PEDALS to reveal the birth of the Moon

TIME HORIZON 10 YEARS

When did the Moon form, and how has it developed over billions of years? The proposed PEDALS (Passively Expanding Dipole Array for Lunar Sounding) project should be able to teach scientists a lot about this, by mapping from the surface deep underground to depths of several kilometres. To achieve this, the PEDALS hub would either land or drop onto the surface of the Moon, then ‘shape memory’ materials would help unroll each of four rolled ‘tethers’ in a cross pattern to create a dipole antenna array with arms up to 200 metres long. Each tether would in fact include hundreds of dipole arrays able to send radio waves into the ground at different frequencies.

Dipole antennas have the benefit of simplicity in design, with fewer parts that might break – a major advantage when you are far away from Earth. Such arrays have already been used for studying other worlds. In 2018, the Mars Express space probe orbiting Mars revealed a lake some 1.5km under the ice caps of the planet’s south pole: the only evidence so far of liquid water on Mars. That probe’s 40-metre-long array is far smaller than PEDALS, so that the resolution of the proposed Moon recordings would be higher, allowing scientists to understanding the crust’s structure, and to identify any significant subsurface voids.

One key challenge will be landing PEDALS on the Moon’s surface without human assistance. The Moon has no atmosphere, so parachutes offer no solution. Instead scientists are proposing that the array could be folded up and then encased in an airbag-like cushion to be landed or dropped on the Moon’s surface.