Tomo Usui on the Mission to the Martian Moon Phobos
Tomo Usui is a professor in the Department of Solar System Sciences at the Institute of Space and Astronautical Science of the Japan Aerospace Exploration Agency. He leads the science team for the next Japanese sample return mission, which will launch in 2024 and return a sample from the surface of Phobos in 2029. He describes how this feat will be accomplished, and what we hope to learn about Phobos, Mars, and the early Solar System.
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Podcast Illustrations
All images courtesy of JAXA unless otherwise noted.
Phobos, taken by NASA’s Mars Reconnaissance Orbiter in 2008. The larger of Mars’s two moons, it is 13 miles across. The most prominent feature is Stickney, the large crater in the lower right. The series of troughs and craters are thought to have formed when material ejected from impacts on Mars later collided with Phobos. It is hoped that a portion of the sample returned to Earth by the Martian Moons Exploration (MMX) mission will include some of this material.
Courtesy of NASA
Artist’s impression of the MMX spacecraft in front of Phobos with Deimos, Mars’s second moon, at upper right.
Courtesy of NASA
The spacecraft consists of propulsion, exploration, and return modules. The total launch mass is 4,000 kg, of which 1,900 kg is propellant. Mounted on the platform supported by the landing gear are two cameras, an infrared spectrometer, a mass spectrometer, a laser ranger (LIDAR), dust sampler, and neutron and gamma ray instrument.
MMX is equipped with two sampling systems - a core sample to access samples 2 or more cm below the surface, and a pneumatic sampler that samples the surface material, which is hoped to include ejecta from Mars.
Artist’s rendering of the rover that will be deployed by MMX by allowing it to free-fall onto the surface from a height of a few meters. The rover, which weighs 25 kilos on Earth, will weigh just 1.5 grams on Phobos.
Courtesy of CNES
Capsule for holding the sample to be returned to Earth. The capsule is 16 inches in diameter and has a heat shield enabling it to survive re-entry into the Earth’s atmosphere. The image shows a capsule from the prior Hayabusa 2 mission to the asteroid Ryugu. The capsules for the Phobos mission will be very similar in shape but larger in size and able to carry 50 grams of sample.
Hayabusa 2 capsule with its parachute as it landed in the Australian desert in December 2020. The capsule, to the right of the parachute, is 16 inches across.
The capture theory of the origin of Phobos posits that Phobos originated as an asteroid farther away from the Sun in the asteroid belt and was captured by Mars when its orbit brought it close to the planet.
The impact theory of the origin of Phobos invokes the collision of a massive body with Mars, which threw off ejecta that coalesced to form Phobos as well as Mars’s second moon, Deimos. By obtaining an isotopic fingerprint of the sample returned from Phobos, we will be able to tell whether Phobos has more affinity to an asteroid or to Mars, and therefore determine which origin theory is more likely.
The capture curation facility is a clean-room laboratory with instruments for analyzing the sample. The capsule is disassembled and opened inside the vacuum chamber shown in the image, where it can be manipulated without risk of contamination.