The PSR (Pre-Shipment Review) of MEGANE is complete

Article by Ogawa Kazunori, JAXA lead of MEGANE

On August 10, 2023, the Johns Hopkins University (JHU) Applied Physics Laboratory (APL) conducted a Pre-Shipment Review (PSR) for MEGANE, one of the observation instruments onboard the Martian Moons eXploration (MMX) spacecraft.

Group photo taken in front of the MEGANE flight model in the clean room at APL. The instrument on the table in the centre is the MEGANE flight model, and behind this are standing officials from APL, NASA and JAXA. On the far right is the MEGANE Principal Investigator (PI) David Lawrence (APL). On the far left is Brian Schratz (APL), who is leading the technical coordination between APL and JAXA. Eighth from the left is Dennis Harris (NASA), who leads NASA MEGANE, and sixth from the right is the MMX Project Manager Kawakatsu Yasuhiro (JAXA).

MEGANE is the acronym for the “Mars-moon Exploration with Gamma-rays and Neutrons” and is a gamma ray and neutron spectrometer that can measure the energies of naturally produced gamma rays and neutrons. The instrument consists of a gamma ray detector made from high-purity germanium (HPGe), and a neutron detector consisting of two helium-3 filled gas proportional counters (GPCs), and a control unit which has the electronic circuits needed to control these.

Germanium is sensitive to ionising radiation such as X-rays and gamma rays. When a gamma ray interacts the semiconductor crystal of HPGe to which a high voltage has been applied, a quantity of electrons equivalent to the energy of the gamma ray is generated, which can then be detected as an electrical signal. Helium-3 is a stable form of helium with two protons and one neutron in the atom’s core.  Neutrons passing through the GPCs will interact with helium-3 atoms to produce tritium (hydrogen-3: a form of hydrogen with two extra neutrons) and a proton. The positively charged proton causes the surrounded gas to ionise and become electrically charged, which can be measured as an electric pulse where the amplitude is proportional to the neutron energy. The result is a device that can detect and measure the energy of both gamma rays and neutrons.

When galactic cosmic rays, which travel through the cosmos, collide with an airless world such as Phobos, they react with the nuclei of the atoms that make up the moon’s surface and produce neutrons and gamma rays that are released into space. Neutrons and gamma rays are also spontaneously emitted by certain types of atomic nuclei (radioactive isotopes) without an external trigger. These emitted gamma rays and neutron have specific energies that depend on the kind of atomic nucleus from which they were released. As a result, MEGANE can determine the type, ratio and distribution of the atomic elements that make up the surface of Phobos by measuring the energy of the emitted gamma rays and neutrons from the orbit of the MMX spacecraft.

MEGANE is therefore expected to play an important role in the scientific goals for MMX which are to elucidate the origin of the Martian moons, and the history of the evolution of the Martian sphere, as well as assist in selecting a landing site for the spacecraft to gather samples to return to Earth, by being able to gather direct and comprehensive information on the kinds of material that make up Phobos over a wide area of the moon.

A poster of MEGANE was hanging in the lobby of APL and the Japanese team members took a photograph together! The MEGANE logo designed by APL can be seen on the poster. On the left is MMX Project Manager Kawakatsu Yasuhiro, centre is Sub-Manager Ootake Hisashi, lower-right is Machimoto Eru who is in charge of international coordination, and top-right is Ogawa Kazunori (article author) who is in charge of JAXA MEGANE.

As part of the MMX project, the United States agreed to provide a gamma ray and neutron spectrometer as part of the international cooperation between JAXA and NASA. APL then took the responsibility for the development of MEGANE under supervision from NASA. APL is a large research centre in the US based in Laurel, Maryland, about one hour’s drive from the US capital, Washington DC. It is one of the leading institutions in the USA in terms of size and capabilities in the field of developing planetary probes and their onboard equipment. MEGANE was developed at the APL campus, and the PSR was also held on the same site.

The PSR is a review meeting that is held before the flight model of an observational instrument is finally transported and integrated with the spacecraft. In the review, it was confirmed that there were no problems with the integration of the instrument in terms of attributes such as the functionality, performance, test results, quality and documentation. When developing observation instruments to be mounted on a spacecraft, a review meeting such as this one is held at the turning point of each development phase to decide if development can proceed to the next phase. The PSR review meeting is the last of such milestone reviews and indicates that the flight model is now nearly complete. During the COVID-19 pandemic, Japan had to participate remotely at judgement meetings that were held overseas, but this time four members of the JAXA team from Japan were able to participate on-site in the MEGANE PSR.

The MEGANE flight model will undergo final adjustments at APL, and soon will be transported to Japan to be integrated with the MMX spacecraft which is currently being developed at Mitsubishi Electric’s Kamakura Works.

Further information:

MMX News: Spectacles to see a moon: NASA select ‘MEGANE’ instrument for MMX