Europa Clipper Set to Revolutionize Jovian Science

The following is a joint article between Nik Alexander and Scarlet Dominik.

A bold new traveler is in its final preparations for a groundbreaking voyage to Jupiter, ready to carry out a mission that will fundamentally redefine the search for life in our solar system and beyond. NASA’s Europa Clipper mission, a 5 billion dollar Jovian orbiter, will aim to answer questions about one of the most tantalizing candidates for life in the solar system: the gas giant’s icy moon of Europa. This mission, set to launch no earlier than October 13th aboard a SpaceX Falcon Heavy, is the first in a new generation of explorers – ones who will get up close and personal with these worlds to help the agency seek out new life on strange new planetoids. The road to launch has not been easy as the agency has struggled with leveled out budgets and programmatic difficulties, but the excitement in the scientific community is palpable. With Europa Clipper’s European companion JUICE already underway, all that remains for the American team is the countdown to launch.  

On Tuesday, September 9th, 2024, NASA leadership held a press conference to discuss the final decision point for NASA’s Europa Clipper mission. Prior to the September 9th press conference, there had been some discussion of alternate missions and even a potential delay for the mission due to potentially faulty components embedded deep within the spacecraft’s systems. Articles had been published questioning whether the spacecraft would even survive at Jupiter, with radiation tolerances representing a large degree of uncertainty. For several weeks, the mood surrounding the mission was grim – an air of uncertainty threatened to derail this once in a lifetime opportunity to explore this strange world, a mini solar system in its own right. 

In attendance at the September 9th conference was SMD Associate Administrator Dr. Nicola “Nicky” Fox, a longtime champion for the agency’s science and exploration missions, who proudly exclaimed that “Europa Clipper is ready for its five and a half year journey to explore Jupiter’s moon Europa in detail. The highly anticipated flagship science mission is expected to proceed as planned with launch NET October 10th, 2024.”

Program scientist Curt Neiber expressed that the journey to the launchpad had been tremendously difficult, with each year seemingly more so than the last. Despite the challenges, the Europa Clipper team has made significant strides in advancing this historic mission towards launch. Speaking about the team, Neiber reiterated: “Through all those years, the one thing we never doubted was that this is going to be worth it… I was worried that we were asking the impossible of this team… and the team completely delivered… They saved this mission this summer.” With the mission in its final steps before launch, the team can look forward to a journey to Jupiter that seeks to answer some of the solar system’s oldest questions.

Europa has had an important role in the scientific study of the heavens since the birth of modern astronomy. On January 7th, 1610, Galileo Galilei, the father of modern astronomy, first observed what he called the Medicean Stars. The Medicean Stars were a collection of small stars that huddled by and traveled alongside Jupiter through the sky, changing their positions around it from night to night. The realization by Galileo that these stars were “satellites” which orbited Jupiter was one of the first major blows to geocentrism, the long-prevailing model of the universe which placed Earth as its center. Today we know the Medicean Stars by a different name: the Galilean Moons; and the second innermost among them, which Galileo called simply “Jupiter II,” we know as Europa.

The Copernican Revolution, which saw the rapid adoption of heliocentrism as the widely-accepted model of the universe, brought with it a somewhat philosophical concept. The aptly named Copernican Principle assumes that an observer of the universe viewing it from Earth is not standing in a privileged position, but an average one. In cosmology this principle has seen substantial success: the Sun (which Earth is in the immediate proximity of) is not the true center of the universe either, it’s one star of hundreds of billions in a galaxy which is itself one out of hundreds of billions. When applied to other fields, such as physics or chemistry, this principle remains appropriate. So far as we can tell, the laws of physics are still the same on other planets, and chemistry is still governed by the same fundamental rules across the cosmos.

There is a standing exception of sorts to this principle in that Earth is the only place in the universe known to host life (though, truthfully, the Copernican Principle wasn’t established with biology in mind). Ever since Mariner 4 returned the first images from the proximity of Mars in 1965, the first of their kind from any world beyond the Moon, an assumption that the rest of the solar system is “dead” has lingered over scientific understanding. However, discoveries in recent decades are increasingly challenging this idea. Europa once again finds it among the ranks of the worlds responsible for this shift.

