave Instrument (SWI), a dual channel heterodyne spectrometer observed both targets in two far-infrared bands around 500 and 250 µm wavelength in order to characterize and calibrate the overall performance of the instrument, including its receiver frontend, spectrometer backend and telescope mechanisms. In addition, the commanding pipeline and operations processes of the instrument were also tested close to its full range of flexibility using the relevant pipelines. In this paper we provide a contextual description of physical and functional characteristics of SWI, its operational principles and in-flight calibration activities during LEGA.

Context . Detecting and characterising plumes that may insert subsurface water into the tenuous atmospheres of the icy Galilean moons is a key objective for the Submillimetre Wave Instrument (SWI) on board ESA’s JUpiter ICy moons Explorer (JUICE) mission. However, faint spectral signatures and complex observational geometries during distant moon monitoring pose significant challenges. Aims . Our aim with this study is to assess the feasibility of detecting water plumes on Ganymede with SWI during the planned moon monitoring observations while orbiting Jupiter. We investigated how observational geometry and plume physical properties might influence the spectral signatures of a gas-phase water plume observed by SWI. Methods . We used the LIne Modelling Engine (LIME) software package, a 3D non-local thermodynamic equilibrium radiative transfer code, to simulate and analyse the ortho-H ₂ O spectral line at 557.936 GHz for different observational geometries. We focused on an observing distance of approximately 1.2 × 10 ⁶ km from Ganymede, corresponding to the moon monitoring phase planned for SWI as part of the JUICE mission. We modelled key plume parameters, including temperature, production rate, and velocity fields, along with different SWI viewing geometries, to assess their impact on plume detectability and to develop strategies for optimising observations. Results . Simulations indicate that SWI can detect water plumes on Ganymede under favourable conditions, even for water production rates as low as 10 ²⁷ s ⁻¹ . Detection is most likely if the plume is observed at the limb of the moon or when located at 90° from the subsolar point. In these geometries, the plume contributes with strong emission line wings with temperature contrasts up to ∼4 K for production rates around 10 ²⁹ s ⁻¹ . Such line wings can be detected with a signal-to-noise ratio of 7 with a 10-minute integration using the high-resolution Chirp Transform Spectrometer backend of SWI during the modelled moon monitoring observations. Increased plume temperatures and velocity distributions enhance line intensity and broadening, potentially enabling plume identification even at lower production rates.

