Exploit each Occasion: Making Science at Performance Checks with the JANUS Instrument onboard ESA/JUICE S/C during its Cruise towards Jupiter

The ESA JUICE mission. ESA’s Jupiter Icy Moons Explorer (JUICE) [1] has been successfully launched from Europe's Spaceport in French Guiana on April 14^th, 2023. A suite of remote sensing, geophysical and in situ instruments are accommodated onboard, with the aim to i) explore Jupiter’s complex environment in depth, ii) study the wider Jupiter system as an archetype for gas giants across the Universe, and iii) characterize in detail the three large ocean-bearing moons (Ganymede, Callisto and Europa), as both planetary objects and possible astrobiological habitats. Before reaching the Jovian system in July 2031, JUICE will cruise ~8 years inside the inner Solar System (see Figure 1), performing 4 planetary fly-bys (the first one ever attempted with both the Moon and Earth, then with Venus and two more with Earth). JUICE instrument operations during such fly-bys are heavily constrained for technical reasons and to give full priority to navigation and mission safety. Nevertheless, such events are good opportunities to perform calibration checks and experience typical fly-by operations that will be undertaken with Galilean satellites. In addition to these planned fly-bys, the JUICE spacecraft will be routinely operated in specific time windows to perform instruments and probe “checkout operations” (see Figure 2). In particular, some of these “checkout windows” without attitude constraints (i.e., above 1.4 AU Sun distance) can be exploited to optimize observation of small bodies of our Solar System, in view of similar observations to be performed on minor targets in the Jupiter system. These data will therefore increase the scientific output of the whole mission at no resources expense, by properly choosing the pointing towards targets of planetary interest. These observations, if properly planned, could significantly contribute to explore the Cosmic Vision’s main theme “How does the Solar System work?”

 

The JANUS instrument onboard JUICE. The JANUS (Jovis, Amorum ac Natorum Undique Scrutator) instrument [2] onboard JUICE is the multispectral camera enabling imaging in the 340-1080 nm wavelength range. JANUS allows  imaging of many different targets: from the icy satellites, to Io, to small inner and irregular moons, the rings and Jupiter itself. While in the Jupiter system, the JUICE trajectory will allow icy Galilean satellites observations down to closest approaches of a few hundred km, resulting in spatial sampling  up to a few m/pixel. All the other targets will be observed from distances above a few 10⁵ km, i.e. spatial sampling above several km/pixel. 13 filters from the near ultraviolet to the near infrared will guarantee good spectral coverage with bandwidths from several tens of nm down to 10 nm.

 

Science during checkout windows and coordination with ground-based campaigns. We propose to exploit the checkout windows during the JUICE cruise phase to perform cometary science with JANUS, adding no cost science to the plans of testing instrument’s capability to detect faint sources (representative for exospheres and rings in Jupiter system) and to characterize unresolved objects (representative for Jupiter small moons). Among small bodies orbiting our Solar System, comets are thought to provide the most direct information on how planets form in all protoplanetary disks [3]. The observation of comets provides key information to investigate the processes of formation and evolution of pebbles forming planetesimals at different heliocentric distance of the primordial Solar Nebula, the pebbles themselves probably originating from different regions of the solar protoplanetary disk [4]. JANUS checkout pointing campaigns of comets of several dynamical groups could allow these targets to be observed at phase angles (i.e., angle Sun-Target-Observer) hardly available from Earth. In particular, optical imagery of cometary coma as a function of phase angle has a key importance in investigating the intimate nature of cometary dust [5]: size, size distribution, shape, and composition of dust particles can be investigated by means of observations taken in different broadband filters as those potentially provided by JANUS, and by means of comparison with data obtained e.g. with ground-based telescopes, with different observing geometries.

 

Here we present the results of a “test” we performed for the Checkout #4, foreseen for the week of 23 February 2026, where the Pointing Campaign #1 will be executed and the spacecraft pointing will not undergo strict thermal constraints as during cruising in the inner Solar System. To this aim, we designed a specific tool, J-SCOUT (Search for Cruise-phase Observation Utility Tool) [6] to query the JPL Horizon database [7] and take an “instant picture” of the small bodies complex within the Solar System at the checkout date(s), considering the latest Consolidated Report on Mission Analysis (CreMA 5.0b23) as the baseline mission profile. We designed the possibility to limit the query to the “active bodies” (as defined in the JPL Horizon Database), resulting in investigations of comets pertaining to 6 different dynamical categories. This allowed us to derive a list of potential “campaign cometary targets” for Checkout#4, which will be used as a “testbed” for such an operational plan, aimed at nominal instrument functional and performance verification activities during cruise. The Checkout #4 outcomes will be used to prepare for next checkout(s), for which J-SCOUT will be used to obtain a new list of “JUICE targets” that will be proposed for a campaign of quasi-contemporary, possibly multi-telescope ground-based observations, hence enhancing the synergy among space missions and ground-based telescopes.

 

References. [1] Grasset et al., 2013, PSS 78, 1. [2] Palumbo P. et al., 2025, SSR 221, 3, id. 32. [3] Blum J. et al., 2017, MNRAS 469: S755-S773. [4] Fulle M., 2021, MNRAS: 505, 2, 3107-2112. [5] Bertini et al., 2017, MNRAS 469, S404; [6] Agostini et al., 2022, PSS 216, 105476. [7] https://ssd.jpl.nasa.gov/horizons/

Figure 1 – Trajectory of JUICE spacecraft through the inner Solar System, before reaching Jupiter system in July 2031. The yellow stars show the Earth gravity assists and the Venus fly-by. The grey circular band show the extension of the asteroid Main Belt extension.

Figure 2 – JUICE mission profile (heliocentric and geocentric distance) with periodic payload checkout (including the pointing campaigns)