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Ko, a 32-year-old computer scientist, was accused of breaking security protocol during training at a Russian space training center, according to the ministry. Ko was named as South Korea's main candidate in September.
"Russia's Federal Space Agency recommended the replacement as Ko repeatedly violated training protocol," Lee Sang-mok, head of the ministry's space technology bureau, told a press conference.
Ko took a book out of the training center without permission and sent it to his home in South Korea. He later returned the book, saying he mistakenly sent it home together with other personal belongings, Lee said. Last month, he again violated regulations by acquiring a spacecraft pilot's instructions he was not authorized to get, which forced the Russian space agency to kick him out.
"The Russian space agency has stressed the importance of abiding by the rules as even small mistakes and disobedience can bring about grave consequences in space," Lee told reporters.
He said Ko seemed "overzealous" in becoming the country's first astronaut, which prompted him to seek to study material that he was not authorized to read. Ko, who was selected from 36,000 candidates, along with Yi, after a series of rigorous physical and mental tests, will remain at the Russian space center in a mission backup role.
Instead of the disgraced male scientist, Yi is scheduled to board the Soyuz craft on April 8, becoming the first space person from South Korea, which is eager to join the space race. She will stay in space for up to eight days aboard the International Space Station, conducting experiments.
The mission will make South Korea the world's 35th country and Asia's sixth to send an astronaut into space. Of the hundreds of people who have gone into space so far, only 49 were women, mostly from the United States.
Yi, currently employed by the Korea Aerospace Research Institute, earned a master's degree in mechanical engineering at the state-run Korea Advanced Institute of Science and Technology. In February, she received her Ph.D. in bioengineering.
The switch of astronaut, which came just less than a month before the planned launch, was very unusual, according to institute officials, who raised doubts about the Russian demand. Four astronauts have previously been dropped before launches, but all of them were rejected due to "medical problems," according to the institute.
"Was Ko's mistake grave enough for him to be removed?" asked one official, speaking on condition of anonymity. "He was selected as the primary candidate after a six-month period of training and tests," he said.
"This incident shows that South Korea has a long way to go to become a space power," he said, citing an ambitious yet stalled project to develop and launch a rocket called the Korea Space Launch Vehicle-I with help from Russia.
South Korea and Russia signed an agreement to cooperate on the KSLV-I project in October 2004, but it has yet to make significant progress due to Russia's reluctance to transfer space technology, according to ministry officials. Seoul originally planned to launch the KSLV-I rocket late last year, but it has been delayed to December this year.
South Korea is stepping up its space projects, highlighted by the launch of a lunar orbiter by 2020 and sending a probe to the moon by 2015, in a bid to follow its rival China. It aims to complete development of a KSLV-II 300-ton rocket at a cost of 3.6 trillion won (US$3.7 billion dollars) within 10 years.
South Korea also plans to launch a large satellite weighing about 100 kilograms every three or four years, and at least two smaller satellites every year. It is also scheduled to open the Naro Space Center, the country's first, on the southern tip of the peninsula this year.