Giant planet atmospheres are continuously affected by external sources, including cometary impacts, interplanetary dust particles (IDPs), and material originating from rings and satellites, that modify their composition over long timescales. At Jupiter, the last major comet impact occurred in July 1994, when the 21 fragments of comet Shoemaker–Levy 9 (SL9) collided with the planet in its southern hemisphere over the course of one week (Noll et al. 1996). This rare event provided a unique opportunity to investigate the shock-induced chemistry recombining jovian atmosphere and cometary material, leading to the detection of several new species in Jupiter’s stratosphere (e.g., CO, CS, and HCN; Lellouch et al. 1996).On longer timescales, monitoring these newly formed species months and years after the impacts provides valuable constraints on the chemical and dynamical evolution of exogenic material in Jupiter’s atmosphere. First, the temporal evolution of their abundances traces the chemical loss processes. For instance, Cavalié et al. (2023) reported an HCN loss of a factor of 2–8 in 2017 ALMA observations compared to the 1995–1998 period. Second, their horizontal and vertical transport constrains atmospheric dynamics and mixing as a function of longitude, latitude, and altitude. At first located at the impact sites, the longest-lived species progressively spread in longitude over a few months, in latitude over a few years (e.g., Lellouch et al. 2006; Moreno et al. 2003), and vertically from pressures near 0.1 mbar initially to several mbar (Cavalié et al. 2023; Rodriguez-Ovalle et al. 2025). Owing to horizontal mixing, these molecules were expected to become uniformly distributed across Jupiter’s disk on a timescale of about 10 years (Moreno et al. 2003). This prediction was confirmed in 2017 for CO, which exhibited a constant latitudinal distribution (Cavalié et al. 2023).However, HCN exhibits an unusual distribution from the mid-latitudes to the polar regions (Cavalié et al. 2023; Rodriguez-Ovalle et al. 2025). This behaviour is unexpected because HCN and CO share the same cometary origin and should therefore display similarly uniform distributions. Two remarkable features were identified by Cavalié et al. (2023). First, they observed an HCN depletion towards the poles, coincident with the position of the auroral regions. This depletion is seen vertically at pressures greater than 0.1 mbar and has been tentatively attributed to adsorption onto aerosols forming and growing at these pressure levels in the polar atmosphere (Friedson et al. 2002). Second, and more surprisingly, HCN exhibits a high-altitude abundance enhancement reaching up to 20 times the low-to-mid latitude abundance. This enhancement is spatially confined to the southern auroral hot spot (Sinclair et al. 2023) at 0.01 mbar, i.e., above the pressure levels where depletion is observed in the polar regions. Such an HCN enhancement could result from local production through ion–neutral chemistry under auroral conditions.In this study, we aim to monitor the temporal evolution of SL9-produced species in the stratosphere of Jupiter, in order to (i) follow their horizontal and vertical transport to constrain the mixing properties at the probed levels; (ii) constrain the chemical losses over time ; and (iii) assess their distribution in the polar regions and understand the auroral related chemistry.We use mapping observations of Jupiter performed with Atacama Large Millimeter/submillimeter Array (ALMA) recorded on three occasions in September 2022, i.e., almost 30 years after the SL9 impacts. We detect the HCN (J=4-3), CO (J=3-2), and CS(J=7-6) emission lines emanating from the stratosphere of Jupiter all along the planetary limb. While HCN and CO were already observed with ALMA in 2017, we present the first high resolution map of CS. Here, we will present radiative transfer calculations and vertical profile retrievals performed at various latitudes to illustrate the horizontal and vertical distributions of the targeted species. We will compare our results with those obtained from observations performed over the last 30 years.References:Cavalié et al. 2021, A&A 647, 8Cavalié et al. 2023, Nat. Astron. 7, 1048Friedson et al. 2002, Icarus 158, 389Lellouch 1996, IAU 156Lellouch et al. 2006, Icarus 184, 478Moreno et al. 2003, P&SS 51, 591Noll et al. 1996, Cambridge University PressRodriguez-Ovalle et al. 2025, A&A 696, 173Sinclair et al. 2023, Planet. Sci. J. 4, 7