Though the icy composition of Europa’s surface was detected as far back as 1972, it would not be until the twin Voyager spacecraft visited the Jupiter System in 1979 that planetary scientists got their first detailed glimpse at Europa’s geology. Europa immediately stood out for its remarkable linear features, aptly named lineae, and its relatively crater-free surface when compared to its neighbors, Ganymede and Callisto. The innermost of the Galilean Moons, Io, is also remarkably crater-free, the reason for which was also discovered by the Voyager probes: Io is heavily volcanically active, and constantly resurfaces itself with fresh material from its mantle. Taken as a whole, one explanation was that Europa may also be geologically active, with ice taking the place of rock, and with liquid water deep below its surface taking the place of magma.

The idea of a stable body of liquid water was a tantalizing prospect for scientists curious about possible biospheres beyond Earth. Even if Europa’s so-called subsurface ocean is quite different from our own, icy moons like Europa are seemingly far more common than worlds like Earth; an environment suitable for life existing below Europa’s surface would have substantial implications regarding the prevalence of life in the universe.

For this reason, decades later when NASA’s Galileo spacecraft, the first spacecraft to orbit Jupiter, received its first mission extension in 1997, it was dedicated to the study of Europa. Across eight close flybys of Europa, Galileo delivered unprecedented data on Europa’s geology, and for the first time detailed maps of how Europa distorts the magnetic field produced by Jupiter. Galileo’s data on how Europa’s interior interacts with Jupiter’s magnetic field lines is considered one of the strongest pieces of evidence in favor of Europa’s subsurface ocean to date. The evidence was so tantalizing that when Galileo’s fuel was depleted and it came time to retire the mission in 2003, the spacecraft was intentionally deorbited into Jupiter in the hopes of preventing possible contamination of Europa by hitchhiking microbes from Earth.

Undeniable, direct evidence of Europa’s subsurface ocean has eluded planetary scientists thus far (though indirect evidence through archival data and computer modeling has only mounted) but it is commonly believed that the detection of cryovolcanic plumes would be a smoking gun for the case. To-date, no spacecraft which has visited the vicinity of Europa has detected an active eruption from its surface. However, there have been several observations of possible plumes made by NASA’s Hubble Space Telescope. More recently, Hubble detected evidence of persistent water vapor in the extremely thin atmosphere of Europa, though only in one hemisphere.

Most recently, NASA’s Jupiter Near-polar Orbiter (Juno) conducted a flyby of Europa, returning the first imagery from its proximity since Galileo. However, Juno is a mission focused primarily on the study of Jupiter and its interior and its instrumentation was not designed with Europa in mind. Nevertheless, the encounter provided detailed pictures of Europa’s surface, including features that provide further support and clues towards the nature of Europa’s active geology. To truly unravel Europa’s mysteries and understand its true potential for hosting life, however, will require a mission wholly dedicated to studying this icy moon.

Ever since Voyager discovered the first clues which pointed towards a potential ocean beneath Europa’s surface, a longing to investigate this moon has persisted among scientists. The excitement towards Europa even permeated in pop culture, with the 1984 film 2010: The Year We Make Contact famously ending with an implication that Europa was inhabited. The early 2000s were littered with ambitious plans to send dedicated spacecraft to assess the habitability of Europa either by orbit or by landing on its surface. The excitement and motivation ultimately culminated in a mission to Europa being the second-highest priority flagship recommendation in the 2013-2022 Planetary Science Decadal Survey after a new Mars rover (which ultimately became the Mars Perseverance Rover). 

The ultimate form of this mission would not be established until 2013, when NASA’s Jet Propulsion Laboratory produced a series of three preliminary studies of varying capability and cost for the purpose of exploring Europa. Europa Clipper was among them, and by far the cheapest. Ultimately, it was decided that Europa Clipper was the most reasonable option to proceed with, balancing risk, cost, and science value.

