SpaceX achieved the first successful touchdown of its prototype Starship rocket during the latest test flight of the next-generation launch vehicle in south Texas on Wednesday, after four previous landing attempts ended in explosions. The feat marked a key milestone for the private rocket company of billionaire tech mogul Elon Musk in its development of a reusable heavy-lift launch vehicle to eventually carry astronauts and large cargo payloads to the moon and Mars. The Starship SN15 blasted off from the SpaceX launch site in Boca Chica, Texas, along the Gulf Coast and reached its planned maximum altitude of 6 miles, then hovered momentarily before flying nose-down under aerodynamic control back toward Earth. Maneuvering itself back into vertical position under rocket thrust as it approached the ground, the 16-story, three-engine vehicle descended to a gentle touchdown on its landing gear. “We are down, the Starship has landed,” SpaceX principal integration engineer …