Using X-rays to Map an Asteroid

NASA's OSIRIS-REx spacecraft will launch September 2016 and travel to the near-Earth asteroid Bennu to harvest a sample of surface material and return it to Earth for study. But before the science team selects a sample site, they can find out a bit about Bennu's elemental make-up.

To determine the composition of Bennu's surface, the Origins, Spectral Interpretation, Resource Identification, Security-Regolith Explorer (OSIRIS-REx) team equipped the spacecraft with an instrument that will identify which elements are present on the asteroid and measure their abundance.

The Regolith X-ray Imaging Spectrometer, or REXIS can image X-ray emission from Bennu in order to provide an elemental abundance map of the asteroid's surface.

"REXIS is different from the other imaging instruments on OSIRIS-REx because we're going to determine what Bennu is made of at the level of individual atomic elements," said Richard Binzel, REXIS principal investigator and instrument scientist at the Massachusetts Institute of Technology (MIT), Cambridge. "We're sniffing the atoms on the surface of Bennu."

To do that, REXIS gets a little help from the sun. Atoms on Bennu's surface absorb incoming solar X-rays that are emitted along with the solar wind. This causes electrons in the atom to move to a higher energy level. However, because these excited electrons are unstable, they quickly de-excite and drop back down to their original energy level and emit their own X-ray in turn. This process is known as fluorescence.

"You have all this energy coming in, and it kicks electrons up to the next energy level, but the electrons quickly decay back down and emit X-rays of precisely that same energy," said Josh Grindlay, REXIS co-principal investigator and deputy instrument scientist at Harvard University, Cambridge, Massachusetts. "The net result is a glowing surface on Bennu."