Which European Space Agency orbiter reached Mars in December 2003?

The European Space Agency orbiter that reached Mars in December 2003 was Mars Express.

Launched on June 2, 2003, Mars Express entered orbit around Mars on December 25, 2003. It was ESA’s first planetary mission and carried instruments to investigate the Martian surface, atmosphere, interior and subsurface. Its radar instrument, MARSIS, helped study buried layers and evidence of water-related materials.

The mission also carried the Beagle 2 lander, which was intended to search for signs of life and study the landing site. Beagle 2 reached the surface but did not fully deploy its solar panels; the orbiter, however, continued a highly productive mission. Mars Express is easy to confuse with Venus Express, another ESA mission based on a similar spacecraft design, or Rosetta, ESA’s comet mission.