The Jupiter Icy Moons Explorer (JUICE) is the first L-class mission of the Cosmic Vision 2015-2025 program of the European Space Agency. JUICE was launched in April 2023 and embarked on an 8-year interplanetary transfer to reach Jupiter and its system. It will perform its Jupiter orbit insertion in July 2031 and start its nominal science phase. JUICE will orbit the planet about 60 times until its Ganymede orbit insertion (GOI) at the end of 2034. This Jupiter tour will comprise 2 close flybys of Europa, and many more flybys of Ganymede and Callisto. After starting the Jupiter tour in the equatorial plane of the planet, JUICE will spend many months in inclined orbits reaching inclinations of up to ~ 35 degrees before going back to the equatorial plane in preparation of GOI.The payload of JUICE comprises 10 science instruments. One of the remote sensing instruments is the Submillimetre Wave Instrument (SWI), which was designed to study the composition, chemistry and dynamics of the atmospheres of Jupiter and the Galilean Moons. It will also investigate the surface properties of the moon surfaces. SWI is an off-axis single dish telescope with 29 cm diameter equipped with two perpendicular pointing mechanisms that allow to point ±72° along track and ±4.3° cross track from the nadir direction of the JUICE platform during the Jupiter tour. It is a heterodyne spectrometer and radiometer that can be tuned independently and simultaneously in two submillimetre bands (530-638 GHz and 1066-1286 GHz). The highest spectral resolution is achieved with the Chirp Transform Spectrometers that cover 1 GHz bandwidth with 10000 channels. SWI is thus a miniaturized observatory with all the inherent complexity implied for observation planning.The JUICE Science Operation Center (JSOC) is conducting planning exercises every year before JOI. These exercises serve the purpose of developing the science observation implementation procedures and the relevant software tools (both at the ESA and instrument team levels) that will ensure smooth operations during the science phase. In parallel, the four JUICE science working groups are continuously consolidating and extending the JUICE science objectives. They also identify multi-instrument synergies and propose coordinated observation campaigns.The latest planning exercise occurred in the first semester of 2026 and focused on two equatorial orbits, namely orbits 37 and 38, that include a close Ganymede flyby. It spans from 23 November to 20 December 2033, with the flyby occurring on 27 November 2033. The flyby inbound is mostly on the nightside and the outbound on the dayside, with a closest approach at ~900 km altitude. The orbits also provide opportunities for Io, Europa and Callisto observations with apparent sizes sufficiently large so that enable to map them.The implementation of the JUICE observing plan follows a multi-step approach (observation plan, pointing timeline, instrument timeline) coordinated by JSOC. A significant effort by JSOC and the instrument teams concerned the identification and implementation of common/complementary observations in the design the spacecraft pointing profile to maximize the science return of the mission in these orbits and flyby. Following the latest JUICE science synergy workshop (ESAC, January 2026), several coordinated campaigns have been implemented in the 2026 planning exercise and helped making the implementation of the timeline more efficient. Of particular interest for SWI are the following coordinated campaigns: zonal winds, distant monitoring and auroral monitoring for Jupiter, and a coordinated mapping of the Ganymede dayside on the flyby outbound leg.For what concerns SWI, the observation sequences pre-defined are complemented with other SWI observations, that can be either prime or ride-along observations. For Jupiter, those include: (i) temperature and winds observations with Jupiter limb scans, (ii) searches for isotopes and new species with limb stares and scans, (iii) temperature and composition measurements by means of regional mapping observations (including the auroral regions). During the flyby Ganymede, SWI will perform surface and exosphere regional mapping observations from the largest distances during the flyby and a series of limb stares and scans to investigate the composition, temperature and winds of its exosphere a few hours from closest approach. Moreover, SWI aims to conduct a monitoring of the Galilean Moons surfaces and exospheres on a daily basis, dedicating about 1 hour per day of operations, to investigate the various processes acting as sources and sinks for the exospheres. Finally, SWI will support observation campaigns initiated and led by other instruments and the latest planning exercise proposed several examples of those, like: (i) stellar and Earth radio occultations for which SWI will observe the temperature, composition and winds of the target at the latitudes/longitudes of the ingress/egress, and (ii) Io and Europa closest distance observations.

The Submillimeter Wave Instrument (SWI) aboard the JUpiter Icy Moon Explorer (JUICE) satellite conducted a series of dedicated observations of the Moon and Earth atmosphere as part of an instrument characterization and calibration campaign during the Lunar-Earth Gravity Assist maneuver in mid-August 2024. The JUICE satellite achieved an altitude of approximately 700 km during its closest approach, while subsequent moon observations were conducted at distances of around 500,000 km. The lunar atmosphere was observed in both nadir and limb geometries in the two bands of SWI (600 and 1200 GHz). Although no direct signature of the water line was detected in the data, these observations allowed upper limits to be placed on the neutral water column density. We proceed to summarize the measurement, modeling approach, and its underlying assumptions, followed by a discussion of the implications for the moon's water cycle.

