Sensor head on the Mars-bound APXS Instrument
The sensor head on the Alpha Particle X-ray Spectrometer instrument was installed during testing at NASA's Jet Propulsion Laboratory. The instrument is part of the Curiosity rover, which will fly on NASA's Mars Science Laboratory mission. For perspective, the sensor head is 7.8 centimeters, or about 3 inches tall.
Image credit: NASA/JPL-Caltech