The XGRS (X-Ray/ Gamma Ray Spectrometer) detector is a gas filled proportional counter measuring X-ray fluorescence spectra reflected from the surface of the asteroid Eros. The excitation source is the Sun, which produces sufficiently intense X-ray radiation to enable analysis of low atomic weight elements like Mg, Al and Si.
Oxford Instruments Analytical Oy supplied NASA’s NEAR (Near Earth Asteroid Rendezvous) mission with four XGRS detectors. Three of these measured the elemental composition of the asteroid surface and the fourth monitored the X-rays emitted by the Sun.
NEAR exceeded expectations and was the first spacecraft in history to land on the surface of an asteroid!
The NEAR spacecraft was launched on February 17, 1996 from Cape Canaveral Florida. It reached Asteroid Eros’ orbit after four years on February 14, 2000. NEAR orbited the asteroid for one year measuring the composition of the surface and physical properties of Eros. NEAR was successfully steered to land on the surface of Eros on February 12, 2001, from where it still sent measurement results to the Earth.
More about NEAR project:
JHUAPL NEAR Project
HowStuffWorks - Project NEAR