The Mars-moon Exploration with GAmma rays and NEutrons (MEGANE) Investigation for the Martian Moon eXploration (MMX) Mission: Progress Towards Launch

Introduction:  The Mars-moon Exploration with GAmma rays and NEutrons (MEGANE) investigation will use both a Gamma-Ray Spectrometer (GRS) and Neutron Spectrometer (NS) to measure the elemental composition of Mars’ moon Phobos [1].  The MEGANE investigation is funded by NASA to participate in the Martian Moons eXploration (MMX) mission led by the Japanese Aerospace Exploration Agency (JAXA).  MEGANE has three primary science goals, which are:  1) Determine whether Phobos is a captured asteroid or the result of a giant impact; 2) Study surface processes on airless bodies in Mars’ orbit; 3) Support MMX sample return by informing landing site selection and documenting the context of the MMX returned samples.  Specifically, MEGANE will use gamma-ray and neutron spectroscopy to measure the concentrations of the following elements at Phobos’ surface:  H, O, Mg, Si, K, Ca, Fe, and Th.  MEGANE flight hardware has been delivered to the MMX spacecraft contractor, Mitsubishi Electric Corporation (MELCO), has been installed on the MMX spacecraft exploration module, and has completed its first full functional test on the spacecraft.  Here, we present the status of the MEGANE investigation as the MMX mission proceeds towards launch in the fall of 2026.

 MEGANE Instrumentation: MEGANE has three hardware components (Figure 1): the GRS, NS, and the Data Processing Unit (DPU).  The GRS consists of a high-purity Ge (HPGe) gamma-ray sensor surrounded by a borated plastic scintillator that serves as an active anticoincidence shield (ACS) and fast neutron detector.  The NS consists of two ³He sensors that measure thermal (neutron energy E_n < 0.4 eV) and epithermal neutrons (E_n > 0.4 eV).  The DPU provides power, commanding, signal processing and data throughput functions.  The MEGANE flight model (FM) hardware was delivered to JAXA and MELCO in December 2023.  After a series of post-delivery tests to show that the hardware met requirements (Figures 2–4), the three components were handed over to JAXA and installed on the MMX exploration module in the spring of 2024.  

MEGANE Post-delivery Activities:  After the installation of the MEGANE components on the MMX exploration module, a series of increasingly complex tests were carried out to exercise various parts of the MMX (and MEGANE) hardware. These tests culminated with an initial electrical test that exercised the full functionality of MEGANE while installed on the MMX spacecraft with the other MMX instruments operating.  With this test, we demonstrated full instrument functionality that meets performance requirements. This performance includes a demonstration of all commanding capabilities, operation of all sensors, and exercising the complete data pipeline flow that goes from the spacecraft to JAXA to the MEGANE Science Operations Center at Johns Hopkins Applied Physics Laboratory.  During this test, the HPGe sensor had higher than expected leakage current, and we are working various mitigations to reduce this leakage current and its impact to GRS performance. 

The MEGANE hardware is currently participating in various spacecraft environmental tests, including a recently completed thermal-vacuum test, as well as system level vibration and electromagnetic interference tests planned for later in 2025.  Final post-environmental MEGANE testing is planned to take place at the Tanegashima launch site prior to launch in the fall of 2026.  

In addition to spacecraft testing, the MEGANE team is working closely with JAXA colleagues to carry out planning for spaceflight operations of the MEGANE instrumentation [3].  MEGANE composition measurements will be conducted during two separate low-altitude operations phases – one phase prior to landing and one after landing.  The primary composition measurements will be obtained when the MMX spacecraft is at an altitude of less than one Phobos radius.  Other MEGANE operations include background measurements when the spacecraft is at high altitudes above Phobos, charged particle environment monitoring with the ACS and NS sensors, and an initial hardware checkout during the Earth-to-Mars cruise phase. 

 Summary: MEGANE flight model hardware has been delivered to and installed on the MMX spacecraft, and has demonstrated its full operation.  Final operations planning and system testing is ongoing prior to the fall 2026 launch of MMX. 

References: [1] Lawrence et al., Earth and Space Science, 6, 10.1029/2019EA000811, 2019; [2] Peplowski et al., NIM A, 10.1016/j.nima.2025.170542; [3] Kuramoto et al., Japan Geosci. Union Meeting 2025, PPS09-01, 2025.