Mars Odyssey

Mars Odyssey holds a remarkable distinction: it is the longest-operating spacecraft ever sent to Mars. Launched by NASA in April 2001 and arriving that October, it has spent well over two decades studying the planet and relaying data — a workhorse whose longevity has made it a permanent part of the Mars exploration infrastructure.

Odyssey's most important early discovery concerned water. Using a gamma-ray spectrometer and neutron detectors, it mapped the distribution of hydrogen just beneath the Martian surface, revealing vast quantities of water ice locked in the shallow subsurface, especially at high latitudes. This finding transformed the search for accessible water — the single most critical resource for any future human settlement — and helped guide the choice of landing sites for later missions, including the Phoenix lander, which dug down and confirmed the ice directly.

The spacecraft also mapped the mineral composition of the surface and measured the radiation environment that future astronauts would face on the journey to and stay on Mars. Just as valuable has been its enduring role as a communications relay, forwarding data from surface rovers and landers back to Earth. For missions like Spirit, Opportunity, and Curiosity, Odyssey has been a vital link, extending the scientific return of the entire Mars program.