NASA's next Mars helicopters go looking for ice, and the antenna has to fold on landing

The ground-penetrating radar antenna for SkyFall, the trio of Mars helicopters NASA is preparing as a follow-on to Ingenuity, has cleared a round of testing at the Jet Propulsion Laboratory ^[1][2]. That matters less as an aeronautics milestone than as a resource-survey one: the instrument exists to find frozen water in the top few meters of Martian regolith, which is the layer orbiters cannot resolve ^[2][9].

Ingenuity, which flew from 2021 to 2024, settled the question of whether controlled flight works in a rarefied atmosphere, and showed that an aerial view is useful to surface missions for things like route and site selection ^[5][6]. SkyFall's question is narrower and more operational. Satellites can map large ice deposits a few dozen meters down but are effectively blind to shallow deposits around 5 meters, and shallow is precisely what a crew would need ^[9]. "The only way to detect shallow subsurface ice remotely is to fly close to the ground," Adrian Tang, SkyFall's ground-penetrating radar lead instrument scientist at JPL, said in a NASA press release quoted by phys.org; flying low and slow, he said, could resolve the fine layering where dry soil gives way to ice ^[10].

Close is where the engineering bill arrives. Three vehicles, one larger and two smaller, each carrying four instruments broadly similar to Ingenuity's, will project radar across 500 to 2,500 MHz to depths of several meters, with the long wavelengths doing the penetrating and the short ones resolving the uppermost layers ^[7][11][8]. That is a 5:1 continuous band ^[1]. A conventional antenna for it would be about 48 cm long and would need a clear view of the ground, while minimum clearance between helicopter and surface is about 15 cm ^[12][13] - a factor of roughly three between what the band asks for and what the landing gear permits ^[2]. At 500 MHz the free-space wavelength is about 60 cm, so even the 48 cm baseline is already a compressed radiator ^[3].

The team selected a Vivaldi: flat, curvilinear, back-to-back curved blades, inherently wideband, and manufacturable in sheet metal, printed circuit board or metallized fabric ^[14][15]. The standard version still would not fit under 15 cm, so it was shrunk further, helped by the fact that dry Martian regolith attenuates radio far less than terrestrial soil, so the design could be tuned narrowly for shallow surveying ^[16]. The interesting property is not the geometry but the duty cycle. "It is about 1 1/2 times longer than the helicopter's legs," said Christine Gebara, the ground-penetrating radar mechanical lead at JPL. "During landing, the Vivaldi has to bend out of the way - and if it lands on a rock, it bends even further. But when the helicopter takes off again, the antenna must spring back into place for data collection" ^[17]. With dozens of flights expected, the antenna has to do that repeatedly without losing its in-flight shape ^[17].

The solution is a materials stack rather than a mechanism: polyester and layers of Vectran, the same fabric used in the airbags that cushioned Spirit and Opportunity, plus flexible fiberglass tape springs and a lightweight magnesium mount ^[18][4]. Qualification began with paper designs and mathematical models before moving to hardware ^[19].

What to watch: how many bend-and-recover cycles the tested article actually accumulated, whether gain and impedance stay within budget after deformation, and whether rock strikes rather than flat landings were in the test matrix. The account also flags radio communications as an unresolved challenge for this mission profile ^[20], and gives no launch date or cost ^[4].