Ares IV was a Terran spaceship which, in 2032, was part of Humanity's fourth manned mission to Mars, following the Ares III landing in 2031. The mission was under the direction of the International Space Agency, in conjunction with NASA. It was commanded by Lieutenant John Kelly, with mission specialists Rose Kumagawa and Andrei Novakovich.
On October 19, 2032, while Kelly was alone in Martian orbit and Kumagawa and Novakovich were on the surface, Ares IV encountered turbulence in orbit, and Kelly reported an unidentified object approaching. Contact was lost with Ares IV at 0922 hours. Three weeks later, when Ares V reached Mars to retrieve Kumagawa and Novakovich, there was no trace of the Ares IV command module and it was assumed lost. (VOY episode: "One Small Step"; Star Trek Magazine Issue 162: "Blast Off!")
In 2376, the USS Voyager encountered a graviton ellipse in the Delta Quadrant, in which they discovered the remains of the Ares IV command module and of John Kelly, solving the centuries-old mystery of its disappearance. (VOY episode: "One Small Step")