Europa Clipper, though more efficient cost-wise than other concepts thrown around in the early 2010s, still represents a flagship planetary science mission. Europa Clipper is large: its solar arrays, designed to power the spacecraft and its instrumentation all the way out at Jupiter, give it a twenty-two meter wingspan. At launch Europa Clipper will weigh just over 6,000 kilograms, making it the heaviest interplanetary probe ever built. Europa Clipper carries a payload of nine different instruments designed to characterize the surface, interior, and near-space environment of Europa in unprecedented detail. One such instrument is REASON (Radar for Europa Assessment and Sounding: Ocean to Near-surface), recognizable as a series of spokes jutting from the spacecraft’s twin solar arrays. REASON is a multi-frequency radar suite expected to penetrate up to 30 kilometers into Europa’s icy crust. The radar experiment is  intended to characterize the structure of Europa’s crust, attempt direct detection of Europa’s ocean or possible subsurface reservoirs on Europa, and potentially identify landing sites for a future follow-up mission. Other instruments include the Europa Thermal Emission Imaging System (E-THEMIS), which will attempt to identify regions of cryovolcanic activity, the Europa Imaging System (EIS), which will map 95% of Europa’s surface in unprecedented spatial resolution, and multiple spectrometers for characterizing the composition of Europa’s surface and extremely tenuous atmosphere.

Now ready for its flight, NASA’s Europa Clipper has once again placed the distant Europa in a position to radically shift our perspective of Earth and the wider universe.

Europa Clipper’s journey to Jupiter begins on launch day – no earlier than October 13. Originally scheduled for launch on the 10th, the team at Kennedy Space Center was forced to hunker down in order to avoid damage from the catastrophic Hurricane Milton, which made landfall as a Category 3 storm just days prior. The spacecraft, encapsulated in its payload fairing, will ascend to orbit atop a SpaceX Falcon Heavy rocket from Launch Complex 39A, the same launch pad used to send the Apollo astronauts to the Moon. The vehicle will fly in a unique, fully expendable configuration, with the mission requiring all of the available performance of the rocket to deliver Europa Clipper to its final departure orbit. From there, the Merlin Vacuum or MVac upper stage engine will ignite once more, sending the spacecraft towards the outer Solar System. The mission will use a 5.5-year trajectory to the Jovian system, with gravity-assist maneuvers at Mars (February, 2025) and Earth (December, 2026), before arriving at Jupiter in 2030. The spacecraft’s cruise and science phases will overlap with the European Space Agency’s JUICE spacecraft, which was launched in April 2023 and will arrive at Jupiter in July 2031. Europa Clipper is due to arrive at Jupiter fifteen months prior to JUICE, despite a launch date planned eighteen months later, owing to a more powerful launch vehicle and a faster flight plan with fewer gravity assists.

When Europa Clipper arrives at Jupiter, the mission will enter a dynamic operational phase, living up to its namesake. This “Clipper” trajectory will see the spacecraft in a state of constant flybys at Jupiter, utilizing the gravitational spheres of influence of Jupiter’s moons to bring its orbit down towards Europa. This unique trajectory serves several purposes, notably protecting the spacecraft from the harsh radiation environment of Jupiter. The mission team estimates that the spacecraft will encounter over 2.8 megarads of radiation, and while much of the spacecraft is shielded from this dangerous environment, minimal time spent in these high intensity regions is optimal. This trajectory also allowed for the resolution of the transistor issue encountered prior to launch – negating the concerns of the “tiger team” that convened. Through careful trajectory planning, it was discovered that any decay observed during these high radiation periods would actually begin to heal itself once exposed to a less intense environment, a testament to the mission planners and their thorough review of the concept.

In prior concepts, known as the Europa Orbiter mission, the spacecraft would have spent roughly 30 days in close orbit of Europa, but would likely have its lifetime limited by available onboard propellant and radiation concerns. The mission’s namesake trajectory, however, mitigates a considerable portion of this risk. Through a series of pre-planned flybys, the mission team can minimize time spent in this dangerous realm while still performing close passes of Europa – maximizing science output. This also serves to minimize the energy required to get close to Europa, utilizing the various moons in the system as an efficient highway towards the ice world. The nominal mission will see the spacecraft perform 44 flybys of Europa, utilizing its suite of science instruments to identify potential environments which may support life below the surface. The mission is unique in its scope, in that it aims to act as an orbital scout for places where life could thrive – a type of mission not attempted since the Viking era.

