A Close Encounter With a Mysterious Moon

Moons of Mars

Mars is kept company by two cratered moons—an inner moon named Phobos and an outer moon named Deimos. Credit: NASA/JPL-Caltech/University of Arizona

Asaph Hall, an American astronomer, discovered two small moons circling the planet ESA’s Mars Express spacecraft made with Phobos in the run-up to Halloween this year. The recent flyby of the larger Martian moon offered the perfect opportunity to test one of the 19-year-old spacecraft’s latest upgrades.

Mars Express HRSC Image of Phobos

The High Resolution Stereo Camera (HRSC) onboard the ESA spacecraft Mars Express took this image of Phobos using the HRSC nadir channel on March 7, 2010, HRSC Orbit 7915. This image has additionally been enhanced photometrically for better bringing features in the less illuminated part. Resolution: about 4.4 meters per pixel. Credit: ESA/DLR/FU Berlin (G. Neukum), CC BY-SA 3.0 IGO


The MARSIS (Mars Advanced Radar for Subsurface and Ionosphere Sounding) instrument on Mars Express was originally designed to study the internal structure of Mars. As a result, it was designed for use at the typical distance between the spacecraft and the planet’s surface, which is more than 250 km (155 miles).

However, it recently received a major software upgrade that allows it to be used at much closer distances. This new capability could help to shed light on the mysterious origin of the moon Phobos.

Water Under Martian Surface

Artist’s impression of water under the Martian surface. Credit: Illustration by Medialab, ESA 2001

“During this flyby, we used MARSIS to study Phobos from as close as 83 km (52 miles),” says Andrea Cicchetti from the MARSIS team at INAF. “Getting closer allows us to study its structure in more detail and identify important features we would never have been able to see from further away. In the future, we are confident we could use MARSIS from closer than 40 km (25 miles). The orbit of Mars Express has been fine-tuned to get us as close to Phobos as possible during a handful of flybys between 2023 and 2025, which will give us great opportunities to try.”

“We didn’t know if this was possible,” says Simon Wood, Mars Express flight controller at ESA’s ESOC operations center, who oversaw the upload of the new software to the ESA spacecraft. “The team tested a few different variations of the software, with the final, successful tweaks uploaded to the spacecraft just hours before the flyby.”

Mysterious origins

MARSIS, famous for its role in the discovery of signs of liquid water on the Red Planet, sends low-frequency radio waves toward Mars or Phobos, using its 40-meter-long (130-foot-long) antenna.

Most of these waves are reflected from the body’s surface,…

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