JUICE is running, Europa Clipper is built and in checkout for launch this year… and the small-body community is onboard.
Space Science Reviews- no lightweights- had been running a “collection” (articles aligned in theme, but not date like in the print era) on JUICE, europe’s JUpiter ICy moons Explorer mission. Launched successfully a year ago, the probe will enter Jovian orbit in 2031, make flybys of Ganymede and Callisto, and take two passes of Europa before settling into Ganymede orbit. Now it’s Europa Clipper’s turn:
Pappalardo, R. T. Buratti, B. J Korth, H. et al. Science Overview of the Europa Clipper Mission May 2024 Space Science Reviews vol. 220 art. 40 s11214-024-01070-5
The “clipper” in Europa Clipper refers to the multiple, fast flybys the probe will take to study its eponymous satellite of Jupiter. Had the mission simply orbited the second-Galilean-moon from Jupiter, the radiation would fry it quickly. Radiation, in part natural to Jupiter, in part pumped by Io emissions, decreases with orbital radius from the planet. That’s why JUICE can enter Ganymede’s gravity well, and study it at leisure, but only study Europa with two flybys. (And even those two flybys will be after the mission has already knocked out some high-value list items.) Europa Clipper will use the rest of its long, lazy orbits around Jupiter to radio its flyby data back to the ground, and perform secondary science too.
What’s this secondary science of the two probes, you ask? Any given orbit, the non-flyby moons can be observed at low to moderate resolutions, which still has value. Io, certainly, is an interesting target (most volcanically active body in the Solar System), and merits time-series data (looking for changes and cycles). Jupiter’s clouds are swirling as we speak, and at any probe distance there’s something, somewhere to be tracked. Same for Jupiter’s ionosphere, fields, etc.- having two probes allows a deeper picture of what the planet’s EM environment is doing. The planet has a ring system- far dimmer than Saturn’s, which is why we don’t see it in amateur telescopes (or even most professional ones). Viewing from different angles and illuminations tells us of the ring material, even if we can’t actually make out individual ring particles.
And of course, there are Jupiter’s lesser satellites, and lesser lesser moons. Amalthea orbits closely and cannot be flown-by, but the same cameras that can keep tabs on Io can also improve our (still poor) understanding of this body. Same with the irregular moons; Jupiter has a whole flock of relatively tiny, distant objects in its gravity well. Their small sizes and far orbits make them difficult camera subjects, but again the view is poor from Earth too, so why not.
This is the relevance and value that gets the two missions onto this blog. Amalthea is small, oblong, and icy (we’ve estimated its density- very low), like many small bodies. Many of the irregular satellites have inclined, eccentric, even retrograde (backwards) orbits, indicating origin by capture- they are small bodies. Jupiter caught them with its enormous gravity, and turned them into more moons. Directly or indirectly, the secondary satellites tell us of the small-body population in the middle Solar System. And even on Callisto, Ganymede, and to an extent Europa, the cratering record indicates the past population of the middle Solar System.
And let’s not forget that the two craft have to get to Jupiter first…by passing through the Main Belt. Both craft take long, indirect trajectories to Jupiter, using flybys through Earth’s gravity field (plus that of an occasional other planet) to compensate for their lack of sufficient launch vehicle. Had either mission taken a logical, Hohmann transfer to Jupiter, the rocket implied by that trip would be so huge, it would blow the budget and likely get the project cancelled. The gravity-assist option is longer and less obvious, but cheaper and ultimately enabling. Europa Clipper found a good trajectory with minimal flight delay, but JUICE requires eight years and numerous loops around the inner Solar System to boost itself that far outward.
Here awaits opportunity- and I don’t mean multipoint observations coordinated with a Mars rover, but that’s happened too. There’s precedent. Galileo- the predecessor mission, not the titan of science- was compelled by NASA policy to search for any asteroid it might observe along its similar, looping cruise phase. The result was our first “mission” (in deed if not in name) to an asteroid, and it was a two-for-one. Cassini followed (even if its flyby was far and low-res), and Rosetta, an explicit comet mission, still bagged two asteroids on its way out. (An instrument on Rosetta even took data on the Deep Impact collision, from a second point of view.) Even NEAR Shoemaker, designed from the start as an asteroid probe, found a second one to pass by. New Horizons found no secondaries it could target… beforehand. After launch, a Kuiper Belt Object was discovered, roughly along the post-Pluto trajectory, and tracked well enough for the probe to make a small course correction burn. New Horizons, designed to study Pluto even at high speed, was given a good, sharp camera (LORRI- LOng Range Reconnaissance Imager). This let it still do observations, beginning rather early as probe missions go, and continuing rather late.
JUICE, and now Europa Clipper, are given somewhat LORRI-like, telescopic instruments. So is Lucy- in fact, an instrument named L’LORRI. This is no quadruple-accident; all must deliver results even in nonideal flybys, as I mentioned. The same telescopic grasp means better capabilities for secondary asteroids; certainly better than the Galileo camera, or even the Cassini second-generation version. JUICE, EC, and of course Lucy also have complementary instruments, mainly a near-infrared imager. The resolution is poorer, but extending the new color data into wavelengths past the human eye allows a deeper understanding of composition and class. This complement includes some lines/bands we have issues with on the ground, due to scattering and noise from Earth’s atmosphere.
So far as has made print (or e-print), I can’t see any explicit targets given. JUICE, for one, keeps repeating ‘in consideration’ or similar. The possibility of (223) Rosa was mentioned multiple times, but consideration is not commitment: no plan is, de facto, a no-plan. Still, there are over one million small bodies (that we know of), while the discovery rate is still high (even before Vera Rubin, along with others like WFST, Flyeye, plus Euclid). JUICE and EC have nontrivial reach, longer than Cassini. And we’ve got some leeway until 2031.