Russia's space agency has so far failed to contact its $13-million unmanned Phobos-Grunt Mars probe, which it lost contact with immediately after launch early on Wednesday -- and may end up as the 18th in a long string of unsuccessful Russian missions to the Red Planet.

"So far all efforts to communicate with the craft have been unsuccessful," lead mission scientist Alexander Zakharov of Moscow's Space Research Institute told Reuters.

The probe is stuck in a dangerously low orbit, officials said, and if uncontacted could eventually crash down to Earth. Should the craft fail, it will join a surprisingly long string of Russian missions to Mars that have failed, even as U.S. rovers have logged hundreds of hours on the Red Planet. Meanwhile, NASA plans to launch a $2.5 billion rover designed to assess the planet's suitability for life later this month, toward the end of a launch window for Mars flights that comes every 780 days.

If Phobos-Grunt eventually crashes back to Earth, its cargo may become a problem: The craft carries a full payload of toxic hydrazine and nitrogen tetroxide fuel and small cargo of radioactive cobalt-57.

It is unclear how much of it will survive the fiery plunge through the atmosphere.

17 failed missions to Mars since 1960



* 1960 Korabl 4 Didn't reach Earth orbit

* 1960 Korabl 5 Didn't reach Earth orbit

* 1962 Korabl 11 Earth orbit only; spacecraft broke apart

* 1962 Mars 1 USSR Radio Failed

* 1962 Korabl 13 Earth orbit only; spacecraft broke apart

* 1964 Zond 2 Radio failed

* 1969 Mars 1969A Launch vehicle failure

* 1969 Mars 1969B Failure Launch vehicle failure

* 1971 Kosmos 419 Achieved Earth orbit only

* 1971 Mars 2 Orbiter arrived, but no useful data and Lander destroyed

* 1971 Mars 3 Orbiter obtained approximately 8 months of data, returned only 20 seconds of data

* 1973 Mars 4 Flew past Mars

* 1973 Mars 6 Occultation experiment produced data and lander failure on descent

* 1973 Mars 7 Missed planet; now in solar orbit.

* 1988 Phobos 1 Orbiter lost en route to Mars

* 1988 Phobos 2 Lost near Phobos

* 1996 Mars 96 Launch vehicle failure

(Source: NASA, Missions to Mars)