Which SpaceX product launch in 2015 marked the first successful landing of an orbital class rocket's first stage, revolutionizing space technology economics?

In the realm of space exploration, certain product launches redefine what's possible in aerospace technology. SpaceX has been at the forefront of commercial space innovation, but do you remember which specific product launch marked their historic breakthrough in reusable rocket technology? This question tests your knowledge of pivotal moments in scientific product development that changed the economics and possibilities of space travel forever.

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Falcon 9 Flight 20, delivering Orbcomm OG2 satellites and landing at Landing Zone 1
Falcon Heavy test flight carrying Elon Musk's Tesla Roadster
Dragon 2 spacecraft's first unmanned test flight to the ISS
Starlink satellite constellation initial deployment mission