In the conference mentioned in the opening of this article, however, Curt Neiber made sure to emphasize in response to a question that Europa Clipper is not setting out with the goal of definitively finding life on the Jovian Moon. There is no such thing as a simplistic instrument capable of finding life with full confidence, especially not from space. This is a lesson NASA learned the hard way in 1976, when the twin Viking landers touched down on Mars and performed a series of experiments designed to detect the presence of microbial life on Mars. The experiments produced contradictory and ultimately non-conclusive results which remain the subject of controversy to this day. Today NASA’s primary order of operations is to first perform detailed surveys and investigations into the present nature of a targeted world and investigate its possible past ahead of sending missions designed to more precisely approach the question of extinct or extant life. Ultimately, this is Europa Clipper’s purpose: to prove or disprove the presence of an ocean on Europa and assess its depth, distribution, possible energy sources, and perhaps even its chemistry.

One of the best possible manners by which Europa’s habitability could be measured is by direct measurements of material from Europa’s subsurface ocean, which may erupt from Europa’s surface in the form of cryovolcanic geysers. While no spacecraft has observed an active eruption from Europa’s surface while in its vicinity, identifying once and for all the nature of cryovolcanism on Europa and where it might be located is a goal of the Europa Clipper mission. Should these geysers exist, they would present new opportunities for Europa Clipper and hypothetical future missions to Europa to explore and investigate its ocean without need of a landing. It has also been emphasized that even if Europa Clipper is unable to identify active plumes, the wealth of valuable data to be gathered by the mission will remain staggering.

Europa Clipper won’t be the only science mission heading towards Jupiter: ESA’s aforementioned Jupiter Icy Moons Explorer (JUICE) spacecraft is already on its way. Launched in April of 2023, JUICE is a mission with a broader focus within Jupiter’s system, with particular focus on Ganymede, the largest of the Galilean Moons. JUICE will attempt to characterize the potential for habitable environments on Europa, Ganymede and Callisto, before eventually entering orbit of Ganymede for detailed analysis. Both spacecraft have different strengths, and are planned to be operational around Jupiter simultaneously. Teams from both missions meet annually to coordinate operations, in the hopes that when both spacecraft arrive next decade, they’ll be able to maximize returns through cooperation. This continues an important legacy of international cooperation with other nations during interplanetary missions, dating all the way back to the Cassini-Huygens mission to Saturn and its moon Titan. 

In many ways, Europa Clipper can be seen as the first of a rising new generation of outer solar system missions. The Galileo and Cassini missions performed broad-stroke surveys of their respective gas giant systems, exploring a great variety of targets and uncovering new questions along the way. Now missions such as Europa Clipper and Dragonfly are following in their footsteps to hone in on specific questions raised by the missions which came before.

Should Europa Clipper perform well, it will no doubt pave the way for its own successors in the future. The idea of a dedicated Europa Lander has long been discussed in the planetary science community, one which would follow after Europa Clipper. Should Europa Clipper confirm a potentially habitable body of water within Europa, a Europa Lander would take the question further, asking: does Europa host extant life? Of course, this is based on a positive result at Europa, which isn’t necessarily guaranteed. Should Europa show promising signs, however, the outer solar system suddenly becomes an incredibly target rich environment. From moons next door to Europa, to the moons of Saturn, Uranus, Neptune, all the way to Pluto and the Kuiper Belt, it’s believed that a great number of icy bodies may similarly host subsurface oceans of their own. This interest is already brewing, much as it did around Europa back in 2011, with a mission to Saturn’s moon Enceladus now one of the top priorities of the latest Planetary Science Decadal Survey. The so-called Enceladus Orbilander was the third highest priority following Mars Sample Return and Uranus Orbiter and Probe.

Many missions aim to answer questions and pose new ones – Europa Clipper aims to usher in a scientific revolution of its own. For centuries, human beings have stared at Jupiter and wondered what mysteries lurk in this strange miniature model of the solar system – a not too distant analogue for worlds that could exist around other stars. Now, the vehicle stands ready to fly, defiant of weather and other constraints. For the mission team, this is the culmination of decades of work, hundreds of thousands of hours of science and engineering and policy work determined to make this mission happen. The future of Jovian science, in the hands of Europa Clipper, is about to be much brighter.

Edited by Beverly Casillas.