A Russian capsule carrying mice, lizards and other small animals returned to Earth on Sunday after spending a month in space for what scientists said was the longest experiment of its kind.

Fewer than half of the 53 mice and other rodents who blasted off on April 19 from the Baikonur Cosmodrome survived the flight, Russian news agencies reported, quoting Vladimir Sychov, deputy director of the Institute of Medical and Biological Problems and the lead researcher.

Sychov said this was to be expected and the surviving mice were sufficient to complete the study, which was designed to show the effects of weightlessness and other factors of space flight on cell structure. All 15 of the lizards survived, he said. The capsule also carried small crayfish and fish.

The capsule's orbit reached 575 kilometers (345 miles) above Earth, according to the news agencies, which said this was far higher than the orbit of the International Space Station.

Russian state television showed the round Bion-M capsule and some of the surviving mice after it landed slightly off course but safely in a planted field near Orenburg, about 1,200 kilometers (750 miles) southeast of Moscow.

"This is the first time that animals have flown in space for so long on their own," Sychov said in the television broadcast from the landing site. The last research craft to carry animals into space spent 12 days in orbit in 2007.

The mice and other animals were to be flown back to Moscow to undergo a series of tests at Sychov's institute, which is part of the Russian Academy of Sciences.