Space debris raises concerns

Parts of Skylab hit the Australian outback in 1979.

(CNN) -- About two-thirds of Mir is expected to burn up when it re-enters the Earth's atmosphere. The remaining 40-50 tons of the craft is expected to land in the ocean. The debris could fall in fragments as small as 40 pounds or as large as 1 ton.

Russian space officials are aiming for a drop zone about 4,000 miles long and 100 miles wide in the south Pacific Ocean between New Zealand and Chile. Japan would be the last populated area under Mir's intended path. Japan said it planned to send its own experts to the ground control center near Moscow to monitor the situation, although Russian space officials said Japan is under no significant threat.

If the Russian ground controllers lose control of Mir, it could hit anywhere between 51 degrees north latitude to 51 degrees south latitude, an area encompassing most major cities.

Disposal of space debris has generated fears in the past:

•  In 1978, a Soviet defense satellite plunged out of control and left radioactive debris in the Canadian Arctic.

•  In 1979, NASA said its Skylab space station would land in the Pacific Ocean, but parts of it smashed into the Australian outback.

•  In 1991, Mir's predecessor, Salyut 7, fell on the Andes Mountains.

No one was hurt in any of these cases.

The Associated Press contributed to this report.