"The Proton is a heavy rocket, which uses highly toxic heptyl as fuel, whereas Soyuz is a medium-class booster using environmentally friendly fuel - kerosene and liquid oxygen," the spokesman said, adding that the two rockets are also produced by different plants - in Moscow and in Samara, respectively.

He said the preparation for the launch, set for September 14, is ongoing.

The Foton-M is a joint project of the European Space Agency and the Russian Federal Space Agency, and is primarily used "for physics and materials science experimentation in weightlessness."

The chairman of the Kazakh National Aerospace Agency said the Russian government failed to send a special commission to visit the site of the crash.

Talgat Musabayev said only members of an expert team from the upper house of the Russian parliament, the Federation Council, were working at the scene.

Kazakhstan said last Friday it wants to ban rocket launches from its Baikonur space center, which Russia rents, whenever the Kazakh president is near the launch site.

The Proton-M rocket, which was launched from the Baikonur space center at 2:43 a.m. Thursday Moscow time (10:43 p.m. GMT Wednesday), experienced an engine malfunction and second-stage separation failure 139 seconds into its flight, and came down in the central Kazakh steppe, 50 kilometers (30 miles) southeast of the town of Zhezkazgan.

Possible environmental contamination from the booster's highly toxic fuel was a particular concern at the time, and a team was been sent to the crash site to determine the extent of any pollution.

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