In the universe of planetary exploration, some experiments are so bold that even Earth’s biggest laboratories can’t hold them literally. As NASA’s Europa Clipper flew by Mars in March 2025, it wasn’t merely taking a gravity shortcut on its billion-mile journey to Jupiter. It was taking a one-time, unrepeatable opportunity to test its most important science instrument: the radar for seeing through ice called REASON.
The gamble was high. REASON, which stands for Radar for Europa Assessment and Sounding: Ocean to Near-surface, is set to see through the thick, mysterious ice shell of Europa and search for hints of a buried ocean an ecosystem that could support the building blocks of life. But first, it had to test its mettle in the harsh conditions of deep space at Jupiter’s moon. Earth-based testing, though thorough, had encountered physical constraints. “To test the ‘echo,’ or the bounceback of REASON’s signals, however, they’d have needed a chamber about 250 feet (76 meters) long nearly three-quarters the length of a football field,” NASA’s Jet Propulsion Laboratory described. Rather, the final hardware was only able to be tested in the sterile environment of an indoor clean room, component by component, never as the fully integrated system.
That’s why the Mars flyby was not just a navigation maneuver it was do-or-die for the engineering of REASON. As the spacecraft hurtled from 3,100 miles (5,000 kilometers) to a mere 550 miles (884 kilometers) above the Martian surface, the radar sent and received signals for 40 minutes, reflecting radio waves off the volcanic plains below. The payoff:60 gigabytes of pristine data, a storehouse that attested to every subsystem performing as intended. “We got everything out of the flyby that we dreamed,” said Don Blankenship, REASON principal investigator at the University of Texas at Austin. “The goal was to determine the radar’s readiness for the Europa mission, and it worked. Every part of the instrument proved itself to do exactly what we intended.”
The REASON radar architecture is as ambitious as its mission. It uses two sets of thin antennas, each stretched out from the spacecraft’s enormous solar arrays, themselves as large as a basketball court required to catch sunlight in the faint region of the Jovian system, where solar radiation is only 1/25th as powerful as on Earth. The antennas, about 58 feet (17.6 meters) long, emit dual-frequency (9 and 60 MHz) signals tailored to investigate shallow and deep subsurface realms. This design aims to take advantage of ice’s high transparency at such frequencies, a concept tested and proven in decades of terrestrial and Martian radioglaciology.
The Mars test was a once-in-a-flight confirmation of the fully integrated hardware of the radar, a situation that could not be replicated on Earth. As Trina Ray, Europa Clipper’s JPL deputy science manager, explained the moment: “The engineers were excited that their test worked so perfectly. All of us who had worked so hard to make this test happen and the scientists seeing the data for the first time were ecstatic, saying, ‘Oh, look at this! Oh, look at that!’ Now, the science team is getting a head start on learning how to process the data and understand the instrument’s behavior compared to models. They are exercising those muscles just like they will out at Europa.”
They are getting those muscles ready just like they will out at Europa. For planetary radar scientists, data from Mars is a treasure trove for sharpening signal processing methods and modeling radar propagation in planetary ice. What has been learned at Mars, where instruments such as MARSIS and SHARAD have shown signal attenuation, dielectric constants, and subsurface structure complexities, is directly applied to strategies for deciphering echoes from Europa’s ice shell.
On Mars, for instance, the observation of bright basal reflections caused controversy about whether they were evidence for liquid brines or solid conductive phases controversies that were finally laid to rest only with careful cross-frequency analysis and modeling of signal attenuation as a function of ice purity, dust concentration, and temperature. Europa’s ice poses similar difficulties. This radar will not only have to differentiate between pure ice, brine inclusions, and the potential ice-ocean interface but also will have to do it considering the moon’s unique thermal and compositional gradients. “Ice is very transparent to radar unless the radar hits something different. That is useful on Earth for measuring glacier and ice shelf thickness, and how they change over time,” said Gregor Steinbrügge, a JPL planetary scientist and REASON investigation scientist.
And at Europa, the stakes are even higher: the radar’s capability to chart the thickness of the ice shell, scout for water pockets, and image the boundary with the underlying ocean could drastically change our concept of habitability elsewhere in the universe.
The Mars flyby success is a critical milestone in a mission that started with the launch of Europa Clipper in October 2024. The spacecraft, now 280 million miles (450 million kilometers) from Earth, will get another gravity assist from Earth in December 2026 before arriving at Jupiter in 2030. From there, it will conduct a series of 40 close flybys of Europa, flying its radar as low as 16 miles (25 kilometers) above the moon’s surface a position that has never before been attained by an ice-penetrating radar in the outer solar system. As Europa Clipper continues on, the engineering achievements of the Mars flyby do more than bring reassurance they set the stage for the mission’s most ambitious scientific objectives. The performance of the radar, confirmed now in deep space, promises to unlock a new frontier in the pursuit of life within our solar system.
