Asteroid mining is a potential trillion-dollar industry, but before any prospectors start fitting their mules for spacesuits, surveying is going to be more important than extraction. To help find out if its worth going to a particular asteroid, scientists from Vanderbilt and Fisk Universities, NASA's Jet Propulsion Laboratory (JPL), and the Planetary Science Institute are developing a new gamma-ray spectroscope that's capable of scanning asteroids, moons, and other airless bodies for gold, platinum, rare earths, and other valuable minerals.

The team's work is based on the fact that everything in the Solar System is bombarded around the clock by high-energy cosmic rays. When these strike an object, a cascade of secondary particles fans out beneath the surface. The neutrons in this shower collide with atoms and generate gamma rays, which are supplemented by other gamma rays produced by radioactive decay. These gamma rays then radiate out of the object and into space. Since these rays are a form of electromagnetic radiation like light, they can carry information about substances they pass through, which can be recorded by a spectroscope.

A gamma-ray spectrogram can be read like a visible light spectrogram and, like the latter, it can reveal the presence and concentration of the elements the rays have passed through. The problem is, planetary gamma-ray spectroscopy is slow and requires cryogenic sensors that are impractical to include on spacecraft. What's needed is something with high resolution that's also cheap and works at normal temperatures.

What the team settled on was a new gamma ray detector made from the recently discovered europium-doped strontium iodide (SrI2). This transparent crystal forms the core of the spectroscope, which operates on the principle of a scintillator counter. As gamma rays pass through the crystal, it gives off flashes of light, which are recorded and analyzed.

"The gold standard for gamma-ray spectroscopy is the high purity germanium (HPGe) detector," says Fisk Professor of Physics Arnold Burger. "However, it requires cryogenic cooling so it is very bulky. It also needs vacuum-tube technology so it consumes too much energy to run on batteries. SrI2 isn't quite as good HPGe, but it is more than adequate to do the job and it is compact enough and its power requirements low enough so that it can be used in spacecraft and even placed on robotic landers."

The team says that the device is still in the early days of development and that it will require much more testing as to its robustness and ability to withstand radiation damage before the SrI2-based instrument can be rated for operation in space. But if it pans out, the new spectroscope could be deployed in everything from CubeSats to planetary rovers. In fact, the team has already built a prototype of a CubeSat version of the gamma-ray spectrometer (pictured above) using off-the-shelf components. It weighs only 1 lb (450 g) and consumes about 3 W of electricity, yet can do the job of a full lab system.

"Space missions to the Moon, Mars, Mercury, and the asteroid Vesta among others have included low-resolution spectrometers, but it has taken months of observation time and great expense to map their elemental surface compositions from orbit," says Professor of Astronomy Keivan Stassun. "With our proposed system it should be possible to measure subsurface elemental abundances accurately, and to do it much more cheaply because our sensors weigh less and require less power to operate. That is good news for commercial ventures where cost, power and launch weight are all at a premium."

The team's research was published in SPIE Newsroom.

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