The Submillimeter Wave Instrument (SWI) aboard the JUpiter Icy Moon Explorer (JUICE) conducted a series of targeted observations of the interstellar comet 3I/ATLAS, discovered on July 1, 2025, by the ATLAS survey in Chile. As the third confirmed interstellar object to traverse our solar system—following 1I/ʻOumuamua and 2I/Borisov—3I/ATLAS offers a unique opportunity to study the composition and physical properties of material from outside the Solar System. JUICE-SWI executed dedicated observations of the comet between November 2 and November 19, 2025, during which the SWI instrument performed four 30-minute observations at a distance of approximately 80 million kilometers. The observations targeted the ground-state rotational transitions of ortho- and para-water in the 600 GHz and 1200 GHz frequency bands, using frequency-switching mode to maximize on-source integration time. We present the data reduction, calibration procedures, and results from the SWI observations. These include derived rotational line intensities and the water production rate. We discuss the implications of the measurements in the context of interstellar comet composition, comparing them with other observations. The results provide early insights into the volatile content of an interstellar visitor and contribute to our understanding of the chemical diversity of cometary material across stellar systems.

Context . The collision of comet Shoemaker-Levy 9 (SL9) with Jupiter in 1994 changed the chemical composition of the Jovian stratosphere for decades. New molecules were detected minutes after the impacts (HCN, CO, H ₂ O, CS, etc.) and some are still present today. They were deposited in the stratosphere at pressures lower than 0.1 mbar and were most probably formed by shock-induced chemistry recombining Jovian and cometary material. However, the question of the origin of these molecules is still not completely understood. One way to address this open question is to determine the isotopic composition of the new molecules. Isotopic ratios have long been measured in the Solar System. They present a variety of values depending on the object or the molecule and therefore trace different reservoirs of material. Derivations of carbon and nitrogen isotopic ratios in HCN four years after SL9 showed atypical depletions in the heavier isotopes that had never been observed before in the Solar System. These results suggested an unusual cometary composition or an unknown fractionation mechanism in the hot and shocked air parcels. Aims . We aim to measure carbon and nitrogen isotopic ratios in HCN to shed light on the puzzling results of 1998. Methods . With Atacama Large Millimeter/submillimeter Array data from 2017 and radiative-transfer calculations, we derived the abundance of two HCN isotopologues, H ¹³ CN and HC ¹⁵ N, at pressures probed from 0.03 to 1.8 mbar. Results . We find ¹² C/ ¹³ C = 73 ± 5 and ¹⁴ N/ ¹⁵ N = 245 ± 29, respectively (0.76–0.87) and (0.80–1.00) times the terrestrial references, and (0.69–0.87) and (0.42–0.70) times the solar-Jovian bulk values. In contrast to the strong depletions reported in 1998, our values are instead compatible with an enrichment in the heavier isotopes relative to the Jovian bulk. Conclusions . We interpret these enrichments as the direct signature of the cometary contribution in HCN and/or as 23 years of chemical evolution in the Jovian atmosphere.

The Jupiter Icy Moons Explorer (Juice) embarked in 2023 on a 8-year interplanetary journey to Jupiter and its icy moons. The Submillimetre Wave Instrument (SWI) is one of the ten science instruments aboard the spacecraft. SWI is a sophisticated and first-of-its-kind payload visiting the outer solar system, featuring dual-band tunable receivers, two independent pointing mechanisms, and spectrometers capable of high resolution (up to a resolving power of 10 ⁷ ). It is designed to support the diverse science objectives of the Juice mission targeting Jupiter's middle atmosphere, icy-moon's exospheres as well as near sub-surface thermophysical properties. For this purpose the Juice mission adopts a complex trajectory tour within the Jovian system, which further necessitates a sophisticated, mission-driven operations concept for SWI. This presents significant planning, operations and commanding challenges which are described in this paper in the context of the Lunar and Earth Gravity Assist (LEGA). After the development and ground calibration of the instrument, the SWI Team has designed a comprehensive calibration strategy applicable during the Cruise Phase of Juice. Among the various opportunities for calibration, including the Near-Earth Commissioning Phase and more than ten Payload Checkout Windows, the LEGA offers the means not only to improve the calibration of the instrument, but also to validate the operational strategy of future icy moon flybys.