That’s easy: Rockets capable of reaching space. There have been lots of other significant advances in optics, detectors, robotics, communications and so forth. But if you don’t have a rocket that can send your hardware into space, you can’t explore space… simple as that. People like Tsiolkovsky, Goddard and Von Braun are the ones who really made space exploration possible.
Stephen W Squyres is Goldwin Smith professor of astronomy at Cornell University, principal investigator for the Mars Exploration Rover mission, and co-investigator for the Mars Express mission and for the High Resolution Imaging Science Experiment on the Mars Reconnaissance Orbiter.