While one of curium's isotopes has a very long half-life relative to its neighbours – Cm-247 at 15.6 million years – it is not the one most often used. That title goes to Cm-244, the source for alpha particle X-ray spectrometers on Martian rovers like Curiosity, which is why Rainbow Dash is here. She is perching over Rocknest 3, which was examined by Curiosity's APXS in October 2012.
