A Russian capsule filled with 45 mice and 15 newts along with other small animals returned from a month's mission in orbit on Sunday with data scientists hope will pave the way for a manned flight to Mars.
Russian Mission Control said the Bion-M craft landed softly with the help of a special parachute system in the Orenburg Region about 1,200 kilometres (750 miles) southeast of Moscow.
The capsule was also carrying snails and gerbils as well as some plants and microflora. Rossiya state television said not all the animals survived but provided no other details.
"This is the first time that animals have been put in space on their own for so long," said Vladimir Sychov of the Russian Academy of Sciences' institute of medical and biological research.
The TsSKB-Progress space research centre's department head Valery Abrashkin said on the day the mission took off in April that the study was aimed at determining how bodies adapt to weightlessness "so that our organisms survive extended flights."
The space mission has been widely praised by Russian state media as a unique experiment that no other country has yet pulled off.
France's Centre National d'Etudes Spatiales (CNES) space centre said 15 of the 45 mice came from a French research lab that is cooperating with the study.
CNES life science department head Guillemette Gauquelin-Koch said the project took "a further decisive step in human adaptation to weightlessness."
Another flight with other animals would probably take place next year, she added.
"They might be microorganisms," she said.
A field research lab was deployed near to where the capsule landed to quickly test the animals' response to their journey and return to Earth.
Officials said the small menagerie composed of dozens of individual cages would now be flown to Moscow for more tests.
Scientists said the animals were needed because they were subject to the kinds of experiments that are impossible to conduct on the humans who are currently operating the International Space Station (ISS).
They added that the mice would have posed a health risk if simply placed on board the ISS for a month.
The experiment's designers said the tests primarily focused on how microgravity impacts the skeletal and nervous systems as well organisms' muscles and hearts.
The animals were stored inside five special containers that automatically opened after reaching orbit and closed once it was time to return.
Also on board were over two dozen measuring devices and other scientific objects, some of which were stationed outside the capsule to measure radiation levels.
The capsule spun 575 kilometres (357 miles) above Earth.
Russia has long set its sights on Mars and is now targeting 2030 as the year in which it could begin creating a base on the Moon for flights to the Red Planet.
But recent problems with its once-vaunted space programme -- including the embarrassing failure of a research satellite that Moscow tried sending up to one of Mars's moons last year -- have threatened Russia's future exploration efforts.
Russia's trials and tribulations are watched closely by other space-faring nations because the Soyuz rocket on which the animals went up represents the world's only manned link to the constantly-staffed International Space Station.