On Sept. 15, 1968, the Soviet Union launched the Zond 5 spacecraft on a mission to loop around the moon and return to Earth. There were no cosmonauts aboard, but there were several smaller vertebrates — including the first two tortoises to ever go to space! Other passengers included wine flies, mealworms, plants and seeds. These were the first animals in deep space and the first to travel around the moon. Photos: Pioneering Animals in SpaceThe Soviet Zond 5 rentry capsule in the ocean (left), Soviet scientists measure Zond 5 tortoises after their circumlunar flight in September 1968. (Image credit: RSC Energia)After looping around the moon, the Zond 5 spacecraft turned around and came back to Earth. It was supposed to land in Kazakhstan on Sept. 22, but the spacecraft's guidance system failed, and it splashed down in the Indian Ocean instead. The Soviets did recover the capsule and found that the tortoises and other critters were still alive, although they had lost some weight since the start of the mission.At left: The Soviet Union's Zond 5 mission launches on the first circumlunar flight on Sept. 15, 1968 from Baikonur, Cosmodrome. At right: A view of the Earth from deep space from Zond 5. (Image credit: RSC Energia)Why it matteredIn September of 1968, the Space Race to the moon was in full swing. NASA was just three months away from the historic Apollo 8 mission, which sent the first astronauts around the moon in daring first-of-its-kind mission. But with Zond 5, the Soviet Union scored an early win for the first circumlunar flight that launched from the Earth to the moon, then returned for a landing (or splashdown, in this case). In fact, the flight may have spurred NASA to launch Apollo 8. The Soviet Zond 5 rentry capsule (left) and a declassified U.S. reconnaissance photo of an N-1 moon rocket on the launch pad in September 1968. (Image credit: NASA/RSC Energia/NRO)"The flight of Zond 5 caused concern in the U.S. that the Soviets would place cosmonauts aboard the next Zond and complete the first crewed circumlunar mission. At about this time, a U.S. Corona reconnaissance satellite photographed an N-1 rocket on its pad at Baikonur," NASA wrote in a report, adding that NASA's then-chief James Webb likely saw those intelligence reports. "The apparently successful Zond 5 mission coupled with the photos of the Soviet lunar booster on the launch pad may have contributed to the decision to send Apollo 8 on its circumlunar mission in December 1968, bringing the moon landing one step closer."Zond 5 launched from Baikonur Cosmodrome atop a Proton rocket and took three days to reach the moon. The two tortoises on board were, appropriately, Russian steppes tortoises, NASA has said. After looping around the moon, the critters spent a few more days in transit for the trip home. It was a guidance system error that led the Zond 5 capsule to splash down in the Indian Ocean instead of landing on land as planned. But the landing on Sept. 21 wasn't the end of the road for the Zond 5 tortoises and other animals. The Soviet recovery ship Vasily Golovnin plucked the Zond 5 capsule from the ocean and returned to port in what is now Mumbai, India on Oct. 3, weeks later! They wer ethen flown to Moscow on an Antonov-12 cargo plane before the capsule was finally opened to find the tortoises alive and in good health. You can still view the Zond 5 capsule at the RSC Energia museum in Korolev. Cassini's Grand Finale death dive at Saturn in 2017Artist's illustration of NASA's Cassini spacecraft plummeting through Saturn's atmosphere on Sept. 15, 2017. (Image credit: NASA)A bunch of space traveling tortoises and other animals aren't the only notable space event that happened today in history. On Sept. 15, 2017, NASA's Cassini spacecraft died in a blaze of glory as it crashed into Saturn after 13 years orbiting the ringed planet. NASA ended the mission in a fiery plunge because Cassini was running out of fuel and the space agency wanted to ensure it wouldn't crash into one of Saturn's icy moons, like Enceladus, and contaminate them. They called it Cassini's Grand Finale.Launched in 1997, the $3.26 billion Cassini mission entered orbit around Saturn in 2004 and dropped a European Huygens probe onto the cloudy moon Titan in early 2005. The mission was expected to last until 2008, but was extended twice, ultimately stretching through to September 2017. During its mission life, Cassini discovered a trove if new Saturn moons, found a strange hexagon at the planet's north pole and tracked mesmerizing details of the planet's many rings. "Congratulations to you all," Earl Maize, the Cassini program manager, said to the mission team after the spacecraft signal was lost."This has been an incredible mission, an incredible spacecraft, and you're all an incredible team. I'm going to call this the end of mission."Want more space history? Check out our full On This Day In Space Story archive and watch our On This Day In Space videos on YouTube.