Tuesday, 25 August 2015

ESA's Europa Flyby Plans

While excitement builds for NASA’s flyby mission for Europa, ESA’s plans for the Jupiter Icy Moons Explorer (JUICE) are now in the implementation phase (known as B2), following successful mission adoption in November 2014.  JUICE will conduct two close flybys of Europa over a couple of weeks in October 2030, before moving onto a wider jovian orbit to complete a reconnaissance of the rest of the jovian system, ending up in orbit around Ganymede in 2032-2033.  JUICE’s sophisticated instrument suite of cameras, spectrometers (UV, near-IR and sub-mm), field and plasma instruments, laser altimeter and sub-surface radar will study Europa’s surface and sub-surface with a clarity, resolution and sensitivity far in excess of any previous exploration, including the ill-fated Galileo mission.  JUICE aims to understand the composition of non-ice material on the surface (particularly those related to habitability of the sub-surface ocean); search for liquid water below the surface; and to study any active processes.

However, the harsh radiation environment of Europa, coupled with the desire to study Jupiter and Ganymede over a 3-year mission, means that JUICE’s brief foray close to Europa will only occur once, with two quick flybys to limit the radiation dose.  Indeed, particle fluxes are 20 times higher at Europa than they are at Ganymede.  We chose to focus these two flybys on regions of high interest for geology, chemistry and astrobiology, including those where Galileo image suggested potential recent activity - chaotic terrains where exchange of materials between the surface and subsurface might have been possible, providing potential insights into the nature of Europa’s subsurface ocean.  Two flybys will no-doubt leave us wanting many more, but it should be remembered that Ganymede is the primary target of this mission.

Regions of interest and trajectory for the two JUICE
flybys of Europa, from the JUICE Definition Study Report.
JUICE will fly on the anti-jovian side of Europa (the far side, from Jupiter’s perspective), with a closest approach of 400 km.  We need to be on the far side for the radar sounding to work.  The far side must also be sunlit for the remote sensing experiments.  This will provide regional (500-1000 m resolution) and local (50 m resolution) imaging for the study of geological processes; the potential for radar sounding down to a maximum penetration depth of 9 km (depending on the surface properties) with a vertical resolution of 50 m or larger; and laser altimetry with a vertical resolution of less than 5 m.

Plans for the Flybys

The spacecraft will approach Europa from the trailing hemisphere (longitude of 90 degrees) that receives the largest radiation dose from the co-rotating jovian magnetosphere; closest approach will be over the anti-jovian point (longitude 180 degrees); and departure is then over the leading hemisphere (longitude 270 degrees).  One flyby will occur over the northern hemisphere (up to latitudes of 45 degrees), the second will be over the southern hemisphere.    JUICE will zip past at 3.6-3.9 km/s.  

The Europa flybys represent the most highly packed observational sequence of the JUICE mission.  Observations will start many hours before the closest approach, using regional imaging and spectroscopic mapping to compare the geology and composition of the leading and trailing hemispheres. JANUS will lead 3 spacecraft slews during the inbound and outbound phases, MAJIS will lead 2, with other instruments riding along to view both the nadir and limb of the moon.   All instruments will be operating simultaneously when the spacecraft is within a distance of 150,000 km of Europa, placing one of the toughest requirements on the design of the JUICE spacecraft.  

During the 2 hours surrounding the closest approach, JUICE will switch from the power-optimised yaw-steering mode to the inertial pointing mode.  After more spacecraft slews, the instruments become purely nadir-pointing for the ±30 minutes around closest approach, and then active instruments (laser altimetry and sub-surface radar) operate for the ±7 minutes surrounding closest approach.  Depending on data volume constraints, distant Europa observations for plume searches will be performed two days either side of the closest approach, and distant views of Europa will continue to be used to study materials ejected from Europa’s surface, such as plume activity from the southern pole.  

Eight regions of interest have been identified, seven of which are on the trailing hemisphere to be viewed during approach.  I’ll try to describe each of these regions in a future blog post, and why we think they’ll be exciting to explore.  The plans continue to be formulated as JUICE moves through the implementation phase, under the guidance of the working groups for the SWT (science working team), but a comprehensive summary of the plans can be found in the JUICE Definition Study Report (Red Book) here:

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