What is considered the largest and most complex international scientific project in history?

The International Space Station (ISS) is widely recognized as the largest and most complex international scientific project in history due to its collaborative nature involving multiple space agencies, including NASA (United States), Roscosmos (Russia), ESA (European Space Agency), JAXA (Japan), and CSA (Canada). The construction of the ISS began in 1998 and has continued to expand, involving contributions from various countries and a myriad of scientific disciplines.

The ISS serves as a microgravity laboratory where scientific research across various fields, such as biology, human biology, physics, astronomy, and meteorology, is conducted in a unique environment. Its enduring operation and the continuous international collaboration exemplify how nations can work together toward common scientific goals, making it a pivotal element in the history of space exploration and international cooperation.

Other options, while significant in their own rights, do not match the scale, complexity, and collaborative effort represented by the ISS. The NASA Mars Rovers and Voyager I are remarkable achievements in their exploration missions but are more singular in focus, lacking the extensive international cooperation seen in the ISS. The Lunar Gateway is still in development and, while important, does not yet hold the historical status achieved by the ISS.