Zond 5

Zond 5 (Russian: Зонд 5, lit. 'Probe 5') was a spacecraft of the Soviet Zond program. In September 1968 it became the first spaceship to travel to and circle the Moon in a circumlunar trajectory, the first Moon mission to include animals, and the first to return safely to Earth. Zond 5 carried the first terrestrial organisms to the vicinity of the Moon, including two Russian tortoises, fruit fly eggs, and plants. The tortoises underwent biological changes during the flight, but it was concluded that the changes were primarily due to starvation and that they were little affected by space travel.

The Zond spacecraft was a version of the Soyuz 7K-L1 crewed lunar-flyby spacecraft. It was launched by a Proton-K carrier rocket with a Block D upper-stage to conduct scientific studies during its lunar flyby.

Out of the first four circumlunar missions launched by the Soviet Union there was one partial success, Zond 4, and three failures. After Zond 4's mission in March 1968, a follow-up, Zond 1968A, was launched on 23 April. The launch failed when an erroneous abort command shut down the Proton rocket's second stage. The escape rocket fired and pulled the descent module to safety. In July, Zond 1968B was being prepared for launch when the Block D second-stage rocket exploded on the launchpad, killing three people, but leaving the Proton first-stage booster rocket and the spacecraft itself with only minor damage.

The Zond 5 mission was originally planned to fly cosmonauts around the Moon, but the failures of Zond 1968A and Zond 1968B led the Soviets to send an uncrewed mission instead, from fear of the negative propaganda of an unsuccessful crewed flight.

Two Russian tortoises (Agrionemys horsfieldii) were included in the biological payload, weighing 0.34–0.4 kilograms (0.75–0.88 lb) each pre-flight. Along with the tortoises, insects and micro-organisms were part of the crew. Soviet scientists chose tortoises since they were easy to tightly secure. There were also two tortoises used as control specimens and four more in a vivarium. Twelve days before launch, the two space-bound tortoises were secured in the vehicle and deprived of food and water; the control tortoises were similarly deprived. The food deprivation was a part of pathomorphological and histochemical experiments. The biological payload also included fruit fly eggs; cells of wheat, barley, pea, pine, carrots and tomatoes; specimens of the wildflower species Tradescantia paludosa; three strains of the single-celled green algae Chlorella; and one strain of lysogenic bacteria. The purpose of sending a variety of terrestrial lifeforms was to test the effect of cosmic radiation on them. However, the test subjects were not analogous to humans, because the choice of life forms were all extremophiles with a substantially higher radioresistance. The Russian Academy of Sciences stated that a mannequin equipped with radiation sensors occupied the pilot's seat.

Kazan Optical and Mechanical Plant had developed the AFA-BA/40 imager, which was installed on the spacecraft, giving it the ability to image the Earth. Zond 5 also contained proton detectors. Zond 5 could transmit some of its data back to ground stations, although data stored onboard and collected after return to Earth has less noise.

Zond 5 launched on 14 September 1968 at 21:42.10 UTC, from Site 81 at the Baikonur Cosmodrome. The thrust of the third-stage rocket was terminated at 160 kilometres (99 mi), which was the start of a 251-second coast. Block D, the upper-stage rocket, ignited and burned for 108 seconds, placing the spacecraft into a parking orbit of 191 by 219 kilometres (119 mi × 136 mi). Fifty-six minutes into the parking orbit the Block D fired a final time for the trans-lunar injection. After this maneuver, the launch was announced to the world. Mission Control discovered a problem with Zond 5's attitude and traced the cause to a contaminated star tracker. Heat caused some of the interior coating to outgas, which delayed an attitude correction on the way to the Moon. The maneuver was performed 325,000 kilometres (202,000 mi) from Earth using the Sun and the Earth as reference points.

On 18 September, the spacecraft flew around the Moon, although it did not orbit it. The closest distance was 1,950 kilometres (1,210 mi). On the way back from the Moon, another star tracker failed. The spacecraft also erroneously switched off the guided reentry system. Eight ships were deployed to the Indian Ocean prior to launch, as a precaution in case the spacecraft could not reach Soviet territory; only three of them had rescue helicopters on board.