Didymos and its small moon Dimorphos are household names in the space community for a dramatic reason. In September 2022, NASA’s DART spacecraft slammed into Dimorphos, shortening the moonlet’s orbital period and demonstrating, for the first time, that humanity can alter the trajectory of an asteroid. Hera’s job is to survey the aftermath in exquisite detail, and JuRa is central to that effort. The DART experiment proved that a kinetic impactor can move an asteroid, but how well it works depends entirely on what the asteroid is made of and how it is assembled. A monolithic boulder would respond very differently to a hit than a loose pile of gravel barely held together by its own gravity. Right now, nobody knows which description fits Dimorphos, and JuRa is designed to find out.

The physics behind the measurement is elegantly simple in principle and fiendishly complex in practice. Every radar measurement is governed by the complex dielectric permittivity of the materials the wave traverses. The real part of the permittivity controls wave velocity, while the imaginary part, expressed as the loss tangent, governs how quickly the wave dissipates. JuRa transmits at 60 MHz with a 20 MHz bandwidth, corresponding to a wavelength of five meters in free space. At that wavelength, sand grains and gravel simply do not scatter the signal, so the wave propagates as if through a continuous medium. But blocks of rock several meters across, separated by voids, scatter the wave in all directions, destroying its coherence. That scattering regime is exactly what scientists expect inside a rubble pile, and it is precisely what JuRa will map: the returned power, quantified as a backscatter coefficient, becomes a proxy for the size, contrast and arrangement of the constituent blocks and voids.

Turning raw echoes into a three-dimensional picture of the asteroid’s guts is where the engineering wizardry comes in. Because JuRa’s crossed-dipole antennas have almost no directivity, a single acquisition illuminates nearly the entire asteroid at once, mixing echoes from many locations that arrive simultaneously. Only the diversity of observing geometries along Juventas’ orbit allows image reconstruction, in a small-body analogue of Synthetic Aperture Radar processing. The formalism is described by the Ewald sphere: each measurement samples one shell of the three-dimensional spectrum of the asteroid’s permittivity-contrast map, and an inverse transform can, in principle, reconstruct the interior. In practice, the fast spin of Didymos, the tumbling of Dimorphos, surface refraction and severe undersampling of the orbit geometry mean that standard SAR shortcuts fail. The JuRa team has therefore built a brute-force, fully three-dimensional processor that evaluates the geometry separately for every point of the imaged body, together with advanced diffraction-tomography methods already validated on asteroid analogues in anechoic-chamber experiments in Marseille.

The observing campaign itself is a delicate dance. Juventas will be released by Hera in mid-January 2027, and after commissioning, including the high-stakes deployment of JuRa’s four antenna booms, the CubeSat will settle into a sun-synchronous terminator orbit. From the end of January to the end of March 2027 it will circle the binary system at a radius shrinking from about 3.3 kilometers to about 2 kilometers, sharing data with Hera through an inter-satellite link whose bandwidth imposes strict limits on telemetry. Scientists expect JuRa to penetrate 100 meters or more below the surface, deep enough to characterize the rubble-pile structure of both bodies. Then, in a fittingly audacious finale, Juventas will land on Dimorphos at the end of March 2027, and JuRa may operate during descent and possibly afterward, using the round-trip propagation time through the moonlet for an absolute measurement of its average permittivity.

What the team hopes to learn is specific and ambitious. For Dimorphos, JuRa aims to reveal the aggregate structure at scales from roughly 20 centimeters to 20 meters, the expected size range of constituent blocks and voids, and to detect whether fine dust or gravel fills the gaps, which would discriminate between macro- and micro-porosity. Dimorphos is thought to be more porous than Didymos, with a macro-porosity near 35 percent and an estimated mean density of 2400 kilograms per cubic meter, and its pre-impact surface showed an apparently uniform accumulation of boulders with no obvious fine debris. JuRa will test whether that homogeneity extends into the interior or whether layers, large blocks, sub-aggregates or compaction zones hide beneath. Such information will directly constrain models of the DART impact, though the team notes that the instrument’s expected 10 to 15 meter resolution will not resolve the local mass redistribution near the crater itself.

Didymos tells its own story. Likely a rubble pile assembled from the debris of a collisional disruption of the Baptistina family’s parent body some 140 to 320 million years ago, it spins once every 2.26 hours, dangerously close to its disruption spin barrier. Its top-shaped profile, with a smooth equatorial ridge and rough, boulder-strewn polar regions, suggests a cohesive interior of roughly 10 pascals covered by weakly cohesive regolith that is progressively shed to feed the moonlet. Comparing the radar textures returned from the primary and the moon will reveal whether segregation processes operated during Dimorphos’ formation, helping scientists distinguish between gradual mass shedding and catastrophic fission, and testing how binary asteroids are born and how stable they remain.

Building a radar that fits inside one unit of a six-unit CubeSat required an unconventional development philosophy. The payload, developed between July 2020 and its delivery to the platform in July 2024 by a consortium led by EmTroniX in Luxembourg, TU Dresden, IPAG in Grenoble and the Polish antenna specialist Astronika, embraces the ‘New Space’ approach: commercial automotive-grade electronics, carefully tested for radiation sensitivity, triple-modular redundancy for critical functions, and deliberately limited redundancy elsewhere. Instead of the classical chirp of planetary radars, JuRa transmits binary phase-shift-keyed codes, including a 13-symbol Barker code compressed by optimal filters, granting the versatility to operate at distances from tens of kilometers down to a ground-penetrating-radar mode on the asteroid’s surface. Full linear polarization, using two independent dipoles, two transmit and two receive channels, provides both partial redundancy and the radiometric accuracy needed for coherent tomographic processing. Cruise commissioning has already shown the instrument behaving well, with noise levels even lower than in laboratory tests.

JuRa’s success will matter far beyond one asteroid. The same team is already adapting the design into the Apophis Radar for ESA’s RAMSES mission, launching in April 2028 toward the near-Earth asteroid Apophis. And for planetary defense writ large, the stakes could hardly be higher: if a hazardous asteroid ever needs deflecting, emergency planners will need to know whether they are pushing a coherent rock or rearranging a cosmic gravel heap. In early 2027, a radar smaller than a cereal box will give humanity its first honest look inside the building blocks of the solar system, and possibly its best insurance policy yet.

Subject of Research: Radar sounding of the internal rubble-pile structure of the binary asteroid system Didymos and Dimorphos by the Juventas Radar on ESA's Hera mission.

Article Title: JuRa: The Juventas Radar on Hera Mission to Probe Internal Structure of Didymos and Dimorphos Asteroids

Article References: Herique, A., Plettemeier, D., Rogez, Y., Berquin, Y., Ciarletti, V., Fa, W., Eyraud, C., Haynes, M., Heggy, E., Kechouindi, A., Kobayashi, T., Kofman, W., Laabs, M., Nolbert, D., Pettinelli, E., Pursiainen, S., Rochat, S., Schmidt, C., Virkki, A., … Ulamec, S. (2026). JuRa: The Juventas Radar on Hera Mission to Probe Internal Structure of Didymos and Dimorphos Asteroids. Space Science Reviews, 222(7), Article 74. https://doi.org/10.1007/s11214-026-01324-4

Image Credits: AI Generated

DOI: 10.1007/s11214-026-01324-4

Keywords: Hera mission, Didymos, Dimorphos, JuRa, Juventas CubeSat, radar tomography, asteroid internal structure, rubble pile, DART impact, planetary defense, synthetic aperture radar, ESA