The goal of MMX is to try and figure out where Phobos and Deimos came from. Did something big hit Mars and knock off a bunch of material, and that material lumped together to form the moons? Or are they captured asteroids of some sort?

"Surprisingly, we have no idea," said David Lawrence, a physicist at Johns Hopkins University serving as the principal investigator for the NASA instrument aboard MMX.

Lawrence told me that if MMX finds lots of volatiles—lighter materials like water, carbon, and sulfur—then this supports the theory Phobos and Deimos are captured, primordial objects. If the moons are bone-dry, they may have come from Mars, and could be a time capsule for early Mars conditions. (In principle, the two moons could have different origins, though Lawrence said this is unlikely.)

MMX will get its answers in two ways: analysis of the Phobos sample back on Earth, and a suite of seven science instruments that will remotely scan the moons. One of those instruments is a NASA-provided gamma ray and neutron spectrometer called MEGANE ("meh-gah-nay"), which means "eyeglasses" in Japanese.

"We're putting on gamma ray-neutron glasses to look at Phobos and tell us its composition," said Lawrence.

Cosmic rays, most of which come from outside the solar system, strike Phobos and produce gamma rays and neutrons. By measuring those particles, MEGANE can determine what elements are present on Phobos. The instrument works best when it is within a one-body radius of its target; for Phobos, this is about 11 kilometers, Lawrence said. MMX will visit Deimos as well, but the spacecraft won't be close enough to get good measurements with MEGANE.

"We need to hang out for a while to get good data," he said.

NASA's most recent Decadal Survey lists a Phobos or Deimos surface exploration mission as a priority under the agency's Discovery program, which funds low-cost planetary science missions. When Japan went looking for an international partner to build a gamma ray and neutron spectrometer, NASA took notice, deeming it a "standalone mission of opportunity" and putting out a call for proposals with a cost-cap of $20 million. The winning proposal came from John Hopkins University's Applied Physics Laboratory, for an instrument styled after spectrometers on the MESSENGER and Psyche missions. The Applied Physics Laboratory will build MEGANE in conjunction with the Lawrence Livermore National Laboratory in California.