LLNL gamma-ray sensor en route to Phobos aboard Japan's MMX mission

A Lawrence Livermore National Laboratory germanium gamma-ray sensor launched Oct. 19 (Oct. 20 Japan time) on an H3 rocket from the Tanegashima Space Center, bound for Phobos to help select the landing site where Japan's Martian Moons eXploration spacecraft will collect samples for return to Earth in 2031.

The detector is LLNL's third germanium sensor to fly in space, following instruments on NASA's MESSENGER mission to Mercury, launched in 2004, and NASA's Psyche mission to a metal-rich asteroid, whose sensor is still in flight. A fourth sensor from the same detector family is scheduled for Titan, Saturn's largest moon, aboard NASA's Dragonfly rotorcraft lander.

The MMX sensor is a component of the Mars-moon Exploration with GAmma rays and NEutrons (MEGANE) instrument, the spectrometer NASA contributed to the mission and built in partnership with the Johns Hopkins Applied Physics Laboratory. MEGANE — pronounced meh-GAH-ney, the Japanese word for "eyeglasses" — is one of 13 instruments on MMX. LLNL describes the mission as the world's first sample return to the Martian sphere; the spacecraft will also observe Deimos, Mars' smaller second moon.

What science question does MEGANE address?

"The primary goal of the mission is to understand the mysterious origins of Phobos and Deimos," said Morgan Burks, the LLNL physicist who developed the sensor with Geon-Bo Kim and Nathan Hines, in a statement. "There are two competing theories: one is that they are captured asteroids and the other is that they were ejected from Mars."

The evidence is split. According to LLNL, remote sensing data show Mars' moons resemble the carbon-rich material found in asteroids in the outer belt. But captured objects would likely orbit elliptically, and Mars' moons instead follow near-circular, equatorial paths — an orbital signature that points toward formation from an impact on Mars, the same mechanism thought to have formed Earth's moon. Elemental composition measurements are the discriminating dataset.

How does the instrument read chemistry from orbit?

Germanium gives planetary scientists the sharpest gamma-ray measurements of a surface's chemistry, but it only works at extremely cold temperatures. The MEGANE sensor operates at 90 kelvins, or minus 183°C.

On an airless body like Phobos, galactic cosmic rays — high-energy particles from outside the solar system — constantly strike the surface and knock neutrons loose from atoms in the soil. Those neutrons strike other nuclei, which shed energy as gamma rays at energies specific to each element. The sensor records the energy of every gamma ray to reconstruct the elemental makeup of the body.

Keeping germanium cold in space requires a mechanical cooler. LLNL counts its Psyche instrument as the fourth generation of its germanium spectrometers, after Cryo-3, the MESSENGER sensor and GeMini, a compact field spectrometer in which engineers paired an ultraminiature mechanical cooler with infrared shielding to reach cryogenic temperatures without liquid nitrogen — a cryogen that is difficult to use outside a lab. The Psyche and MMX instruments also carry cryocoolers.

Budget and mass limits shaped the MMX design directly. "MEGANE faced tighter resource constraints on MMX than the team's instrument aboard Psyche," Burks said. "This impacted every decision from our choice of a low-mass, low-power cryocooler to radiator design."

What did the Psyche flyby prove?

The Psyche mission, launched in October 2023, will study its asteroid's composition to better understand the cores of rocky planets like Earth. After 2.6 years in deep space, the LLNL sensor ran during a Mars gravity assist on May 15, a slingshot maneuver that carried the spacecraft past the planet at more than 12,000 mph within 2,864 miles of the surface.

NASA said that altitude was too far to measure gamma rays from Mars. Burks nonetheless called the flyby "an invaluable dress rehearsal" that let the team test "all aspects of the instrument performance and data pipeline," according to an LLNL report published in July. The flyby data will help gauge the sensor's sensitivity to iron, nickel, silicon and sulfur — elements the team expects to find at Psyche. The spacecraft arrives at the asteroid in August 2029 and will orbit it for at least 26 months.

How do Titan and Phobos differ as targets?

On Dragonfly, the LLNL detector will exploit Titan's ambient cold for passive cooling rather than a cryocooler while measuring the surface's elemental composition. Titan's thick atmosphere blocks cosmic rays, so the instrument will carry its own radioactive neutron source to trigger the gamma rays. Dragonfly launches on a SpaceX Falcon Heavy in July 2028 and reaches Titan in 2034.

MMX enters Mars orbit in August 2027 and transfers to Phobos the following month, with the sample return capsule set to land on Earth in 2031.